The growth and gonadal maturation of the African catfish, Clarias gariepinus (Burchell) broodstock fed differently heated soybean-based diets

An investigation was carried out to assess the growth and gonadal maturation of Clarias gariepinus (Burchell) fed differently heated soybean diets in concrete tanks. Four hundred and eighty male and female C. gariepinus (182 ± 10 g for females and 208 ± 5 g for males) were randomly distributed in groups into hapa nets. Iso‐nitrogenous (310 g kg^?1 crude protein) and iso‐caloric diets (355 Kcal kg^?1) prepared from raw soybean (D0) and soybean autoclaved for 5, 10, 15, 20, 25 and 30 min labelled D0, D5, D10, D15, D20, D25 and D30, were fed to the fish for 84 days. The fish meal based diet tagged DFM served as control. The male and female broodfish fed the fish‐meal‐based diet and diet D25 had significantly higher (P < 0.05) growth performance, gonad maturation, gonadosomatic index, relative fecundity and percentage egg fertilization and hatchability than the fish fed the other diets. The females had significantly higher (P < 0.05) weight increases and healthier condition over their male counterparts fed the same diets. This study showed that diet D25 was found to be the best substitute for fish meal that provided adequate nutrients required for the formation of genital products that produced strong offspring in C. gariepinus broodstock culture.     

A method for the quantification and optimization of hydrodynamics in culture tanks

A method was developed to quantify hydrodynamic mixing parameters, and to optimize the physical environmental conditions, in culture tanks. Improved mixing will result in better tank water quality, more efficient use of available volume by the culture animals (leading to optimal stocking densities and better feed management) and possibly reduced water pumping requirements. Experiments were conducted to determine the influence of a range of flow rates, residence times, water depths and stocking densities on hydrodynamics in juvenile turbot (Scophthalmus maximus (L.)) tanks. Decreases in water depth resulted in significant improvements in mixing and the efficiency with which the water was used, as indicated by reductions in dead volumes. A depth of less than 9.4 cm at a flow rate of 2 l min^–1 was expected to minimize dead volumes in the tank. This indicated that mixing was better in shallower tanks. Within the range 0–13 l min^–1, increased flow rate improved mixing at a constant depth of 9 cm (and water volume of 18.54 l) though increased flow rates greater than about 2.5 l min^–1 produced only small improvements in mixing. Within the range 0–50 fish per tank (equivalent to a mean stocking density of 0–1.84 kg m^–2), stocking density did not significantly influence mixing in tanks with a depth of 9 cm and flow rate of 2 l min^–1. Such depth reductions, for demersal species, may be a useful means to either decrease water use without reducing residence time, or alternatively to increase the flushing rate without increasing water use, at a given stocking density. The large changes in the efficiency with which the tanks were used, which were achieved with ease, indicates that attention to water mixing can give positive benefits to a wide range of land-based farm operators. Care must be taken when adjusting tank hydrodynamics, that water quality is maintained and that biological parameters such as stress levels, sunlight effects and feed management are optimal.     

Growth performance of pengba,Osteobrama belangeri (Valenciennes, 1844) and water quality changes during fingerling rearing with varied stocking density in large outdoor concrete tanks

Fingerling rearing of pengba, Osteobrama belangeri, was conducted at varied densities for 90 days in 12 outdoor concrete tanks (10 m × 5 m × 1 m). The tanks, grouped into four triplicated treatments, were stocked with fry at 20, 30, 40 and 50 fry m^‐3; and designated as control, T‐1, T‐2 and T‐3 respectively. Tanks were filled up to 90 cm depth. Evaporation loss compensated fortnightly. Fish sampling and monitoring of water quality parameters done at fortnight intervals. Survival varied between 71.5% and 84.0% in treatments. Both survival and total length significantly reduced with increasing densities from control to T‐2 (p < .05), while no differences were observed between T‐2 and T‐3 (p > .05). Similarly, significant reductions in final body weight and specific growth rate were observed with increased densities and these values further reduced in T‐3 than T‐2 (p < .05). Such results corroborated the inverse relationship between stocking density and growth. Gross biomass yields in two higher densities (T‐2 and T‐3), despite their lower survivals, were significantly higher than the two lower densities. The numbers of fingerlings harvested were 35, 74 and 112% higher than the control in T‐1, T‐2 and T‐3 respectively. The lowest size obtained even at the highest density (T‐3) was of 7.0 ± 1.0 cm and 3.99 ± 0.17 g, which can be considered as ideal grow‐out stocking size of pengba. Therefore, the study showed feasibility of stocking up to 50 fry m^‐3 for rearing from fry to fingerling stage. However, the other lower densities may be used in case of requirement of larger fingerlings.     

Microsatellites markers to depict the reproductive and genetic patterns of farmed gilthead seabream (Sparus aurata): illustration by a case study on mass spawning

In the absence of breeding strategy, natural spawning constitutes the breeding ground for fish farmers to empirically manage their commercial broodstock. In this context, we used six microsatellite markers to characterize the genetic pattern of six commercial seabream broodfish tanks having a common history spanning four generations. The progeny of one tank single‐day mass‐spawning event, reared in two different environments, was used to estimate the genetic parameters for body weight. Limited genetic differentiation was observed among broodfish groups. A panel of nine loci allowed us to unambiguously assign 95.4% of the offspring (1692) and identify 37 parents (65% of the total broodfish). The limited effective population size (N_e = 15.3) was due to the elevated variance of parental contributions and to broodfish failing to contribute to the progeny. The fluctuation of the allele frequency highlighted the risks of genetic drift and reduction in the heterozygosity in the next generations. Heritability for body weight was moderate at commercial size (0.40 ± 0.10) and the high genetic correlation at later stages laid the groundwork for precocious selection criteria for growth. The discussion opens on the opportunity to use mass spawning for selective breeding.     

Effect of stocking density on growth performance,survival, production,and financial benefits of African sharptooth catfish (Clarias gariepinus) monoculture in earthen ponds

ABSTRACT

The present on-farm study assessed the effect of different stocking densities on growth, production, and financial benefits of African sharptooth catfish (Clarias gariepinus) in earthen ponds for 180 days. Low stocking density (LSD), medium stocking density (MSD), and high stocking density (HSD) of 3, 6, and 9 fish m^?2, respectively, were tested. C. gariepinus stocked at LSD and MSD showed significantly higher weight gain, specific growth rate, and final mean weight than those cultured at HSD (P < .05). There was a significant linear relationship between the stocking density and the yields and financial variables (P < .05). Net fish yields were significantly higher for C. gariepinus reared at HSD and MSD than those stocked at LSD (P < .05). Similarly, C. gariepinus raised at MSD and HSD generated significantly more profit than those cultured at LSD (P < .05). Results demonstrated that farmers can achieve high net yield and financial benefits by stocking C. gariepinus at HSD of 9 fish m^?2.     

Observations on swim bladder stress syndrome in Arctic charr (Salvelinus alpinus), induced by inadequate water depth

In two separate experiments, juvenile Arctic charr (Salvelinus alpinus) were held at three different water depths (12, 24 and 37 cm) and at three different stocking densities (10, 50 and 100 kg/m³). Mortalities from swim bladder stress syndrome (SBSS) were noted over 2 months. Low water depth caused high mortality while stocking density did not induce the stress syndrome to any notable degree. An analysis of catecholamines in the blood plasma of a small number of fish indicated elevated dopamine levels in stressed fish.     

Effects of three culture densities on growth and survival of <Emphasis Type="Italic">Octopus vulgaris</Emphasis> (Cuvier, 1797)

The purpoase of this research was to test the effects of three culture densities on the growth and survival of Octopus vulgaris. A total of 141 sub-adult octopuses (1,175.4 ± 194.9 g) were randomly distributed in nine tanks of 2,000 l each (3.6 m × 1.1 m, and 50 cm water depth). Three tanks were stocked with a low initial density of 4 kg/m³, while three other tanks were stocked with an initial density of 8 kg/m³, and the remaining three tanks were stocked at an initial density of 15 kg/m³. Octopuses were all fed frozen squid (Loligo gahi) at 5% body weight per day (%BW/day). The experiment lasted for 70 days. Water temperature varied between 20 ± 2°C, and salinity varied between 36 ± 1 ppt. During the entire experiment, dissolved oxygen was always >75%, and ammonia was always lower than 0.1 mg/l. No differences in growth or growth rates (between 0.9 and 1.1%BW/day for the three densities) were found. Nevertheless, mortality was significantly lower for the low density compared to the other two densities tested. Maximum densities in the culture tanks (>25 kg/m³) were attained in the higher culture densities after 56 days of the experiment.     

Design, loading, and water quality in recirculating systems for Atlantic Salmon (Salmo salar) at the USDA ARS National Cold Water Marine Aquaculture Center (Franklin, Maine)

The Northeastern U.S. has the ideal location and unique opportunity to be a leader in cold water marine finfish aquaculture. However, problems and regulations on environmental issues, mandatory stocking of 100% native North American salmon, and disease have impacted economic viability of the U.S. salmon industry. In response to these problems, the USDA ARS developed the National Cold Water Marine Aquaculture Center (NCWMAC) in Franklin, Maine. The NCWMAC is adjacent to the University of Maine Center for Cooperative Aquaculture Research on the shore of Taunton Bay and shares essential infrastructure to maximize efficiency. Facilities are used to conduct research on Atlantic salmon and other cold water marine finfish species. The initial research focus for the Franklin location is to develop a comprehensive Atlantic salmon breeding program from native North American fish stocks leading to the development and release of genetically improved salmon to commercial producers. The Franklin location has unique ground water resources to supply freshwater, brackish water, salt water or filtered seawater to fish culture tanks. Research facilities include office space, primary and secondary hygiene rooms, and research tank bays for culturing 200+ Atlantic salmon families with incubation, parr, smolt, on-grow, and broodstock tanks. Tank sizes are 0.14 m³ for parr, 9 m³ for smolts, and 36, 46 and 90 m³ for subadults and broodfish. Culture tanks are equipped with recirculating systems utilizing biological (fluidized sand) filtration, carbon dioxide stripping, supplemental oxygenation and ozonation, and ultraviolet sterilization. Water from the research facility discharges into a wastewater treatment building and passes through micro-screen drum filtration, an inclined traveling belt screen to exclude all eggs or fish from the discharge, and UV irradiation to disinfect the water. The facility was completed in June 2007, and all water used in the facility has been from groundwater sources. Mean facility discharge has been approximately 0.50 m³/min (130 gpm). The facility was designed for stocking densities of 20–47 kg/m³ and a maximum biomass of 26,000 kg. The maximum system density obtained from June 2007 through January 2008 has approached 40 kg/m³, maximum facility biomass was 11,021 kg, water exchange rates have typically been 2–3% of the recirculating system flow rate, and tank temperatures have ranged from a high of 15.4 °C in July to a low of 6.6 °C in January 2008 without supplemental heating or cooling.     

Vulnerability to a small-scale commercial fishery of Lake Tana's (Ethiopia) endemic Labeobarbus compared with African catfish and Nile tilapia: An example of recruitment-overfishing?

In 1986 a motorised, commercial gillnet fishery was introduced in Lake Tana, Ethiopia's largest lake (3050 km²) in addition to the artisanal, predominantly subsistence fishery conducted from reedboats. The three main species groups targeted by this fishery are a species flock of endemic, large Labeobarbus spp., African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus). The commercial gillnet fisheries was monitored during 1991–1993 and in 2001 (CPUE, effort allocation, catch composition, yield) and the development of the stocks of the three species groups was determined using an experimental trawl program during 1991–1993 and 1999–2001.In 1991–1993 the vast majority of fishing activities took place in the southern Bahar Dar Gulf (71%). Each species group contributed roughly one third to the total catch. In 2001, 41% of the effort was allocated to the north-eastern shores of Lake Tana. Both the CPUE and the contribution of O. niloticus to the total catch had doubled. However, the proportion of large specimen (>50 cm TL C. gariepinus; >20 cm FL O. niloticus) in the trawl surveys decreased significantly. A three-fold decline in abundance of the anadromous Labeobarbus species occurred. The 15 large labeobarb species are long-lived, ecologically specialised endemics. The seven riverine spawning Labeobarbus species form aggregations in the river mouths in August–September, during which period they are targeted by the commercial gillnet fishery.A sharp decrease in abundance by ca. 75% of the migratory riverine spawning Labeobarbus species in the sublittoral and pelagic zones of the lake, areas where no fishing takes place and the collapse of juvenile Labeobarbus (between 5 and 18 cm FL: by 90%) during the 1990s suggest recruitment-overfishing. To prevent the possible extinction of the unique Labeobarbus species flock all fishing effort should be severely restricted near the river mouths and on the upstream spawning areas during August–September (peak breeding period) to protect the vulnerable spawning aggregations.     

Larval rearing of hilsa shad,Tenualosa ilisha (Hamilton 1822)

Hilsa, Tenualosa ilisha has received much attention for culture due to decline of the natural population. Lack of knowledge on larval rearing is the bottleneck for its culture. This study was aimed at developing larval rearing protocols for hilsa shad. Hilsa larvae (4 days old, 4.76 ± 0.06 mm/0.49 ± 0.01 mg) were stocked in fibreglass‐reinforced plastic tanks (1.7 m³ water volume) at 300, 600 and 1,200 nos/m³ in triplicates in three experimental systems viz., E‐I (circular, 0.567 m water depth), E‐II (circular, 0.962 m water depth) and E‐III (rectangular, 0.567 m water depth) and reared for 46 days. The larvae were supplied with Chlorella vulgaris, Brachionus calyciflorus, mixed phytoplankton and mixed zooplankton during 4–50, 6–25, 8–50 and 26–50 days of their age respectively. In each system, higher (p < 0.05) fry survival at 300 nos/m³ than in higher densities indicates density dependent stress. Circular tanks showed higher survival (13.3%–61.31%) than in rectangular tanks (6.88%–27.26%) in each stocking density, indicating the importance of tank shape for rearing. Water depth affected fry survival in circular tanks (E‐I and E‐II) at 300 nos/m³; at 0.962 m depth, survival was higher (61.31%, p < 0.05) than that of 0.567 m depth (49.93%). Good fry survival was achieved through feeding the larvae initially with Chlorella followed by co‐feeding with Brachionus, mixed phytoplankton and zooplankton and rearing in circular tanks at 300 nos/m³ densities at 1 m depth. This first‐ever larval rearing protocol is useful for mass production of fry to support hilsa aquaculture in future.     

The effects of stocking density on gonad growth, survival and feed intake of adult green sea urchin (Strongylocentrotus droebachiensis)

In this study the effects of different stocking densities on survival, gonad growth and feed intake in adult green sea urchin (Strongylocentrotus droebachiensis) were examined. Two size groups of sea urchins with respective initial mean weight of 45 g (S) and 70 g (L) were used. The experiments were carried out in 755 l plastic tanks divided into chambers by vertical fibreglass lamellae. In the two experiments stocking densities of 6, 12, 14 and 16 kg m^− 2 (Exp I) and 3, 6 and 8 kg m^− 2 (Exp II) were used. Sea urchins reared individually in compartmentalised raceways were used as controls in both experiments. All groups were fed a formulated moist feed ad libitum for 60 days at 8 °C (± 0.5). Mortality was observed in all groups, except for the control group and the lowest density group of large sea urchins (3 kg m^− 2, L). Both mortality and occurrence of injuries increased significantly with increasing stocking density in both experiments and for both size groups. In the highest density groups mortality was 60% (L) and 80% (S). There was a significant increase in gonad index during the experimental period in both experiments. Increased stocking density had a significantly negative effect on gonad growth in both size groups while feed intake was unaffected. The results show that adult S. droebachiensis can maintain high survival rate and high gonad growth at stocking densities up 6 kg m^− 2 in holding facilities of the design used in the present study.     

Effect of different broodstock densities on the reproductive performance of Nile tilapia, Oreochromis niloticus (L.), in a recycling system

The present study was conducted to evaluate the effect of different stocking densities on the seed production of Nile tilapia, Oreochromis niloticus (L.), under intensive recycling hatchery system conditions. Males and females with mean body weights of 163.2 and 105.0 g, respectively, were stocked at three broodstock densities (4, 8 and 12 fish m^–2) at a male:female ratio of 1:3 in 1 × 1 × 0.43 m (W × L × H) fibreglass tanks. The tanks were illuminated at 2500 lux for 18 h day^–1 and the water temperature was maintained at 29 ± 1 °C. Effluent from spawning tanks was recycled through a biological filter with 10–15% replacement of new water per day. The experiment lasted for 126 days. The results showed that breeders stocked at 4 fish m^–2 had significantly higher (P < 0.05) mean values for total seed production, seed kg^–1 female day^–1, seed female^–1 day^–1, seed m^–2 day^–1 and spawning synchrony than at 8 and 12 fish m^–2 broodstock densities. The mean percentage of seeds in the yolk-sac and swim-up fry stages was highest at 4 fish m^–2 broodstock density. However, the recovery rate was not affected by broodstock density. It is recommended that further research should be conducted to determine whether weight m^–2 , number m^–2 or age of broodstock should be the basis for stocking broodstock.     

The effect of stocking density on yield,growth, and survival of Asian river catfish (<Emphasis Type="Italic">Pangasius</Emphasis><Emphasis Type="Italic">bocourti</Emphasis> Sauvage, 1880) cultured in cages

Asian river catfish (Pangasius bocourti Sauvage, 1880) were cultured at five different stocking densities in cages (submerged volume 1 m³) suspended in a dugout pond from August to November 2009. Pangasius bocourti fingerlings (mean weight 27.09 ± 0.54 g) were stocked at densities of 12, 25, 50, 100, and 200 fish m⁻³. At the end of 3 months, the harvest weights (gross yields) were, respectively, 2.05 ± 0.30, 5.20 ± 0.31, 10.60 ± 0.42, 19.98 ± 0.78, and 42.37 ± 0.41 kg m⁻³. The mean fish weights among the stocking densities of 25, 50, 100, and 200 fish m⁻³ were not significantly different, but were significantly higher than that of the 12 fish m⁻³ density. The specific growth rates among high stocking densities of 50, 100, and 200 fish m⁻³ were not significantly different; however, they were significantly higher than those of the low stocking densities of 12 and 25 fish m⁻³. Asian river catfish performed poorly at the lowest density. The results indicate an initial lower stocking threshold for Asian river catfish of above 5.20 kg m⁻³. The Asian river catfish cultured in small cages placed in a pond reached the desirable market size (>200 g) within a 90-day grow-out period. The results show that the maximum yield for Asian river catfish during a 3-month production cycle was not reached.     

Improved hatchery‐rearing techniques for juvenile production of blue swimming crab,Portunus pelagicus (Linnaeus, 1758)

The effects of tank colour, larval stocking density, antibiotic administration and water exchange on survival and moulting of blue swimming crab, Portunus pelagicus, were determined. Circular 4‐m³ experimental larval‐rearing tanks were used in triplicate for all treatments. White, dark grey, blue and brown were tested as tank background colours. The stocking densities tested were 10, 20, 40, 60, 80 and 100 larvae L^?1. The effect of oxytetracycline was estimated by comparing a treatment with oxytetracycline to a treatment without oxytetracycline administration. The daily water exchange rates tested were 0%, 25%, 50% and 100% of the tank volume. In all treatments, the larvae were fed with Artemia nauplii, rotifers and encapsulated Spirulina. The highest percentage survival was observed in the dark‐grey tanks when the stocking density of larvae was 20 larvae L^?1. No larva reached the juvenile crab size in white tanks. No significant difference in survival was found between treated and non‐treated larvae with oxytetracycline when the daily water exchange rate was more than 50%.     

Influence of hormonal induction and broodstock feeding on longfin yellowtail Seriola rivoliana maturation,spawning quality and egg biochemical composition

To optimize broodstock management for Seriola rivoliana, a survey over 5 years was performed to evaluate the effects of successive hormonal inductions with GnRHa or dietary regime on spawning quality. Running males and females with an oocyte diameter > 500 μm were injected with GnRHa. The spawning quality was compared among consecutive years and different moments along the spawning season. Besides, three different feeding regimes were tested. Spawning quality parameters were not significantly affected by time along the spawning season, whereas they were clearly influenced by diet. Particularly, broodfish fed the mackerel regime (M) showed a significantly higher number of eggs than other treatments. Besides, dietary protein content significantly improved broodstock fecundity of S. rivoliana. Moreover, hatching rates were also higher in broodfish fed mackerel and the dietary docosahexaenoic acid (DHA) levels significantly increased this parameter. Despite polar lipids were not significantly affected, neutral lipids fatty acid composition of S. rivoliana eggs showed higher 18:2n‐6 in eggs, whereas ARA content was not affected. The results of this study denoted the high fecundity and good spawning quality of S. rivoliana broodfish and suggested the importance of high protein, energy and DHA levels in broodstock diets.     

Reproductive performance and the growth of pre-stunted and normal Nile tilapia (Oreochromis niloticus) broodfish at varying feeding rates

A 119-day experiment was conducted to investigate the effects of feeding rate and reproductive performance of stunted (S) and non-stunted or normal (N) Nile tilapia (Oreochromis niloticus). Both the groups were reared in tanks re-circulated with bio-filtered water and fed with floating pellets (30% crude protein) twice daily. Seed were harvested weekly from the mouths of incubating females. The study showed that early stunting with subsequent high feeding rate can improve both growth and reproductive output in female Nile tilapia. Broodfish type and feeding rate showed significant (P < 0.05) effects on both the frequency of spawning and the seed output. In general, seed output from normal broodfish increased linearly over the experimental period at all the feeding rates. However, seed output from stunted broodfish showed a linear increment for 3% feeding rate, exponential increment for 2% but quadratic for 1% showing decline after 10th week of the trial period. Results also showed that trends of seed output from stunted broodfish increased linearly with the increase in feeding rate showing that optimum rate could be higher than 3%. While from normal group the relationship was quadratic; increasing from 1%, peaked at 2% and declined at 3% feeding rate. Final GSI of the stunted females was significantly (P < 0.05) higher than that of normal females. The GSI of stunted fish showed a decreasing trend with the increased feeding rate. Both the broodstock groups fed at 1% biomass grew linearly whereas at 2 and 3%, they grew exponentially. As compared to the normal, stunted broodfish had significantly (P < 0.05) higher fat content in viscera although similar levels were in carcass and ovary. Carcass fat content was significantly (P < 0.05) lower in fish fed at 1% biomass but significantly (P < 0.05) higher in the ovary and viscera of fish fed at 3% biomass. This study shows that tilapia hatchery operators could manipulate the seed production according to the seasonal demand by using appropriate broodstock stunting and feeding strategies.     

Tilapia wild spawning control through predator fishes: Israelitrial with red-drum and hybrid bass

Tilapia wild spawning is a nuisance in warm freshwater aquaculture growout ponds. To cope with this problem two experiments were carried out with predatory fish that do not reproduce in fresh water. One experiment tested the capacity of hybrid bass (Morone saxatilis × M. chrysops) and red-drum (Sciaenops ocellatus) as predators of wild spawning of hybrid tilapia (Oreochromis niloticus × O.aureus), and the other compared predation effectiveness of red-drum of different sizes and stocking densities.Both hybrid bass and red-drum effectively reduced tilapia wild spawning and improved by 15–20% tilapia performance and food conversion ratio. These effects were obtained stocking small red-drum (20 g) or large red-drum (60–80 g) or bass (135 g) at stocking densities of 500–1000 predators/ha, together with 15000 tilapia/ha of 65–75 g. Hybrid bass stocked at 750/ha and large red-drum stocked at 500/ha presented over 90% survival. Red-drum at higher stocking density and/or lower stocking weight presented reduced survival (40–60%). Red-drum of all examined stocking weights presented better growth rates when stocked at 500/ha than at higher densities.     

Environmentally conditioned,year‐round volitional spawning of cobia (Rachycentron canadum) in broodstock maturation systems

Year‐round control of the spawning cycle of cobia (Rachycentron canadum) has been established by using water temperature manipulation. To compare the effectiveness of using this method to induce volitional spawning in cobia, two 80 m³ recirculating aquaculture systems (RAS) were used. Temperatures in one of the maturation tanks (‘Mat 1’) were maintained between 27 and 29°C for 12 months of the 15.5‐month study period. Temperatures in the second maturation tank (‘Mat 2’) were allowed to fluctuate naturally throughout the year and ranged from 20 to 32°C. A total of 101 spawning events occurred in the tanks between the spring of 2008 and the summer of 2009 (3 April 2008 to 17 June 2009). Of the 38 total spawning events in Mat 1, 17 of them (44.7% of all Mat 1 spawning events) occurred during the off‐season (fall and winter). The egg viability rates did not differ significantly (P > 0.05) between on‐ and off‐season spawns in Mat 1. Conversely, cobia broodstock exposed to natural water temperatures (no environmental manipulation) in Mat 2 followed the natural pattern of warm water (>26°C) dependence, limiting egg production to spring and summer seasons. This method of water temperature manipulation allows for effective control of the cobia reproductive cycle without compromising egg viability.     

Stocking density for the intensive rearing of juvenile crayfish, <Emphasis Type="Italic">Pacifastacus leniusculus</Emphasis> (Astacidae), using <Emphasis Type="Italic">Artemia</Emphasis> nauplii to supplement a dry diet from the onset of exogenous feeding

Recent advances in intensive rearing of astacid juvenile crayfish have greatly improved the results. This challenges the current application possibilities of the studies performed previously, and new research on density is required. A 100-day experiment was carried out under controlled conditions to evaluate density effects on survival and growth rates of juvenile crayfish in optimal conditions of feeding. Juvenile stage 2 Pacifastacus leniusculus were stocked in fibreglass tanks (1 m², 200 l water) at 20 ± 1°C and fed a dry diet for salmonids supplemented with restricted amounts of Artemia nauplii. Stocking densities were 100, 300, 600 and 1,000 crayfish m⁻². Mean survival rate was reduced significantly with increased stocking density, ranging from 86.33% (100 m⁻²) to 39.13% (1,000 m⁻²). All checks showed that at the lowest initial density (100 m⁻²) animals grew significantly faster those at higher densities, recording a final carapace length of 15.28 mm and weight of 1.08 g. Among the treatments of 300, 600 and 1,000 m⁻² no differences were found either in carapace length or in weight throughout the experimental period, with a final mean growth of 14 mm carapace length and 0.72 g weight. The final proportion of animals with chelae autotomy rose significantly with increasing stocking density, ranging from 14.44% (100 m⁻²) to 41.45% (1,000 m⁻²). This study shows that diet is a decisive factor for stocking successfully high densities under controlled conditions and provides useful information to set adequate densities in accordance with the production objectives.     

Nursery rearing of the Asian catfish, Clarias macrocephalus (Günther), at different stocking densities in cages suspended in tanks and ponds

Growth and survival of hatchery‐bred Asian catfish, Clarias macrocephalus (Günther), fry reared at different stocking densities in net cages suspended in tanks and ponds were measured. The stocking densities used were 285, 571 and 1143 fry m^?3 in tanks and 114, 228 and 457 fry m^?3 in ponds. Fish were fed a formulated diet throughout the 28‐day rearing period. Generally, fish reared in cages in ponds grew faster, with a specific growth rate (SGR) range of 10.3–14.6% day^?1, than those in cages suspended in tanks (SGR range 9–11.3% day^?1). This could be attributed to the presence of natural zooplankton (copepods and cladocerans) in the pond throughout the culture period, which served as additional food sources for catfish juveniles. In both scenarios, the fish reared at lower densities had significantly higher SGR than fish reared at higher densities. In the pond, the SGR of fish held at 228 and 457 m^?3 were similar to each other but were significantly lower than those of fish held at 114 m^?3. The zooplankton in ponds consisted mostly of copepods and cladocerans, in contrast to tanks, in which rotifers were more predominant. Per cent survival ranged from 85% to 89% in tanks and from 78% to 87% in ponds and did not differ significantly among stocking densities and between rearing systems. In conclusion, catfish nursery in cages suspended in tanks and ponds is density dependent. Catfish fry reared at 285 m^?3 in tanks and at 114 m^?3 in ponds had significantly faster growth rates than fish reared at higher densities. However, the desired fingerling size of 3–4 cm total length for stocking in grow‐out culture can still be attained at stocking densities of 457 m^?3 in nursery pond and 571 m^?3 in tanks.