Fry Length Research Articles

Growth Performance of Mirror Carp (Cyprinus carpio var. specularis, Lacepède, 1803) Fry Fed with Varying Protein Content Feeds

An experiment was conducted in aquaria for seven weeks to examine the effect of varying dietary protein level on growth, FCR (food conversion ratio) and survival of mirror carp fry. Similar weight (2.29 ± 0.08g) and length (4.21 ± 0.05 cm) of mirror carp fry were stocked in the aquaria (1 fry/L) and fed laboratory prepared feeds containing 25% (T1), 30% (T2), 35% (T3), 40% (T4), and 45% (T5) protein. In terms of growth and FCR, diet containing 30% protein performed significantly (p<0.01) better than those of other tested diets. Survival rate, on the other hand, dropped significantly (p<0.01) when the stocks were fed diets containing over 30% protein. These findings suggest that about 30% protein in diet appears to be sufficient for obtaining optimum growth of mirror carp fry.DOI: http://dx.doi.org/10.3126/on.v10i1.7747

Growth and Survival of Fingerlings of a Threatened Snakehead, Channa striatus (Bloch) in Earthen Nursery Ponds

Nursery rearing of a high-valued threatened snakehead fish, Channa striatus was studied in relation to varying stocking densities in earthen ponds. The experiment was conducted for eight weeks in nine earthen nursery ponds having an area of 0.012 ha with an average depth of 0.8 m. Fry produced from natural propagation was first reared in indoor cemented cisterns with hatched Artemia cyst as live food for 10 days and then stocked at 150,000, 200,000 and 250,000 ha-1 in treatment-1 (T1), treatment-2 (T2) and treatment-3 (T3), respectively. At stocking, mean length and weight of fry was 1.17±0.18 cm and 0.15±0.03 g, respectively. Fry in all the experimental ponds were fed with a supplementary feed comprising of fish meal (50%) and mustard oil cake (50%) at the rate of 5-8% of the estimated body weight. In addition, chopped trash fishes were supplied at the rate of 5% of the estimated biomass daily, till harvest. The physicochemical parameters of water and plankton were monitored simultaneously and were within the acceptable range for fish culture. Growth parameters (final weight, final length, weight gain, length gain and specific growth rate) and survival of fingerlings were significantly higher in T1 than those in T2 and T3, respectively. Feed Conversion Ratio (FCR) was significantly lower in T1 followed by T2 and T3 in that order. Significantly higher survival of fingerlings was obtained in T1 than those in T2 and T3. Overall, highest growth and survival were obtained from T1 where stocking density of fry was 150,000 ha-1. Hence, of the treatments evaluated, stocking density of 150,000 fry ha-1 appears to be the most efficient stocking density for rearing of C. striatus fingerlings in earthen nursery ponds. This study represents the first successful attempt to produce fingerlings of the threatened C. striatus in nursery ponds, the findings of which might immensely be helpful towards the protection of snakehead from extinction as well as for its conservation, stock enhancement and rehabilitation.

Effect of Selected Diets on the Growth and Survival of Snakehead Fish (Channa striatus) Fry

Twenty-five days feeding trial was conducted to evaluate the growth and survival of Channa striatus fry fed with three types of fresh foods (bloodworm, trash fish and Acetes shrimps). Ten fry were stocked in each aquarium (46x46x37 cm) and fed ad libitum twice a day (1000 and 1700 h). All treatments were triplicate. The fry fed trash fish showed a significantly (p<0.05) higher weight gain percentage (376.50±20.74%) than those fed with Acetes shrimp (233.05±10.18%) and bloodworm (199.08±17.25%). The final mean total lengths of fry fed with trash fish, bloodworm and Acetes shrimps were 7.91±0.23, 7.28±0.23 and 7.21±0.17 cm, respectively. Fry fed with trash fish also showed the best Specific Growth Rate (SGR) value (6.24±0.17% day-1) followed by Acetes shrimp (4.81±0.12% day-1) and bloodworm (4.33±0.22% day-1). The best Food Conversion Ratio (FCR) value (3.63±0.27) was found among fish fed with trash fish compared to those fed Acetes shrimp (7.41±0.88) and bloodworm (11.48±1.51). It could be concluded that trash fish was an excellent feed for C. striatus fry in term of growth, weight gain and FCR.

Parental Male Effects on Landlocked Fall Chinook Salmon Progeny Survival

AbstractThe Lake Oahe, South Dakota, population of landlocked fall‐run Chinook salmon Oncorhynchus tshawytscha is maintained entirely by hatchery propagation and exhibits relatively poor egg survival during hatchery incubation. This study was undertaken to determine the influence of male gametes on embryo survival. Eggs from an individual female were subdivided and subsequently fertilized with milt from four discrete males. This was repeated with three additional females using the milt from the same four males. This entire procedure was then replicated three times, using four new females and four new males each time, for a total of 16 males and 16 females. The eggs from each unique cross were then incubated discretely. There was no significant effect of spawning males on subsequent embryo survival to the eyed stage of egg development. Swim‐up fry length and weight were also not significantly affected by male parentage. In contrast, there was a significant maternal effect on eyed egg survival, and swim‐up fry length and weight, which varied significantly among progeny from individual females. These results suggest that the relatively poor survival exhibited by Lake Oahe landlocked fall Chinook salmon eggs during hatchery incubation is largely a function of initial egg quality from spawning females.

Effect of different protein levels of fry feed on the production of quality tilapia (Oreochromis niloticus) fry

A 4-week long feeding experiment was conducted to evaluate the effect of different protein levels of quality Tilapia (O. niloticus) fry production in Agro-3 Fish Hatchery and Culture Farm, Boilor, Trishal, Mymensingh. Four days old 1247 fry (length 0.8 cm and weight 0.012 g) were stocked at the density of 1560/m2 in twelve synthetic hapas. Four different treatments (T1, T2, T3 and T4) in three replications were used having different level of protein containing diets (T1,=35.95%, T2=40.43%, T3=45.61% and T4=50.85%) in the experiment. Feeds were supplied to fry at 4 h intervals during 24 h cycle upto satiation level. Sampling was done at 7 days interval throughout the experiment. The best growth was obtained at 35.95% protein level in T1 treatment. Therefore, a follow up second experiment with feeds having protein levels 30.12%, 33.42%, 36.19% and 39.01% encircling the best performed diet (35.95% protein) of the first experiment was conducted. Significantly (P&lt;0.05) higher mean weight gain (g), SGR (%/day), PER, survival rate (%) and lower FCR value were found at 36.19% protein containing diet. Thus, from the results of the two experiments conducted in two consecutive years it can be said that the feed containing approximately 36% protein is the best for the production of quality tilapia fry. If farmers use 36% protein instead of 40% then it will be more economic because fish meal is costly. Further experiment can be carried with feed containing 36% protein from cheaper ingredients other than fish meal to reduce the feed cost. DOI: http://dx.doi.org/10.3329/jbau.v9i2.11053 J. Bangladesh Agril. Univ. 9(2): 359–364, 2011

Stocking density effects on growth and production of the threatened silurid catfish,Mystus cavasius(Hamilton) fingerlings in nursery ponds

The fingerling-rearing experiment of the threatened catfish, Mystus cavasius was carried out at different stocking densities in earthen nursery ponds. Twelve-day-old fry were stocked at 200 000 ha−1 in treatment-1 (T1), 250 000 ha−1 in treatment-2 (T2) and 300 000 ha−1 in treatment-3 (T3) respectively. The mean length and weight of fry at stocking was 1.24 ± 0.25 cm and 0.11 ± 0.04 g respectively. Fry in all the experimental ponds were supplemented with SABINCO nursery feed for the first 14 days and starter-I feed for days 15–56. The physico-chemical parameters and plankton population of pond water were within the suitable level for fish culture. Growth in terms of final weight, final length, weight gain, length gain and specific growth rate and survival of fingerlings were significantly higher in T1 than those in T2 and T3. Feed conversion rate was significantly lower in T1 followed by T2 and T3 in that order. Significantly higher number of fingerlings was produced in T3 than that in T2 and T1. Even then, consistently higher net benefits were obtained from T1 than those from T3 and T2. Among the treatments evaluated, 200 000 fry ha−1 was the best stocking density considering the highest growth, production and net benefits of fingerlings of M. cavasius in nursery ponds.

The influence of triiodothyronine (T3) on the early development of piracanjuba (Brycon orbignyanus)

This paper reports the triiodothyronine's (T3) effects on the early growth and survival of piracanjuba (Brycon orbignyanus) produced from fertilized eggs hormone exposed. The study was carried out in two phases. In the first phase, eggs divided in 6 batches were immersed in T3 solutions: 0.01; 0.05; 0.1; 0.5 ppm; 1 ppm and control (no T3). After a 15-min immersion, eggs were transferred to incubators where larvae were kept up to 72 h after hatching. Larval weight, length and yolk sac volume were determined every 12 h. Sixty and 72 h after hatching, larvae exposed to 0.5 ppm T3 were significantly heavier than the others, and those exposed to 1 ppm T3 showed the lowest weight. The yolk sac absorption was not affected. In the second experimental phase, the resulting fry from the first phase were stocked into 3 boxes per treatment (5 larvae L(-1)) and fed with plankton, fish larvae and feed prepared in the hatchery (48% CP) in the first 3 days, plankton and feed from the 4th to the 10th day and only feed in the next (last) 5 days. Fry weight, length and specific growth rate were determined at 1, 5, 10 and 15 days. Survival was calculated in the last day. In the 15th day, fry length did not differ among treatments but the weight of the control group was higher. Higher survival in the T3-treated groups suggested lower predation among fry. The results allowed us to conclude that there was no expressive effect of T3 on the growth, but it improved the survival of the piracanjuba progeny.

Short Communication: Stocking Density-Dependant Growth and Survival of Asian Sun Catfish, Horabagrus brachysoma (Gunther 1864) Fry During Hatchery Rearing

The present study was conducted to determine the optimum stocking density of Horabagrus brachysoma fry during fingerling production in the hatchery. For this purpose, four density levels (400, 800, 1200 and 1600 fry/0.6 m3) were considered. The experiment of fry rearing continued for a period of six weeks. The total length and wet weight of fry were significantly highest (P < 0.05) at 400 fry/0.6 m3 density compared to the other three density treatments during the entire rearing period. At harvesting, the length and weight of fry stocked at 1200 and 1600 numbers/0.6 m3 were similar to each other, and both were significantly lower (P < 0.05) than those of fish stocked at 800/0.6 m3. The decreased SGR, percent weight gain, and survival were also observed at the two highest density treatments. But the total biomass was observed to be highest (P < 0.05) at 1600/0.6 m3 density compared to those of the other three densities. If individual size and number of individuals for stocking are not constraints, the maximum number of surviving fry from a minimum rearing space was achieved at a stocking density of 1600 fry/0.6 m3 tank.

Growth and Survival of Channel Catfish,Ictalurus punctatus,Fry Fed Diets with 36 or 45% Total Protein and All Plant or Animal Protein Sources

The basic nutrient requirements for channel catfish, Ictalurus punctatus, are well known, with anecdotal evidence suggesting that catfish fry grow faster and have better survival when fed an animal protein diet. However, the ability to grow channel catfish as small as 11 g on all plant diets and a lack of published data showing the superiority of fish or animal proteins compared to nutritionally equivalent plant proteins for catfish fry indicates that it may be possible to raise channel catfish fry on diets with only plant protein sources. Therefore, an experiment was conducted to compare the effects of 36 and 45% animal protein diets and 36 and 45% all plant protein diets on catfish fry growth and survival. Experimental diets were formulated to contain: 36% all plant protein (primarily from soybean meal); 36% animal and plant protein (≥45% of crude protein as fish meal); 45% all plant protein (primarily from soy protein concentrate and soybean meal); and 45% animal and plant protein (≥60% of crude protein as fish meal). The catfish were fed at a rate of 20% of body weight daily for 28 d using 24-h automated feeders. Mean ending weights and lengths of catfish fry were not significantly different (P > 0.05) for any treatment. Mean mortality was also not significantly different (P > 0.05) among diets. Regression analysis of growth rate and analysis of variance of final weights revealed that there was no significant difference in growth rate for any of the four diets. These results indicate that growth is not limited in channel catfish fry fed all plant protein diets, and that there is no apparent advantage to the inclusion of animal protein in diets for channel catfish fry.

The effect of egg size and nutrient content on larval performance: implications to protracted spawning in northern pike (Esox lucius Linnaeus)

Variation in developmental rate from fertilization to swim-up, and body size at swim-up, may affect the growth and survival of young-of-the-year fish. Fish egg size (diameter) is often positively correlated to adult female size, but whether increased egg size equates to higher egg nutrient content and subsequently improved embryo/larval performance, remains unclear. Artificially fertilized northern pike eggs from individual females (total length 400–800 mm) were cultured under temperature controlled laboratory conditions to test the hypothesis that female body size positively influences egg size and the amount of specific nutrients (carbon, nitrogen, phosphorus, and fatty acids) allocated to eggs. We further hypothesized that greater egg nutrient content would positively influence egg survival, developmental rate, and the size of swim-up fry. These hypotheses were investigated in the context of two different northern pike spawning strategies (early season tributary vs. late season deep shoal spawning). Nutrients were allocated conservatively in northern pike eggs, showing very little variation in nutrient concentrations, but the total mass of all egg nutrients increased linearly with egg dry mass. Neither egg dry mass nor nutrient content (concentration or mass), were related to either egg diameter or female body size. The mass of individual egg nutrients was, however, strongly correlated with egg dry mass (r 2 range 0.62 to 0.99 for individual nutrients) and positively related to the total length of swim-up fry (r 2 = 0.58). The eggs of late spawning pike had significantly greater dry mass (average = 3.02 mg/egg) and developed more rapidly to swim-up (average = 17.89 days) than did those of early spawners (average = 2.28 mg/egg, 19.05 days). Our results indicate that egg quality may be best assessed by egg dry mass, which was correlated with greater nutrient mass and increased swim-up fry body length, as opposed to egg diameter or female body size which showed no correlation to egg nutrient composition and egg/larval performance.

Effects of copper toxicity on growth, reproduction and metal accumulation in chosen ornamental fishes

The 96h LC50 values of copper for Carassius auratus and Xiphophorus helleri were 0.30 and 0.36ppm. Sublethal concentrations of copper affected specific growth rate and reproductive performance in both species. Metal accumulation in muscle and gonad tissues was linearly increased with increase in sublethal levels of copper and exposure period. Young ones accumulated same or more amount of metal found in gonad tissue of parent fish and it might be due to the direct transfer of copper from gonad to young ones during development. Metal accumulation caused significant (P<0.05) reduction in mean diameter and weight of eggs in C. auratus and mean body length and weight of fry in X. helleri. Ovo-viviparous fish is more severely affected by copper toxicity than oviparous fish.

Raising of Stocking Materials of Indian Major Carps in Pen Enclosures in Selected Floodplain Wetlands of Assam, India

Raised stocking materials, fingerling of Indian major carp seeds (Catla catla, Labeo rohita Cirrhinus mrigala) in pens, installed in two floodplain wetlands of Assam viz. Goruchora (Golaghat district) and 46-Morakollang (Morigaon district). The pens were stocked at 30,000 fry•ha-1, in a ratio of 3:2:1 of C. catla, L. rohita and C. mrigala fry, respectively. The average initial length and weight of fry at the time of stocking for C. catla, L. rohita and C. mrigala were 5.45 cm (1.80 g), 4.51 cm (1.15 g) and 4.65 cm (1.12 g), respectively. The fish stocks inside the pens were given supplementary feeds, mustard oil cake and rice polish (1:1) at 5% body weight. The pens were harvested after a rearing period of 145 days with an average growth of C. catla (19.0 cm, 85.1 g), L. rohita (15.8 cm, 52.8 g) and C. mrigala (15.8 cm, 42.9 g) in Goruchora wetland, while in 46- Morakollang wetland it was recorded as 17.0 cm (63.8 g) for C. catla, 15.4 cm (55.9 g) for L. rohita and 14.0 cm (41.5 g) for C. mrigala for further grow-out in the lakes proper. During the period of experimentation, the fluctuations in water quality parameters like water temperature (17.6-29.0o C), transparency (72-102 cm), dissolved oxygen (3.60-16.50 mg•l-1), pH (7.12-9.53), total dissolved solids (46.6-111.3 mg•l-1) and specific conductance (102-232 µS•cm-1) were monitored, too. The production in terms of fish biomass (fingerlings) was estimated at 2106 and 1780 kg•ha-1 in Goruchora and 46- Morakollang wetlands, respectively. The cost-benefit ratio was calculated as 1.40 (Goruchora) and 1.77 (46-Morakollang), which can be considered as significant, economically.

Evaluation of Hatchery versus Wild Sockeye Salmon Fry Growth and Survival in Two British Columbia Lakes

AbstractWe evaluated the relative success of sockeye salmon hatchery fry stocking in two British Columbia–Alaska transboundary lakes (1,622‐ha Tatsamenie Lake and 492‐ha Tahltan Lake). Fry stocking began in the late 1980s and is still under way. During the study period, survival patterns in the two lakes were different. At Tatsamenie Lake, wild egg‐to‐fry survival was higher than hatchery egg‐to‐fry survival (11.3% versus 4.3%) and wild egg‐to‐smolt survival was higher than hatchery egg‐to‐smolt survival (5.8% versus 2.5%). We found no relationship between fry survival and stocking date, spawner abundance, or food availability in Tatsamenie Lake, but we did find a significant positive relationship between early‐spring fry length and egg‐to‐fry survival. Also, we found that in net‐pen experiments, larger fry survived better. From this, we concluded that hatchery fry stocked into Tatsamenie Lake may suffer from size‐mediated mortality and that net‐pen supplemental feeding could be used to overcome this problem. However, because eggs used for hatchery production come from the lake, fry stocking can only be justified when hatchery fry survival exceeds wild fry survival. At Tatsamenie Lake, this goal has not yet been attained. At Tahltan Lake, wild egg‐to‐fry survival was lower than hatchery egg‐to‐fry survival (1.5% versus 6.3%) and wild egg‐to‐smolt survival was lower than hatchery egg‐to‐smolt survival (3.6% versus 12.8%). Hatchery fry release date and food availability both failed to explain these differences in fry and smolt survival at Tahltan Lake. However, there was a strong negative relationship between total female escapement and wild egg‐to‐smolt survival. This supported the results of an earlier investigation, which suggested that Tahltan Lake was spawning‐site limited. We concluded that at Tahltan Lake, hatchery fry stocking increased smolt numbers, is biologically justified, and should be continued.

Morphometric model and laboratory analysis of intracohort cannibalism in giant grouper Epinephelus lanceolatus fry

During larviculture of giant grouper, Epinephelus lanceolatus , intracohort cannibalism usually results in mass mortality. Thus, in the present study, we attempted to develop a cannibal- prey length relationship from morphometric measurements for giant grouper fry and to verify it with pairwise predation experiments. Based on measurements of morphometric characteristics (i.e. mouth width, body depth, and total length), a model of prey length (mm) to cannibal length was constructed: TL prey = 0.83 TL cannibal - 2.48. According to the equation, approximately 30% is a threshold in total length differences to use for grading giant grouper fry. The threshold was supported by our pairwise experiments using 136 pairs; we found that 33 out of 36 cannibalism incidents occurred only when the prey was of equal or smaller size than that predicted by the equation. In aquaculture practice of this species, we thus suggest that when the length of larger fry exceeded that of smaller fry by more than 30%, the potential cannibals should be removed.

Production of monosex male black crappie, Pomoxis nigromaculatus, populations by multiple androgen immersion

Monosex populations can solve the problem of stunted black crappie populations in small impoundments and can increase the potential of black crappie as an aquaculture species. In this study, we investigated the effective mode and duration of exogenous androgen administration to produce monosex male black crappie populations. We conducted two experiments using the synthetic androgens, 17α-methyltestosterone (MT) and trenbolone acetate (TBA). In the first experiment, the same age (45 days old), two different size fry with mean total lengths (±S.D., N=50) of 20.1±1.4 and 26.1±2.0 mm were either fed 60 mg MT/kg diet for 45 days or immersed in a 1 mg MT/l solution on 10 occasions for 5 h/day every 3–5 days between 45 and 86 days post-hatch (dph). In the second experiment, a different cohort of fry was immersed in a 1 mg TBA/l solution for 5 h/day every 3–5 days either on seven occasions between 40 and 66 dph or on 10 occasions between 45 and 86 dph. Mean total lengths (±S.D., N=50) of fry at 40 and 45 dph were 20.3±0.9 and 21.6±1.2 mm, respectively. Both modes of MT administration were ineffective for altering sex ratios in larger black crappie fry. Oral administration of MT to smaller fry resulted in 23% intersex fish. MT administration via short periodic baths of smaller fry produced 96% male populations. TBA was as potent as MT to induce masculinization in black crappie: both immersion treatments produced all male populations. Results of the present study indicated that initial size of fry as well as initial age should be considered when selecting appropriate size black crappie fry for hormone treatments. Monosex male populations in black crappie can effectively be produced through a series of (seven or less) short (5 h) immersions of young fry (40−45 days old and 20−21 mm) in a 1 mg/l aqueous solution of MT or TBA. Besides high effectiveness, immersion method will be the preferred mode of androgen administration to black crappie because it reduces variation in hormone uptake associated with the oral administration of steroids and provides more flexibility in feeding during the period of gonadal differentiation.

Effect of dietary protein level on the reproductive performance of female swordtails Xiphophorus helleri (Poeciliidae)

A study to determine the effect of increasing levels of dietary protein on swordtail (Xiphophorus helleri), a popular freshwater live bearer, was carried out. Five isocaloric semi-purified diets containing 20%, 30%, 40%, 50% and 60% dietary protein were used. Broodstock performance was evaluated based on growth parameters, proximate content and fry production of female broodstock. Results showed that while the 20% and 30% protein produced the lowest specific growth rate (SGR) values, there was no significant difference between 40% and 60% dietary treatments. The 20% dietary treatment also displayed lowest protein content in both ovaries and muscle of female fish. Fry production was highest from females fed with 50% and 60% protein, followed by the 30% and 40% protein while the diet containing 20% protein produced lowest number of fry. A significant correlation was also obtained between number of fry produced and the weight of female fish, indicating that size is a major factor influencing production. Relative fecundity was lowest for the 20% protein diet followed by the 30–40% and 40–60% protein diets. There were no significant differences in both weight and length of fry produced among the dietary treatments. Proximate analysis of fry also did not show any trend with different dietary protein levels fed to female broodstock. Dietary protein requirement of female swordtail during reproduction is therefore crucial for both the somatic growth and reproduction processes in order for fry production to be optimized. Based on our results, we suggest that a minimum of 30% protein be included in the diet of female swordtail broodstock.

Starvation Resistance in Lake Trout Fry

Newly hatched fry were acclimated to 7 or 12°C and either fed daily (controls) or denied food for varying lengths of time and then fed daily until the end of the study (day 91 at 7°C and day 43 at 12°C). Growth was reduced by delays in the onset of feeding of 27 or more days at 7°C and 7 or more days at 12°C. Mortality of fry unfed for more than 34 days at 7°C, or more than 21 days at 12°C, was higher than among controls. Daily mortality increased with the length of the food deprivation period and did not cease immediately when food was made available, but reached zero by the end of the study. Mortality among unfed fry reached 50% in about 59 days at 7°C and 32 days at 12°C. Study results permitted calculation of the “point-of-no-return” (PNR) mortality, which included the mortality that occurred during the period of food deprivation, and also the delayed component of mortality that was directly attributable to starvation and that occurred after food was made available. The PNR for 50% mortality for food-deprived fry occurred after 52 days at 7°C and 24 days at 12°C. Thus, both measures of mortality indicate that lake trout fry would be highly resistant to death by starvation in the thermal habitat they would be expected to occupy in the Great Lakes. We conclude that a more likely adverse effect of reduced food availability would result from a reduction in growth rate that extends the length of time fry remain small and vulnerable to predation by adult alewives and other non-native fishes with which they associate.

Phase-correction of turbulent distortions of an optical wave propagating under conditions of strong intensity fluctuations.

Phase correction of a plane wave and a spatiolimited beam propagating through a turbulent layer of atmosphere were considered. The required adaptive corrector element size and the system bandwidth were found by numerical simulation. These requirements were determined to be the same as for a weak-intensity scintillation approximation. The size of the required segmented mirror element was found to be equal to Fried length r0, whereas the tolerable time lag was r0/V, where V is the wind velocity. However, the local slope sensors then became impractical, as did tip-tilt correction over the corrector subapertures.

Dispersion of Atlantic salmon (Salmo salar) fry from competing families as revealed by DNA profiling

Minisatellite-based DNA profiling was used to investigate the dispersion of synchronously spawned families of Atlantic salmon (Salmo salar L.) fry from artificial nests in a natural stream. By the end of the summer, i.e., 17 weeks after hatching, detected dispersion was mainly downstream and less than 1 km. Within this distance, three families that had been stocked together showed different patterns of dispersion, with the relative abundance of each family changing systematically with distance downstream from the nest, but with no monopolization of any area or habitat type by any one family. The length of fry also changed systematically with distance downstream, with the patterns of change depending on family. For each family, fry were larger closer to the nest. Changes in habitat type had a common effect on the density and length of fry from all the families.

Development and evaluation of a stress challenge testing methodology for assessment of Nile tilapia (Oreochromis niloticus, Linn.) fry quality

A decline in the quality of the freshwater fish seed available to small-scale farmers in Asia has been identified as an important constraint on the development of aquaculture in the region. There is a need for simple and reliable methods of determining the quality of seed at the farm gate or hatchery. The present study was carried out, in the field, to develop and evaluate a stress challenge test for Nile tilapia (Oreochromis niloticus, Linn.) fry as the effects of stress on fish performance have been well documented and similar tests are used elsewhere in the sector. Formalin and iodized salt were used as reference toxicants. LC50 values were estimated using mortality figures resulting from exposure of marketable-sized fry to saline and formalin solutions. The two test concentrations, 24 ppt for saline and 500 p.p.m. for formalin, were used as reference toxicants in acute, static bioassays to challenge fry reared under different conditions during the post sex-reversal, nursery period. Using existing farm practice as controls the conditions were: 50% increased stocking density, formalin treatment, 40% reduction in feed rate and no supplementary feed. Fry length proved to be a very significant co-variable; adjusted means showed no significant difference between treatments. The importance of only marketing tilapia fry weighing over 0.2 g was emphasized. It was recommended that the challenge testing approach could be modified, but that it was vulnerable to unquantifiable and uncontrollable factors arising from management and the environment, which may have altered responses to the stressor.