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.     

Effect of light intensity and photoperiod on Nile tilapia Oreochromis niloticus L. seed production

The objective of this study was to compare the effect of six light intensity/photoperiod combinations (2500 lux/18 h, 2500 lux/15 h, 2500 lux/12 h, 500 lux/18 h, 500 lux/15 h and 500 lux/12 h) on seed production in the Nile tilapia Oreochromis niloticus L. Each combination represented a treatment and was triplicated in 0.4‐m³ fibreglass tanks within a recirculating water system. Water temperature was maintained at 29.0 ± 1.0 °C. Males and females with mean body weights of 116.8 and 91.6 g, respectively, were stocked at a rate of eight fish/tank with a male to female ratio of 1:3. Seeds (eggs, sac fry and swim‐up fry) were collected every 2 weeks. The experiment lasted for a total of 120 days. The results showed that the 2500 lux/18 h treatment produced significantly greater (P < 0.05) total seed/tank (4944), seed kg^?1 female day^?1 (50.9), seed m^?2 day^?1 (40.3) and seed/ female day^?1 (6.7) than treatments with medium or short photoperiods (15 and 12 h day^?1 respectively) or lower light intensity (500 lux). The degree of spawning synchrony and percentage of the sac and swim‐up fry stages were significantly higher in the 2500 lux/18 h treatment than in the other treatments. Under the conditions tested in this study, seed production and spawning synchrony in the Nile tilapia may be improved by subjecting breeders to a light intensity of 2500 lux and a photoperiod of 18 h day^?1.     

Design and performance of recirculating aquaculture system for marine finfish broodstock development

Tropical and subtropical climatic conditions in India present an ideal and unique opportunity for being the leader in tropical marine finfish aquaculture. However, the problem persist due to non-availability of marine finfish seed for the culture. In response to this problem, broodstock development of different tropical marine finfishes for seed production was started. The present study was undertaken to design a recirculating aquaculture system (RAS) and studying their performance in managing the various water quality required for the marine finfish broodstock development and breeding. The design of RAS, developed in the present study, included a broodstock tank, egg collection chamber, electrical pump, rapid sand filter, venturi type protein skimmer and biological filter. Two RAS were designed, one was stocked with a demersal fish species, orange spotted grouper (Epinephelus coioides) and the other was stocked with a pelagic fish species, Indian pompano (Trachinotus mookalee) at the rate 1 and 0.5 kg/m³ with a sex ratio of 1:1 and 1:2 (female: male) respectively. Various physio-chemical parameters, viz, total ammonia nitrogen (TAN), nitrite, nitrate, pH, alkalinity, temperature, free carbon dioxide (CO₂) and dissolved oxygen (DO) of both tank water were analyzed to assess the performance of recirculating aquaculture system in maintaining the water quality. Gonadal development of the fishes was assessed and the spawning performance was recorded and finally, economic performance of the system was also evaluated. During the entire experimental period, mean monthly total ammonia nitrogen was less than 0.07 and 0.06 mg L⁻¹ and mean monthly nitrite was less than 0.02 and 0.01 mg L⁻¹ in orange spotted grouper and Indian pompano RAS tanks respectively. The pH (7.8–8.2), DO (>4 mg/L) and alkalinity (100–120 mg/L) were found to be in optimum range in both recirculating aquaculture systems. Carbon dioxide was found to be nil during the entire experimental period in both the systems. In fact these levels were comparable or less than that is reported as the permissible limits for broodstock development. Indian pompano and Orange spotted grouper matured and spawning was obtained with production of fertilized eggs round the year. Economic evaluation showed the price of 10,000 fertilized eggs of orange spotted grouper to be US $ 1.33. The design of RAS devised in the present study is efficient in controlling and maintaining optimum water quality for broodstock development of both demersal and pelagic finfishes. The fishes stocked in RAS attained final maturation and round the year spawning was obtained.     

Effects of controlling temperature and light duration on seed production in tilapia, Oreochromis spilurus (Günther)

An experiment was conducted to compare the seed production of tilapia, Oreochromis spilurus (Günther), under ambient water temperature and photoperiod, and under controlled water temperature (29.0 ± 2.0 °C), and photoperiods of 13 and 14 h day^?1. Male and female breeders with average weights of 155.4 and 78.7 g, respectively, were stocked in nine 1.04 X 1.04 X 0.40 m (L X W X H) fibreglass tanks. Each tank was stocked with two males and six females. Seed collection was carried out biweekly for 300 days. The results showed that the seed kg^?1 female day^?1, seed m^?2day^?1, total seed production per tank and spawning rates of females were highest during the 14 h day^?1 photoperiod (112.4, 77.4, 24 724 and 42.1%, respectively), followed by ambient spawning conditions (104.0, 57.9, 18 356 and 36.6%, respectively), and lowest during the 13 h day^?1 photoperiod (49.5, 36.5, 12 021 and 25.7%, respectively). Under ambient spawning conditions, peak spawning occurred in May. Monthly seed production was affected by changes in temperature and light duration. In both controlled photoperiods, the peak of seed production was observed during the first month after stocking. In all treatments, spawning declined gradually after the peak, which can be attributed to exhaustion of the breeders. The findings of this study indicate that seed production of O. spilurus in Kuwait can be extended beyond the restricted spawning season of 6-7 months by maintaining water temperature at 29.0 ± 2.0 °C and photoperiod at 14 h day^?1.     

Improving spawning synchrony in the Nile tilapia, Oreochromis niloticus (L.)

Abstract. The development stage and number of seed (eggs and fry) removed from individual incubating Oreochromis niloticus (L.) broodfish at 10-day interharvest intervals (IHI) was compared with the harvest of free-swimming fry from undisturbed fish held in a tank within the same system. Spawning synchrony and productivity were compared for (1) total exchange of females for ‘conditioned’ females after each harvest, (2) exchange of only spawned females, and (3) return of females back into the spawning tank. Improved synchrony and seed production were positively related to removal of eggs and fry from incubating females and could have important implications for broodstock management in commercial hatcheries. Allowing fish to remain undisturbed in clear water tanks may lead to inhibition of spawning activity related to the formation of hierarchies. Individual weight loss by females during incubation was correlated with stage of seed harvested and thus with timing of spawning during the interharvest interval.     

Parentage assignment of stocked black sea bream <Emphasis Type="Italic">Acanthopagrus schlegelii</Emphasis> in Hiroshima Bay using microsatellite DNA markers

The genetic contribution of 51 broodstock, comprising 29 females and 22 males, reared at Hiroshima City Marine Products Promotion Center for the production of stocked black sea bream was monitored during two consecutive years using seven microsatellite DNA loci. The high discrimination ability of these markers was reflected in the polymorphic identification content (PIC=0.831), the exclusion probability (Q≈1), and the low probability of identity index (I=3.635⁻¹⁰). The total number of breeders contributing to the mating process was estimated at 32 (62.7%) in 2000 and 30 (58.8%) in 2001. On pedigree reconstruction, 69.3% of the offspring were successfully assigned to a single broodstock pair. Loss of alleles accounted for 16.9% during seed production; nevertheless, 90.9% of males and 69.0% of females participated in the mating process. Based on microsatellite genetic tagging, 58.9% of the fish sampled during the two months after release were identified as hatchery stock, presenting no significant differences from wild conspecifics in either fork length or body weight.     

Hatchery broodstock conditioning of the blue mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> (Linnaeus 1758). Part I. Impact of different micro-algae mixtures on broodstock performance

Blue mussel (Mytilus edulis) broodstock collected from the Irish Sea during wintertime (November) was conditioned with three different microalgae diets. Positive flagellates (PF) treatment consisted of Pavlova lutherii, Isochrysis galbana (T-Iso), and Chaetoceros calcitrans (1:1:1). Positive diatoms (PD) treatment consisted of Pavlova lutherii, Chaetoceros calcitrans, and Skeletonema costatum (1:1:1). Broodstock animals in the PF and PD treatments were fed a total of 2.4 × 10¹¹ algae cells per day. Animals in the negative flagellates (NF) treatment received only 1/8th of the total amount of algae of the PF diet. The conditioning diets had an impact on spawning success and broodstock fecundity but not on hatching rate, which was similar in all three groups. The best results were obtained with the PD diet where 84% of the conditioned animals spawned and females released 5.0 × 10⁶ eggs on average. Animals belonging to the PF and NF treatments released, on average, only 3.6 × 10⁶ and 1.6 × 10⁶ eggs, respectively. Although the amounts of algae provided to the broodstock animals had no effect on the hatching rate, the D-larvae resulting from the NF treatment were smaller in size than the larvae from the other treatments. Biochemical analysis of the different broodstock groups at the end of the experiment revealed higher carbohydrate levels in group NF than in PF and PD, supporting the theory that gametogenesis is mainly supported by the energy from the glycogen reserves. As far as we are aware this is the first study describing hatchery broodstock conditioning of blue mussels under fully controlled conditions.     

Hatchery production of Florida red tilapia seed in brackishwater tanks: the influence of broodstock age

Abstract. Production of Florida red tilapia seed (eggs, sacfry and fry) in 34-m² above-ground tanks using recirculated brackishwater (12 ppt) was studied over a 5-month period on Lee Stocking Island (Exuma Cays, Bahamas). Four tanks were stocked with year class 1 (YCI) breeders, while 2 tanks were stocked with year class 2 (YCII) breeders at densities of 200 and 240/tank (5·9 and 7·1/m²), respectively, and at a sex ratio of 3 females to 1 male, beginning in November 1988. Seed production was measured by removing free-swimming fry from tanks and eggs and sacfry from the mouths of brooding females, and assessing the number of each at 15 to 16-day intervals from February to June 1989. Average seed production (seed/m²/day) was significantly higher in YCI (52·3) than in YCII (36·0) broodstock, indicating the advantages of replacing broodstock with yearling breeders each year.     

Effect of geographic origin, temperature and timing of broodstock collection on conditioning, spawning success and larval viability of Ruditapes decussatus (Linné, 1758)

Culture of Ruditapes decussatus is clearly limited by the availability of seed, as this production proceeds almost exclusively from natural recruitment. Artificial spawning and larval rearing programs could provide an alternative source of spat. This study was designed to evaluate the effect of different conditioning temperatures on the broodstock maturation, spawning success and larval viability of two geographically (north and south of the Iberian Peninsula) distinct populations of European clam (R. decussatus) collected at different periods of the year in order to create “optimal” artificial spawning and larval rearing programs. Two batches of clams from each population were collected in October and February, and conditioned at 18 ± 1°C, 20 ± 1°C and 22 ± 1°C. Of the three variables analysed the timing of broodstock collection was the most determining factor for gametogenic development, spawning and larval rearing. Geographic origin and conditioning temperature also greatly affected the spawning. The results also showed that the February conditioning was more effective than October and that the best conditioning temperatures were 20 ± 1°C and 22 ± 1°C for the northern and southern populations, respectively. These results suggest that the efficient conditioning temperature for each population of the same species is related to the seasonal temperature regime from their geographic origin. Larval viability and growth performance seemed to be independent of the broodstock conditioning.     

Spawning induction and seed production of Eastern little tuna,Euthynnus affinis (Cantor, 1849), in the post‐ and pre‐spawning seasons by hormonal treatment in a semi‐closed recirculation system with elevated temperature

We previously established a method for spawning induction in Eastern little tuna (ELT) Euthynnus affinis (Cantor, 1849) by administering a gonadotropin‐releasing hormone analog (GnRHa) during the natural spawning season in Japan (August–October). In order to establish seed production of ELT in the off‐spawning season, we first conducted three spawning induction trials by GnRHa administration from October 2011 to January 2012 using ELT broodstock (2 years old; three females and four males) maintained in a 10‐m³ tank with a semi‐closed recirculation system and static elevated temperature. Average water temperature and daily egg production in three trials lasting 11–15 days were 27.0 ± 0.09°C and 268 173 eggs (Trial 1), 27.0 ± 0.11°C and 277 9098 eggs (Trial 2), and 25.5 ± 0.39°C and 291 113 eggs (Trial 3) respectively. Mean fertilization rate and mean hatching rate were 70.4% and 60.5% (Trial 1), 83.9% and 79.6% (Trial 2), and 62.5% and 57.4% (Trial 3) respectively. We also succeeded in producing ELT larvae in the pre‐spawning season (April–July), although the quantity and quality of larvae produced were inferior to those produced in other calendar months. In trials involving periodic GnRHa administration during the off‐spawning seasons, hatched larvae were obtained in the 10‐m³ tank after six of nine administrations in the 2011–2012 off‐spawning season and in 16 of 19 administrations in the 2012–2013 off‐spawning season. The findings of this study demonstrated that hormonal treatment and thermal control could be used to extend the spawning period in ELT, potentially allowing larval production in the post‐ and pre‐spawning seasons.     

Shifting the natural spring–summer breeding season of the Australian freshwater crayfish Cherax quadricarinatus into the winter by environmental manipulations

Ninety sexually mature Cherax quadricarinatus females were exposed to various combinations of photoperiod and temperature for 2 months during the summer. Females were randomly assigned to either “winter” “semi-winter” or “summer” simulation treatments. In the “winter” treatment, crayfish were exposed to a simulated winter photoperiod (gradual decrease from 14L:10D to 10L:14D, 4 weeks at short day length followed by gradual increase to 14L:10D) and temperature (gradual decrease from 27 to 15 °C, held for 4 weeks, and then gradual increase to 27 °C). In the “semi-winter” treatment, crayfish were exposed to a simulated winter photoperiod and a summer temperature (27–29 °C). In the “summer” treatment, the crayfish were exposed to summer water temperatures and a photoperiod of 14L:10D. Following the 2 months of conditioning, the females were stocked for 7 months in small groups with males under environmental conditions similar to those of the “summer” treatment. All females were individually tagged and molting and spawning events were monitored. Females exposed to “semi-winter” conditioning in the summer, demonstrated a threefold increase in the rate of first spawning during the winter (December–February) compared with the other females. Crayfish breeders can easily implement these findings since shifting the breeding season into the winter only requires shortening of the photoperiod in the summer. The stocking of ponds in the spring with large nursed juveniles that hatched from eggs spawned in the winter, would allow the attainment of market size at the end of the limited growout season in temperate zones.     

Effects of photoperiod on growth and spawning efficiency of Nile tilapia (Oreochromis niloticus L.) broodstock in a recycling system

This study was conducted to evaluate the effects of photoperiod on the growth, feed conversion ratio (FCR) and spawning performance of Nile tilapia (Oreochromis niloticus) broodstock. Duplicate groups of two males (66±3.04 g) and six females (50.5±1.58 g) were stocked in 0.4 m³ fibreglass tanks in a recirculating water system at a male:female ratio of 1:3. The fish were subjected to four photoperiod treatments: 24:0 light:dark (L:D), 18L:6D, 12L:12D and 6L:18D. Light intensity was kept constant at about 2500 lx throughout the study. The fish were fed a commercial tilapia diet (35% crude protein and 16.6 MJ GE kg⁻¹) at a daily rate of 2–3% of tank biomass, twice a day, for 130 days. Males grew significantly faster than females under all photoperiod regimes. The best growth rate and FCR were attained at 18L:6D, followed by 24L:0D, 12L:12D and 6L:18D respectively. The number of eggs per female, number of eggs per spawn and number of spawnings per female were all significantly higher in the 12L:12D treatment than in all other photoperiod cycles. Interspawning intervals and days elapsed per spawn were also shorter in the 12L:12D treatment. The time to first spawning was slightly longer in the 24L:0D and 6L:18D than in 12L:12D and 18L:6D light phases. The 18L:6D and 6L:18D photoperiods produced the lowest spawning performance. It is concluded that a 12L:12D photoperiod regime should be adopted for maximum fecundity, seed production and spawning frequencies of Nile tilapia broodstock reared in intensive, recirculating systems. If maximum reproduction is desired, a near‐natural day length photoperiod should be used.     

Effects of protein and lipid concentrations in broodstock diets on growth, spawning performance and egg quality of yellowfin sea bream (Acanthopagrus latus)

A study was conducted to determine the effects of dietary protein and lipid concentrations on growth, spawning performance and egg quality of yellowfin sea bream, Acanthopagrus latus. Nine diets representing a combination of three protein concentrations (40, 50 and 60%) and three lipid concentrations (15, 20 and 25%) were tested with three replicates. Each replicate was stocked with 10 fish with a sex ratio of 1:1 and the average weights were 415 g and 236 g for females and males, respectively. Fish were fed to satiation twice daily. The weight gain of A. latus broodstock was not significantly (P > 0.05) different among fish fed diets with the various protein and lipid concentrations. Except for relative fecundity and survival rate of 3-day-post-hatch (3DPH) larvae, spawning performance of A. latus was not significantly affected by different concentrations of dietary protein or lipid. Relative fecundity was found to be significantly elevated by dietary lipid at 20% concentration. However, survival rate of 3DPH larvae was highest from broodstock fed diets with 40% protein and 25% lipid, 50% protein and 15% lipid and 60% protein and 15% lipid concentrations. Egg diameter generally increased with increasing dietary protein from 40 to 60% at each lipid concentration. Total length of hatchlings followed an inverse relationship with dietary protein regardless of the lipid concentration. The oil globule diameter (OGD) of hatchlings significantly increased with decreasing dietary protein from 60 to 40%. Yolk sac length (YSL) and OGD of hatchlings was generally greatest at 15% dietary lipid concentration irrespective of protein concentrations. Total length of 3DPH larvae was greatest from broodstock fed diets with 50% protein and 20% lipid, respectively. Body protein content of broodstock was not significantly affected by dietary protein and lipid concentrations. However, increasing dietary lipid generally resulted in increasing body lipid content regardless of the dietary protein concentration. Protein and lipid contents of eggs, hatchlings and 3DPH larvae were significantly affected by dietary protein and lipid concentrations of the broodstock diets. The present study revealed that the best reproductive performance of A. latus broodstock was achieved at 40% dietary protein and 20% dietary lipid concentrations based on relative fecundity, fertilization rate, hatchability and survival of 3DPH larvae.     

Effect of dietary essential fatty acids on level of oestradiol‐17β and vitellogenin,reproductive performance and larval quality of striped catfish (Pangasianodon hypophthalmus) in out‐of‐spawning season

A study was conducted to determine the effect of essential fatty acid in striped catfish (Pangasianodon hypophthalmus) broodstock diets on a level of oestradiol‐17β and vitellogenin, reproductive performance and larval quality in out‐of‐spawning season. Five female and five male broodstock were stocked into a net cage (3 × 5 × 1.5 m³) and fed with pelleted diets containing 1.5% fish oil and different amounts of corn oil (CO) in diet formulation, that is 0%, 1%, 2% and 3% respectively. Gonadal development, oestradiol‐17ß and vitellogenin concentrations were examined every 2 weeks. Oestradiol‐17ß and vitellogenin concentrations showed an increase during the maturation process. The significantly highest percentage of female mature, fecundity, hatching rate, larval production and larval survival rate were obtained from fish fed with 2% corn oil in the diet. This result revealed that the administration of 2% corn oil in the diet as a source of n‐6 (linoleic acid) and 1.5% fish oil as a source of n‐3 (linolenic acid) can improve the reproductive performance of striped catfish broodstock in out‐of‐spawning season.     

Effects of dietary protein and energy levels on spawning performance of Nile tilapia (Oreochromis niloticus) broodstock in a recycling system

The effects of dietary protein and energy level on spawning performance of Nile tilapia broodstock and hatchability of their eggs were studied. Nine diets containing three protein (30, 35 and 40%) and three energy (14.6, 16.7 and 18.8 MJ GE/kg) levels were prepared. The diets were fed to duplicate groups of broodfish (average weight of 60.5 ± 4.6 g for males and 39.4 ± 3.1 g for females) at 2–3% of their body weight, twice a day, for 150 days. At low protein level (30%), time to first spawning was significantly longer than at 35 and 40% protein levels, and increased with increasing dietary energy (decreasing protein-to-energy (P/E) ratio). At 35 and 40% protein levels, time to first spawning was not affected by dietary energy. Inter-spawning intervals (ISI) showed irregular patterns in relation to dietary protein and energy, however, at 40% protein the females tended to spawn at shorter intervals, regardless of dietary energy. Spawning performances, including total number of spawnings per tank, number of spawnings per female, absolute fecundity and average number of eggs per spawn, were all significantly lower (P < 0.05) at 30% CP than at higher protein levels. At all protein levels, increasing dietary energy from 14.6 to 18.8 MJ GE/kg (and decreasing P:E ratio) resulted in a significant decrease in fish fecundity. Egg size was not significantly affected by dietary protein and energy. At all energy levels, egg hatchability increased with increasing dietary protein levels (P < 0.05). Eggs produced from broodstock fed 30 and 35% CP, exhibited significantly lower hatchability (P < 0.05) and needed less time for hatching and yolk-sac absorption, and resulted in lower larval length than those fed a 40% protein diet. However, increasing energy level at each dietary protein level did not significantly affect these parameters. These results revealed that the best spawning performance of Nile tilapia broodstock reared in a recycling system was achieved at 40% dietary protein and 16.7 MJ GE/kg, with a P/E ratio of 23.6 g/MJ.     

Dietary supplementation of probiotics and influence on feed efficiency,growth parameters and reproductive performance in female rainbow trout (Oncorhynchus mykiss) broodstock

The aim of this study was to determine the effects of dietary probiotics on reproductive performance, growth, haematological and biochemical parameters of female rainbow trout broodstock. A total of 60 broodstocks with an average initial weight of 2,267.4 ± 54.9 g were divided into four groups and three replicates in raceway ponds. Fish were fed with diets containing 0 (control), 1 × 10⁹ (P1), 2 × 10⁹ (P2) and 4 × 10⁹ (P3) CFU probiotic per kg diet twice a day for 8 weeks before spawning season. Results revealed that higher levels of dietary probiotics (P3) enhanced protein efficiency ratio and decreased feed conversion ratio (p < .05). There were no significant differences in haematological and other growth parameters among different treatments. According to the results, probiotic‐fed treatments had the lowest levels of cholesterol (237.7 ± 4.3 mg/dl) and the highest levels of total protein (11.3 ± 0.3 g/dl) and albumin (6.9 ± 0.3 g/dl), so that differences were found between P3 and control groups in all cases (p < .05). Based on the observations from reproductive parameters, the highest egg diameter, working and relative fecundities, fertilization and hatching rates, eyed eggs survival and alevins survival rates up to the absorption of the yolk sac were observed in P3 treatment (p < .05). Moreover, eyeing, hatching and yolk sac absorbing stages in P3 treatment were earlier than those in other treatments (p < .05). The results of present study demonstrated that probiotic supplementation improved reproductive performance and some growth and biochemical parameters in female rainbow trout broodstock and the best performance was observed in fish fed 4 × 10⁹ CFU probiotic per kg diet. Due to an increase in egg production and egg size in probiotic‐fed treatments, this feed additive can be used as a suitable ingredient to increase high‐quality egg production in rainbow trout breeders.     

Growth and production of the Amazonian tambaqui in fixed cages under different feeding regimes

Experimental culture of the native Amazonian fish tambaqui, Colossoma macropomum, in fixed cages was carried out over a period of 8 months, in Lake Urubu (Rio Grande do Norte, Brazil), to assess the viability of fixed cage culture of tambaqui and to test the influence of diet on growth rates. Nine synthetic net cages (1 m³) were each stocked with 45-day-old fish (mean weight 3 g; mean total body length 51 mm) at a density of 34 fry m^–3. During the first 2 months of culture, fish were fed a balanced formulated feed on an as-fed basis at the rate of 5% body weight day^–1. During months 3–8 this continued for fish in treatment 1 while those in treatment 2 were fed tropical regional fruits, on a wet weight basis at the rate of 5% body wt day^–1. Fish in treatment 3 were given no supplementary feed. Monthly biometric measurements were made on all fish. Fixed cage fish culture was shown to be a viable and simple technique. Survival in all treatments was 100%. With balanced supplementary feed, production was 14.4 kg m^–3, compared with 4.9 kg m^–3 and 2.1 kg m^–3, respectively, in the treatments where fish were fed with fruits and were not given any supplementary feed.     

Maturing, Spawning and Egg Collecting of the White Shrimp Penaeus vannamei Boone in a Recirculating System

A specially designed recirculating system was used to maintain P. vannamei broodstock for maturation, mating and spawning in the same tank. Two groups of different size shrimp were stocked in this system in Phase I (8 May to 5 September 1989) and Phase II (5 September to 5 December 1989). Results showed that this recirculating system was able to maintain good water quality keeping broodstock maturing, mating and spawning with high stocking density (10 shrimp/m²) and low mortality (6%). Seventy-eight percent of spawned eggs were collected by a submerged egg collector. Mating success was very poor (3.4%) in Phase I even though there was normal spawning frequency. Mating success and nauplii production were much improved in Phase II. During Phase II 104 male and female broodstock housed in a 6 m³ tank produced 1.6 ± 10⁶ nauplii (with 70% mating success) in three months. This study illustrated that broodstock size was a critical factor of successful mating and fertilization for P. vunnamei , and also, reduction of handling stress and lowered disturbance of the shrimp greatly enhanced the mating success.     

Off-Season Spawning of Eurasian Perch Perca Fluviatilis

This study was designed to determine the effects of the selection for body weight and of two final warming durations during an out-of-season reproductive cycle on gonad recrudescence, plasma steroid levels and spawning in Eurasian perch Perca fluviatilis males and females. Two different bodyweight classes (200–250 and 350–400 g) and two different warming periods (1 or 2 months) were studied.At the end of the chilling period, a significant effect of the selection for body weight was observed on gonad development whereas fish were the same age. According to higher levels in testosterone (12 ng mL^–1), a better development was recorded for smaller breeders (P < 0.05). A short water warming period up to 14°C (1 month v.s. 2 months) allowed higher rates of females with completed gonadogenesis (33–38% v.s. 7–8%) and spawning (31–33% v.s. 0%) (P < 0.05). Spontaneous out-of-season spawning (17 ribbons) and larvae were obtained. Relative fecundity was about 100 eggs g^–1 of female body weight. Five ribbons were partly fertilized with a rate ranging from 5 to 90%. These studies have shown that delayed spawning in Eurasian perch can be obtained by a temperature manipulation, but results are not sufficient because of low stimulation rate of females, low spawning rates and high heteroge-neity of fertilization rates. The effects of other environmental factors like photoperiod should be tested in further experiment to improve the protocol inducing out-of-season spawning production.     

The effect of stocking density on growth,survival and agonistic behaviour of African catfish

A high rate of sibling cannibalism is one of the principal obstacles in the rearing of larvae and juveniles of the African catfish Clarias gariepinus. This paper examines the underlying behavioural components of agonistic behaviour under culture conditions. Ten day old catfish larvae were stocked at 30 larvae l^–1 in three tank designs with different surface areas and equal volumes. Stocking densities were 1.2, 0.6 and 0.3 fish cm^–2 bottom surface. Growth did not differ between treatments. The highest mortality over 30 days was recorded at the medium density (0.6 larvae l^–1). Fish stocked at this density showed the highest rate of aggression, while there was no difference in aggression between the highest and the lowest stocking densities. The results indicate that stocking density should be at least as high as 1.2 larvae cm^–2 bottom surface area to obtain high production at best survival rates.