Mean Specific Growth Rate Research Articles

Dietary potentials of the edible larvae of Cirina forda (westwood) as a poultry feed

An experiment was conducted to determine the performance of broiler chicks to the replacement of fishmeal with the larvae of Cirina forda. Three diets, namely Diet A (100% C. forda larvae and 0% Fish meal); Diet B (50% C. forda larvae and 50% Fish meal); and Diet C, which was the control (100% Fish meal and 0% C. forda larvae) were compounded and fed to the birds. The results showed that the consumption rate, the mean weight gain and specific growth rate of birds fed with Diets A and B did not differ significant (P > 0.05) when compared with the control. The potentials of the edible C. forda larvae as a protein source, which can replace and elicit similar growth trends in broiler chicks like the protein present in conventional fish meal, are discussed.

Tilapya Balığı (Oreochromis niloticus L.) Yavrularının Balık Unu Yerine Farklı Oranlarda Soya Unu İlave Edilen Yemlerle Beslenmesinin Büyüme Parametrelerine Etkisi

1 Abstract: In this study, the effect of replacement of fish meal by defatted soybean meal with the dietary lysine supplementation in prepared diet was investigated in Nile tilapia fingerlings. Three different experimental diets (all containing mean 35.8% crude protein and 2.6 DE) at a various ratios of soybean meal has prepared. Diet 1 (Group 1; Control) Protein from fish meal is 25.13 % of crude protein. Control (grup 1) diets were composed of fish meal completely. Experimental diets group 2 and 3 were composed of 30% and 50% defatted soybean protein respectively instead of fish meal. The experiment was carried out in a closed rearing system with recirculated filtered water for 12 weeks. Highest mean weight gain (MWG) and specific growth rate (SGR) were obtained group 1 fed with diet 1 (646,15 % ; 2.39) and was significantly better than all other levels (P 0.05 statistical significance level. In addition, all parameters except FCR and MWG were not significantly different between group 2 and group 3 (P<0.10). Based on all measured criteria, these suggest that 50 % of fish protein from fish meal can totally be replaced by defatted soybean meal without adverse effect on fish performance.

Comparison of Hatchery Performance, Agonistic Behavior, and Poststocking Survival between Diploid and Triploid Rainbow Trout of Three Different Utah Strains

AbstractThe hatchery performance (growth, feed conversion, and survival) of rainbow trout Oncorhynchus mykiss was compared between diploid and triploid fish from three Utah strains: Fish Lake‐DeSmet (FD), Sand Creek (SC), and Ten Sleep (TS). For FD, specific growth rates were slightly higher for triploids (2.79%/d) than for diploids (2.60%/d), but final mean weight at 108 d did not significantly differ. For SC and TS, there were no significant differences in final mean weight or specific growth rates after 138 or 122 d, respectively. Feed conversion ratios and fish mortality in the raceways did not differ significantly between diploids and triploids, except for SC for which mortality rates were slightly higher for triploids (4.0%) than diploids (2.1%). Possible differences in agonistic behavior between diploids and triploids were assessed by video observation of size‐matched pairs (diploid‐diploid, diploid‐triploid, or triploid‐triploid). There were no significant differences between diploid and triploid fish of any strain in the number of chases or counterattacks. To determine whether there were any differences in susceptibility to stocking and handling stressors, each strain was transported at high density for 4 h, then stocked into (1) tanks with ambient hatchery well water (control), (2) tanks with either high pH and high temperature (FD) or high temperature (TS and SC), or (3) the tail end of the raceway from which it came. There were no significant differences in mortality after 96 h between diploid and triploid fish for any of the three strains for any of the treatments. Poststocking survival of FD in a small, spring‐fed lake was assessed by gill netting in the spring after fall stocking. No significant difference in overwinter survival was noted between diploid and triploid FD. Overall, the data indicate that triploid SC, FD, and TS rainbow trout perform equally well in the hatchery as diploid fish.

Tambaqui growth and survival when exposed to different photoperiods

The use of different photoperiods (light) were investigated during tambaqui (Colossoma macropomum) juvenile growth under captivity. Light intensity tested was: continuous dark (24hrs without light), natural photoperiod simulation (10hrs of light and 14hrs without light) and continuous light (24 with light). No mortality was recorded among treatments. Significant differences was observed after 50 days of experiment among mean fish weight, fish kept under a continuous darkness showed a better specific growth rate (6.02%) when compared to control fish (natural photo period, 3.67%). Fish exposed to continuous light presented the lowest mean specific growth rate (2.04%). It is possible to improve tambaqui juvenile weight gain performance when kept under continuous darkness.

P33 In vitro perfusion biofilm model for the growth of oral microbes associated with oral malodour

Objectives Oral malodour is mainly due to the generation of volatile sulphur compounds (VSC) by oral anaerobes present in the tongue biofilm. The aim of this study was to evaluate the use of a biofilm system to model the ecology and VSC physiology of tongue biofilm microcosms. Of particular interest was the use of the model to maintain a high diversity of bacterial groups and study their response to environmental changes including exposure to VSC substrates and inhibitors.Methods Biofilms were grown using a Sorbarod perfusion model modified by supply of anaerobic gas to grow oral anaerobes. Sorbarods were inoculated using tongue scrape suspension containing 3 × 108 cfu ml−1 and pre‐incubated anaerobically for 24 h at 37°C to promote cell adhesion. After 24 h, the biofilm system was perfused at 12 ml h−1 with BHI (0.74 g L−1) supplemented with haemin, cysteine and/or DTT. Biofilms and eluates (released cells) were sampled up to 96 h and numbers of broad bacterial groups enumerated by selective media. VSC production was measured using a HalimeterTM.Results Repeated analysis of biofilms showed that a quasi‐steady state was achieved between 48 and 96 h. The mean specific growth rate was 0.018 h−1. Ecological analysis showed all broad groups were maintained at a constant proportion reflecting the original inoculum for both biofilms and eluates. VSC specific activity was upregulated when biofilms were perfused with appropriate VSC‐substrates (cysteine, glutathione and methionine). In situ exposure of the biofilms to pulses of putative inhibitors (Zn, H2O2 and chlorhexidine) at a range of concentrations typically used in vivo gave dose‐dependent inhibition of VSC production (for Zn and H2O2) and reduced viable numbers by one to two log‐fold (H2O2 and chlorhexidine).Conclusions The modified Sorbarod perfusion model is suitable for maintaining high diversity biofilms representative of the tongue flora in terms of both ecological behaviour and response towards VSC substrates and inhibitors.

Growth performance of juvenile Russian sturgeon (Acipenser gueldenstaedtii Brandt &amp; Ratzenburg, 1833) at two stocking densities in net cages

Summary The growth performance of juvenile sturgeon (Acipenser gueldenstaedtii Brandt & Ratzenburg, 1833) was studied in experimental net cages (2 m · 2m · 2 m) in freshwater Sapanca Lake. Fertilized eggs were imported from Krasnodar, Russia in January 2001. Two experimental stocking densities were employed (12 and 8 ind. m )3 ). Sturgeon were hand-fed a pelleted commercial trout feed twice daily throughout the 203-day experiment period. Fish were grown at ambient temperature (12.7–28.5� C) and natural photoperiod. Initial and final mean body weights were 279.5 g (±31.27) and 1112.8 g (±234.77) in the first group, and 271.1 g (±28.21) and 1140.5 g (±213.31) in the second group, respectively. Results were a fourfold biomass density increase in both trials from an initial 3.48 to 12.4 kg m )3 (high density) and 2.2 to 8.8 kg m )3 (low density). Mean specific growth rate (SGR) was determined as 2.69% day )1 in both groups. Mean condition factor (CF) at the end of the experiment was 0.36 ± 0.05 in the high-density group and 0.41 ± 0.05 in the low-density group. Mean feed conversion ratio (FCR) was calculated to be 5.7 and 5.8, respectively. Survival was 92.7% in the first group (high density) and 96.1% in the second group (low density). The results demonstrated that sturgeon juveniles can be raised in cages and that growth performance (CF, SGR and FCR) and mortality of sturgeon were not significantly different at the two stocking densities (P > 0.05) tested.

Competition between Hatchery‐Reared and Wild Juvenile Chinook Salmon in Enclosures in the Sacramento River, California

AbstractWe conducted two types of experiments in the upper Sacramento River, California, to test the effects of hatchery‐reared juvenile Chinook salmon Oncorhynchus tshawytscha on the emigration, growth, and survival of their wild counterparts. In 3 years of displacement experiments, emigration rates from 8‐m2 enclosures into downstream traps were similar between control enclosures that contained 40 wild fish and treatment enclosures to which 33 or 40 hatchery fish were also added. The mean number of wild fish in enclosures at the end of experiments differed by less than one fish between treatments and controls during all 3 years, indicating that hatchery fish prompted few wild fish to emigrate. In 2 years of competition experiments wherein fish could not emigrate, the enclosures contained wild fish (40 fish), wild fish plus hatchery fish (40 wild fish plus 33 or 40 hatchery fish), or wild fish at the same total density as the treatment with wild plus hatchery fish (73 or 80 wild fish). During 2001, survival and specific growth rates of wild fish were similar among treatments, probably because resources were not limiting. During 2002, survival was similar among treatments, but the mean specific growth rate was 0.008 g/d higher in the treatment with 40 wild fish than in the treatment with wild plus hatchery fish (95% confidence interval (CI), 0.005–0.011 g/d). These data indicated a negative effect on wild fish growth of adding hatchery fish. The specific growth rate in the high‐density wild fish treatment was intermediate and 0.003 g/d higher than in the treatment with hatchery fish (95% CI, −0.0005 to +0.006 g/d), providing some evidence that hatchery fish had a greater negative effect on wild fish growth than an equal density of wild fish did.

Comparison of Hatchery Performance Attributes among Three Strains of Age‐0 Coaster Brook Trout

AbstractThe hatchery performance attributes of three strains (Lake Nipigon [Nipigon], Siskiwit Bay [Siskiwit], and Tobin Harbor [Tobin]) of fingerling coaster brook trout Salvelinus fontinalis were compared during a 12‐week laboratory experiment. Despite similar initial sizes, the final mean length and weight of Nipigon fish (94.1 mm and 9.9 g) were significantly greater than those of the Siskiwit (86.7 mm and 7.7 g) and Tobin (90.4 mm and 7.2 g) strains. There were no differences in mean specific growth rates in length and weight or mean daily growth rate in length among the strains. However, there was a difference in the mean daily absolute growth rate in weight, the Nipigon and Siskiwit strains (0.10%/d and 0.08%/d, respectively) displaying a greater rate of growth than Tobin Harbor (0.07%/d) fish. Mean survival rates (range = 97–98%) and relative weights (range = 100.1–101.1 g) did not differ significantly among the three strains. Based on our results, we recommend that these strains of fingerling coaster brook trout be reared similarly in hatchery environments and that strain selection for rehabilitation stocking purposes consider hatchery residence time and the natural genetic variation of the source population of broodstock fish.

Hatchery Performance Attributes of Juvenile Lake Sturgeon Fed Two Natural Food Types

AbstractThe use of stocking programs to rehabilitate depressed populations of lake sturgeon Acipenser fulvescens requires the optimization of early life stage growth in hatchery settings. We evaluated the performance of juvenile lake sturgeon fed different proportions of two natural food types—adult brine shrimp Artemia spp. and larval bloodworms Chironomus spp.—over a 10‐week laboratory experiment. The following five diet treatments were provided: (1) 100% bloodworms; (2) 75% bloodworms : 25% brine shrimp; (3) 50% bloodworms : 50% brine shrimp; (4) 25% bloodworms : 75% brine shrimp; and (5) 100% brine shrimp. Lake sturgeon growth was positively related to bloodworm content in the diet and was significantly different among treatments. Although the initial sizes of fish were similar, mean fork length and wet weight were greatest for the 100% bloodworm treatment (183.1 mm and 37.5 g, respectively) and lowest for the 100% brine shrimp diet (118.4 mm and 9.5 g) by the end of the study. Mean specific and absolute growth rates in length (0.93% and 1.28 mm/d, respectively) and weight (2.61% and 0.45 g/d) were greatest for the 100% bloodworm diet, followed by the other treatments in order of decreasing amount of bloodworms in the diet. Percent weight gain (525%) and food conversion efficiency (67%) were also greatest for the 100% bloodworm diet, followed by the other treatments in order of decreasing dietary bloodworm content. Whole‐body protein (13.9%), lipid (2.5%), and gross‐energy (3.77 kJ/g) content were greatest for fish fed 100% bloodworms, with these proximate‐composition measures also ranked in order of decreasing dietary bloodworm content for the other treatments. We recommend that juvenile lake sturgeon reared in hatchery environments be fed high proportions of bloodworms to maximize growth, body size, and lipid content for rehabilitation stocking programs.

Polyculture of milkfish Chanos chanos (Forsskal) and the red seaweed Gracilariopsis bailinae (Zhang et Xia) in brackish water earthen ponds

Growth, net production, and survival rates of milkfish cultured with Gracilariopsis bailinae at two stocking density combinations (T1– 30 fingerlings 100-m−2 pond+1-kg G. bailinae 4-m−2 net cage, T2– 30 fingerlings 100-m−2 pond+2-kg G. bailinae 4-m−2 net cage) in brackish water earthen ponds over four culture periods were determined. The control (T3) was stocked at 30 fingerlings 100-m−2 pond. Specific growth and production rates of G. bailinae were also calculated. There were no significant differences in mean growth, survival, and net production rates of milkfish between the three treatments. Irrespective of stocking singly or in combination with G. bailinae, significantly higher mean growth and mean production rates for milkfish were obtained during the third culture period of year 1 than those obtained from the other culture periods. Survival rates were not significantly different among the four culture periods. There were no significant differences in mean specific growth and mean net production rates between the two stocking densities of G. bailinae. Significantly higher mean specific growth and mean net production rates of red seaweed were also obtained during the third culture period of year 1 than those obtained from other culture periods. The production of milkfish and red seaweed was higher during the dry season. Growth rates of milkfish was positively correlated with temperature and salinity, while net production rates were positively correlated with temperature and total rainfall, but was inversely correlated with dissolved oxygen. G. bailinae growth and net production rates were positively correlated with water temperature and salinity. Results show that milkfish can be polycultured with G. bailinae grown in net cages in brackish water ponds at stocking density combination of 30 fingerlings 100-m−2 pond+1-kg G. bailinae 4-m−2 net cage.

Growth, survival, haemolymph osmolality and organosomatic indices of the western king prawn (Penaeus latisulcatus Kishinouye, 1896) reared at different salinities

The growth, survival, haemolymph osmolality and organosomatic indices of the western king prawn, Penaeus latisulcatus, (2.95±0.26 g mean initial weight), were measured when reared at 10, 22, 34 and 46 ppt salinity for 60 days. Mean final weight, total length and specific growth rate (SGR) of the prawns were highest at 34 ppt. Rearing at different salinities did not alter moult increments and intermoult periods of the prawns. One hundred percent mortalities were observed at 10 ppt.Survival was highest at 22 ppt and food conversion ratios were significantly lowest (P<0.0.5) in prawns cultured at 22 and 34 ppt. Haemolymph osmolality increased with the increase in medium salinity and physiological age of the prawns. Isosmotic points calculated from regression between haemolymph and medium osmolality were 28.87, 29.46 and 31.73 ppt at 0, 20 and 60 days of culture, when body weights were 2.95±0.26, 4.02±0.47, 5.79±0.64 g respectively. After 60 days of culturing, hepatopancreatic moisture levels increased with the increase in salinity, whereas tail moisture levels remained unchanged. Wet and dry hepatosomatic indices were highest at 22 ppt. Wet tail muscle index was highest at 34 ppt, whereas dry tail muscle index did not change by any salinity levels. The results suggest that the optimum salinity range for rearing western king prawns ranges from 22 to 34 ppt and 10 ppt was unsuitable for culture.

Evaluation of the Use of Visible Implant Tags in Age-0 Atlantic Cod

The visible implant elastomer (VIE) tag and the soft visible implant alphanumeric (VIAlpha) tag were evaluated as methods for tagging age-0 Atlantic cod Gadus morhua. Tank experiments on laboratory-bred Atlantic cod (82–141 mm) lasted 150 d. The VIE tag proved to be a good alternative when put on the dorsal part of the fish, with no tag loss and good tag visibility. When put on the ventral part of the fish, however, many VIE tags were covered by growing silvery scales. Retention of soft VIAlpha tags injected behind the eye was quite low (67%), but larger fish had higher tag retention than smaller fish. Survival of experimental fish was high (96%). Mean specific growth rates ranged from 0.46% to 0.51% per day. Both VIE and soft VIAlpha tags were used to individually tag age-0 Atlantic cod (85–290 mm) in the wild. Skin pigmentation of wild Atlantic cod made reading soft VIAlpha tags difficult, while VIE tagging on the dorsal part of the fish still worked well. We suggest that VIE tags are a good alternative for short-term demographic studies on age-0 Atlantic cod, while low retention rates cast doubt on the use of soft VIAlpha tags on these small fish.

Tank Performance of Larval Saugeyes (Walleye × Sauger) Produced Out‐of‐Season and during Regular Season Spawning

AbstractIn the north‐central region of the United States, hybrid saugeyes (female walleye Sander vitreus [formerly Stizostedion vitreum] × male sauger S. canadense) have been recognized as an important species for recreational fisheries and as a potential aquaculture commodity. We compared the performance of larval saugeyes that were produced out of season with larvae produced during the regular spawning season using intensive rearing systems. Fertilized out‐of‐season and regular season saugeye eggs that were produced by crossing two Spirit Lake, Iowa, female walleyes and two Mississippi River male saugers were obtained from the Rathbun Hatchery, Iowa Department of Natural Resources, Moravia. Out‐of‐season spawning was induced by light and photoperiod manipulation. Eyed eggs were air‐shipped overnight at 10°C. First‐feeding larvae (19,000 fish/tank) were stocked in six 800‐L cylindrical tanks provided with turbid water to prevent cannibalism and with surface sprays to secure swim bladder inflation. The fish were fed exclusively with commercial diets (Fry Feed Kyowa B‐400 and B‐700 and Bio‐Oregon starter diet). The specific growth rate (SGR), condition factor, and survival were compared between saugeyes reared out of season (50 d) and during the regular season (39 d). The mean SGR and survival of saugeyes from the out‐of‐season experiment were significantly lower than those of saugeyes from the regular season experiment (P &lt; 0.05), although the condition factor of the fish did not differ between the experiments. Significant differences in SGR were also observed between tank replicates, suggesting possible differences in the feeding responses of saugeyes within a tank system. Large discrepancies (40–43%) were observed between actual survival at the end of the rearing period and predicted survival based on counts of dead larvae siphoned from the tank bottom, suggesting the occurrence of cannibalism and the deterioration of larvae bodies prior to tank cleaning. The actual survival (12–19%) of larval saugeyes in both experiments shows some promise in the intensive rearing of saugeyes exclusively on dry feeds.

Growth pattern and growth dependent mortality of larval and pelagic juvenile North Sea cod Gadus morhua

We investigated growth patterns and evidence of growth dependent survival for a pop- ulation of Atlantic cod Gadus morhua using analysis of their otolith microstructure. Central concen- trations of a population of cod larvae and juveniles in the north-eastern North Sea were sampled twice (in April and May 2001), and larval/juvenile growth history from each of the sampling sequences was outlined. Growth rate was estimated by fitting a Laird-Gompertz equation to lengths- at-age, and we found the mean specific growth rate in length at age 20 d was 3.2% d -1 , declining to 1.9% d -1 at an age of 90 d. Otolith radius and larval standard length were highly correlated, and otolith growth was used as a measure of larval somatic growth. The larvae were divided into 3 groups dependent on their hatch-date, and for each hatch group, the same period of past growth was com- pared between fish sampled in April and May. A 2-way repeated-measurement ANOVA revealed a significant higher past growth of fish sampled in May in 2 of the 3 hatch-groups, implying a higher mortality of the slow growing larvae. Additionally, otolith size at age differed significantly between the April and May sampling of the oldest larvae. Using the obtained information we illustrate the consequences of being larger at age by nonparametric spline functions. These show up to 10-fold differences in the relative survival probabilities between cod of smallest or largest size at age, suggesting that mortality can be strongly dependent on growth.

Quality of Nephrops as food for Atlantic cod (Gadus morhua L.) with possible implications for fisheries management

Abstract Nephrops was found to be of low quality as food for cod. In a laboratory experiment the mean specific growth rate of 1 kg cod was 0.184 and 0.415% d−1 when fed to satiation on Nephrops and capelin, respectively. This large difference in growth rate resulted not only from less intake of Nephrops (1.19 kg cod−1) than capelin (1.55 kg cod−1) but also because more Nephrops (4.6 kg) than capelin (2.2 kg) were required to produce each kilogramme of cod. Higher food conversion ratio was consistent with lower fat content of Nephrops (1.3%) than capelin (9.2%) but the exoskeleton also reduced the digestion rate of Nephrops. In the groups where Nephrops and capelin of equal mean weight were offered simultaneously, 40% of the diet consisted of Nephrops during the first week and 10% during the final seven weeks of the experiment. At the end of the experiment, condition factor, liver index, and gonadosomatic index were significantly lower for cod fed on Nephrops (0.967, 5.7, 7.1, respectively) than for those fed on capelin (1.086, 15.8, 11.2, respectively). These results suggests that predation by cod on Nephrops might be reduced by regular release of capelin or other similar food in the distributional areas of Nephrops.

Nutritional Value and Digestibility of Fermented Shrimp Head Waste Meal by African Catfish Clarias gariepinus

Feeding trials and digestibility studies were conducted in recirculatory systems to assess the effect of replacing fish meal with 0, 5, 10, 20, 30 and 40% fermented shrimp head waste meal (FSHM) in practical diets for African catfish Clarias gariepinus. The different levels of FSHM were mixed with other ingredients to formulate 40% protein diets (Diets 1-6). The diets were fed to fingerlings of C. gariepinus (12.0± 0.03g) at 3% body weight per day for 84 days. The effect of the diets on the growth, carcass and mineral compositions of the fishes were evaluated. Results showed no significant variation (p>0.05) in apparen t digestibility coefficient (ADC) of nutrients ADC and ADC , protein efficiency ratio (PER), food protein energy conversion ratio (FCR) and hepasomatic index of the fishes fed all the diets. However, the fishes fed diets (1-5) had similar (p>0.05) mean weight gain (MWG) and specific growth rate (SGR), which differed significantly (p<0.05) from the MWG and SGR of the group of fishes fed diet 6. Carcass yield and mineral depositions differed marginally in the fishes fed the various diets. Comparative costs analyses indicated that the best profit margin would be realized by replacing fish meal with 30% FSHM in the diet of the fish.

Growth, feed utilization and body composition of black catfish, Rhamdia quelen, fry fed diets containing different protein and energy levels

In order to evaluate the protein and energy requirement of black catfish fry, eight experimental diets containing four protein levels (30%, 34%, 38% and 43%) and two lipid levels (8% and 14%) were tested. For 30 days, 0.3 g fish were fed the experimental diets to approximate satiation, twice a day. Growth was significantly affected by both protein and lipid content of the diets. At the same protein level, final mean weight, weight gain and specific growth rate (% per day) were significantly improved ( P<0.01) by increasing the dietary lipid (energy) content. At the same dietary lipid content, final weight, weight gain and SGR obtained with diets containing 38% and 43% protein were similar and significantly ( P<0.01) higher than those obtained with diets containing 30% and 34% protein. Feed conversion ratio was improved by increasing the dietary protein content, obtaining a value of 0.95 (fed/gain) with diets 38/14 and 43/14. Protein efficiency ratio (PER) was not significantly affected by the different dietary treatments for each dietary lipid or protein level tested, reaching a mean value of 2.80 in fish fed high lipid diets. Results obtained in the present study suggest that for black catfish fry, the optimum dietary protein content is 37% with an estimated optimum digestible protein (DP)/DE ratio of 23.6 mg kJ −1.

The marine survival and growth of wild and hatchery‐reared Atlantic salmon

Summary Catches of Atlantic salmon Salmo salar L. decreased in the 1980s and 1990s over its entire area in the North Atlantic and smolts were often released for stock enhancement. However, there are questions about their survival and performance relative to fully wild fish. This paper reports on the survival and sea growth of River Imsa salmon released from 1981 to 1999 as 1‐ and 2‐year‐old hatchery and wild smolts. Survival was significantly higher for wild than hatchery fish. Hatchery salmon released as 2‐year‐old smolts had lower survival, were captured more in coastal than freshwaters, grew more slowly and attained maturity younger than corresponding 1‐year‐old smolts. The survival rate of hatchery fish released as 2‐year‐old smolts, but not 1‐year‐olds and wild smolts, decreased during the 1980s and 1990s. Growth rates at sea, adult size and the proportion of multi‐sea‐winter fish of all three groups also decreased over time. Catches in coastal relative to freshwaters were higher for two‐ than one‐sea‐winter fish. Salmon captured in coastal water were greater in length than those captured in rivers. Mean specific growth rate at sea was similar for wild and hatchery salmon released as 1‐year‐old smolts, and higher than in hatchery fish released as 2‐year‐olds. The proportion of two‐sea‐winter salmon correlated positively with the specific growth rate in the first year at sea. Total capture of wild adult salmon in rivers and Norwegian home waters each year correlated positively with the specific growth rate in the first year at sea. The same correlation held for hatchery fish released as 2‐ but not 1‐year‐old smolts. Synthesis and applications. The coastal fishery was size‐selective in reducing the size and age of salmon. Releases of 1‐year‐old smolts were financially more profitable than those of 2‐year‐olds. Decreasing production of River Imsa salmon since 1981 was chiefly caused by reduced sea‐age at maturity and growth rate at sea of both hatchery and wild fish. A counteracting measure would be to reduce the size selectivity of the salmon fisheries.

Effect of dietary protein level on growth and survival of juvenile freshwater crayfishCherax quadricarinatus(Decapoda: Parastacidae)

Protein requirements of juvenile freshwater crayfish Cherax quadricarinatus, with initial mean weight of 1.08 ± 0.34 g, were studied. Juveniles were fed experimental diets containing seven levels of crude protein (CP) (20, 25, 31, 37, 43, 49, and 55%) and 18.73–21.45 kJ g−1 of gross energy. The highest mean weight and specific growth rate (SGR) (P < 0.05), with values of 9.6 g, and 3.64% day−1, respectively, were achieved by feeding a diet of 31% CP. This diet resulted in a survival rate of 80% over 60 days of culture at 27 °C. The higher protein level tested (55% CP), yielded a significantly (P < 0.05) lower mean weight of 6.4 g. However, there were no significant differences in SGR among the 20, 25, and 49% CP diets. Feed Conversion Ratio (FCR) was close to 1 for all treatments except the lowest (20%) and highest (55%) CP. Optimum protein requirements were estimated by fitting mean weight gain data to a quadratic model (y = 1.142+0.484 − 0.0071x2, r2 = 0.952, P < 0.05). Optimal growth occurred with 34.2% CP, but the results suggest that optimum protein level for 1-g juvenile C. quadricarinatus is 31% in least-cost diets. The information obtained will contribute to the formulation of diets that will enhance production of C. quadricarinatus.

The effects of density on the growth and survival of different families of juvenile Penaeus japonicus Bate

Variations in the growth and survival of six families of juvenile (initial mean weight=4.16 g) Penaeus japonicus were examined at two densities (48 and 144 individuals m −2) in a controlled laboratory experiment. Survival was very high throughout the experiment (95.4%), but differed significantly between densities and rearing tanks. Family, sex and family×density interaction did not significantly affect survival. Mean specific growth rate (SGR) of the shrimp was 18% faster at the low density (1.93±0.05% day −1) than at high density (1.64±0.03% day −1). However, there was a small but significant interaction between family and density indicating that growth of the families was not consistent at both densities. The inconsistent growth of the families across the two densities resulted in a change in the relative performance (ranking) of families at each density. Sex, rearing tank and rearing cage also affected growth of the shrimp. Mean SGR of the females (1.79±0.03% day −1) was 5% faster than males (1.70±0.03% day −1) when averaged across both densities. Shrimp grew significantly faster in rearing tank 3 than rearing tank 1 or 2 at both densities. Results of the present study suggest that family×density interaction could affect the efficiency of selection for growth if shrimp stocks produced from shrimp breeding programs are to be grown across a wide range of densities.