Age, growth and mortality of greater amberjack from the southeastern United States

Rings on 323 sectioned otoliths (sagittae) were used to determine the ages of greater amberjack, Seriola dumerili, sampled from headboats and commercial handline vessels operating out of ports from North Carolina through the Florida Keys. Age and growth determinations were difficult. Rings were identified and counted on 71% of the samples, but measurements could be made on only 48%. The oldest fish examined was 17 years old and was 1552 mm total length (TL). Back-calculated mean lengths at annulus formation were 366, 873, 1207 and 1471 mm TL for ages 1, 5, 10 and 15, respectively. The von Bertalanffy equation describing theoretical growth for TL was L_t = 1648(1 − e^{−0.119(t + 1.230)}), where T = years. The equation using fork lengths (FL) was L_t = 1514(1 − e^{−0.115(t + 1.178)}). The weight-length relationship for greater amberjack was W = 7.2 × 10⁻⁸L^2.700, where W is the whole fish weight (in kg), and L is the total length (in mm). Conversion of total length to fork length may be obtained using the equation: FL = −4.807 + 0.887(TL), and fork length to total length: TL = 20.110 + 1.111(FL). Greater amberjack are fully recruited to the fishery by age 8. Estimates of total instantaneous mortality (Z) ranged from 0.60 to 0.65 depending on the year.     

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.     

Feeding has the largest effect on the ammonia excretion rate of the southern rock lobster, Jasus edwardsii, and the western rock lobster, Panulirus cygnus

The total ammonia nitrogen (TAN) excretion of spiny lobsters Jasus edwardsii and Panulirus cygnus, was determined in relation to temperature, body weight, emersion, daily rhythm and feeding. Temperature and body weight had large influences on the rate of TAN excretion. Exponential relationships were found between temperature (T) and TAN excretion of both species. These were described by the following equations: J. edwardsii Log₁₀ TAN=0.041T−3.57 (r²=0.979, F=143.2, P=0.001), P. cygnus Log₁₀ TAN=0.057T−3.90 (r²=0.987, F=302.2, P<0.001). TAN excretions of both species were positively correlated to body weight (W), and the relationships were described by the following equations: J. edwardsii Log₁₀ TAN=0.473 log₁₀ W−1.704 (r²=0.42, F=14.05, P=0.001), P. cygnus Log₁₀ TAN=0.499 log₁₀ W−1.346 (r²=0.69, F=44.18, P<0.001). TAN excretion increased significantly when lobsters were re-immersed after a 30 min period of emersion. However, it returned to pre-emersion levels by the second hour of re-immersion. Daily rhythm resulted in a significantly higher nocturnal TAN excretion rate for J. edwardsii; no daily rhythm was observed for P. cygnus. Feeding had the largest influence on TAN excretion, with maximum increases of 6.28 (J. edwardsii) and 5.60 (P. cygnus) times the pre-feeding level. TAN excretion rates remained significantly higher than the pre-feeding levels for an extended period (26 h, J. edwardsii; 30 h, P. cygnus). Implications for the use of purging tanks in lobster holding facilities and for the design of biofiltration systems are discussed.     

Effects of body weight, water temperature and ration size on ammonia excretion by the areolated grouper (Epinephelus areolatus) and mangrove snapper (Lutjanus argentimaculatus)

The effects of body weight, water temperature and ration size on ammonia excretion rates of the areolated grouper Epinephelus areolatus and the mangrove snapper Lutjanus argentimaculatus were investigated. Under given experimental conditions, L. argentimaculatus had a higher weight-specific ammonia excretion rate than E. areolatus. Weight-specific ammonia excretion rates of fasted individuals of both species showed an inverse relationship with body weight (W, g wet wt.), but a positive relationship with water temperature (t, °C). The relationships for total ammonia nitrogen (TAN) were: E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=21.4·exp^0.11t·W^−0.43 (r²=0.919, n=60); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=121.5·exp^0.12t·W^−0.55 (r²=0.931, n=60). Following feeding, the weight-specific ammonia excretion rate of E. areolatus increased, peaked at 2 to 12 h (depending on temperature), and returned to pre-feeding levels within 24 h. A similar pattern was observed for L. argentimaculatus, with a peak of TAN excretion being found 6 to 12 h after feeding. Stepwise multiple regression analysis indicated that weight-specific TAN excretion rates of both species increased with increasing temperature and ration (R, percent body wt. d⁻¹): E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=22.8·t−28.8·R−378.2 (r²=0.832, n=24); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=22.9·t−25.4·R−216.4 (r²=0.611, n=24). The effect of body weight on weight-specific postprandial TAN excretion was not significant in either species (p>0.05). This study provides empirical data for estimating ammonia excretion of these two species under varying conditions. This has application for culture management.     

A study of appetite, digestion and growth in juvenile green turtle (Chelonia mydas L.) fed on artificial diets

One-year-old immature green turtles (500–900 g) were kept at 25°C in recirculating seawater. When deprived of food for 108 h and then offered floating trout pellets, they consumed a satiation meal (S g) described by the equation S = 0.025 W^0.96 where W is wet body weight in g. When food was offered at different deprivation times after satiation, maximum meals were taken after 60–72 h and corresponded to 2.5% body weight. Longer deprivation (108 h) led to reduced intake of 1.9–2% body weight. Gastric emptying was measured using X-radiography or chromic oxide to label the faeces. Complete stomach emptying required 110 ± 11 h S.D. and maximum appetite occurred when the stomach was estimated to be 80–95% empty. The average time for a meal to be voided from the alimentary tract was 176 ± 16 h S.D. Diets containing 40–50% protein and 4.2–5 kcal/gram were assimilated with apparent efficiencies of 76 ± 6% S.D. and 86 ± 6% for energy and protein nitrogen, respectively. A preliminary estimate for an energy budget for these animals based on a 23 day growth period accounted for 90% of the daily food intake as:

100 I39M + 32E

where known daily intake (I calories) is compared with measures or estimates of metabolic rate (M), growth (G) and excreted wastes (E). Although this balance is similar to that of herbivorous fish, the turtle has slower feeding and digestion rates than its teleost counterparts.     

Long‐term Growth Study of Male and Female Green Sea Urchins,Strongylocentrotus droebachiensis,under Constant Light and Temperature Regime

Juvenile green sea urchins, Strongylocentrotus droebachiensis, with an initial mean test diameter of 9.5 mm (±1.2 SD) were held in the laboratory, in individual compartments and at constant temperatures (8.5 C) (n = 90) for a period of 99 wk. The animals were supplied with flow‐through sea water, and fed the Nofima sea urchin manufactured feed ad libitum. Growth, survival, feed intake, feed conversion ratio (FCR), and gonad index (GI) of the sea urchins were monitored during this period. The mortality was 11% throughout the experimental period. The test diameter (TD) increased significantly with time, best described by a polynomial equation (TD = 0.8756 + 0.584Time − 0.002Time², R² = 0.995). Sex was included as dummy variable, but did not contribute significantly; hence, there were no differences in growth trajectories between males and females. The study showed there were no significant differences in body wet weight or FCR between males and females. However at the end of the experiment the female urchins had a significantly higher GI of 31.1% (±4.4), compared with the male urchins which had an average GI of 26.8% (±5.4). In conclusion, the present study showed that male and female green sea urchins have the same growth capacity and feed convention ratio but by the end of the experiment the gonad index was significantly higher in female than male urchins.     

A comparison of the biological characteristics of Sarotherodon niloticus (L.) with those of S. aureus (Steindachner) and other tilapia of the delta and lower Nile

The tilapia species occurring in the lower Nile are Sarotherodon niloticus, S. aureus, S. galiaeus and Tilapia zillii. The distinguishing characteristics between the previously confused S. niloticus and S. aureus are summarised; from these there is no evidence of hybridization of the two species in natural populations in Egypt. The scales are used to estimate the growth rates of tilapia species in two coastal lakes and in both, S. niloticus grows faster than S. aureus after the first year. The possible factors causing the growth checks on the scales are considered.

The spawning season of S. niloticus appears to attain a discrete peak in April–May, whilst the spawning season of S. aureus extends from May to September with at least two actual spawnings within this period. Natural spawning cycles are compared with those observed in fish ponds.

A more extended spawning period of S. aureus may explain the reduced growth rate of the species after the first year. The spawning cycle of all species coincides with the temperature regime of the water bodies. The fecundities of S. aureus and S. niloticus are similar and are described by log F = log 1.33 + 2.23 log L, which suggests that small fishes produce more eggs per g body weight than large.

The salinity tolerance of the Nile tilapia can be ranked as T. zillii > S. galilaeus > S. aureus > S. niloticus. Evidence on chronic and acute effects of salinity are reviewed and upper estimates for salinities giving unimpeded growth are deduced as being, T. zillii 29‰, S. galilaeus 15–20‰, S. areus 10–15‰ and S. niloticus 5–10‰.     

Influence of environmental variables on the distribution of Squatina guggenheim (Chondrichthyes, Squatinidae) in the Argentine–Uruguayan Common Fishing Zone

The environmental variables affecting the spatial distribution of Squatina guggenheim Marini, 1936, an important commercial resource, are unknown. Here the influence of temperature, salinity and depth on the distribution of S. guggenheim in the Southwest Atlantic was analyzed. The species–habitat associations were evaluated for the S. guggenheim population as a whole as well as segregated by sex and by size classes (group 1, 23–44 cm; group 2, 45–74 cm; group 3, 75–91 cm L_T). Sampling took place during four bottom trawl surveys conducted in the Argentine–Uruguayan Common Fishing Zone (AUCFZ) during spring (October 1997) and fall (March 1997, March–April 1998, May–June 1998). The sharks were captured using an Engel bottom trawl and the hydrographical information was obtained using a CTD. Bottom depth was measured with an echo-sounder. The distribution of the population was affected (P ≤ 0.05) by temperature during spring and fall, while it was affected by salinity only in spring. S. guggenheim showed preference for temperatures ranging from 7.0 to 18.5 °C and salinity values ranging from 33.4 to 33.8. Generally, the distribution of males was affected (P ≤ 0.05) by temperature during both seasons, while females were influenced by temperature only in spring. The distribution of both sexes was affected by salinity in spring but not in fall. The distribution of adults sharks (group 3) was more affected by temperature and salinity than that of juveniles sharks (groups 1 and 2). Depth had no effects on the distribution of adults (males or females); however, juveniles belonging to size-group 2 were associated with depth during both spring and fall. High population catches were associated to thermal fronts during all surveys suggesting that these oceanographic structures play an important role as feeding areas for S. guggenheim. A population strategy of spatial segregation by size was proposed and discussed.     

The life-history patterns of Barbus bocagei sclateri (Günther, 1868) in a tributary stream of the Guadalquivir River basin, southern Spain

Abstract– The age, growth and reproduction of Barbus bocagei sclateri (Günther, 1868) were studied from November 1987 to September 1989 in a tributary stream of the Guadalquivir River basin. The maximum ages observed were 8 + in males and 11 + in females. The seasonal growth began in April and lasted 4–6 months. The males matured during their third year of life (2 +) and females during their fifth/sixth year (4 +/5+). There was a significant difference in the overall sex ratio of 1. 3: 1 (961 males to 740 females). Reproductive activity started in April/May and lasted until June/July. The end of the reproduction period coincided with a fall in the percentage of oxygen saturation. During this reproductive period, females spawned 2 batches of eggs. The relationship between fecundity (Fec) and fork length (FL, mm) was: Fec= 7.54 × 10⁻⁴ FL^3.06 The maximum contribution to the fecundity of the population was observed in the 6 + female group. The reproductive effort was also maximum in this age. Compared with the rest of the European barbel populations studied, the life-history patterns of this stock are characterized by low annual growth, early maturity, reduced longevity and high fecundity.     

Population age and sex structure of Aphanius fasciatus Nardo, 1827 (Pisces: Cyprinodontidae) in the Mesolongi and Etolikon lagoons (W. Greece)

The population structure of Aphanius fasciatus in the Mesolongi and Etolikon lagoonal system was studied, using 5794 fish. Significant differences were observed in the number of individuals of each sex, the age and size composition and survival. The overall males to females sex ratio was 1:2.44, although there was seasonal variation. During the reproductive period the percentage of males in the population decreased significantly, while after reproduction and during recruitment they increased. The females in each age class were larger than the males (40.33 mm and 36.72 mean total length for females and males, respectively). The survival rate of females was greater than males (0.73 for females and 0.60 for males). The strategy of this species is to invest in female individuals.     

Reproductive Performance of Macrobrachium rosenbergii Females in Captivity

Abstract.— The breeding frequency, fecundity, egg hatchability, larval output and viability of pond-reared, mature Macrobrachium rosenbergii females were individually followed up during 180 d. Sexually mature prawns were maintained under controlled laboratory conditions (28.7 C and 12-h light/d) in a 190-L freshwater recirculation system and fed a formulated diet. Ovarian development, moulting and spawning events were checked daily. At least six consecutive moults were recorded for each of the 18 females. The duration of the intermoult period averaged 27.5 d and was not affected by ovarian development or spawning, but intermoult periods followed by spawning had significantly lower growth rates. From a total of 126 moulting events recorded, egg laying successfully followed 76 (60.3%) of them. The number of eggs per spawn (NES) varied from 26,587 to 74,775 for females weighing 20.0 to 55.8 g. The relationship between NES and female size (W; in g) was found to be NES = 484 + 1454W ( r ²= 0.74). Results suggest that the number of viable larvae produced per egg clutch may be increased by in vitro incubation, as the losses of eggs, which usually occur under in vivo incubation, are prevented. The present study illustrates that under adequate and stable rearing conditions, M. rosenbergii females are able to spawn up to five times during 180 d, in comparison to three to five times per year as reported for wild prawn populations.     

Adding value to live, commercial size soft-shell clams (Mya arenaria L.) in Maine, USA: results from repeated, small-scale, field impoundment trials

The soft-shell clam (Mya arenaria L.) fishery in the state of Maine, USA, is worth $5–10 million annually and is primarily based on the sale of live individuals. More than 80% of the catch is sold for the “steamer clam” market that is highly seasonal due to supply and demand. Prices paid to harvesters for live clams throughout the year increases by as much as 70% during a 4-month period between early spring and late summer. If clams harvested in the spring could be held until late summer, a value-added product could be developed in this fishery. From April to August 1996, at an intertidal and subtidal location in eastern Maine, I tested whether it was biologically feasible to impound commercially harvested clams (shell LENGTH=44–75 mm) at densities between 720 and 850/m² using cages and nets (1.12 m²) to protect animals from predators. Survival was 91.7% (n=12) and 91.6% (n=6) at the intertidal and subtidal site, respectively. There was no discernible shell growth during this period and there was no difference between initial and final clam weights. The methodology was transferred to commercial harvesters in two Maine communities: Wiscasset (1997 and 1998) and Perry (2000). Impounded clams ( =21.8 kg/cage; n=18) lost, on average, 5 kg/cage in 1997 and 1998, whereas no significant net loss in wet weight occurred in cages deployed in 2000 ( =23.2 kg/cage; n=3). Differing harvesting and handling methods of individual clammers, prior to impounding clams, likely explains the variation in weight lost during the impounding periods. The difference in price per live kilogram between the beginning of the impounding period and the August sale date resulted in an average gain of $13.60/cage for clammers in 1997 and 1998 and $57.73/cage in 2000. Simple culture techniques can be used to increase the value of the live harvest of soft-shell clams along the coast of Maine. Clam impoundments may be a way for communities that co-manage the public clam resource with the state's marine resource agency to generate funds to pay for traditional management schemes, as well as enhancement programs that employ hatchery-reared juveniles. In addition, an indirect benefit of clam impoundments might be to create spawner sanctuaries since animals are impounded during the time when gamete release occurs.     

A rapid ELISA method for detecting specific antibody level against nodavirus in the serum of the sea bass, Dicentrarchus labrax (L.): application to the screening of spawners in a sea bass hatchery

The use of an indirect elisa for the detection of the sea bass, Dicentrarchus labrax (L.), antibody to nodavirus is described. The sera of 110 adult sea bass (78 females and 32 males) maintained in captivity were analysed, and the females were individually classified in seropositive (16%) and seronegative (56%) groups, while some fish (28%) with low but detectable antibody levels were not classified. The proportion of seropositive males was smaller (3.1%) than the females. Repeated serological examination of 18 individually labelled females (spawners) revealed no changes over 5 months. The immunization of sea bass females with heat-killed nodavirus induced antibody titre as reflected by corresponding changes in elisa optical density readings. The antibody level increased 4 weeks post-immunization and was still detectable after 41 weeks. These preliminary results suggest the possibility of detecting virus-positive or virus-negative animals in attempts to reduce and prevent the vertical transmission of the virus in sea bass hatcheries.     

Survival and growth of hatchery-reared individuals of the European lobster, Homarus gammarus (L.), in field-based nursery cages on the Irish west coat

At present, one of two strategies is employed by fisheries managers for enhancing wild stocks of homarid lobsters using hatchery-reared individuals. The first is repeated releases of large numbers (>5000 at a time) of postlarvae (stage IV and V; carapace length [CL]=5–7 mm) to selected bottom locations. This option exists primarily because these programs lack space, time, and/or the finances to rear animals to larger sizes that would most likely have initially higher survival rates. The second is to rear animals in the laboratory for 5–8 months to stage XII+ (CL=12–16 mm) and then release small numbers (<1000) of these relatively large juveniles. To date there has been no attempt to release large numbers of relatively large juveniles because the costs are too prohibitive.

We have developed a low-cost, low-maintenance, field-based nursery caging system for rearing cultured lobsters, Homarus gammarus (L). Individuals (780 and ranging in CL from 5.2 to 7.2 mm) were reared in pre-fouled and unfouled containers (360 cm³) fabricated from an extruded plastic netting (3.2 mm aperture) and in pre-fouled plastic petri dishes (200 cm³) that were deployed in five near-bottom cages for 10 months (September 2000 to June 2001) at two subtidal sites located in a shallow, relatively exposed embayment on the west coast of Ireland. Animals apparently were able to survive and grow by suspension feeding on the plankton and/or foraging on the fouling community that settled on and within individual containers. Mean recovery rate (±95% CI) was independent of a priori fouling treatments, but was site-specific (42.1±7.9% and 27.8±13.7%; n=5). These rates are minimal estimates of survival because we found that at least 20% of the animals were capable of escaping from the mesh containers. Mean recovery in petri dishes that prohibited emigration was 53.3±37.02% at one site and 75.0±23.1% (n=5) at the other. These recovery rates compare favorably with survival rates of fed conspecifics held in the laboratory over the same time (54/81=66.7%). At the end of the experiment, animals in field cages had mean CLs that were significantly smaller than the fed controls. Because of costs incurred with maintaining small lobsters under laboratory conditions, results of this short-term, manipulative field experiment indicate that field-based nurseries represent an economically viable, third option for managers of lobster stock enhancement programs.     

Sex reversal in salmonid culture: Part III. The production and performance of all-female populations of brook trout

Oral administration of 17β-oestradiol at 20 mg/kg diet for a period from 0 to 60 days following “swim-up” resulted in the sex reversal of male brook trout and the production of essentially all-female populations. The gonads of these animals were indistinguishable from the ovaries of untreated fish.Oral administration of this hormone at the same concentration but for the period from 0 to 40 days resulted in a population which comprised 67% females, 21% males and 12% bisexual animals.Within the control group, the growth rate of males was significantly greater than that of females. Growth in the all-female stocks produced by dietary oestradiol administration (0–60 days) was significantly lower than that of the untreated population taken as a whole but was identical to that of the control females.The growth of brook trout during the first year was closely comparable to that of rainbow trout reared under identical conditions. During the second year, the growth of the rainbow trout exceeded that of the brook trout. At 19 months of age, all the animals in the control and oestradiol treated brook trout populations were fully mature; maturity in the rainbow trout was substantially confined to males.     

Selectivity in a trawl codend during haul-back operation—An overlooked phenomenon

The selectivity of a 99 mm trawl codend was assessed using a codend cover fitted with a MultiSampler, which was acoustically triggered to take separate samples at three different phases of the haul. The first sample was collected during towing, the second during haul-up and the third at the surface. A total of 18 hauls were conducted with a commercial fishing vessel west of Scotland. It was demonstrated that escapes take place not only during the tow but also in the short period when the trawl is hauled back from the seabed and when the codend is at the surface. For haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus) and Norway lobster (Nephrops norvegicus), respectively, the mean percentages escaping at the surface were 16, 12 and 38% of the total escape while 17, 8 and 28% escaped during the haul-up phase. Compared to towing, the escape rate (no./min) increased for haddock by a factor 2.7 during haul-up and by a factor 1.7 at the surface, whereas the escape rates of whiting were similar for the three phases. The escape rate of Norway lobster increased by a factor of approximately 7 for both the haul-up and surface phases, compared to the towing phase. The selectivity parameters L50 (50% retention length) and SR (selection range = L75–L25) were estimated and compared for the three different phases and for the whole haul for haddock, whiting and Norway lobster. For all three species there was no significant (P > 0.05) difference in L50 between the three phases of the haul. There was also no significant difference for whiting and Norway lobster when comparing the SR of the three phases, whereas the SR was significantly lower for haddock when comparing the surface phase with towing and haul-up. The estimate of L50 when towing was about 6 cm lower for haddock and whiting and 9 mm for Norway lobster compared to the selection curve estimated conventionally for the whole haul. Finally, the effect of sea state, duration and codend catch on the selectivity parameters were estimated for the individual phases and for the whole haul. A significant effect of at least one variable was found in all phases.     

The dynamics of an exploited population of a siluroid catfish (Schilbe intermidius Reupell 1832) in the Cross River, Nigeria

We studied the breeding biology, growth, mortality, recruitment pattern and yield per recruit of a freshwater siluroid catfish (Schilbe intermedius) in the Cross River, Nigeria. Monthly variation in mean gonadosomatic and mean condition indices shows that the species breeds twice in a year (March and September). We used 12 consecutive months length–frequency data together with FiSAT software to study the population dynamics of this catfish which is exploited by artisanal fishermen. Fitting the seasonalized von Bertalanffy growth function to our length–frequency data gives the following growth parameters: L_∞=27.5 cm total length, K=0.29 yr⁻¹, C=0.5, WP=0.46. Using the seasonalized length converted catch curve, we estimated the instantaneous total mortality coefficient Z as 1.85 yr⁻¹. The instantaneous natural mortality coefficient M was 0.81 yr⁻¹ while the instantaneous fishing mortality coefficient F was 1.04 yr⁻¹, giving the current exploitation rate E=0.56. From the analysis of probability of capture of each length class we estimated the length at first capture L_c to be 10.9 cm. The relative yield per recruit analysis predicted the maximum exploitation rate E_max=0.54. The current exploitation rate E is more than this predicted maximum E_max. Thus, the species is on the verge of being overexploited.     

Production of a high percentage of male offspring in growth-enhanced transgenic tilapia using Oreochromis aureus ZZ selected pseudofemales

Gene transfer has offered a new tool for the development of improved fish strains for aquaculture. Monosex fish populations could minimize the impact of genetically modified organisms in the environment. In Oreochromis aureus, the use of pseudofemale spawners (sex-reversed male with a female phenotype) is an alternative technique for producing genetically male tilapia offspring. O. aureus fry were treated with 17β-estradiol at 100 mg/kg of food for 45 days. We obtained 77.1% females and 45.9% in the control group. Females randomly taken from the treated group were crossed with normal males. Fry from pseudofemales producing more than 90% male progenies were submitted to 17 β-estradiol treatment to obtain F₂ pseudofemales. The results of the sex-reversal were low and variable ranging between 66.0 and 84.3% females. F₂ pseudofemales were crossed with transgenic males from the F70 line (O. aureus × O. urolepis hornorum) and non-transgenic (O. aureus) males. The sex ratio of progeny of F₂ pseudofemale deviated significantly (P < 0.01) in favor of males in the crosses with transgenic (90.2%) and non-transgenic (89.3%) males compared to the results observed with normal females (51.0 and 52.3%, respectively). The mean fry production with pseudofemales (per m²/day) was similar to the normal females in the crosses with transgenic and non-transgenic males. To our knowledge this is the first report on the production of a near monosex population in genetically modified fish.     

Individual growth variation and its relationship with survival in juvenile Pacific oysters, Crassostrea gigas (Thunberg)

In order to study individual growth variability and its relationship with survival in juvenile Crassostrea gigas, parental oysters were sampled at four sites located along the French Atlantic coast and bred under controlled hatchery conditions. Four groups of larvae were obtained by crossing five males and five females from each of the four sites, and a fifth group by crossing these 20 males and 20 females together in a pool. Larvae were reared under conditions allowing the maintenance of a maximum variability of size and gave five experimental groups. Oysters were individually monitored for growth and survival from 3 to 10 months after fertilization. The individual growth performances were relatively stable over time and no noticeable compensation for growth occurred. Early growth rate was a very good predictor of size later in life: 66% of variation in the live weight at 10 months could be explained by variation in the initial growth rate calculated between 3 and 4 months. A significant group effect was observed on survival and on growth rate. Mortality mostly occurred between 3 and 5 months and appeared to affect the slow-growing animals more. However, two groups can be distinguished among those which died during the experimental period, one which showed a decrease in weight and the other whose growth was similar to surviving oysters. These results are discussed in the light of usual oyster farming practices and selective breeding. This revised version was published online in August 2006 with corrections to the Cover Date.     

Induced Maturation of Striped Bass Morone saxatilis Exposed to 6, 9, and 12 Month Photothermal Regimes

Photoperiod and temperature manipulation were used to induce out-of-season spawning in striped bass Morone saxatilis Maturational status of 40 age V striped bass, held in five environmental chambers exposed to compressed 6 mo and 9 mo cycles and a 12 mo reference cycle, was monitored by monthly egg and gonadal (ultrasound) diameter measurements. The primary effect of shortened photothermal and reproductive cycles was a reduction in egg and ovary diameters, and the number of ovulating females. Egg diameters were directly related to cycle length, averaging 845 μm, 946 μm, and 1,073 μm for females held on the 6, 9, and 12 mo cycles, respectively. Ovary diameters of females held on the 6 mo cycle were smaller (19.6 mm) than those held on the 9 mo (27.4 mm) and 12 mo (24.5 mm) cycles. Ovary diameter was highly correlated with egg diameter ( r = 0.91). The number of ovulating females decreased with cycle length, totalling 38%, 88%, and 100% in the 6, 9, and 12 mo cycles, respectively. However, no differences in fecundity (58,452 eggslkg) or fertility (39%) were detected between photothermal treatments or tank-spawned and strigspawned females. When held for a second 6 mo cycle, none of the females spawned repetitively. Males held on the 6 mo cycle had smaller testicular diameters (16.2 mm) than those held on 9 mo (23.6 mm) and 12 mo (19.2 mm) cycles. Testicular diameter of spermiating males (15.7 nun) was significantly larger than that of non-spermiating males (6.9 mm) in all cycles. The low number of spawning females and small egg, ovarian, and testicular diameter offish held on the 6 mo cycle suggest that subjecting striped bass brood stock to an abbreviated cycle length (<9 mo) constrains maturation development and reproductive success.