Effects of temperature and salinity on the survival and development of mud crab, Scylla serrata (Forsskål), larvae

The combined effects of temperature and salinity on larval survival and development of the mud crab, Scylla serrata, were investigated in the laboratory. Newly hatched larvae were reared under 20 °C temperature and salinity combinations (i.e. combinations of four temperatures 25, 28, 31, 34 °C with five salinities 15, 20, 25, 30, 35 g L⁻¹). The results showed that temperature and salinity as well as the interaction of the two parameters significantly affected the survival of zoeal larvae. Salinity at 15 g L⁻¹ resulted in no larval survival to the first crab stage, suggesting that the lower salinity tolerance limit for mud crab larvae lies somewhere between salinity 15 and 20 g L⁻¹. However, within the salinity range of 20–35 g L⁻¹, no significant effects on survival of zoeal larvae were detected (P>0.05). The combined effects of temperature and salinity on larval survival were also evident as at low salinities, both high and low temperature led to mass mortality of newly hatched larvae (e.g. 34 °C/15 g L⁻¹, 34 °C/20 g L⁻¹ and 25 °C/15 g L⁻¹ combinations). In contrast, the low temperature and high salinity combination of 25 °C/35 g L⁻¹ resulted in one of the highest survival to the megalopal stage. It was also shown that at optimal 28 °C, larvae could withstand broader salinity conditions. Temperature, salinity and their interaction also significantly affected larval development. At 34 °C, the mean larval development time to megalopa under different salinity conditions ranged from 13.5 to 18.5 days. It increased to between 20.6 and 22.6 days at 25 °C. The effects of salinity on larval development were demonstrated by the fact that for all the temperatures tested, the fastest mean development to megalopa was always recorded at the salinity of 25 g L⁻¹. However, a different trend of salinity effects was shown for megalopae as their duration consistently increased with an increase in salinity from 20 to 35 g L⁻¹. In summary, S. serrata larvae tolerate a broad range of salinity and temperature conditions. Rearing temperature 25–30 °C and salinity 20–35 g L⁻¹ generally result in reasonable survival. However, from an aquaculture point of view, a higher temperature range of 28–30 °C and a salinity range of 20–30 g L⁻¹ are recommended as it shortens the culture cycle.     

Integrated rearing system approach in the farming of mud crab,shrimp, fish,oyster and periphyton in bracksihwater pond

Diversification of production systems has been identified as one of the major strategies for the long‐term sustainability of brackishwater aquaculture. On the context of diversification, a 180‐day culture experiment was carried out where four farming systems were tested: monoculture of mud crab, Scylla serrata (T1), polyculture of mud crab, with mullet Mugil cephalus and shrimp Penaeus indicus (T2), polyculture of mud crab with mullet, shrimp and oyster Crassostrea cuttackensis (T3), polyculture of mud crab with mullet, shrimp, oyster along with periphyton net (T4) covering 10% of the water spread area. There was no significant difference in the growth performances of mud crab, shrimp and oyster among the treatment but a significantly higher (p < .05) final average body weight (ABW) of M.cephalus was recorded in T4 (121.19 ± 2.09 g). Production considering only the fed animals (excluding oyster) was significantly (p < .05) higher in T4 (2,115 kg/ha) compared to other treatments. The overall water quality parameters do not vary significantly (p > .05) among the treatment ponds. Although there was no significant difference in total ammonia nitrogen (TAN) and nitrite‐N, highest value was recorded in T2 (56.23 ± 39.47 µg/L and 185.53 ± 110.71 µg/L) and T1 (55.78 ± 32.26 µg/L and 180.83 ± 103.65 µg/L). The results from the current experiment based on water quality and growth of animals showed that there is no negative interaction between the mud crab and other animals stocked in the polyculture pond which indicated that polyculture provides an efficient and sustainable utilization of resources.     

Stepwise function of natural growth for Scylla serrata in East Africa: a valuable tool for assessing growth of mud crabs in aquaculture

Predicting growth is critical in aquaculture, but models of growth are largely missing for mud crab species. Here, we present the first model of natural growth in juvenile and adult mud crabs Scylla serrata from East Africa using a stepwise growth function based on data on intermoult periods and growth at moult from field mark‐recapture, pond and laboratory studies. The results showed a sigmoid growth pattern in carapace width and suggest that S. serrata in East Africa will reach 300 g and sexual maturity ~9.9 months after settlement, and a commercial size of 500 g after 12.4 months. Analyses of the literature identified several issues with the common praxis to compare standard growth measures between aquaculture studies with different initial size or growing periods. Using the new growth function to estimate the proportional difference between modelled and obtained growth as an alternative method, we show that growth rates of S. serrata cultured in cage systems, which are dominant in East Africa, was <40% of the estimated natural growth and growth obtained in pond systems. The analysis also indicated that growth rates of S. serrata in Southeast Asia was over 50% higher compared with similar culture systems in East Africa, and that different species of mud crabs had large differences in growth rates. This study shows that growth in the present mud crab aquaculture systems in East Africa is below their expected potential. Further work is needed to identify the factors behind this observation.     

Domestication of the mud crab <Emphasis Type="Italic">Scylla serrata</Emphasis>

The significant decrease in wild mud crab population highlights the need to manage the resources and domesticate crabs. This paper presents the initial results of the domestication of mud crab Scylla serrata aimed at producing good-quality captive broodstock. The analysis of the genetic structure of the base population was done as a prerequisite for domestication. Adult S. serrata from the northern to southern parts of the Philippines (Cagayan, Camarines, Samar, and Surigao) were obtained for genetic diversity analysis and domestication. Analysis of molecular variance showed that differences in the genetic variability between the four populations were not significant. Moreover, no significant deviation from Hardy–Weinberg Equilibrium was observed in each sample population and even in pooled populations. Body weight was positively correlated with the carapace width. Second spawning occurred 41–46 days after the first spawning and 34 days from second to third spawning. However, there was a decrease in the number of zoea in repeat spawnings. Twenty-four first-generation (F₁) families were produced from the four sites. The duration from spawning of the base population (P₀) to attainment of broodstock size F₁ was 10–14 months. Four second-generation (F₂) families were produced after 11–12 months. Up to the F₂, crabs tested negative for six viruses: white spot syndrome virus, infectious hypodermal and hematopoietic necrosis virus, gill-associated virus, yellow head virus, Taura syndrome virus, and infectious myonecrosis virus. The reproductive performance of P₀ was comparable to the succeeding generations. Several families were obtained from one population in a year. However, due to the cannibalistic behavior of crabs, more space is required for the nursery and grow-out phase. The domestication of S. serrata is the first study done on any mud crab species in the Indo-west Pacific region. The initial results would serve as guide to understand and eliminate the barriers to mud crab domestication. The breeding technology developed from this study will support the production of good-quality seedstock for farming.     

Effects of Starvation and Feeding on Lipid Class and Fatty Acid Profile of Late Stage Mud Crab,Scylla serrata,larvae

Lipid class and fatty acid (FA) analysis were conducted on newly molted, fed, and starved zoea V and megalopa of the mud crab, Scylla serrata (S. serrata). Larvae starved for 4 d showed a substantial decrease in total FA content, from 49.67 μg/mg to 13.94 μg/mg ash‐free dry weight (AFDW) at the zoea V stage, and from 38.47 μg/mg to 10.40 μg/mg AFDW at the megalopa stage. This depletion indicates that S. serrata larvae effectively utilize stored lipid reserves for energy during periods of food deprivation. Megalopa subjected to longer starvation periods, however, did not utilize lipid as the major energy source after day 4, suggesting increased reliance on protein catabolism during prolonged starvation. At both larvae stages the major FAs were 18:1n‐9, 16:0, 20:5n‐3 (eicosapentaenoic acids, EPA), 18:3n‐3 (linolenic acid, LNA), 18:0 and 22:6n‐3 (docosahexaenoic acid, DHA) and this FA profile persisted in both fed and starved larvae. The highly unsaturated fatty acids (HUFA), EPA, DHA, and arachidonic acid (20:4n‐6, AA) were not conserved in tissue during starvation, indicating that HUFA requirements might be lower for S. serrata larvae than shown for other crustaceans. Similarly, a high level of LNA in newly molted zoea V and megalopa were rapidly depleted in unfed larvae, indicating that this FA had an important role as an energy reserve. Throughout the study, FAs from the polar lipid fraction dominated larvae tissues, while FAs from the neutral lipid constituted the largest accessible energy reserve during starvation (depleted from 23.05 to 1.23 μg/mg AFDW in zoea V, and from 19.00 to 1.27 μg/mg AFDW in megalopa). The results of this study provide new insight into lipid utilization of S. serrata larvae during development, an important step toward development of formulated diets for use in mud crab hatcheries.     

Effects of acute and chronic toxicity of unionized ammonia on mud crab, Scylla serrata (Forsskål, 1755) larvae

While the effects of ammonia on fish and prawn larvae are well documented, little is known of its effect on mud crab (Scylla serrata) (Forsskål, 1755) larvae. Two experiments were conducted in 5 L hemispherical plastic bowls, containing 3 L of ultra‐filtered and settled seawater and various larval stages of mud crab to (1) determine the acute median lethal concentration (LC₅₀) of unionized ammonia and (2) to determine the chronic effects of unionized ammonia on survival and percentage moulting to zoea and megalop stages. The larval stages that exhibited the highest tolerance to ammonia over 24 h were zoea 1 (LC₅₀ of 4.05 mg L^?1 of unionized ammonia) and zoea 5 (LC₅₀ of 6.64 mg L^?1 of unionized ammonia). The megalop stage had the lowest total ammonia LC₅₀ at both 24 and 48 h, making it the larval stage most susceptible to total ammonia. Exposure to 6.54 mg L^?1 of unionized ammonia resulted in 100% death of all larvae within 24 h. The tolerance of S. serrata larvae to total ammonia did not appear to increase with ontogenetic development. The results indicate that the concentrations at which total ammonia produces an acute or chronic response in mud crab larvae are far higher than those experienced in current larval production systems (0–0.5 mg L^?1 of total ammonia) used as industry standards in Australia.     

Influence of binder type on leaching rate and ingestion of microbound diets by mud crab, Scylla serrata (Forsskål), larvae

Microbound diets (MBD), composed of nutrients held within a matrix or binder, were shown to be readily ingested by larvae of the mud crab, Scylla serrata (Forsskål). In an effort to improve experimental MBD developed for S. serrata larvae, this study determined the effects of various binders on estimated larval ingestion of MBD as well as their leaching rates. Microbound diets with the same dietary compositions were bound with either agar, alginate, carrageenan, gelatin or zein. All diets contained ¹⁴C‐labelled rotifers and were fed to Zoea I, III, V larvae and megalopae. The ¹⁴C content of larvae fed ¹⁴C‐labelled MBD for 2 h was used to estimate the relative ingestion rates of MBD. For all larval stages tested, the estimated mean larval ingestion of MBD did not differ significantly between binder types (P>0.05). Determination of the amount of ¹⁴C‐labelled nutrient leaching from MBD bound with various binders after 30, 60 and 240 min of immersion showed that for all types of binders, ¹⁴C leaching occurred primarily within the first 30 min of immersion. Zein‐bound MBD consistently showed the lowest numerical leaching rate among the five binders tested for all immersion periods examined, and the differences compared with other binders were often significant, particularly after 60 and 240 min of immersion. Larval feeding experiments showed that MBD prepared with binders showing higher rates of leaching were not utilized to a greater degree by S. serrata larvae. Furthermore, unnecessary leaching from MBD particles wastes dietary nutrients and is likely to impact on water quality. On this basis, our results suggest that because of its consistently lower leaching rates, zein is probably a more suitable binder for MBD developed for S. serrata larvae.     

Optimal first feed organism for South African mud crab <Emphasis Type="Italic">Scylla serrata</Emphasis> (Forskål) larvae

It is not known whether rotifers or Artemia nauplii are the best first food for South African mud crab Scylla serrata larvae. In order to test this, larvae were fed with five different test diets. These were rotifers for the first 8 days and newly hatched EG^® type Artemia nauplii (San Francisco Bay) from day 6 onwards (treatment R6A); newly hatched EG^® type Artemia nauplii throughout the rearing period (treatment EG); newly hatched Vinh-Chau strain (Vietnam) Artemia nauplii throughout the rearing period (treatment VC); decapsulated cysts of EG^® type Artemia throughout the rearing period (treatment DECAP); or decapsulated cysts supplemented with low densities of Artemia EG type Artemia nauplii (treatment MIX). Two experiments were conducted approximately 1 month apart using larvae from two different female crabs. Although results showed it is possible to rear S. serrata larvae through metamorphosis on Artemia nauplii exclusively, larval performance (development, survival and successful metamorphosis) was enhanced by the inclusion of rotifers as a first feed.No significant difference in performance was recorded between larvae fed on the two strains of Artemia nauplii. Larvae fed on decapsulated cysts in treatments DECAP and MIX performed poorly, but there were indications that decapsulated cysts and other inert diets may have potential as supplements to live food in the rearing of S. serrata larvae.     

Studies on pathogenicity and prevalence of white spot syndrome virus in mud crab, Scylla serrata (Forskal), in Zhejiang Province, China

Mud crab, Scylla serrata (Forskal), is the most commercially important marine crab species in China. In recent years, serious diseases have occurred in major mud crab culture regions in SE China. PCR detection of white spot syndrome virus (WSSV) in diseased mud crabs collected from Zhejiang Province during 2006–2008 showed a prevalence of 34.82%. To study the pathogenicity of WSSV to mud crab, healthy mud crabs were injected intramuscularly with serial 10‐fold dilutions of a WSSV inoculum. The cumulative mortalities in groups challenged with 10^?1, 10^?2, 10^?3 and 10^?4 dilutions were 100%, 100%, 66.7% and 38.9% at 10 days post‐injection, respectively. All moribund and dead mud crabs except the control group were positive for WSSV by PCR. Based on the viral load of the WSSV inoculum by quantitative real‐time PCR, the median lethal dose (LD50) of WSSV in S. serrata was calculated as 1.10 × 10⁶ virus copies/crab, or 7.34 × 10³ virus copies g^?1 crab weight. The phenoloxidase, peroxidase and superoxide dismutase activities in haemolymph of WSSV‐infected moribund crabs, were significantly lower than the control group, whereas alkaline phosphatase, glutamate‐pyruvate transaminase and glutamic‐oxaloacetic transaminase were higher than in the control group. WSSV was mainly distributed in gills, subcuticular epithelia, heart, intestine and stomach as shown by immunohistochemical analysis with Mabs against WSSV. The epithelial cells of infected gill showed hypertrophied nuclei with basophilic inclusions. Numerous bacilliform virus particles were observed in nuclei of infected gill cells by transmission electron microscopy. It is concluded that WSSV is a major pathogen of mud crab with high pathogenicity.     

Status of Biological Studies and Aquaculture Development of the Mud Crab, <Emphasis Type="Italic">Scylla serrata</Emphasis>, in China: An Experimental Ecological Studies

This is an initial paper in a series of overviews of biological research and aquaculture development of the mud crab, Scylla serrata, in China. Results of experimental ecological studies on mud crabs are reported here. As a result of these experimental studies, results that are important for mud crab culture were also discovered and these include, methods to condition and manage broodstock, determination of ecological conditions that are suitable for embryonic development, and the influence of temperature, salinity, diet and larval density on development and survival of larvae. Results of this work will be useful in establishing a good method for artificial mass culture of larvae.     

Induction of moulting in hatchery‐reared mangrove crab Scylla serrata juveniles through temperature manipulation or autotomy

The effects of temperature and autotomy of chelipeds on survival, growth and moulting of mangrove crab (Scylla serrata) juveniles were investigated under laboratory conditions for 60 days. Hatchery‐produced crabs with 2.0–2.3 cm internal carapace width (1.7–2.2 g body weight) at intermoult stage were exposed to one of four temperature treatments (constant 29, 32 or 35°C, or ambient [24–31°C]) or subjected to cheliped autotomy. All crabs held at 35°C had 100% mortality due to incomplete moulting during first moult. The mean survival of crabs at termination was 58%, 64% and 50% for ambient temperature, 29 and 32°C respectively. Specific growth rate (SGR) of crabs in the ambient and 29°C were comparable but significantly lower than those at 32°C. The moult interval of the crabs was significantly shorter in treatments with constant water temperature of 29 and 32°C compared with ambient temperature. The survival of crabs with intact chelipeds was comparable with those with one or two autotomized chelipeds. Crabs with intact or one autotomized chelipeds had significantly higher SGR than crabs with both chelipeds autotomized in the first moult. On the second moult, however, high SGR was observed in crabs with two chelipeds autotomized. The moult interval was significantly shorter in the autotomized crabs compared with crabs with intact chelipeds. The results suggest that the optimum water temperature for rearing S. serrata juveniles ranges from 29 to 32°C. Likewise, autotomy of chelipeds can promote moulting without adversely affecting survival of crabs.     

Ready-to-Serve Crab Sandwich Spread in Retort Pouch: Product Development and Process Optimization

A ready-to-serve sandwich spread was developed from the meat of mud crab, Scylla serrata, by thermal processing in retortable pouches in an overpressure retort at temperatures 111.1, 116.1, and 121.1°C; and F₀ values 5, 6, and 7 min. Process optimization was done by evaluating the samples for texture, color, commercial sterility, thiobarbituric acid (TBA) value, and sensory evaluation. All samples were found to be acceptable based on these quality evaluations, and together with principal component analysis (PCA), the sample processed at 116.1°C, 6 min was found to be the best, with cook value of 84.29 and total process time of 42.59 min.     

INTERNATIONAL STUDY ON Artemia:1 XXI. INVESTIGATIONS INTO WHY SOME STRAINS OF Artemia ARE BETTER FOOD SOURCES THAN OTHERS. FURTHER NUTRITIONAL WORK WITH LARVAE OF THE MUD CRAB,Rhithropanopeus harrisii

In a series of studies conducted by members of the International Study on Artemia, differences in nutritional quality were documented between geographical strains of the brine shrimp Artemia. Concomitant with, and possibly related to nutritional effectiveness are differences in biometric traits and biochemical composition. Since these initial experiments could only hint at causal relationships, a series of experiments were carried out to further our understanding of cause and effect. In one experiment we tried to ascertain whether the factors responsible for poor survival in larvae fed some brine shrimp strains are cumulative throughout larval development or whether there are critical periods when larvae are more sensitive. Our results indicate that the factors contributing to poor growth and survival are cumulative. In a second experiment, we contaminated Brazil Artemia with diel-drin, chlordane and combinations of the two and fed them to mud crab (Rhithropanopeus harrisii) larvae. The levels of contamination were chosen to approximate those suspected of contributing to the poor food value of some brine shrimp strains. In this experiment, however, diel-drin and chlordane contamination appeared to have no effect on growth and survival of R. harrisii larvae. In a third experiment, we fed Utah strain Artemia on Isochrysis galbana for 24 hours and then fed these brine shrimp to mud crab zoeae. Previous work had shown that this regime made Utah Artemia more suitable as a food source for a prawn. However, in our experiments, it did not improve survival of R. harrisii.     

Use of a <Emphasis Type="Italic">Noctiluca</Emphasis>-killing bacterium <Emphasis Type="Italic">Marinobacter salsuginis</Emphasis> strain BS2 to reduce shrimp mortality caused by <Emphasis Type="Italic">Noctiluca scintillans</Emphasis>

The ability of an algicidal bacterium Marinobacter salsuginis strain BS2, isolated from shrimp pond water, to reduce shrimp mortality was investigated under laboratory conditions. When two species of shrimp (Penaeus monodon and Litopenaeus vannamei) (body length 1.5–1.8 cm) were cultured together with the dinoflagellate Noctiluca scintillans, nearly 80 % of the shrimps died within 7 days. However, when bacterial strain BS2 was also added to the culture, N. scintillans was killed within 48 h, and shrimp survival rates on the 7th day improved from 23 to 87 % for both P. monodon and L. vannamei. The bacterium BS2 alone had no effect on shrimp condition. Under conditions of increased dissolved oxygen, the effect of using BS2 was greater, and shrimp survival rates improved even more dramatically, from 26 to 98 %. These studies provide the first evidence that the use of killing bacteria, isolated from shrimp culture water, can suppress harmful algal blooms (HABs) and thus restore the efficiency of shrimp production. The control of HABs in this way in shrimp culture farms would be a major benefit for shrimp production.     

Comparative study on the embryonic development of three mud crabs Scylla spp.

Morphological changes in the embryos, egg size and development, incubation period and morphological structures of newly hatched zoea of three mud crab Scylla species were determined. The three species exhibited similar embryonic development composed of 10 stages. The mean egg diameter of Scylla serrata was significantly larger (P<0.05) at the prehatch stage. The mean egg diameters of Scylla tranquebarica and Scylla olivacea were similar (P>0.05). The incubation period was the longest in S. serrata and the shortest in S. olivacea. There was a positive relationship between egg size and larval size, as S. serrata exhibited the largest egg size and first zoea. However, no correlation was detected between egg size at prehatch and lengths of the morphological structures of the newly hatched zoea. The three species exhibited similar lengths of cephalic structures, but S. olivacea had significantly shorter (P<0.05) abdominal structures. The duration of spawning from ablation was the shortest in S. tranquebarica and the longest in S. olivacea. The study is relevant to aquaculture and fisheries management of Scylla species.     

Induced out-of-season spawning of the mud crab, Scylla paramamosain (Estampador) and effects of temperature on embryo development

Treated with combined bilateral eyestalk ablation and maintenance of water temperature at 22.5±1.5 °C, mud crab Scylla paramamosain females with mature ovaries were induced to produce eggs outside the natural spawning season in subtropical southern China. Newly extruded eggs from a crab were incubated in vitro at 10, 15, 20, 25, 27, 30, 35 °C, respectively, and the embryonic development was closely monitored. Abnormal cell division was observed at temperatures 10 and 35 °C. At 15 °C, development remained at the gastrula stage by day 32 post‐spawn, at which time the experiment was terminated. Hatching of in vitro incubated eggs occurred between 20 and 30 °C. An increase in incubation temperature from 20 to 25 °C reduced the incubation duration by 14 days, 2.6 times of that measured for a similar 5 °C increase from 25 to 30 °C. Embryonic development of S. paramamosain was divided into stage 0–10, and the duration of each stage was recorded for each incubation temperature. The information obtained allows accurate prediction of hatching time of female crabs incubated under variable temperatures. Larvae hatched from in vitro incubated eggs were reared to reach first juvenile crab stage and their dry weights were similar to those of larvae hatched naturally.     

Successful rearing of whiteleg shrimp Litopenaeus vannamei larvae fed a desiccation‐tolerant nematode to replace Artemia

The nematode Panagrolaimus sp. was tested as live feed to replace Artemia nauplii during first larval stages of whiteleg shrimp Litopenaeus vannamei. In Trial 1, shrimp larvae were fed one of four diets from Zoea 2 to Postlarva 1 (PL1): (A) Artemia nauplii, control treatment; (NC) nematodes enriched in docosahexaenoic acid (DHA) provided by the dinoflagellate Crypthecodinium cohnii; (N) non‐enriched nematodes; and (Algae) a mixture of microalgae supplemented in C. cohnii cells. In Trial 2, shrimp were fed (A), (NC) and a different treatment (NS) with nematodes enriched in polyunsaturated fatty acids (PUFAs) provided by the commercial product S.presso^®, until Postlarva 6 (PL6). Mysis 1 larvae fed nematodes of the three dietary treatments were 300 μm longer (3.2 ± 0.3 mm) than control larvae. At PL1, control shrimp were 300 μm longer (4.5 ± 0.3 mm) than those fed DHA‐enriched or PUFAs‐enriched nematodes. No differences were observed in length and survival at PL6 between control larvae and those fed DHA‐enriched nematodes (5.1 ± 0.5 mm; 33.1%–44.4%). Shrimp fed microalgae showed a delay in development at PL1. This work is the first demonstration of Panagrolaimus sp. suitability as a complete substitute for Artemia in rearing shrimp from Zoea 2 to PL6.     

Fishery biology of mud crabs <Emphasis Type="Italic">Scylla</Emphasis> spp. at Iriomote Island,Japan: species composition,catch, growth and size at sexual maturity

The fishery biology of mud crabs Scylla spp. was examined using baited traps and gill nets from September 2001 to August 2005 at Iriomote Island, Japan. To elucidate the growth of the crabs, artificially produced S. serrata juveniles were released and recaptured at the study site. The sizes at which 50% of females and males of S. serrata reached sexual maturity (SM₅₀) were estimated as an external carapace width (ECW) based on the morphology of the abdomen and the chela respectively. Two species, S. serrata and S. olivacea, were identified in the area with S. serrata being the dominant species (>95% of the catch). Changes in the mean ECW and the results of the release and recapture experiments suggested that the recruitment of young crabs to the fishery occurred from December/January to April/May. The SM₅₀ of females and males occurred at 132.4 and 150.7 mm ECW respectively. The body size composition of S. serrata revealed that immature crabs comprised approximately 40 and 65% of the catch for females and males respectively. To maintain a sustainable fishery for S. serrata, a minimum landing size based on the SM₅₀ estimates should be implemented as a fishing regulation.     

Apparent digestibility of some nutrient sources by juvenile mud crab, Scylla serrata (Forskal 1775)

As part of preliminary work aimed at the development of a formulated diet for the mud crab, Scylla serrata, an experiment was conducted with juvenile mud crabs (95.65±2.17 g) to determine apparent digestibility coefficients (ADC) for cellulose, fish meal, shrimp meal, blood meal, soybean meal, wheat flour and cod liver oil. Apparent digestibility coefficients for dry matter (ADC_dm), energy (ADC_energy) and protein (ADC_protein) were in the ranges 70.0–95.7%, 77.4–97.1% and 57.7–97.9% respectively. Soybean meal had the highest ADC_dm and wheat flour had the lowest value (P<0.05), while the ADC_dm for fish meal, blood meal and shrimp meal were not different (P≥0.05). Similarly, soybean meal had the same ADC_energy as that of fish meal, but higher than those of cod liver oil, blood meal and shrimp meal (P<0.05). Moreover, the ADC_protein for blood meal or shrimp meal were not significantly different from fish meal (P≥0.05); nevertheless, they were lower than that of soybean meal and higher than that of wheat flour (P<0.05). Of significant interest was the ADC_dm (78.0%) and ADC_energy (77.4%) for cellulose, which indicates that plant‐based nutrient sources may well be a useful component of formulated diets for mud crabs.     

Effect of temperature on growth and survival of maraena whitefish Coregonus maraena (Bloch 1779) larvae in controlled conditions

This 28‐day study investigated the effect of three rearing temperatures, 11, 15 and 19°C, on survival and growth of maraena whitefish fry in a recirculating aquaculture system. Three groups of larvae in three repetitions were reared in recirculating system. Each group comprised 200 larvae. Feeding level was fixed at 500–700 Artemia sp. metanauplii per fish per day. Larvae were fed fresh live brine shrimp at 10 ml/tank every 3 hr. Significantly higher body weight (p = 0.00), total length (p = 0.00), larval yield (p = 0.00) and condition factor (p = 0.00) were obtained at 19°C compared to 15 and 11°C, as well as at 15°C compared to 11°C. Significantly higher survival (p = 0.00) was observed in larvae reared at 11 and 15°C compare to 19°C and no significant differences were observed between 11°C compared to 15°C. No significant differences in size heterogeneity among treatments were found (p = 0.46). In larviculture, the optimal assessed temperature for growth of maraena whitefish was 19°C, with highest survival observed at 11°C, at the end of this 28 days trial. The findings in this study apply to the particular study location and may not be applicable more broadly.