Survival of Clarias gariepinus fry in earthen ponds: Effects of composts and leaks

To inform decisions on improving the yields of African catfish Clarias gariepinus fingerlings in earthen ponds, the hypothesis that composts and leaks were partly responsible for usually low and variable fry survivals was tested, through comparison of treatments and simple regression. The occurrence of amphibians was significantly higher (P < 0.05) and survival of fry was significantly lower (P < 0.02) in ponds with composts than in those without. The survival of fry and fingerlings in earthen ponds was negatively correlated with their night leaking rates and regression analysis yielded the equations y = − 13.31x + 82.56 for fry and y = − 6.97x + 95.29 for fingerlings, where y is the survival of fry or fingerlings in a holding pond and x is the leaking rate of the pond. Realizing how negatively the existence of composts and leaks could affect the survival of fry and fingerlings, fertilizing unfenced ponds with composts and sterilizing nursery ponds by drying were proscribed.     

Photoperiod influences the growth, food intake, feed efficiency and digestibility of red sea bream (Pagrus major)

Two consecutive trials were conducted to investigate the effects of photoperiod manipulation on growth rate, food intake and feed conversion efficiency (Trial 1), and the digestibility of nutrients and energy (Trial 2) in red sea bream, Pagrus major (body weight 19–120 g). Fish were exposed to four photoperiods (6L:6D, 12L:12D, 16L:8D and 24L:0D) with light intensity 1500 lx on the water surface. The fish were fed with a commercial diet to apparent satiation. In Trial 2, 0.5% chromic oxide (Cr₂O₃) was used as an inert marker. Significantly higher weight gain and specific growth rates were observed in fish exposed to a 24L:0D photoperiod followed by 16L:8D, 6L:6D and 12L:12D photoperiods (P < 0.05). Food intake and feed conversion efficiency (FCE) were also significantly higher in fish exposed to 24L:0D followed by 16L:8D, 6L:6D and 12L:12D photoperiods (P < 0.05). Fish exposed to 24L:0D and 16L:8D photoperiods showed significantly higher lipid and energy digestibility than those exposed to a 12L:12D photoperiod (P < 0.05). The results demonstrated that the enhancement of growth performances under 16L:8D and 24L:0D photoperiods were attributed to improved appetite, greater food intake and higher feed conversion efficiency as well as higher digestibility.     

Hydroacoustic in situ target strength of smelt (Osmerus eperlanus (L.))

Dependence of the in situ acoustic target strength (TS) of smelt (Osmerus eperlanus (L.)) on the total fish length (L) was estimated from TS measurements made with a split-beam 120 kHz echo sounder with reference to fish length–frequency distributions in trawl catches. The studies were conducted in the eutrophic and turbid Lake Hiidenvesi in southern Finland. Smelt dominated the catches-in-number with proportions at different sampling stations ranging from 91.8 to 100%. A TS–length equation of the form TS = alog L − b was fitted through the modes in the TS-distributions and in the respective fish length distributions. The estimated model (±parameter standard error) was TS = 23.4 (±2.24)log₁₀ L − 68.7 (±1.82). With a pre-defined slope of 20, the fitted equation was TS = 20 log₁₀ L − 65.9 (±0.23). The estimated day–night difference in TS–length relationship was not significant (t-test, p = 0.37).     

Breeding and juvenile culture of the lined seahorse, Hippocampus erectus Perry, 1810

All seahorse species worldwide have been placed under CITES Appendix II since 2004, because they have been over-exploited for traditional Chinese medicine and aquarium trades. Aquaculture has been recognized as a long-term solution for sustaining the seahorse trade while minimizing wild collection. In this study, we evaluated the breeding and juvenile culture of an important aquarium seahorse species, the lined seahorse Hippocampus erectus, Perry 1810. Pairing, mating and copulation behavior were observed. Gestation time and brood size were 17.33 ± 2.94 days and 272.33 ± 66.45 individuals/brood, respectively. Growth rates differed among juveniles from different broods. Effects of temperature on the growth rates and survivorship of the juveniles during the first two weeks were compared. The highest growth rate and survivorship of the juveniles occurred at 28–29 °C among the temperatures tested (24–33 °C). Growth rate and survivorship of the juveniles during the first 9 weeks at 28 °C were investigated. The final standard length and survivorship of the juveniles were 6.32 ± 0.52 cm and 71.11 ± 10.18%, respectively, and the relationship between the wet weight and the standard length of the juvenile seahorses can be expressed as: W = 0.0034 L^2.5535 (r² = 0.9903, n = 12, P < 0.01). These findings suggest that H. erectus is a good candidate for commercial aquaculture.     

Substitution of Chaetoceros muelleri by Spirulina platensis meal in diets for Litopenaeus schmitti larvae

The nutritional response of Litopenaeus schmitti larvae to substitution of Chaetoceros muelleri by Spirulina platensis meal (SPM) was evaluated. The substitution levels (S) were 0%, 25%, 50%, 75% and 100%, dry weight basis. Final larval length (FL) ranged from 1.98 to 3.16 mm for the different substitution levels. There was a significant relationship between S and FL, described by the following quadratic equation: FL = 2.853 + 0.01598S − 0.000233S². The substitution level (S) yielding maximum FL was 34.2%. Development index (DI) values ranged from 2.84 to 3.93 and were dependent on substitution level. The corresponding equation was DI = 3.799 + 0.00945S − 0.000189S² (P < 0.01). Maximum DI was obtained at 25.0% substitution. Survival was high (82–87%) and no significant differences were found between treatments. Protein digestibility of either microalgae was high, with 92% for SPM and 94% for C. muelleri, with no significant differences between them. The results in this study indicate that an adequate balance of nutrients in relation to the requirements of the species is critical. To simultaneously improve FL and DI, a 30% substitution of C. muelleri by SPM is suggested. This is equivalent to feeding 0.15 mg larvae^− 1 day^− 1 dry weight basis of a 70% C. muelleri/30% SPM diet, representing 0.078 mg protein larvae^− 1 day^− 1, 0.026 mg lipids larvae^− 1 day^− 1 and 2.732 J larvae^− 1 day^− 1.     

Effect of probiotics, Enteroccus faecium, on tilapia (Oreochromis niloticus) growth performance and immune response

The aim of this study was to analyze the effect of a probiotic bacterium, Enterococcus faecium ZJ4 on growth performances and immune responses of tilapia (Oreochromis niloticus). The tilapias were treated with E. faecium ZJ4 at a final concentration of 1 × 10⁷ cfu ml^− 1 in aquaria water every 4 days. Six aquaria with three replicates for treated and controls were used. After 40 days, the tilapias supplemented with the probiotic showed significantly better final weight and daily weight gain (DWG) than those fed the basal diet (Control) (P < 0.05). There was no remarkable difference (P > 0.05) in the total serum protein, albumin content, globulin concentration and A/G ratio between the treated and control tilapias. The result of lysozyme activity assay was similar to these biochemical indexes. However, the complement component 3 content, myeloperoxidase (MPO) activity and the respiratory burst activity of blood phagocytes were higher (P < 0.05) in E. faecium treated tilapias (trial 1) than the controls.     

Evidence for fine geographical scale heterogeneity in gene frequencies in yellowfin tuna (Thunnus albacares) from the north Indian Ocean around Sri Lanka

Yellowfin tuna are currently considered by the member nations of the Indian Ocean Tuna Commission to constitute a single stock in the Indian Ocean due to a lack of knowledge about yellowfin tuna population structure in this region. Previous studies of Indian Ocean yellowfin tuna based on morphology and fisheries data have hinted at the presence of multiple stocks in the region, and further, that stocks may mix in the north western Indian Ocean around Sri Lanka. To better understand the genetic stock structure of yellowfin tuna in the north western Indian Ocean, we examined genetic variation in 285 yellowfin individuals collected over a period of 4 years from six fishing grounds around Sri Lanka and a single fishing ground in the Maldive Islands. We screened variation in both the mitochondrial ATPase 6 and 8 region (498 bp) and three microsatellite loci. Significant genetic differentiation was detected among sites for mitochondrial DNA (Φ_ST = 0.1285, P < 0.001) and at two microsatellite loci (F_ST = 0.0164, P < 0.001 and F_ST = 0.0064, P < 0.001), while spatial analysis of molecular variance of mtDNA data identified three genetically heterogenous groups namely; western, south eastern and all remaining sites. These results suggest the possibility that genetically discrete yellowfin tuna populations may be present in the north western Indian Ocean.     

Optimal dietary protein requirement for juvenile ivory shell, Babylonia areolate

A growth experiment was conducted to determine the optimal dietary protein requirement for juvenile ivory shell reared in indoor aerated aquaria. Six isoenergetic experimental diets using fish meal, casein and gelatin as protein sources were formulated to contain graded levels of protein (27, 33, 38, 43, 49 and 54% of dry diet, respectively). Triplicate groups of 40 shells (average weight 93.50 ± 1.70 mg) were stocked in 120-l tanks and fed to apparent satiation twice daily for 8 weeks. The results showed that the growth performance and feed utilization were significantly affected by dietary protein level (P < 0.05). Maximum weight gain, mean protein gain, specific growth rate and soft body to shell ratio occurred at 43% dietary protein level (P < 0.05). There were significant differences in protein, lipid, moisture and ash content in soft body; except that ash content in shell was not significantly affected by dietary protein level. Pepsin activity in soft body tissue significantly increased with dietary protein level up to 43%, and trypsin-like enzyme activity increased with dietary protein level up to 49%. However, lipase activity in soft body decreased with increasing dietary protein level. However, no significant differences (P < 0.05) in survival, calcium, phosphorus concentration in the shell and soft body were found among dietary treatments. Quadratic regression analysis of weight gain against dietary protein level indicated that the optimal dietary protein requirement for maximum growth and feed utilization of juvenile ivory shell is 45% of dry diet.     

Effects of Cu-exchanged montmorillonite on growth performance, microbial ecology and intestinal morphology of Nile tilapia (Oreochromis niloticus)

A total of 360 Nile tilapia at an average initial body weight of 3.9 g were randomly allocated to 4 treatments, each of which had three replicates of 30 fish/tank and used to investigate the effects of Cu²⁺-exchanged montmorillonite (Cu-MMT) on growth performance, microbial ecology of the skin, gill and intestine, and intestinal morphology. The dietary treatments were: 1) basal diet, 2) basal diet + 1.5 g/kg MMT, 3) basal diet + 30 mg/kg copper as CuSO₄ (equivalent to the copper in the Cu-MMT treatment group), or 4) basal diet + 1.5 g/kg Cu-MMT. The results showed that supplementation with Cu-MMT significantly improved growth performance as compared to control and fish fed with Cu-MMT had higher growth performance than those fed with MMT or CuSO₄. Supplementation with Cu-MMT reduced (P < 0.05) the total intestinal aerobic bacterial counts and affected the composition of intestinal microflora with a tendency of Aeromonas, Vibrio, Pseudomonas, Flavobacterium, Acinetobacter, Alcaligence, Enterobacteriaceae decreasing as compared with control. Measurements of villus and microvillus heights at different sites of the intestinal mucosa indicated that dietary addition of MMT or Cu-MMT improved intestinal mucosal morphology. However, the microbial ecology of the skin and gill was not affected by the addition of CuSO₄, MMT or Cu-MMT. Supplementation with CuSO₄ had no (P > 0.05) effect on the growth performance, microbial ecology and intestinal morphology. The results showed that Cu-MMT exhibited antibacterial activity in vivo and protected intestinal mucosa from invasion of pathogenic bacterium and toxins, resulting in a positive effect on the growth performance.     

Age, growth, and reproduction of silky sharks, Carcharhinus falciformis, in northeastern Taiwan waters

To examine the age, growth and reproduction of silky sharks, Carcharhinus falciformis, in the waters off northeastern Taiwan, 469 specimens (213 females and 256 males) were collected from August 2000 to January 2002 at the Nanfanao fish market, northeastern Taiwan. The relationship between body weight (W) and total length (TL) for both sexes combined was expressed as: W = 2.92 × 10⁻⁶ TL^3.15 (n = 469, p < 0.01). The relationship between TL and vertebral centrum radius (R) for both sexes combined data was estimated as: TL = 25.979 + 18.197R (n = 250, p < 0.01). Growth bands (including translucent and opaque zones) in precaudal vertebrae formed once a year between December and January and were counted up to 11 and 14 for females and males, respectively. The von Bertalanffy growth function (VBGF) was used to model the observed length at age data. The sexes combined VBGF predicted an asymptotic length (L_∞) = 332.0 cm TL, growth coefficient (k) = 0.0838 year⁻¹, age at zero length (t₀) = −2.761 year (n = 250, p < 0.01). Size at 50% maturity for males was estimated to be 212.5 cm based on the logistic curve, which corresponded to 9.3 years. Females matured at 210–220 cm, which correspond to 9.2–10.2 years. The length at birth was estimated to be 63.5–75.5 cm TL. The number of embryos per litter was 8–10 and sex ratio of embryos was 1:1.     

Density of brown trout, Salmo trutta L., and Atlantic salmon, Salmo salar L., parr after point and scatter stocking of fry

Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., fry were point and scatter stocked in the early part of June at densities of 63–263 fry 100 m⁻² per species in the River Viantienjoki, a small river in northern Finland, and their population densities were assessed in late summer. Both species were always stocked together in similar quantities. Point stocking was used in the first 2 years and scatter stocking in the following 2 years. In point stocking, there was no correlation between the distance from the stocking sites (maximum = 250 m) and parr density in census sites ( r = −0.013 and 0.019 for brown trout and Atlantic salmon, respectively). The stocking density of fry did not influence parr density in August by either method or by species. Stocking density explained only from 11% to 23% of the parr survival depending on the species or stocking method. The mean densities of Atlantic salmon and brown trout parr did not differ significantly from each other at any fishing site ( P > 0.05). Both point and scatter stocking appear to be suitable methods for use in small rivers. The parr densities depend more on the other factors (e.g. habitat quality) than the stocking method, and the choice between methods could be based on the time and labour available.     

Replacement of fish meal by meat and bone meal in diets for large yellow croaker, Pseudosciaena crocea

A growth experiment was conducted to investigate the effects of replacement of fish meal (FM) by meat and bone meal (MBM) in diets on the growth and body composition of large yellow croaker (Pseudosciaena crocea). Six isonitrogenous (43% crude protein) and isoenergetic (20 kJ g^− 1) diets replacing 0, 15, 30, 45, 60 and 75% FM protein by MBM protein were formulated. Each diet was randomly allocated to triplicate groups of fish in sea floating cages (1.0 × 1.0 × 1.5 m), and each cage was stocked with 180 fish (initial average weight of 1.88 ± 0.02 g). Fish were fed twice daily (05:00 and 17:30) to apparent satiation for 8 weeks. The water temperature ranged from 26.5 to 32.5 °C, salinity from 32 to 36‰, and dissolved oxygen content was approximately 7 mg l^− 1 during the experimental period. Survival decreased with increasing dietary MBM and the survival in the fish fed the diet with 75% protein from MBM was significantly lower than other groups (P < 0.05). There were no significant differences in specific growth rate (SGR) among the fish fed the diets with 0 (the control group), 15, 30 and 45% protein from MBM. However, SGR in the fish fed the diets with 60 and 75% protein from MBM were significantly lower than other groups (P < 0.05). No significant differences in feeding rate were observed among dietary treatments. The digestibility experiment showed that the apparent digestibility coefficients (ADC) of dry matter, protein, lipid and energy of MBM were significantly lower compared with those of FM (P < 0.05). Essential amino acid index was found to be correlated positively with SGR in the present study, suggesting that essential amino acid balance was important. Body composition analysis showed that the carcass protein and essential amino acids were not significantly affected by dietary MBM. The lipid and n-3 highly unsaturated fatty acid (n-3 HUFA) in fish muscle, however, significantly decreased with increasing dietary MBM. These results showed that 45% of FM protein could be replaced by MBM protein in diets of large yellow croaker without significantly reducing growth. It was suggested that the reduced growth with higher MBM was due to lower digestibility and imbalance of essential amino acids.     

Effect of gelatinization degree and inclusion level of dietary starch on the characteristics of digesta and faeces in Nile tilapia (Oreochromis niloticus (L.))

Higher faeces removal efficiency can help and reduce the negative impact of faecal waste on water quality and on the environment. Physicochemical properties of feed ingredients/nutrients may change the faeces characteristics and the ratio of removed to non-removed solids. The aim of this study was to assess what physicochemical properties of the diet (viscosity, fermentation and dry matter) influence faeces characteristics and faeces removal using Oreochromis niloticus (L.) as the test animal. Four experimental diets were formulated by the addition of two types of starch differing in gelatinization degree (gelatinized and native) and two levels (high and low) of starch to a basal diet according to a 2 × 2 factorial design. The diets were assigned to 16 tanks with 40 fish each, with four replicates for each diet. Replacement of native starch by gelatinized starch improved faeces removal rate, growth and digestibility (P < 0.01), but reduced fermentation (P < 0.05) at the end of the intestine. Addition of gelatinized starch did not change viscosity and dry matter of the digesta at the end of the intestine. A high level of starch in the diet also increased digestibility, growth and faeces removal percentage (P < 0.05). Fermentation and dry matter content at the end of the intestine were not influenced by a high starch diet, but viscosity was higher at the high level of starch inclusion. Volatile fatty acid levels in the stomach of Nile tilapia were high in the treatments with gelatinized starch. In conclusion, intestinal fermentation induced by native starch had a negative impact on faeces removal efficiency. A higher inclusion level of starch resulted in a higher viscosity of the digesta, leading to higher faeces removal efficiency.     

Seasonal variation in flesh quality, comparison between large and small Atlantic salmon (Salmo salar) transferred into seawater as 0+ or 1+ smolts

The aim of this study was to identify the effect of growth, size and season on the flesh quality of Atlantic salmon (Salmo salar). Following smoltification, two groups of size sorted 0+ and 1+ smolts (four treatment groups in all) were measured for body weight and length in January, June and October 2002. The fish were stored on ice for 4 days before filleted and samples taken for flesh colour, fat/dry content, end pH, gaping score and texture shear force. Large fish upon smoltification grew faster during the first year at sea, while smaller fish grew faster during the second year at sea, resulting in similar weight at slaughter. Season showed the main influence on quality, where fish slaughtered in October had harder texture, higher fat content and redder colour compared to previous samples (P < 0.05). There were only minor differences between the fish slaughtered in January and June (P > 0.15). No significant differences (P > 0.05) were detected as an effect of size or smoltification age when effects of season were accounted for in the statistical model. We conclude that the observed variation in quality was an effect of changes in growth with season. We recommend that actions aimed to halter growth prior to slaughter could be an effective control measure to reduce seasonal quality variations.     

Metabolic scope, swimming performance and the effects of hypoxia in the mulloway, Argyrosomus japonicus (Pisces: Sciaenidae)

The culture of the mulloway (Argyrosomus japonicus), like many other Sciaenidae fishes, is rapidly growing. However there is no information on their metabolic physiology. In this study, the effects of various hypoxia levels on the swimming performance and metabolic scope of juvenile mulloway (0.34 ± 0.01 kg, mean ± SE, n = 30) was investigated (water temperature = 22 °C). In normoxic conditions (dissolved oxygen = 6.85 mg l^− 1), mulloway oxygen consumption rate (M·o₂) increased exponentially with swimming speed to a maximum velocity (U_crit) of 1.7 ± < 0.1 body lengths s^− 1 (BL s^− 1) (n = 6). Mulloway standard metabolic rate (SMR) was typical for non-tuna fishes (73 ± 8 mg kg^− 1 h^− 1) and they had a moderate scope for aerobic metabolism (5 times the SMR). Mulloway minimum gross cost of transport (GCOT_min, 0.14 ± 0.01 mg kg^− 1 m^− 1) and optimum swimming velocity (U_opt, 1.3 ± 0.2 BL s^− 1) were comparable to many other body and caudal fin swimming fish species. Energy expenditure was minimum when swimming between 0.3 and 0.5 BL s^− 1. The critical dissolved oxygen level was 1.80 mg l^− 1 for mulloway swimming at 0.9 BL s^− 1. This reveals that mulloway are well adapted to hypoxia, which is probably adaptive from their natural early life history within estuaries. In all levels of hypoxia (75% saturation = 5.23, 50% = 3.64, and 25% = 1 .86 mg l^− 1), M·o₂ increased linearly with swimming speed and active metabolic rate (AMR) was reduced (218 ± 17, 202 ± 14 and 175 ± 10 mg kg^− 1 h^− 1 for 75%, 50% and 25% saturation respectively). However, U_crit was only reduced at 50% and 25% saturation (1.4 ± < 0.1 and 1.4 ± < 0.1 BL s^− 1 respectively). This demonstrates that although the metabolic capacity of mulloway is reduced in mild hypoxia (75% saturation) they are able to compensate to maintain swimming performance. GCOT_min (0.09 ± 0.01 mg kg^− 1 m^− 1) and U_opt (0.8 ± 0.1 BL s^− 1) were significantly reduced at 25% dissolved oxygen saturation. As mulloway metabolic scope was significantly reduced at all hypoxia levels, it suggests that even mild hypoxia may reduce growth productivity.     

Effect of hyperoxygenation and low water flow on the primary stress response and susceptibility of Atlantic salmon Salmo salar L. to experimental challenge with IPN virus

Intensive salmon smolt production normally includes reduced water flow and hyperoxygenation (added oxygen) of remaining water. There is little information on how different water quality parameters influence the fish health and the susceptibility to infectious diseases. The current experiment was carried out to evaluate if the combination of hyperoxygenation and reduced water flow (hyperoxic) can act as a chronic stressor to salmon in freshwater (FW) in such a way that it increases the susceptibility to IPN virus (IPNV) following seawater transfer. In FW, after 22 days of hyperoxic exposure plasma ion, TBARS and cortisol were measured. The cortisol levels were significantly (p = 0.011) higher in the hyperoxic group compared to controls maintained under normal oxygen saturation and water flow (normoxic), indicating chronic stress. Hyperoxygenation in FW caused decreased plasma [Cl⁻] compared to the normoxic group (p = 0.037), while [K⁺] tended to be higher in the hyperoxic group (p = 0.088). No significant differences were observed in plasma [Na⁺], total osmolality, TBARS or hematocrit, but there was a tendency towards a lower hct in the hyperoxic compared to the normoxic group. In SW the mortality was higher in the hyperoxic group challenged with IPNV (34%) compared to the normoxic group challenged with IPNV (20%) (p = 0.02), and no mortality was observed in the PBS injected fish. The challenged fish showed an overall increase in plasma cortisol day 8, 10, 12 and 14 post-challenge (p = 0.015, p = 0.000, p = 0.046 and p = 0.022 respectively). After SW transfer and challenge, plasma [K⁺] was elevated in both challenged groups, but no consistent trends were found for plasma [Cl⁻], [Na⁺] or total osmolality during the SW phase. There were no significant differences in the gene expression level of IFN 1, Mx and IL 1β prior to challenge, suggesting that the basic expression level of these genes were not affected by hyperoxygenation. IPNV was detected in kidney and pylorus, by immunohistochemistry, cell culture, and RT-PCR in head kidney. This experiment indicates that chronic stress induced by a combination of low water flow and hyperoxygenation increases the susceptibility to IPNV challenge.     

Effects of entrance design on catch efficiency of arabesque greenling traps: a field experiment in Matsumae, Hokkaido

ABSTRACT:   A field experiment was conducted in the Matsumae area of Hokkaido, Japan, during June and July 2002, to investigate the effects of different entrance designs on the catch efficiency of fish traps by fishing with commercial traps (entrance inclination angle [α] = 37°; funnel length of entrance [ L _f ] = 22 cm) and experimental traps. The experimental traps were of the same size and similar design as commercial traps, with different entrance inclination angles (trap E1: α = 46°; E2: α = 27°; E3: α = 0°; all L _f  = 22 cm) or funnel lengths (E4: α = 37°, L _f  = 8 cm). In total, 2200 fish during 200 trap hauls were captured. The catch was significantly higher using both traps E2 and the commercial trap than with trap E3 ( P  < 0.05), and the catch of trap E2 was higher than that of the commercial trap. There were no significant differences in mean fish body length or the frequency distributions of body length among trap types (E1, E2, E3 and commercial). The funnel length of the entrance also affected the catch of traps. Trap E4 had significantly higher catches than the commercial trap ( P  = 0.04) when traps were deployed for a 1-day soak time. Fish body length frequency distributions did not differ between trap E4 and the commercial trap. The results showed that catch can be greatly affected by trap entrance designs.     

Regional size, age and growth differences of red grouper (Epinephelus morio) along the west coast of Florida

Red grouper (Epinephelus morio) were collected from the west coast of Florida, the central area of fishery harvest in U.S. waters, by fishery-dependent sources during 2000–2005. The west Florida shelf was divided into two regions: north (capture locations ≥28°N latitude) and south (capture locations <28°N latitude). Significant differences were found for age, length, and size-at-age by region and by gear; red grouper from the north were significantly younger and smaller on average than those from the south. Regional differences were also noted with respect to age progression; year class trends were only detected in the north. The 1996 year class dominated the landings in 2000–2001 (ages 4 and 5) and the 1999 year class dominated in 2004–2005 (ages 5 and 6). Regional data were fit to a size-modified von Bertalanffy growth model indicating smaller asymptotic length (L_∞) and faster growth rate (k) in the north (north: L_∞ = 800 mm, k = 0.23 mm year⁻¹, t₀ = 1.12; south: L_∞ = 863 mm, k = 0.15 mm year⁻¹, t₀ = 0.05). Mortality estimates derived from catch curves resulted in higher total and fishing mortality in the north for both gears. Alternative explanations of regional differences likely depend on nursery delineation and correlation to periodic environmental events such as red tides and hurricanes; all possibly important factors based upon anecdotal information. Nevertheless, our finding of regional demographic differences in red grouper from the west coast of Florida suggests a more complex population spatial structure for red grouper.     

Occurrence of two forms of Y chromosome in rainbow trout (Oncorhynchus mykiss) males from Rutki strain

Morphologically differentiated sex chromosomes are found in only few salmonid species. Some populations of rainbow trout, Oncorhynchus mykiss, exhibit chromosomal polymorphism related to sex. We found sex-related chromosomal polymorphism in fish from the synthetic Rutki strain, Poland, in approximately 85% (n = 22) of examined males (XY-like) whereas the remaining males (n = 4) possessed chromosomes similar to these observed in females (XX-like). To investigate whether males possessing XX-like chromosomes were genotypic males or genotypic females with altered phenotypic sex, androgenetic progeny of four males (representing both XY-like and XX-like forms, n = 2 + 2) was examined. Androgenetic progeny (F₁) of all four fathers consisted of both phenotypic females and males. F₁ male progeny of two fathers showed XX-like chromosomes whereas F₁ male progeny of the other two fathers possessed YY-like (supermale) chromosomes. F₁ were reared further until they were sexually mature. Two males from each of four F₁ families were used to produce F₂ androgenetic and control F₂ generation. All F₂ individuals, androgenetics and control, were phenotypic males at sexual maturation. The results indicate that males possessing XX-like chromosomes are genetic males and they are not sex-reversed females. Thus, the Y chromosome can exist in different morphological forms in farmed rainbow trout. The Y^S chromosome (shorter form, unlike the X chromosome) has a shorter or absent p-arm and lacks the pericentromeric cluster of AT-rich chromatin and the 5S rDNA sequences that are found in the Y^L chromosome (longer form, like the X chromosome).     

Effects of protein hydrolysate in weaning diets for Atlantic cod (Gadus morhua L.) and Atlantic halibut (Hippoglossus hippoglossus L.)

The study aims to test whether predigested dietary protein enhances the utilization of formulated diets at weaning, and also whether it stimulates intestinal maturation. In this study, Atlantic cod [ Gadus morhua L.; 41 days posthatch (dph)] and Atlantic halibut [ Hippoglossus hippoglossus L.; 63 days postfirst feeding (dpff)] were weaned onto diets with graded levels of protein hydrolysate. By increasing the inclusion of dietary protein as hydrolysate from 0 to 400 g kg⁻¹, cod increased the rate of survival from 7 ± 1% to 18 ± 2% (82 dph; regression, P =  4*10⁻⁷). In halibut, the survival rate decreased from 57 ± 9% to 22 ± 7% as the inclusion of protein in the form of hydrolysate increased from 0 to 450 g kg⁻¹ (119 dpff; regression, P =  8*10⁻⁵). Growth was not affected in any of the species. Results in specific activities of the intestinal enzymes leucine aminopeptidase (LAP) and alkaline phospatase (AP) supported the results in survival in halibut and partly also in cod, by showing increased activities in groups with increased survival ( anova , P <  0.05). The lower optimal level of hydrolysed protein in halibut than in cod is suggested mainly because of a slower feeding practice in halibut, which allows more extensive nutrient leaching before ingestion.