The present study presents a method of assessing and fitting a stock-recruitment relationship. The method is based on a combination of an equilibrium production model and a yield per recruit model. It does not use recruitment data which are, in general, highly dispersed, but production data. Thus, the relationship obtained is not of the parametric form of classical stock-recruitment relationship such as those of the Beverton and Holt (1957) and Ricker (1954), however, in some cases, the resulting stock-recruitment curve resembles models currently used in fisheries science. It is interpreted as an expectation of the recruitment conditionally to stock spawning biomass and stock demographic structure. However, for some realistic values of the parameters, the curve thus obtained describes a loop which suggests that two different recruitment levels may be expected from the same stock spawning biomass level. This theoretical property is illustrated with simulations. As an illustrative example, the method is applied to North-West Atlantic gadoid stocks. The significance of the method, its originality, the validity of the underlying assumptions and their conceptual consequences are discussed.

Les stratégies de pêche, développées par les capitaines des thoniers senneurs vénézuéliens dans la mer des Caraïbes, reposent sur la recherche d'indices visuels indiquant la présence de thons à la surface de l'océan. Une fois que la détection est réalisée, le senneur peut choisir la tactique qui consiste à demander l'aide d'un thonier canneur (pêche à l'appât vivant) afin d'augmenter le taux de réussite d'un coup de senne. Pour prendre en compte la variabilité de différents facteurs, comme la saison, les indices de détection associés aux bancs, l'aide du thonier canneur, les espèces de thons et le poids moyen des poissons présents dans le banc, les probabilités de réalisation de certains événements sont estimées à l'aide de la modélisation Logit, issue de modèles Log-linéaires. Les probabilités de capturer des bancs de gros individus sont maximales lorsque l'albacore est l'espèce dominante et que la pêche a lieu sur banc libre ou sur requin-baleine; cet effet est d'autant plus marqué en saison sèche.En revanche, si le listao est l'espèce dominante, la probabilité de capturer des gros individus est faible, en particulier lorsque la pêche est réalisée sur des baleines. Les résultats de cette modélisation sont intégrés dans un arbre de décision. Si on retient le critère de l'espérance mathématique des gains potentiels, l'aide du thonier canneur ne semble pas être la meilleure des tactiques possibles. Pourtant son impact est positif sur le taux de réussite d'un coup de senne, puisque le pourcentage de calées positives passe de 58 % sans aide à 85 % avec aide. L'efficacité de cette aide dépend du type d'association rencontré. Elle augmente lorsqu'il s'agit de bancs libres ou en association avec des baleines, mais diminue lorsque le banc est associé à un requin-baleine.Une simulation par la méthode de Monte Carlo montre que, malgré l'absence de cette aide, quelques gains importants peuvent être réalisés lorsque l'objet flottant associé est une baleine. En conséquence, le choix tactique qui consiste à demander l'aide d'un thonier canneur et qui est plus fréquent chez les petits et moyens scnncurs que chez les grandes unités, pourrait traduire une attitude de prudence face au risque, sans écarter certaines contraintes physiques de ces senneurs comme leur vitesse. Si on la compare avec d'autres zones de pêche thonière, la mer des Caraïbes se caractérise par une forte proportion de coups de senne faites sur requins-baleines (37 % des coups) et sur baleines (24 %). En contrepartie, le nombre de coups de senne faits sur des épaves flottantes est trèss faible. Ceci pourrait être lié à la nature des courants marins qui entraîneraient ces épaves en dehors de la zone de pêche des senneurs vénézuéliens. La similitude des comportements adoptés face au risque, dans des milicux très différents comme la pêche et l'agriculture, est discutée brièvement.

A protocol for the incubation of Centropristis striata ovarian follicles in vitro from females that spawn repeatedly under laboratory conditions was developed. Isolated follicles underwent maturation after 15–24 h of incubation at 25 °C in 75% Leibovitz L-15 medium, containing relatively low doses of human chorionic gonadotropin (hCG) or homologous pituitary extract (50% response with approximately 2 IU/ml and 0.02 pituitary equivalents/ml, respectively). During maturation in vitro, follicles underwent a remarkable volume increase (about 340%) that was correlated with proteolysis of primarily the largest yolk proteinin the oocyte. The maturation response of follicles to hCG was size-dependent within a range from 400 to 600 m in diameter, with a maximum response displayed by 530–600 m follicles. The response of these follicles to hCG was not inhibited within a range of incubation temperatures from 15 to 28 °C, but the length of time required for the follicles to achieve 50% maturation was negatively correlated with increasing temperatures (r2 = 0.97; p ≤ 0.01). However, the largest follicles (530–600 μm) did not undergo maturation in response to high doses (1 μg/ml) of the major maturation-inducing steroids (MISs) known in teleosts, 17.20-dihydroxy-4-pregnen-3-one (17.20β) and 17.20β.21-trihydroxy-4-pregnen-3-one (20βS), even after a previous incubation period of follicles with 10 IU/ml hCG for 1.3 or 6 h. These observations indicate that C. striata follicles are very sensitive to increasing doses of gonadotropin, and that their responsiveness to this hormone is highly size-dependent within a very narrow range. The preliminary results also suggest that 17.20β and 20βS are not the MIS in this species.

Sperm was collected from testes and from three or four parts of the deferent ducts of 4 to 6 winter-spawning male rainbow trout, about every two weeks over a three month period (January toMarch). Milt volume, spermatocrit, sperm motility, 11-ketotestosterone (11-KT) and 17α-hydroxy,20β dihydroprogesterone (17,20P) levels and ionic composition were determined. Steroids and ions were also measured in blood plasma. The K+ concentrations and steroid levels in the seminal fluid from each part of the ducts did not vary significantly over the period of study. In two out of the three parts of the ducts, Na+ (parts 1 and III) Ca2+ (parts I and III) and Mg2+ (parts I and II) levels were significantly higher at the end of March than two months earlier. Motility of testicular sperm peaked (42%) in mid-February while that of sperm from ducts usually remained stable (80 to 100%) throughout the three months. Seminal fluids and blood plasma were confirmed to be isotonic (300 mmol/kg). Levels of K+, Na+, Ca2+, 11-KT and 17.20P in the plasma and seminal fluid differed significantly (respectively 0.3 vs 34 mmol, 134 vs 116 mmol, 3.4 vs 2.0 mmol, 87 vs 17 to 39 ng/ml, 11 vs 3 to 13 ng/ml), with the Na+ and steroid levels being positively correlated in both liquids. Osmolality, Cl-, Na+, K+, Mg2+ and Ca2+ concentrationsremained constant along the genital tract. In contrast, spermatocrit and 11-KT and 17.20P levels decreased significantly and progressively from testis to the posterior or middle part of the ducts (respectively 97 to 49%, 39 to 17 ng/ml, 13 to 3 ng/ml) while sperm motility (percentage of motile cells and duration of motility) increased (74 to 96% and 41 to 51 s). No significant correlation was found between sperm motility and the studied physicochemical parameters of the seminal fluid.

Heritability of larval growth rate in diferent food environments and genotype-food environment interaction (during larval growth) are estimated in Mytilus chilensis, based on a nested design, involving 24 full-sib families (8 males each mated to 3 separated females) grown under three different algal concentrations. The heritability estimates for larval shell length ranged between 0.2 ± 0.1 and 0.9 ± 0.3 and showed a decreased towards the low food treatment; but the values suggest that selection would be effective. However, since there is a significant genotype-food interaction, selection should be done in each environment in which the animals are expected to perform. The significant interaction term (p<0.001) associated with a significant genetic variability for larval growth suggest the presence of genes in the population whose expression depends upon local food conditions.