Comparative effects of long-term hypoxia on growth, feeding and oxygen consumption in juvenile turbot and European sea bass

When juvenile turbot Scophthalmus maximus and sea bass Dicentrarchus labrax were fed to satiation, growth and food intake were depressed under hypoxia (3·2±0·3 and 4·5±0·2 mg O₂ l^-1). However, no significant difference in growth was observed between fishes maintained in hypoxia and fed to satiation and fishes reared in normoxia (7·4±0·3 mg O₂ l^-1) and fed restricted rations (same food intake of fishes at 3·2 mg O₂ l^-1). Routine oxygen consumption of fishes fed to satiation was higher in normoxia than in hypoxia due to the decrease in food intake in the latter. Of the physiological parameters measured, no significant changes were observed in the two species maintained in hypoxia. This study confirms the significant interaction between environmental oxygen concentrations, feeding and growth in fishes. Decrease in food intake could be an indirect mechanism by which prolonged hypoxia reduces growth in turbot and sea bass, and may be a way to reduce energy and thus oxygen demand.     

Older triploid fish retain impaired reproductive endocrinology in the European sea bass Dicentrarchus labrax

This paper reports on an evaluation of growth, gonadal development and reproductive endocrinology of older triploid (3n) European sea bass Dicentrarchus labrax in comparison with their diploid (2n) counterparts throughout their fifth and seventh annual cycle of life. While older triploids retained impaired reproductive endocrinology, a sexually related dimorphic growth was observed with 3n females attaining the largest sizes. Comparisons of some body indexes showed that 3n females had a significantly lower hepato‐somatic index (I_H) than 2n females but a significantly higher viscero‐somatic index (I_F). In contrast, both male and female triploids showed significantly lower gonado‐somatic index (I_G) than diploids. Accordingly, diploids produced mature gametes but triploids did not, demonstrating that despite the longer time given to triploids for gonadal development, they could not reproduce. Furthermore, older triploids had lower levels of plasma sex steroids (testosterone, T; 11‐ketotestosterone, 11‐KT and oestradiol‐17β, E₂) and luteinizing hormone (LH) than their 2n counterparts with 3n females showing drastic effects of triploidization on their reproductive endocrinology. Vitellogenin (VTG) was undetectable in 3n females. Gonadal content of steroid hormones and Sparus aurata‐type gonadotropin‐releasing hormone (sbGnRH) in the brain and pituitary were also lower in triploids compared with diploids. These results suggest that older 3n D. labrax retain functional sterility in both sexes, and 3n females might reach larger sizes than 3n males and their 2n counterparts in this species.     

Growth, gonadal development and sex ratios of meiogynogenetic diploid sea bass

Gynogenesis showed little effect on general physiology and gonadal development in sea bass Dicentrarchus labrax . Meiogynogenetic fish showed well-developed gonads indicating low occurrence of developmental imbalances even after gynogenesis induction in this species. In addition, the proportion of sexes of meiogynogenetic sea bass was similar to the diploid controls in two independent trials, which did not deviate significantly from a 1:1 male: female sex ratio. Even considering some environmental influence on sex differentiation, as has been previously demonstrated, the fact that the proportion of sexes was similar between gynogenetic and control diploids essentially eliminates the possibility that in the sea bass the females are the homogametic sex. Although the mechanism of sex determination of this species still remains unknown, even after gynogenesis induction, the genetic mechanism of the ZW/ZZ type could probably operate in the sea bass.     

Muscle fibre growth dynamics in diploid and triploid rainbow trout

The effect of triploidy on muscle fibre growth was determined by comparing hyperplasia and hypertrophy of white muscle fibres in all-female, diploid and triploid rainbow trout Oncorhynchus mykiss (100–400 mm total length). Conventional morphometry and protein and DNA concentrations were used to assess muscle fibre hyperplasia and hypertrophy in white muscle samples derived from an anterio-dorsal location. Muscle fibre distributions were significantly different between triploids and diploids in trout <300 mm. The proportion of fibres <20 μm was higher in diploids than in triploids and the proportion of fibres in the 20–40 μm category was higher in triploids than in diploids. This indicates that the hyperplastic fibres of triploids are larger than those of diploids. Larger hyperplastic fibres in triploids are probably due to the combined effect of increased nuclear size in triploids and the relatively high nucleus: cell ratio observed in small muscle fibres. These larger fibres may be less favourable to cellular metabolic exchange because of their smaller surface area to volume ratios, and perhaps account for reduced viability and growth observed in triploids during early life stages. On the other hand, the lack of difference in the distribution of fibres <20 μm between diploids and triploids at larger body size ranges (301–400 mm) imply that triploid trout may have higher rates of new fibre recruitment and growth capacity at these sizes. There was no difference between diploid and triploid trout in the mean size of muscle fibres; however, the number of fibres per unit area was reduced by 10% in triploids. No differences were observed in protein or DNA concentrations in muscle tissues between the two genetic groups. Since triploid nuclei have 1·5 times more DNA than diploid nuclei, this deviation from the expected muscle DNA concentration (1·3–1·4 times more DNA in triploids when the 10% reduction in fibre density is considered) suggests that the number of nuclei per muscle fibre is reduced. In both diploids and triploids, mean fibre size increased with body length while fibre density decreased. Similarly, protein concentration in the muscle tissue increased and DNA concentration declined with increasing body length. Protein/DNA ratio was strongly and positively correlated with fibre size. These results demonstrate that changes in DNA and protein concentrations can be used to assess hyperplasia and hypertrophy in muscle tissues. However, the morphometric procedure provides better insight into muscle fibre growth as it enables the direct visualization and analysis of muscle fibre distribution patterns.     

The influence of temperature on the onset of first maturity in sea bass

Oocyte development was aborted and full maturity did not occur in virgin female sea bass Dicentrarchus labrax unless they remained in water above 10° C during the main period of gonad development. There was no difference in the condition factors of females with high or low I _G values in March, which indicates that retarded gonadal development was not necessarily due to poor body condition or nutritional state. It is suggested that the extended duration of the adolescent phase of female bass around southern Britain is a response to their environment, rather than an intrinsic aspect of the species'biology.     

Effects of constant short and long photoperiod regimes on the spawning performance and sex steroid levels of female and male sea bass

Under constant short photoperiod, the spawning time of 2-year-old sea bass Dicentrarchus labrax was advanced as compared to controls, whereas spawnings were delayed under constant long photoperiod. High plasma levels of 17β-oestradiol (E_2/) and testosterone (T) in females were coincident with the appearance of vitellogenic oocytes in the ovary, while high levels of 11-ketotestosterone (11-KT) and T in males were coincident with the presence of spermiating males. Although plasma levels of E₂ in females and 11-KT in males were low during the remainder of the cycle, levels of T were always >1 ng ml⁻¹ in both sexes, suggesting that T could play an important role during the initial stages of gonadal development. The profiles of E₂ and T in females and 11-KT and T in males exposed to constant short days were similar to those in the control group, but fish which were maintained under constant long photoperiods showed a bimodal pattern of these steroids. The results obtained from fish exposed to constant photoperiod regimes provide further evidence that an endogenous process could be operating to control the reproduction of sea bass.     

Feeding and growth of juvenile sea bass: the effect of ration and temperature on growth rate and efficiency

The effect of ration on the growth of pairs of juvenile sea bass Dicentrarchus labrax fed squid mantle was recorded at four temperatures: 6, 10, 14 and 18) C, covering the range typical of Welsh coastal waters. Initial weight of the fish ranged from 2.8 to 15.9 g. A predictive model for the maximum meal size (M_max) at temperatures between 10 and 18) C, accounted for 95% of the variance in lnM_max. Even when offered excess food, bass at 6) C had a low rate of food consumption [0.19% body weight (BW) day^?1] and lost weight (G=?0.04% day^?1). Predictive regression models for specific growth rate (G) accounted for 86% of the variance at reduced rations and 70% at maximum meals. The relationship between G (calculated for total biomass per tank) and ration was a decelerating curve. G at maximum meals increased with temperature, at lower rations G decreased with temperature. For a pair of bass with a combined weight of 15 g, predicted maintenance ration ranged between 0.7 and 2.3% BW day^?1 and increased with temperature. Maximum meal size was more sensitive to temperature than maintenance ration. At 18) C optimum ration was 7.4% BW day^?1. At lower temperatures, the optimum ration was the maximum meal. The maximum gross growth efficiency was 17.4% at 18) C. Mean absorption efficiency was 94.8%. Ration level had no significant effect on absorption efficiency, which was lowest at 6) C. Condition indices (Fulton condition factor, wet and dry liver—somatic indices and body depth index) increased with meal size at all temperatures except 6) C. An increase in temperature between 10 and 18) C generally resulted in a decrease in condition indices at a given ration. When comparisons were made at a given standard length, gut and carcass weight increased with ration. Visceral fat and gut weight decreased with increased temperature.     

Biochemical and morphological correlates of growth in diploid and triploid rainbow trout

Diploid and triploid rainbow trout contained the same amount of RNA per unit weight of white muscle tissue at a given growth rate. This finding, along with a previous finding that the protein content of triploid muscle tissues is not different from that of the diploids, indicates that triploid nuclei produce more RNA than their diploid counterparts. This increased production of RNA is probably due to the expression of the extra set of chromosomes in triploid nuclei. RNA-based growth correlates and liver somatic index ( I _H) were valid predictors of growth for both triploid and diploid fish. RNA concentration appeared to be better than RNA/DNA and RNA/protein in predicting growth.     

Effect of temperature on swimming performance of sea bass juveniles

At four temperatures ( T= 15, 20, 25 and 28° C) swimming performance of Dicentrarchus labrax was significantly correlated with total length (23–43 mm L _T); r²=0.623–0.829). The relative critical swimming speed ( RU _crit= U _crit L _T⁻¹), where U _crit is the critical swimming speed, was constant throughout the L _T range studied. The significant effect of temperature on the relative critical swimming speed was described binomially: RU _crit=−0.0323T²+ 1.578 T −10.588 (r²=1). The estimated maximum RU _crit (8.69 L _T s⁻¹) was achieved at 24.4° C, and the 90% performance level was estimated between 19.3 and 29.6° C.     

The influence of a power station on the survival of juvenile sea bass in an estuarine nursery area

Trawl surveys in the estuary of the River Medway in the autumns of 1987–1991 have shown that the distribution of juvenile sea bass Dicentrarchus labrax is strongly associated with the warm-water outflow from Kingsnorth power station. In years of low abundance of first-year bass, very few were caught outside the warm-water discharge channel, whereas proportionately more fish of abundant year classes occupied the main river. About 15% of the available juvenile bass population died on the cooling-water intake screens at Kingsworth in the autumn and winter of 1987 and 1988. However, growth and survival of first-year bass in the Medway Estuary may be enhanced by the power station's warm-water effluent, such that overwinter mortality due to inadequate nutritional reserves and low temperatures may be considerably reduced.     

Hormonal regulation of the European sea bass reproductive cycle: an individualized female approach

Fifteen tagged female sea bass Dicentrarchus labrax were sampled weekly from September to April and plasma vitellogenin (VTG), testosterone (T), 17β-estradiol (E₂), and two potential maturation inducing steroids (MISs): 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and 17,20β,21-trihydroxy-4-pregnen-3-one (20βS) assayed. An oocyte sample was obtained via intraovarian cannulation at each sampling time from every female and the stage of development of the most advanced clutch of oocytes determined and related to VTG and hormone plasma levels for each female. The mean number of ovulations per female was 1·75+0·25 when those females that did not present ovulations were excluded and up to 4 ovulations detected in some females. The highest plasma levels of T ( c. 6 ng ml^-1) were observed during postvitellogenesis and the beginning of maturation while maximum plasma levels of E2 (>5 ng ml^-1) were obtained during late vitellogenesis. VTG plasma levels increased throughout vitellogenesis peaking ( c. 2·5 mg ml^-1) at postvittelogenesis. For the first time significant changes of plasma progestogens were detected in European sea bass during the sexual cycle. The highest plasma level of 17,20βP ( c. 1·1 ng ml^-1) was observed during postvitellogenesis while the highest level of 20βS ( c. 1·4 ng ml^-1) coincided with final maturation. These results suggest that 17,20βP and 20βS play a role in the early and final maturation, respectively, in the European sea bass.     

Trypsin and chymotrypsin as indicators of nutritional status of post-weaned sea bass larvae

The aim of the present work was to evaluate to what extent variation in the activity of either trypsin or chymotrypsin could be a sensitive and reliable indicator of nutritional deficiencies during post-weaning stages in a marine fish. Sea bass Dicentrarchus labrax larvae obtained from two different spawns classified as high quality (HQ) or low quality (LQ) on the basis of their hatching variables, were used to evaluate if enzyme production could be conditioned by factors other than those related to the feeding regime. In addition, larvae in each group were either fed normally or received a ration reduced by 60%. Trypsin and chymotrypsin activities were separately measured in larvae at days 3 and 15 after weaning. Changes in mass and final survival were also recorded. Food limitation resulted in a lower final mass only in the HQ larvae. Survival was significantly higher in HQ larvae, but also in larvae suffering food restriction, irrespective of the quality of the spawn. In both groups of larvae, lower food availability was correlated to increased values of trypsin, and particularly, of chymotrypsin. Variations were high enough to support the usefulness of such enzymes as indicators of a modified nutritional status. It is suggested that the noticeable increase in the activity of both enzymes observed in the restrictedly fed larvae could compensate initial deficiencies and explain the good results in growth and survival obtained in LQ larvae.     

Eleven new microsatellites of the sea bass (Dicentrarchus labrax L.)

Eleven polymorphic microsatellites were isolated from the sea bass, Dicentrarchus labrax, using a microsatellite enrichment protocol and selective hybridization with an (AC)₁₂ probe. The loci showed different variation patterns in 21 unrelated sea bass individuals, with a mean number of alleles of 8.6 and a mean observed heterozygosity of 0.68. These microsatellite markers should be useful for population genetic analysis and biodiversity studies of sea bass.     

Growth heterogeneity in rearing sea bass ( Dicentrarchus labrax): test of hypothesis with an iterative energetic model

This study aimed at modeling the relative importance of food intake on growth heterogeneity among cultured sea bass (Dicentrarchus labrax). First, we designed an individual growth model comprising five compartments (Energy intake, Losses, Net Energy, Recovered Energy and Maintenance). This model was calibrated with a first experiment carried out in eight tanks; A total of 130 juveniles (11 g) per tank were fed by a self-feeder (84 days, 20°C, 16L : 8D, 30 g NaCl/l). A second experiment was performed to better understand the relation between individual food intake, individual growth and growth heterogeneity, using the model as a tool for a hypothetico-deductive approach on growth heterogeneity (135 passive integrated transponder-tagged fish, same rearing conditions as above and individual food intake measured by X-ray every 14 days). The tested hypotheses were that food intake was (a) homogeneous, (b) proportional to the fish weight (i.e. to W1.00) X-ray (c) proportional to W0.66 and (d) reflected by the X-ray measurements of food intake. For each hypothesis, a simple linear regression between experimental and simulated results was produced. The Fitness indicators of these analyses, together with their confidence intervals (calculated by bootstrapping), allowed testing the relevance of these hypotheses. The analysis indicated that growth heterogeneity was largely accounted for by individual variations of food intake, as revealed by the X-ray analysis, and that food intake was proportional to W1.00, which suggests a dominance hierarchy where small fish are incapable of feeding maximally.     

Differences in the pattern of antipredator behaviour between hatchery‐reared and wild European sea bass juveniles

Shoals of hatchery‐reared and wild sea bass juveniles Dicentrarchus labrax were tested for differences in their antipredator responses towards a potential live predator, the eel Anguilla anguilla . Eight experimental shoals ( i.e . replicates), each composed of 15 individuals from the same stock of juveniles ( i.e . wild or hatchery), were video recorded for 5 min before and after predator exposure. A set of behavioural variables were measured during the pre‐stimulus and stimulus phases of each test and compared between the two groups of replicates. Results showed that in both hatchery‐reared and wild juveniles predator exposure elicited a significant increase in the mean level of shoal cohesiveness and mean shoal distance from the predator, and a significant decrease in the mean shoal distance from the bottom. Shoals of wild juveniles, however, aggregated more quickly and reached higher shoal cohesiveness within the first 20 s of the stimulus period than shoals of hatchery‐reared fish. During this period, the wild fish also reached the highest peak in shoal cohesiveness, which then decreased gradually towards the levels observed before predator exposure. Another component of the antipredator response, the predator inspection behaviour, was fully developed in both wild and hatchery fish. Wild fish, however, tended to inspect the predator at a closer distance than hatchery fish.     

The impact of seasonal alteration in the lipid composition of broodstock diets on egg quality in the European sea bass

Groups of European sea bass Dicentrarchus labrax L., broodstock were fed two pelleted, lipid-enriched diets [maize oil, low essential fatty acid (EFA) or fish oil, high EFA]. Those fed the high EFA diet for 12 months or only during vitellogenesis (September to February) with the low EFA diet fed for the remaining 6 months, showed improved egg quality and higher hatching rates when compared to two groups fed the high EFA diet during either previtel-logenesis (February to September) or the spawning period (February to April). The higher quality eggs contained higher levels of total n-3 fatty acids, including increased levels of both docosahexaenoic acid (DHA) and eicosapaentaenoic acid (EPA). High levels of triacylglycerol (TAG) were linked with the poorer quality eggs from those groups fed the low EFA diet during the period of vitellogenesis. The present work identifies vitellogenesis as the period in the maturation during which EFA are incorporated most effectively into the developing oocytes.     

Microsatellite polymorphism and population subdivision in natural populations of European sea bass Dicentrarchus labrax (Linnaeus, 1758)

Polymorphism of microsatellite markers was used to study the genetic variability and structure in natural populations of European sea bass Dicentrarchus labrax. The data consisted of six microsatellite loci analysed for 172 individuals from three samples collected in the Golfe-du-Lion (France) and one sample collected in the Golfo-de-Valencia (Spain). Our goals were (i) to assess the level of genetic variability as revealed by these markers, (ii) to estimate the genetic differentiation among natural populations within a restricted area, and (iii) to evaluate how microsatellite loci fit the predictions of the two most widely used mutation models (the infinite allele model and the stepwise mutation model). As expected, our results indicate that the genetic polymorphism is very high when compared with previously used genetic markers, the mean expected heterozygosity per locus ranging between 0.69 and 0.93. We also found that all loci but one fitted the infinite allele model better. Using this model as a lower limit, we could extrapolate from the observed diversity effective population sizes on the order of 35 000 individuals. Our results also suggest that there may be a slight genetic differentiation between the two gulfs (F_ST= 0.007, P < 0.05), indicating that the corresponding populations are likely to be dynamically independent. This finding for a species with high dispersal abilities, if confirmed, has important beatings on fish-stock assessment.     

Development and linkage relationships for new microsatellite markers of the sea bass (Dicentrarchus labrax L.)

Twenty-eight polymorphic microsatellites were isolated from the sea bass, Dicentrarchus labrax, using a microsatellite enrichment protocol and selective hybridization with oligonucleotide probes. Analysis for these markers and 11 recently described microsatellites of D. labrax found linkage between 26 loci and revealed eight linkage groups.     

Isolation and characterization of microsatellite DNA loci in sea bass,Lates calcarifer Bloch

Polymerase chain reaction (PCR)‐based isolation of microsatellite arrays (PIMA) technique was used to isolate seven polymorphic microsatellite loci in sea bass, Lates calcarifer Bloch. A total of 62 samples of wild and cultivated sea bass collected from a few populations within Peninsular Malaysia were used in the study. For seven polymorphic loci, the number of alleles ranged from four to nine and locus heterozygosities ranged from 0.710 to 1.000. The loci will be useful for studying population structure, genetic variability of wild and hatchery stocks of L. calcarifer and broodstock management purposes.