DOI: 10.31038/AFS.2021324

Abstract

An analysis of nutrient flow, based on nitrogen (N) and phosphorous (P), was conducted on an integrated rabbit–fish–rice system (IRFR) system at the Rwasave Fish Farming Station (National University of Rwanda). Rabbits, stocked at 12 per are (1200 rabbits.ha^-1) of pond, were reared over fishponds stocked with one and three male tilapia (Oreochromis niloticus) per m² for pond treatments PT1 and PT2. Effluent fertilised by the rabbits was drawn away from the ponds by pipes installed at the bottom of the ponds to irrigate rice (Oryza sativa L., variety Yuni Yin4) fields. There were six 400 m² ponds and nine 90 m² rice fields; three of the latter were irrigated by canal water and fertilised by NPK (200 kg.ha^-1.crop^-1, 2 applications; 100 kg.ha^-1.crop^-1, one application).

The results showed that rabbit droppings supplied about 27% N and 79% P of the total N and P inputs, fertilising the ponds at a rate of 3.98 kg N and 1.94 kg P.ha^-1.d^-1. The fish recovered 18.5‑7.6% N and 16.9-34.3% P of the total nitrogen (TN) and total phosphorus (TP) inputs. All water quality variables remained within good limits for tilapia aquaculture and nutrient distribution was not dependent on fish density. Large amounts of N and P accumulated in the water, sediment, and effluent fertilised rice fields at a higher rate (118.5 kg N and 27.2 kg P.ha^-1.d^-1) than that of inorganic fertilizers, resulting in a slightly higher rice yield than that induced by NPK and urea.

Tilapia effluent was thus able to substitute inorganic fertilisers completely, allowing savings to the farmers, and showing its potential as a fertiliser for fish and crop production rather than waste to be discharged, polluting the environment by its solids and organic matter component. Further studies involving a thorough analysis of nutrients lost and diversified uses of the nutrient-rich effluent are needed.

Keywords

Integrated aquaculture-agriculture system, Nutrient-rich effluent, Rabbit-fish-rice system, Earthen ponds

Introduction

During recent years, aquaculture has developed worldwide for the purpose of addressing food insecurity and income generation. In aquaculture, ponds are dynamic ecological systems that continuously process and remove large quantities of nutrients and organic material [1]. This has led to large quantities of pond nutrients (total solids and organic matter) being discharged, generally into natural water bodies, where they constitute a major source of water pollution, especially in semi-intensive and intensive aquaculture systems in countries where farmers lack effluent management techniques.

Earlier aquaculture enterprises are either extensive or semi-intensive fish farms, integrated to agriculture (crop and/or livestock) or not (that is stand-alone enterprises). The later are intensive systems or recirculated aquaculture systems (RAS) which, when operating at larger scale, are risky ventures and are not suitable for resource-poor farmers in developing countries [2]. They require formulated fish feeds and operate using high level energy, and high investments. Subsequent to that, these systems cause high risks including water quality deterioration through accumulation of nutrients in water and pond bottom soil, diseases, low profit margin, and lead to impacts such as pollution, environmental destruction, and reliance on pelleted feeds [3]. Extensive and semi-intensives aquaculture rely on fertilsation and use crop by-products for supplements to fish feeds. These are often Integrated agriculture-aquaculture (IAA), less risky systems because they benefit from synegisms from constituent enterprises, have a diversity in produce and environmental soundness [4,5].

The nutrient budget of fish ponds in conventional aquaculture explains that only small portions of inputs are recovered in fish biomass and in water columns, with the largest amounts lost in pond bottom mud [6-8]. During fish harvest by the draining of the ponds, nutrients in the upper layer of mud are carried away in the effluent and released into rivers. The most important nutrient in fertilised fish ponds are N and P which, in key concentrations, are limiting for phytoplankton growth. N and P, in intensive aquaculture systems are also reported the two main pollutants of water [9,10].

The feeding and fertilisation of fish ponds, with fish feed and fertilisers always result in the accumulation of nutrients in the form of fish waste and other organic matter. In this regard, observations made of channel catfish (Ictalurus punctatus) and hybrid catfish (C. macrocephalus x C. gariepinus) ponds, reported an increase of most water quality variables, including nutrients, total solids, organic matter, and a high 5-day biochemical oxygen demand in the remaining 25% of effluent when fish ponds are being drained [11-13]. This last part of pond effluent is potentially harmful to the environment as it often contains more than 50% of the total load of nutrients [14], 35.5% TN and 10.4% TP [15].

However, the aquaculture wastewater, because of its load of nutrients, could be seen as a potential fertiliser for fish farming and agriculture production [16] more generally in integrated agriculture-aquaculture (IAA) systems [2,17] rather than a waste to be discharged and to pollute water bodies (that is, the environment) by solids and organic matter. According to [2,4,18], IAA-farming is characterized by the recycling of nutrients between the farm components and this system allows the intensification of production and income generation while reducing environmental impact [17]. Tropical integrated pond systems are reputed to retain nutrients to a high degree as the latter are re-used by primary and secondary producers, making the system a better nutrient converter than the recirculation aquaculture systems [19].

In the current study, nutrient-rich wastes were dropped directly into fishponds from grass-fed rabbits and pond wastewater (that is, fertilised effluent) was then used as fertiliser for irrigated rice. The flow of bio-resources and nutrients throughout the rabbit-tilapia (O. niloticus)-rice integrated system was investigated, with the rabbits as the major entry point for nutrients for the system [20]. The purpose of this study was to identify the flow of nutrient by quantifying the mass flow of N and P nutrients by using the mass balance approach, assuming that IAA farming allows the recycling of nutrients between farm components.

Materials and Methods

Site and Experimental Arrangement

The present study was carried out at the Rwasave Fish Culture Research Station (SPIR) of the National University of Rwanda (geographic co-ordinates 02° 36’ 10’’ S and 29° 45’ 25’’ E and elevation about 1,625 m above sea level). The University is at Butare in the Southern Province of the Republic of Rwanda.

Figure 1 is a detailed diagramme of the agriculture system implemented at the SPIR combined with an annotated photograph. The experiment used three different living organisms, namely rabbits (Oryctolagus sp.), fish (Oreochromis niloticus L.), and rice (Oryza sativa, var Yuni Yin4); these were all farmed as part of a full IAA system.

Figure 1: A profile of the experimental rabbit-fish–rice integrated system at the Rwasave Fish Culture Research Station. The connections between the rabbit hutch, the fish pond, and the rice field are demonstrated. The pond is here viewed lengthwise.

Rabbit Housing

Rabbit hutches were built and placed over the fishponds to allow all rabbit droppings to fall straight into their respective fishponds. Each hutch was divided into four cages, each 0.7-1 m in height with a 1 m² wire mesh floor. Three hutches were installed over each fish pond of 4 ares (400 m²). The rabbits were placed in their hutches one week before the ponds were stocked with fish and two weeks before the rice was transplanted into the adjoining fields. The rabbits were a local strain of the Oryctologus sp. genus.

(1200 rabbits per hectare) of pond. The mean live weight of each rabbit was 600-800 g. The rabbits were fed with cut grass, brought in from the pond dykes and the station surrounds. Rabbit health was attended to: the only disease that was frequently observed was rabbit gall, and this was treated by subcutaneous injections of ivermectin and sometimes by skin application of motor or crankcase oil.

Fish Ponds and Rice Fields

Before the experiment began, the fishponds were drained, dredged, and dried to minimise any possible effect of prior use. The inlet pipes were blocked by a fine mesh net to avoid wild aquatic species, such as frogs, molluscs, and wild fish, being carried into the pond by the flow of water in the canals that connected the ponds to the Rwabuye River.

Each fishpond was connected to a 90 m² (9 m x 10 m) rice field by a PVC outlet pipe, installed about 10 cm from the bottom of the pond for the irrigation of the rice fields (Figure 1). Two small PVC pipes, perforated along their length (8 m), were installed at a depth of 40 cm in each of the rice fields to drain the pond water after it had seeped through the rice field soil.

The experiment commenced on 24 August 2009, on the day the rice was transplanted. This was two weeks after the rabbit hutches were stocked and the ponds filled with water. The experiment ended on 25 January 2010, the day the rice was harvested.

Experimental Management

The principle purpose of the experiment was to use rabbit droppings to fertilise fish ponds, thereby producing well-grown fish as well as fertilised pond water (effluent) that could then be used to irrigate rice fields; this would simultaneously reduce the discharge of nutrients from the fish culture ponds into the environment.

The experiment consisted of two treatments in a completely randomised design with three replicates [21]. Hand-sexed, monosexual juvenile male Nile tilapia (Oreochromis niloticus L.) were used for their grow-out phase. The six fishponds were each connected to their own rice field of 90 m². The experiment was then conducted in six fish ponds of four ares (400 m²) each and in nine rice fields of 90 m² (9 m x 10 m) each. The following treatments took place in the fishponds:

• -Pond treatment 1 (PT1): three of the six fishponds were stocked with one fish per m² and were fertilised with the droppings from 1,200 rabbits per hectare of pond.
• Pond treatment 2 (PT2): three of the six fishponds were stocked with three fish per m² and were also fertilised by dropping from 1,200 rabbits per hectare of pond.
• With regard to the rice, the following three treatments took place:
• Rice treatment 1 (RT1): three of the nine rice fields were fertilised by chemical fertilisers (NPK: 17:17:17 and urea 45%-N).
• Rice treatment 2 (RT2): three of the nine rice fields were fertilised by the effluent of the fishponds stocked with 1 fish per m² (see PT1).
• Rice treatment 3 (RT3): three of the nine rice fields were fertilised by the effluent from the fishponds stocked with three fish per m² (see PT2).

Dynamic of Nutrients in the Rabbit-Fish-Rice Integrated System

Composition of Rabbit Droppings and Rabbit Feed

Every two weeks, a 24-hour cycle collection of droppings (both rabbit dung and urine) was conducted. The dung, taken from under one cage lodging four rabbits, was collected twice a day, at 07 h and 16 h for night and day excreted amounts respectively. All overnight excretion, both dung and urine, was collected at 07 h. Urine excreted during the day was collected, and the volume recorded, every two hours during the day in order to minimise loss through evaporation; subsequently, it was poured back into the fishpond.

The biochemical composition of the droppings was determined by the laboratory of the Animal Science and Poultry Department, University of KwaZulu-Natal. The analysis determined the concentration of N, P, Ca, gross energy, fibres, fat, ash, and moisture using the ALASA method for feed and plants detailed [22]. The same analysis was carried out for the composition of the rabbit forage, the rabbit pellets, fish carcasses, and rice at harvest. N was determined on a LECO TruSpec Nitrogen Analyser according to Official Method 990-03 and expressed as percentage protein [22].

Water Quality in Fish Ponds

Water quality parameters, including dissolved oxygen, water temperature, pH, and electrical conductivity, were monitored on a daily basis, twice per day, using appropriate manual probes.

Chlorophyll a was determined after filtrating water on Whatman paper microfibre GF/C (retention: 1.2 µm; Ø47 mm) and was analysed using the acetone extraction method [23].

Pond water nutrients, including N and P forms, were analysed from a sample collected fortnightly. A one litre sub-sample was filtered through Whatman filter paper and kept in the fridge for later laboratory analysis. Another litre of non-filtered water was analysed for TN and TP. TN was analysed using the Kjeldhal method (Blume, 1966) and adapted by [24]; TP was determined after hydrolysis into ortho-phosphates by persulfate digestion [25], thereafter with the colorimetric method. Ortho-phosphate analysis was carried out on filtered water following the ascorbic acid method [23]. Inorganic N, NH₄-N (mg/l) and NO₂-N (mg/l), was determined by the colorimetric method, while NO₃-N was analysed using the cadmium reduction method [26].

Water Seeping through Rice Field Soil

A one-litre sample of seepage water was collected at the drains (perforated PVC tubes) that had been installed at a depth of 50 cm under the rice field. The sample was a mixture of water collected from the two drains installed in each. One half of the sample (that is, 500 ml) was filtered for NH₄-N, NO₂-N, NO₃-N, and PO₄-P analysis; the second half was not filtered for TN and TP dosage. Nutrient analysis was done using the same procedures as described above for the pond water nutrients.

Nutrient Analysis in Soil Samples

The analysis parameters used for the soil samples that were collected at the start, the midterm (90 days after transplanting DAT), and the end (153 days DAT) of the experiment were TN, nitrates (NO₃-N), ammonia (NH₄-N), TP, and phosphates (PO₄-P). The soil samples were collected using a soil auger along a “double S” trajectory in the fish pond and a “W” trajectory in the rice field. Samples were taken from a mixture of eight clumps of soil both for fishponds and rice fields.

Soil pH was measured by the electrometric method in a soil-solvent suspension, and cation exchange capacity (CEC) was analysed on a saturated soil as detailed in IITA (1975, adapted by [24]. The TN was analysed using the Kjeldahl method according to INEAC (1959), adapted by [24], and NO₄-N ammonium and NO₃-N nitrates from soil were determined according to the method detailed by McKeague (1978), adapted by [24]. The TP in the pond sediment and rice field soil was analysed using the spectrometric methods detailed [27], and the PO₄-P was determined by extraction followed by the blue-colorimetric method set out [28].

Statistical Analysis

The mean values were compared using a two-way analysis of variance (ANOVA II) for parameters changing over time, especially for water and soil quality. Significant differences among the treatments as shown by ANOVA were further tested using the least significant differences of means at a 5% level (LSD_0.05). Possible relationships between the parameters were highlighted through regressions and correlations analysis using GenStat statistical software (GenStat12.1 Ed^®, 2009 – VSN International Ltd), which was also used for ANOVA.

Results

Rabbit Nutrition, Excretion of Droppings and Discharge of Nutrients

Rabbits were fed forage ad libitum but formulated pellets were supplied only during the first month to supplement the ration in order to help the rabbits adapt to the new environment. The bromatological composition of forage, as well as that for rabbit droppings (both for rabbit fed forages or formulated pellets, is detailed in Table 1.

Table 1: Chemical composition of the rabbit forage, droppings from rabbits fed forage and rabbits fed pellets, highlighting the contribution of the rabbits to the integrated rabbit-fish-rice system. (N: nitrogen, P: phosphorus, K: potassium, Prot: crude proteins, G.E: gross energy, Ca: Calcium).

^жChemical composition [29]; data not available.

The average amount of dung ranged from 0.44 to 0.85 kg.a^-1.d^-1, and the average amount of urine ranged from 0.86 to 1.69 l.a^-1.d^-1. These wastes fluctuated widely over the rearing time, and no significant difference was found between the amounts of droppings (P<0.001) of the various treatments being voided into the fish ponds. Rabbit dung was very rich in basic nutrients for plankton development (Table 1). Rabbit wastes were composed of 1.57 or 1.71% N and 0.62 or 0.34% P content when rabbits were fed grass or formulated pellets respectively. The rabbit urine might be an important source of nutrients in view of its composition: 2.17 % and 1.15 % N and P respectively. Rabbit waste was the major source of organic nutrient in the current IAA, providing about 505.10 kg TN and 245.87 kg TP to fish ponds during the 127 days of pond fertilisation monitoring. The largest amounts of TN (1365 kg) and of TP (65 kg) were sourced from the canal water used to refill the pond during the culture period. However, the waste weight increased significantly in time (P=0.02) with increasing rabbit weight.

The present study identified a good rabbit growth with a daily weight gain of 8.0 g/day while the mean weight changed from 821.03 to 1362.7 g with a survival rate of 85.4%.

Fish and Rice Yields

In treatments TP1 and TP2 (1 and 3 fish.m^-2), total fish yield was 953 and 1939 kg.ha^-1 respectively. Fish mean weight of 104.3 ± 4.2 and 70.7 ± 1.6 g.fish^-1 was obtained in TP1 and TP2 respectively. Recorded rice yields were 5.79, 5.44, and 5.87 t.ha^-1 in RT1, RT2, and RT3 respectively and rice straw biomass was 10.79, 9.97, and 9.70 t.ha^-1. The fish and rice content in N and P nutrients is presented in Table 1.

Pond Water Quality

Table 2 presents the mean and standard error for listed water quality parameters and the major nutrients which characterised the pond water. The overall temperatures that were recorded ranged from 20.4 to 29.9°C and the pH ranged from 6.5 to 8.4. Total alkalinity ranged from 40 to 120 mg CaCO₃ l^-1 in the ponds undergoing the range of treatments. All the parameters of the water quality remained within acceptable limits for pond aquaculture throughout the duration of the experiment. The temperature, DO, pH, and total alkalinity values did not differ significantly between treatments, but the DO, the pH, and the temperature recorded had significantly higher values (P<0.05) in the afternoon than observed at dawn within the same treatment (Table 2).

Secchi disk transparency was significantly higher (P<0.05) in the ponds stocked with one fish/m² than in those stocked with three fish/m².

The daily primary productivity ranged from 0.7 to 2.9 g C/m²/d for PT1 and from 1.1 to 2.8 g C/m²/d for PT2, with no significant difference (P<0.05) between treatments, whereas the chlorophyll a concentrations were significantly higher in PT2 than those in PT1 (P = 0.002, Table 2). Regarding the nutrient concentrations in the pond water, there appeared to be no accumulation of inorganic N (ammonia, nitrites, and nitrates) as toxic levels were not reached in any of the treatments (Table 2). There was no significant difference between the treatments for all nutrients, except for the available P, for which the phosphate concentrations in PT2 were significantly higher than those in PT1 (P<0.05).

Table 2: Physico-chemical parameters characteristics of fish pond water in rabbit-fish-rice integration system [ponds stocked with one (PT1) and three (PT2) fish per m² of pond].

The values presented above are means ± SE of mean. Data with different superscript letters (that is, a and b) in the same row and letters (that is, x, y, and z) in the same column for the same parameter were significantly different (P < 0.05). NS refers to no significant difference, whereas the P values with t denote differences according to time (that is, sampling dates), with dt referring to daytime (07 h 00 to 08 h 00 for a.m. – before noon; 14 h 00 to 15 h 00 for p.m. – after noon) and tdt referring to the differences considered for interaction treatment*daytime. LSD_0.05 is the least significant difference to which means are compared to point out the significance at 5% level.

Nutrients in Water Seeping through Rice Field Soil

Samples of water that had filtered through rice field soil were analysed for N and P forms in order to assess the possible discharge of nutrients from the system to the underground environment. N forms did not differ significantly among the rice fields fertilised by effluent from ponds with one fish per m² (RT2) and those fertilized with effluent from ponds stocked with three fish per m² (RT3) (Table 3). However, water seeping in the fields treated by inorganic fertilizers (RT1) were found to have significantly higher concentrations (P = 0.001) of nitrates (0.30 ± 0.13 mg/l) than water from fields of RT2 (0.20 ± 0.09 mg/l) and RT3 (0.17 ± 0.08 mg/l) treatment. The RT1 N concentrations after seepage were slightly lower than those recorded in the pond effluent and water that flowed to irrigate the rice field. The TP concentration in water from the rice fields undergoing RT1 was significantly higher (P<0.05) than those in water from the rice fields undergoing RT2 and RT3 (which were not significantly different from each other).

Table 3: Concentration of nutrients in the outflow after water has leached into the rice field soil. RT1: rice fields fertilised by NPK and urea; RT2: rice fields fertilised by effluent of fishponds stocked with one fish per m²; RT3: rice fields fertilised with effluent of fishpond stocked with three fish per m².

Data are mean values ± standard error of mean; different superscript letters in the same column denote treatments that are significantly different (P<0.05). The data in parentheses are the minimum and maximum recorded for each treatment; RT denotes rice field treatment; P values with t denote time-based differences (that is, sampling days); P values with trt*t refer to the differences considered for interaction treatment*time.

Nutrients in Rice Field and Fish Pond Soil

The pH, CEC, ammonia-N, TN, TP, and phosphates were analysed to characterise chemically the impact of integrated livestock (rabbits) and aquaculture on the soil at the bottom of the ponds as well as the soil in the rice fields. The results obtained for the start, midterm (90 days), and end (153 days) of the experiment are summarised in Figures 2 and 3. A slight increase over time in pH was observed in both the ponds stocked with one fish per m² (PT1) and those stocked with three fish per m² (PT2), with no significant difference being discerned between them (P<0.05). At the end of experiment, the pH averaged 4.78 and 4.92 in PT1 and PT2 respectively, while the averages were 4.65 and 4.70 respectively at the start of the experiment. The CEC averaged 9.69 and 7.98 meq/100g in PT1 and PT2 respectively, and were significantly different from one another (P<0.05). A decrease was noted in CEC, especially in the ponds stocked with the least fish, from the start of the experiment to the end.

TN concentrations were significantly higher (P<0.05) in PT2 than in PT1 at the end of experiment, but the observed N increase that took place over time within each treatment was not statistically significant (P<0.05). TN concentrations averaged 0.11 and 0.15% in PT1 and PT2 respectively, with a non-significant increase (P>0.05) with time, from 0.09 to 0.12% and 0.11 to 0.18% in PT1 and PT2 respectively.

There were no significant differences between P nutrients (TP and phosphates), nor between the treatments, nor among treatments, over time. As recorded for TN in the fish pond soil of PT1, the concentrations of TP and phosphates were slightly higher at midterm (that is, 90 days after stocking) than at the start and the end of experiment. This was most likely the result of the fish pond water being used to irrigate the fields, as pipes were placed near the sediment-water interface at the bottom of the ponds.

Figure 2: N and P in the bottom soil of fishponds that were fertilised by rabbit droppings and stocked with one fish per m² (PT1) and three fish per m² (PT2) in a rabbit-fish-rice integrated system in Rwanda. Data were collected at the start of the experiment (that is, after 1 day), at midterm (that is, after 90 days), and at the end (that is, after 153 days after transplanting) of the experiment.

Figure 3 presents the nutrient pattern in the rice field soil that emerged during the rabbit-fish-rice integrated system experiment. No significant changes of the parameters were observed within each treatment over the culture time. Only ammonia-nitrogen and CEC increased from the start to the end of experiment. The treatments also did not differ significantly (P>0.05) from one another at the end of the culture time for these parameters. Generally, N nutrient concentrations, TN, and ammonia (NH₄-N), decreased in the midterm point of the experiment and increased at the end of experiment in all treatments. The rice fields fertilised by effluent from highly stocked ponds (RT3) had, however, slightly higher concentrations of N nutrients than did the rice fields receiving effluent from low stocked fish ponds (RT2) and the rice fields fertilised by chemical fertilisers (RT1). The mean concentrations for TN were 0.11 ± 0.05%, 0.13 ± 0.04%, and 0.14 ± 0.04% N in rice field soil for RT1, RT2, and RT3 respectively. Even though no significant differences were observed with regard to TP and phosphates (PO₄-P) between the treatments, changes could be signaled within treatments over time and among treatments at the end of rice culture period.

Figure 3: Chemical parameters in rice fields fertilised were as follows:  RT1: chemical fertilisers (NPK and urea); RT2: effluent from fishponds stocked at one fish per m²; RT3: effluent from fishponds stocked at three fish per m². Data were collected at the start of the experiment (that is, after 1 day), at midterm (that is, after 90 days), and at the end (that is, after 153 days after transplanting) of the experiment.

Phosphate concentrations were higher in RT3 (0.30 ± 0.2 mg P/kg) than in RT2 (0.23 ± 0.09 mg P.kg^-1) and RT1 (0.22 ± 0.08 mg P.kg^-1), and they decreased towards the end of the culture period. Inversely, TP was more highly concentrated in those rice fields that received NPK and urea (RT1) than in those (RT2 and RT3) fertilised by pond water effluent.

TP concentrations were high at the start (359.47 ± 112, 263.67 ± 74.77, and 288.17 ± 138.2 mg P.kg^-1 for RT1, RT2 and RT3 respectively), but then decreased progressively to an average of 275.79 ± 73.8, 248.28 ± 107.1, and 186.06 ± 36.18 mg P.kg^-1for RT1, RT2, and RT3 respectively at the end, with no significant differences recorded between the treatments.

Discussion

Source of Nutrients in the IRFR System

The productivity of all IAA fish ponds depended totally on inputs which originated mainly from on-farm and/or off-farm sources of nutrients – but all external to the fish pond [30,31]. Studies on the use of rabbit droppings and of the resource-flow in the IRFR system [32-34] have confirmed that rabbit droppings (faeces and urine) provide not only a better environment for tilapia but also a major source of nutrients on which the whole system of fish and rice production relied. The rabbit dung composition, in this study, showed that rabbit dung could be a better fertiliser than most other manure.

This study investigated the flow of nutrients (N and P) by means of their mass balance throughout the IRFR without considering a complete nutrient budget of the system. The mass balance of N and P nutrients showed that rabbit waste accounted for about 27.0% N and 79.1% of the P of the total nitrogen and phosphorus input supply. Of this, rabbit urine accounted for 20.0% N and 64.0% P of the total N and P respectively of the total fish pond inputs, thus highlighting the major role of rabbit urine in providing nutrients. The nutrient mass balance explains effectively the nutrient flow and contribution of each resource, but is not a good estimation of nutrient budgets which normally require an accurate estimate of the volume of water being exchanged in the system. This estimation poses difficulties: [35] reported the uncertainty and difficulty of estimating pond seepage and pond evaporation, arguing that methods based on changes in pond depth are prone to error. To avoid such errors, we opted for the nutrient mass balance. This method was also chosen because of the difficulty in determining the exact amount of pond mud, the denitrification, and the ammonia volatilization; in most studies these potential factors of N losses are estimated indirectly. The higher amount of nutrients contained in the influent water is due to the order of magnitude comparable to the organic resource amount as ponds are refilled after rice field irrigation by pond effluent; this confirms the success of such practices in Rwanda. The influent water provided up to 72.99% N and 20.91% P as an off-farm source of the N and P input to the pond; these quantities may have been principally constituted of dissolved organic nitrogen (DON) from the rice and vegetable fields upstream of the fish farming station. According to a study [36] the DON is decomposed slowly by bacteria and therefore accumulates over the rearing time, the DON was reported to be the major form of N [31,37] in fish ponds. Therefore, the nutrients available after decomposition of rabbit droppings by bacteria may have resulted largely from use during phytoplankton development.

Water Fertilisation and Nutrient Distribution

The concentrations of measured variables for water quality (Table 2) suggested good conditions for phytoplankton and tilapia growth. The significant increase during the day for pH and dissolved oxygen confirmed good phytoplankton activity; this activity, on the one hand, removes carbon dioxide by photosynthesis and, on the other hand, enriches the water with oxygen through the same process [12,38,39]. The removal of effluent to fertilise rice fields seems likely to have been the only probable reason for the fluctuation observed in TN and TP concentrations, during which N decreased mainly after the first month.

About 27% of the N and 79% of the P from the rabbit droppings were released in fish ponds, fertilising the pond water at a rate of 3.98 kg N and 1.94 kg P.ha^-1.d^-1; this is a higher rate than that (1.75 kg N and 0.39 kg P.ha^-1.d^-1) reported in an integrated Nile tilapia cage-cum-pond system [40] where tilapia were fed pellet feed and waste fertilised ponds were used to raise fingerlings. This rate is comparable to that (3.71 kg.ha^-1.d^-1 N and lower than 8.06 kg.ha^-1.d^-1 P) observed in caged hybrid catfish waste fertilising open-pond Nile tilapia [8]. The inputs from rabbits provided a pond fertilisation rate equivalent to the application of urea and triple superphosphates (TSP) at the rate of 4 kg.ha^-1.d^-1 of N and 1 kg.ha^-1.d^-1 of P used [7] in an integrated Lotus-Tilapia experiment (2 fish per m²) that resulted in a net fish yield (3345 ± 113.4 kg.ha^-1.y^-1), comparable to that obtained in the present study (2611 kg.ha^-1.y^-1 for 1 fish per m² and 3459 kg.ha^-1.y^-1 for 3 fish per m²). The rabbit droppings thus raised substantially the TN and TP concentrations of pond water from 0.21 to 3.16 mg.l^-1 of TN and 0.01 to 0.66 mg.l^-1 of TP. Supplement material to that from rabbit waste might be canal water, fish waste, plankton die-off, and other external unaccounted sources such as levee and watershed erosion, small leaves from rabbit hutches, and leaves blown into the pond by wind [41,42]. Siddiqui and Al-Harbi (1999) [41], found that tilapia excreted 59-72% of the N and 60-62% of the P constituent in the feed. A study [43] observed that covering the pond edge substantially reduced nutrients in pond and concluded that run-off from the pond dyke was the major source of turbidity in the fish pond.

The effluent from the fertilised pond in this study held high amounts of TN (about 19175-18135 kg N.ha^-1) and TP (3510-4225 kg P.ha^-1 ) following their respective fish stocking rates (1‑3 fish per m²). When used to irrigate rice fields, these effluents provided the rice fields with about thirteen times the amount of N (1478 kg.ha^-1TN) and more than twenty-six times (133 kg.ha^-1 TP) of P obtained in treatment with inorganic fertilizers (NPK and urea). The role of rabbit droppings as pond fertiliser was thus clearly highlighted and the results showed that it was not necessarily dependent on fish stocking density. The reported study was limited in that only harvested fish, rice grain, and rice straw were assessed for N and P as major nutrient output of the integrated system.

N and P Recovered by Harvested Products

The assessment of N and P mass balance showed that with a low fish stocking rate, Nile tilapia recovered lower N and P from inputs than with a high fish stocking rate; this was probably due to the amount of fish waste in these ponds which logically surpassed that present in ponds with a low stocking rate. In low stocking rate ponds (1 fish per m²), Nile tilapia recovered 18.5% N and 16.9% P of the TN and TP of the rabbit droppings inputs, while this recovery was only of 4.9% N and 13.3% P of the total N and total P inputs, including that of the inflow canal water. In higher stocking density rate ponds (3 fish per m²), the fish recovered more (37.6% N and 34.3% P) of the total N and total P of the rabbit droppings input; including the canal water that refilled ponds, this was only 10.2% N and 27.13% P of the total inputs. Whatever the considered source of input, these nutrients recovery rates were higher than many recovered rates reported in various studies (Table 4).

The probable explanation for the differences shown in various recovery rates in Table 4 relies on how quickly each source makes nutrients available to the fish. It is known that when nutrients from feeds are directly used by fish, the recovery rate is higher than when fertilisers are used (Table 4). Tacon et al. (1995) in [44], found that supplementing feed in semi-intensive aquaculture farms improved N recovery, ranging from 5% to 25%, in fish. From Table 4, it can be argued that rabbit droppings must be better used by fish ponds than many other inputs to ensure a better environment for Nile tilapia growth.

Table 4: Nutrient recovery rates by Nile tilapia for various rearing systems.

The rice field component of the system received fertilised pond effluent as organic input for rice growth. The results showed that rice grain accounted for a lower percentage of input in fields fertilised with effluent (0.44-0.50% N and 0.22-0.25 P of the total N and P of the effluent input) than in fields treated with NPK and urea (6.11% N and 6.54% P of the total N and P inputs). The rice straw in fields fertilised by effluents accounted for 0.77-0.79% N and 0.71%-0.88% P, while it accounted for 10.79% N and 23.24% P of the total N and total P inputs. The differences obtained here seem logically to be due to the order of magnitude of each source of input. The amount of nutrients supplied in the effluent was high and therefore remained in the soil, the seepage, and contributed to weed growth during the farming period.

Nutrients in Pond Sediment, Rice Field Soil, and Seepage Water

Nutrient mass losses were difficult to measure precisely because the amount of sediment, infiltrated water, and nutrients accumulated in the rice fields were not quantified but only their concentrations in water and sediment assessed. Changes in sediment concentrations of P and phosphates followed the activity applied in the fish ponds. Normally P is strongly adsorbed by pond soil [6,11] directly from pond water, while N is lost primarily through ammonia volatilisation and denitrification [45-47]. Munsiri et al. (1995) [42] stated that organic matter, N, P, and TP in pond bottom soil accumulates strongly in the upper 10-20 cm of sediment as a result of the fertilisation process due to microbial activity. In the present study, TN and ammonia nitrogen, as well as TP and phosphates, increased up to midterm (that is, 90 days) but thereafter, except for ammonia nitrogen, decreased until the end of the experiment. The observed decrease in P and TN was most likely due to adult fish movement and the various factors causing waves (for example, pressure from the pipe sucking water) which disturb the sediment-water interface, thereby allowing re-suspension of nutrients sucked by the pipe to irrigate the rice field. Overall, no significant changes in nutrient concentrations were observed in the rice field soil, neither among treatments nor over time, and this demonstrated that nutrients were used by growing rice at almost same rate in inorganic or effluent fertilised fields. The ammonia, TP, and TP pattern in rice field soil showed higher concentrations at the beginning of the experiment as a result of fertiliser application and irrigation by pond effluent, but all these nutrients decreased at the end of the experiment. These observations agree [39] whose findings identified an increase of nutrients in the soil during the first 15 days after transplanting (DAT). The decrease in N and P at the end of experiment was probably due to the nutrient uptake by the rice in the growing phase, caused by the strong nitrification processes in the upper layer of the soil (De Dautta et al., 1985 in [39].

The present integrated system was designed in such a way that all effluent used for irrigation could filtrate through rice soil before it reached the environment. Lower concentrations, but not significant (P>0.05), were observed for TN, TP, and phosphates, while nitrates decreased significantly and ammonia increased in the water that reached the under layer of soil. N forms were higher in seepage water than P forms, suggesting that microbial activities on N were more intense at the soil surface and in the pipes in which ammonia concentration increased and surpassed that in effluent water. The seepage water accounted for only between 80 and 88% of TN and TP of the effluent that entered the rice field, between 29 to 40% of the soluble phosphates, and up to 6.0% of the nitrates of the effluents. This observation suggests that nitrates and soluble phosphates were the nutrients most used by the growing rice.

Conclusion

This study analysed the flow of nutrients in the integrated rabbit-fish-rice system and demonstrated that about 27% N and 79% P in pond water were attributable to rabbit droppings (faeces and urine). Rabbit droppings provided a fertilisation rate of 3.98 kg N and 1.94 kg P.ha^-1.d^-1, leading to fish yields comparable to those obtained from using urea and TSP at a rate of 4 kg N and 1 kg P.ha^-1.y^-1, yielding 3 344.6 kg.ha^-1.y^-1 for 2 fish per m² stocking density of male Nile tilapia in an earthen pond.

Fish recovered about 18.5-37.6% N and 16.9-34.3% P of the TN and TP in rabbit dropping input to the pond. The relatively large amount of N and P that passed through pond water made the it especially appropriate for rice fertilisation and could replace totally the inorganic fertilisers used in common practice in the culture of rice. The re-use of tilapia pond effluent, captured from the bottom of the pond, allowed the recycling of the large amount of N and P by providing the growing rice with required nutrients at a high rate (118.5-125.3 kg N and 22.9-21.2 kg P.ha^-1.d^-1). The lack of these nutrients, especially N, is the most limiting factor in irrigated rice fields (De Datta et al., 1988 in [39]. In this way, in an integrated pond effluent and rice culture system, a large amount of N accumulated in the rice field, making the soil able to be better used in for a demanding rotated crop.

This integrated farming seems particularly environmentally friendly and sustainable, thus appropriate for resource-poor farmers in developing countries, such as Rwanda, as it recycles nutrients, thereby reducing the investment costs and the negative environmental impacts of aquaculture.

Further studies are needed to document the best fertilisation/irrigation frequency and to investigate the most efficient use of the effluent nutrients through the expansion of the current integration system. It is also recommended that a study should be made that aims at determining the complete nutrient budget of the integrated rabbit-fish-rice system.

Acknowledgment

The authors wish to thank the Nile Basin Initiative/ATP project for providing financial support for this study. They are also grateful to the workers of the Rwasave Fish Farming and Research Station (SPIR) for their commitment during the experiment. Thanks to Dr. Gatarayiha for his comment regarding statistical analysis applied in this paper. We acknowledge efforts and help by Mrs. Beulah John who proofread this article while she was not feeling good, many thanks.

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DOI: 10.31038/AFS.2021323

Abstract

A dredge survey was conducted in September 2015 to provide the first assessment of sea urchin resources in southern Breiðafjörður (65°07´N, 22°31´W). An underwater photography of the sea floor was undertaken as well. The efficiency of the dredge was assessed by comparing the number of sea urchin/m² seen on photos to the number/m² caught by the dredge. The efficiency varied between sub-areas investigated with average of 29%. The whole area investigated was 9.7 km², consisting of seven sub-areas. Sea urchins were found in significant concentration in all sub-areas with the average density ranging from 1.7 to 6.9 ind/m². The density was correlated with depth and bottom topography. The mean density for all areas combined was 3.5 ind/m², giving a stock size of 2.700 tonnes.

Introduction

In Icelandic waters the green sea urchin (Strongylocentrotus droebachiensis) is the only targeted urchin species. It is common around Iceland but its distribution is very pachy. Harvesting started in 1983 by divers which was not economically feasible and stopped in 1989. In 1993 the fishing started again and now by dredging and peaked in 1994 when 1 500 tonnes were landed. After that the fishery diclined extremely and stopped 1997. More than half of the catches in these years came from Breiðafjörður west Iceland, but the fishery was conducted wiedly. In 2004 exploitation of the stock started again and now only in Breiðafjörður. The landings were minimal (<50 t) until 2007 when it reached 134 tonn. Since then the landings have been 130-400 tonn [1]. The main fishery has alway been in the the southern part of Breiðafjörður and focused on small hot spots. Since 1993 sea urchins have only been harvested by dredging but the selectivity and efficiency of the dredges used in Iceland is unknown.

The information on geographical distibution and size of the stock around Iceland is limited and no estimates of biomass, trends in relative abundance or assessments of sutainable yield existed before 2015. No fishery-indipendent survey has been done until now (2015) and the only data (location, landed catch, fishing effort) that have been abailable are from the fishery. However, some investigations on densities (ind./m²) and population structures in very small areas off Iceland have been carried out by diver sampling. The results have shown patchy distribution, either low densities or high with grate range at the same locations at different time [2-4].

The main objective of the present study was to assess the stock size and distribution in the main fishing area in Breiðafjörður west Iceland and the efficiency of the dredge used.

Material and Methods

In September 2015 a survey was conducted to assess the usable sea urchin stock in southern Breiðafjörður the main fishing area in west Iceland at depth of 8-60 m. The survey was carried out on a commercial sea urchin fishing vessel using a commercial dredge measuring 250 cm in width and with 150 cm long bag. The mask size of the bag was 100 mm. The main fishing area in the fjord was divided into seven smaller sub-areas differing in size, depth and bottom type.

In order to determine the distribution and biomass/abundances two methods were used, bottom photogrpahy and area swept method, conducted at the same time at the same site. An underwater photocamera was used to estimate the density of urchins from photograps. Photographs were taken at 22 sites within four of the seven investigated areas (I, II, VI, and VII) (Figure 1). A total of 160 photos were captured and the sea urchins from the photos were counted. When the area swept method was used, each catch was weighed and the distance covered by the dredge was caluclated. The total catch weight was divided by the size of the area covered in each tow to give abundance in kg/m². Biomass estimates in a given area were calculated from the mean biomass in that area multiplied by the total size of the area. The density of the green sea urchins (no/m²) was calculated by dividing the mean wet weight of the individuals in an area (which differed between areas) into the abundance (kg/m²) of the area. This was carried out for all subareas except area V where the abundance was assessed from previous fishing surveys.

Figure 1: A map of the seven fishing subareas (shaded) investigated in Breiðafjörður. The red dots denote the dredge stations and the blue are photo stations.

The density (no/m²) of sea urchins from the photos and the results from the dredge survey (no/m²) from the same area at the same time were compared and the dredge efficiency assessed as a percent of captured individuals of what was observed from the photographs. The assessed efficiency of the dredge was then used to calculate the stock size within an area and as a whole for all areas combined.

Results

The mean number of sea urchins counted from the bottom photographs within an area, range and the mean number caught by the dredge as well as the efficiency of the dredge is shown in Table 1. The highest mean density (6.6 ind/m²) from photos as well as the smallest catch by the dredge 0.5 ind/m² was in area VI at 55 m depth, resulting in the lowest efficiency (8%). The efficiency was highest (57%) in area VII at 20 m depth, where the lowest density was observed from the photos (1.5 ind/m²). The mean efficiency of the dredge was 29%, ranging from 8-57% between areas (Table 1).

Table 1: Mean number of urchins/m² from photos, range, percentage of photos without urchins, mean number of urchins/m² from catch and mean efficiency of the dredge.

The whole area investigated measured 9.7 km² and contained seven smaller harvesting areas (sub-areas) that differed in size, depth and bottom topography (Figure 1). Sea urchins were widely distributed and found at all stations sampled (91) at 8-60 m depth. The estimated mean abundance, assessed by the area swept method corrected for the dredge efficiency, ranged from 0.14-0.41 kg/m² and the density from 1.7-6.9 ind./m². The standing stock for all areas combined was assessed to be 2.700 tonnes (Table 2).

Table 2: Mean wet weight ± SD of sea urchin in each area. Depth, area size, estimated mean abundance and range (kg/m²), density (no/m²) and total standing stock/biomass (wet weight t) for green sea urchin at 8-60 m depth. The abundance, density and biomass is corrected with the efficiency of the dredge which was calculated as 29%.

*The abundance in area was estimated from previous fishing surveys but the size of the area was measured in the study.

The density of the green sea urchin in the areas differed, but the mean density for all areas combined was 3.5 ind/m² . The maximum was found at the lowest mean depth (11m) at stations III (5.1 ind/m²) and IV, (6.9 ind/m²) at gravel bottom with kelp beds. The size of these areas were moderate giving the maximum biomass in these areas. Average density was observed in area I (3.1 ind/m²) and II (3.4 ind/m²) where the mean depth was 35 and 33 m respectively. Here the bottom was covered with gravel and drifting kelp was observed. These were the smallest areas in the investigation. The lowest density was observed at area VI (1.7 ind. m^-2) (only 2 tows taken) and VII (2.1 ind. m^-2), where the depth varied greatly between stations with a mean of 19 m at area VII. The bottom was gravel and rocky with kelp beds. The size of area V was measured but the density was estimated from previous fishing as beeing 3.4 ind. m^-2 (Table 2).

Discussion

The total green sea urchin stock investigated in the main fishing area in southern Breiðafjörður was estimated to be around 2.700 tonnes. The distribution of the urchins was patchy in seven sub-areas observed, differing in depth, size and density of the urchins. Most of the tows (88%) were taken at 8-35 m depth and the most common bottom type was sand and gravel, although rocky substrata and mud were also observed. The maximum density (area III and IV) was observed at the lowest depth (mean 11 m) in kelp beds at gravel bottom. Kelp beds might indicate a high food supply but nutrition influences growth and reproduction [5]. At greater depths as 35 m (area I and II) the density was rather low, kelp was observed probably drifting, as kelp beds occur at maximum 30 m depth [6]. The lowest density (area VI) was observed in a single tow taken at the gratest depth (50m) on muddy bottom where food supply might be a limited factor.

In Icelandic waters the green sea urchin is most common in the shallow subtidal zone at depts above 50 m but have been observed down to 600 m [7]. The density generally decreases with depth to about 20-30 m which in many areas corresponds to the distribution of kelp [6]. The green sea urchin is most often distributed on a rocky bed but is also found on gravel and sandy bottoms [8] especially where there are strong currents and good food supply [6,9]. Upper depth limits vary with season and wave action that can dislodge the urchins or limit their change to graze on macroalgae. On sedimentary bottoms urchins rely on drift algae and are more sparsely distributed [8]. Sea urchin mainly feed on kelp but are also known for feeding on various bottom species, dead animals and even on lime algae scraping from rocks [10,11]. In this study, the greatest distribution was observed at lower depths in kelp beds. However, at greater depths, drifting algae [12] and detrital kelp [13] can supply food enough for sea urchin to grow and reproduce.

In the present study an indirect method was used to estimate the efficiency of the dredge. The number of sea urchins seen on bottom photographs were compared to the number that was fished by the dredge at the same site just after photographing. The mean efficiency of the dredge for all areas combined was estimated 29%. The efficiency differed between all areas and was related to depth, highest at the moderate depth. Efficiency and selectivity of dredges are influenced by numerous factors such as their design, on operational factors i.e. towing speed, the ratio of warp length versus water depth, duration of the tow, and on environmental factors such as depth, current speed and bottom type. The efficiency can be estimated by various methods as by comparing the abundance, size and biomass of urchins in the dredge catch with those remaining in tracks after dredging (direct method) [14-17] or with that of un-dredged sediments (indirect method) [18]. Efficiency and selectivity can also be assessed by repeatedly fishing the same area until the target species is markedly reduced [19]. Capture efficiency for dry dredges has been estimated for several bottom-dwelling commercial bivalves, primarily scallops with different methods giving different efficiency from 1-40% depending on investigations [14,17,20-22].

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DOI: 10.31038/AFS.2021322

Abstract

In the study, changes in biochemical composition of warty crabs, Eriphia verrucosa between female and male individuals after the reproductive period were evaluated. The warty crabs used as study material were freshly obtained from fishermen after the breeding season (end of July). According to the data obtained in the study where the female and male individuals were evaluated separately, the average moisture, crude protein, crude lipid and crude ash contents in female E. verrucosa were 77.89%, 20.96%, 0.91% and 2.66%, respectively. In male E. verrucosa, it was found to be 76.30%, 21.44%, 0.79% and 2.47%, respectively. In the study, it was determined that the difference between male and female individuals in crude protein and crude lipid values was significant (p<0.05). Although the crude protein content was low in females, the crude lipid content was found to be higher. In other words, it was determined that while the protein ratio decreases in female individuals after the reproductive period, the lipid ratio increases. This suggests that the energy lost due to ovulation during the reproductive period may have been met by the accumulation of fat in the body.

Keywords

Eriphia verrucosa, Post reproductive, Biochemical

Introduction

The vast majority of crab production comes from hunting. Hunting is generally carried out with crab baskets, nets, bottom trawls, dredges and ladles, depending on the environment in which the species lives [1]. Although various parts of the body can be eaten in crabs, white meat in its pincers is most preferred. Crabs are a fishery product that finds very high value in developed countries in terms of edible meat quality and economic value. There is a crab industry in developed countries, and crabs that pass through various processing stages are produced in different products in this industry [2].

Crab meat is indicated as an important nutrient in a balanced diet because it is rich in protein, mineral substances and vitamins and low in fat [3]. Crabs contain high amounts of iron, potassium, selenium, zinc and vitamin B1, which are beneficial for human health [4]. Although sea crabs, which have become an industry loved and consumed in various countries, are among the least known fisheries in terms of human consumption in our country [5], they are consumed with pleasure and intensely by the people of the region.

However, although Eriphia verrucosa is a commercially used and fondly consumed fishery product in Mediterranean countries, it is not preferred in our country due to our traditional nutrition culture [6]. In this study, by investigating the post-reproduction biochemical composition of warty crab (E. verrucosa), which is not economically evaluated in our country but has high economic and nutritional value, and it aimed to promote the increase of the consumption and use of this species in Turkey.

Materials and Methods

Thirty female (average 62 g and 50 cm) and male (average 85 g and 53 cm) warty crabs used in the study were freshly obtained from fishermen at the end of July. The samples were transported immediately to Sinop University, Faculty of Fisheries, Feed Technology and Aquaculture laboratory in ice-filled containers. After length and weight measurements were made, the meats in the pincers were separated manually, homogenized and stored at -80°C until analyzed.

All biochemical analyses in crab meats were made on a wet basis. All analyses [Dry matter (DM), crude protein (CP), crude lipid (CL), and crude ash (CA)] were performed according to the standard methods of the Association of Official Analytical Chemists [7]. Dry matter was detected by drying the samples at 105°C until a constant weight was achieved. Crude lipid (CL) content was determined according to the procedure of the Soxhlet method. Crude protein (CP) content was determined as total nitrogen content by the Kjeldahl method. Crude ash content was measured after samples were treated in a muffle furnace at 550°C for 6 h. All analyses were performed in triplicate.

All analysis results were presented as mean values ± SE. Anderson-Darling and Levene’s tests were used for homogeneity of variances and equality of variance of groups, respectively. The differences between the results were analyzed using one-way analysis of variance (ANOVA), followed by Tukey’s method for multiple comparisons. Arcsine square root transformations of percentage data were conducted for homogeneity of variances before statistical analysis. Differences were considered significant when p<0.05. Statistical analysis was done by using the Minitab 17 statistical software for Windows.

Results and Discussion

In the study, it was aimed to determine the post-breeding biochemical properties of crabs. In the research, male and female crabs were taken from fishermen in July, and biochemical analyzes were made and evaluated. Proximate composition of female and male warty crabs (E. verrucosa) used in the study are given in Table 1. Changes in moisture, protein, fat and ash contents (%) of female and male wart crabs are shown in Figure 1.

Table 1: Proximate composition (%) of female and male warty crabs (E. verrucosa) (wet weight).

Data are reported as mean ± standard errors of three replicate (3). Rows values with the same superscript or no superscript are not significantly different (p>0.05).

Figure 1: Changes in moisture, protein, fat and ash contents (%) of female and male warty crabs.

In the present study, moisture and ash contents of female and male crabs were determined as 77.89 ± 0.12%, 2.66 ± 0.09% and 76.30 ± 0.07%, 2.47 ± 0.17%, respectively. In terms of moisture content, it was determined that the statistical difference between female and male individuals was significant (p<0.05), while the difference between ash contents was not statistically significant (p>0.05). [4] found that warty crabs had a lower moisture content of 74.4% and ash content of 1.8% than the findings of the present study. [8] found moisture and ash contents of warty crab as 78.03% and 1.81%, respectively, in the comparative analysis of biochemical compositions of warty crabs, blue crabs (Callinectes sapidus) and crabs (Cancer pagurus). In terms of moisture, although they show similarity with the present study, reported that the ash content was lower. [9] reported warty crabs moisture and ash contents as 76.13 ± 0.04% and 2.35 ± 0.01%, respectively, and similar results were obtained. [10] determined the moisture and ash content of female and male warty crabs as 75.44%-2.08% and 75.44%-3.41%, respectively. While showing similarities with the present study in terms of moisture content, it was determined that ash content was lower in female individuals and higher in male individuals. Again, [11] reported that female and male warty crabs determined the moisture and ash content as 74.30%-2.00% and 75.47%-1.85%, respectively. [5] reported that in their study comparing the nutritional composition between male and female members of blue crab (Callinectes sapidus) and sand crab (Portunus pelagicus), moisture and ash contents in female and male individuals of blue and sand crabs was 82.03%-2.20% and 80.35%-1.68%, and 80.13%-2.66% and 79.84%-2.36%, respectively.

In the present study, the protein contents of female and male crabs were determined as 20.96%, and 21.44%, respectively. In terms of protein content, the difference between male and female individuals was determined to be statistically significant (p<0.05). [10] reported that they determined the protein contents in female and male wart crabs as 21.29% and 20.80%, respectively. [9] reported that the amount of crude protein was 19.66 ± 0.02 g/100 g in their studies to determine the nutritional composition of 20 warty crabs (154.91 g), regardless of gender. [6] determined the protein content as 17.12% in their study in which they determined the close composition of wart crab during cold storage. [4] reported the crude protein amount as 21.30 ± 0.01% in July in a study they conducted on 113 crabs in order to determine the biochemical compositions of the crabs they obtained from the Karakum region of Sinop during the reproductive period. It has been determined that the findings of this study are consistent with the findings of the present study. [11] reported that female and male warty crabs determined the protein contents as 22.45% and 21.40%, respectively. When the values they obtained were compared with the findings of our study, it was determined that the amount of crude protein was high. This difference is thought to be due to the difference in the research area and the number of samples. [5] reported that the protein contents of male and female blue and sand crabs were 14.26%-16.81% and 15.83%-17.55%, respectively. The difference between the current study and the study is thought to be due to differences in the research region and species.

In the present study, the crude lipid content of female and male crabs was determined as 0.91% and 0.79%, respectively. In terms of lipid content, the difference between female and male individuals was determined to be statistically significant (p<0.05). Ayas and Özoğul [11] reported that female and male warty crabs determined the lipid contents as 0.96% and 1.11%, respectively. Durmus et al. [10] reported that they determined the lipid contents in female and male wart crabs as 0.95% and 0.92%, respectively. Kaya et al. [9] the amount of crude oil 0.66 ± 0.01 g/100 g, Demirbaş et al. [4] determined the amount of crude lipid as 0.40 ± 0.01% in July. It was determined that the lipid contents determined in both studies were lower than in the current study. It is thought that the difference determined between the current study and the study of Demirbaş et al. [4] may be related to the method of obtaining the material to be analyzed. Türeli et al. [5] reported that the protein contents of male and female blue and sand crabs were %1.51-%1.16 and %1.38-%1.33, respectively. The difference between the current study and the study is thought to be due to differences in the research region and species.

Conclusion

As a result, these creatures, which have an important place in the ecosystem and are an important food source due to their high protein value and low fat ratio, need to be investigated more in today’s world where food sources are decreasing day by day and alternative food sources are tried to be created. Efforts should be made to introduce and spread these creatures, which are consumed only in coastal areas in our country, to consumers in every region as food. In addition, basic research should be done about our natural resources in terms of sustainable fisheries and these studies should be made widespread.

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