Even when fishing effort data are not available in a developing fishery, it is often still possible to develop a simple fishery indicator to obtain information about the status of the stock. Using catch data only, Grainger and Garcia, (FAO Fish. Tech. Pap. 359, 1996) showed that the changes over time in the relative rate of catch increase (RRCI) can be used to detect when a stock reaches its over-fishing level, i.e., the year when RRCI falls to zero. The method presupposes that fishing effort increased steadily over the period concerned. We propose a generalization of this method that consists in obtaining a crude estimate of the maximum sustainable yield (MSY) by plotting the trend in catches against the smoothed RRCI. The yellowfin tuna (Thunnus albacares) fishery in the Eastern Atlantic was used to show the bold relationship between MSY estimates obtained from standard equilibrium production models and from this method. Given that it is very difficult to estimate effective fishing effort for skipjack tuna (Katsuwonus pelamis), we show how this simple fishery indicator can be used to obtain proxies of MSY for skipjack fisheries located in the Eastern Atlantic Ocean and in the Indian Ocean.

Each summer the presence of large concentrations of bait fish in the area of the central Azores Islands gives rise to mixed-species feeding aggregations usually at dawn and dusk. The encircling of prey initiated by common dolphins (Delphinus delphis), often mixed with spotted dolphins (Stenella frontalis), results in the formation of a compact ‘ball’ of several thousands prey fish close to the surface. Other dolphins, in particular the bottlenose (Tursiops truncatus), also eat the prey fish, whose high concentration makes them easy to capture. Large tunas (Thunnus thynnus, Thunnus albacares) sometimes participate in the phenomenon. Seabirds (mainly cory’s sheawaters, Calonectris diomedea borealis) are always present throughout the few minutes during which the entire collective food hunt takes place. A model of the phenomenon, based on 15 observations, is proposed. It comprises 4 stages: a preparation phase, an intensification phase, a mature phase, and a dispersion phase. These observations allow a better understanding of the tuna-dolphin aggregation process. They show that it is the tunas that generate and beneficiate from the aggregation with dolphins, rather than the contrary.

We describe daily allocation patterns of fishing effort (hookah diving) of the artisanal fleet in San Felipe, Yucatan (Mexico), using catch, fishing effort, catch per unit of effort, variable costs, quasi rent and distance from port to four fishing grounds as performance variables. Two vessel categories were defined by the presence/absence of a LOng RAnge Navigation (LORAN) system. Hookah divers caught a daily average of four species in 95% of the trips, thus precluding the quantification of effective fishing effort allocated by species. Highest catches and quasi rent were mainly obtained on Thursdays and Fridays, coinciding with the highest catches of Octopus maya and spiny lobster (making up 50% of the total economic revenue) on Fridays in 3 of the 5 weeks analysed. An upper ceiling of daily catch per diver occurred close to the weekend, suggesting a limited capacity in terms of handling time and diving hours, and also a catch level that fulfills daily economic expectations. Both LORAN and non-LORAN vessels preferred to work the nearest ground to port, in spite of higher yield and economic rent from more distant grounds. Nonlinear modelling of fishing effort allocation showed distance from port as the key decision factor. Generalized linear modelling (GLM) revealed significant effects of vessel type and fishing ground, with LORAN vessels having significantly higher catch rates and tending to allocate more effort to distant grounds than vessels without LORAN. GLM performed by species corroborated that Friday was the most productive day concerning spiny lobster and octopus. This day effect in the most valued species also suggests that pressure for higher economic benefits occurs before diminishing (Saturday) or ceasing (Sundays) fishing activities.

Les variations de la diversité spécifique entre les assemblages depoissons démersaux du golfe de Gascogne (océan Atlantique) et du golfe duLion (mer Méditerranée) sont analysées à l'échelle d'unedécennie. L'objectif est d'interpréter la dynamique de ces peuplements dontles niveaux d'exploitation par la pêche diffèrent. Les donnéesutilisées sont issues des campagnes de chalutages réalisées de 1983à 1997 pour l'évaluation des stocks de poissons démersaux. Lesindicateurs de diversité analysés sont les indices N1 etN2 de Hill, la richesse spécifique S, la régularitéJ’, les courbes K-dominance et les stratégiesdémographiques des principales espèces. Chacun des deux peuplements estconstitué de trois assemblages d'espèces (assemblage côtier, duplateau et de la pente continentale). Il n'existe de différence significative nientre les assemblages du golfe de Gascogne, ni entre ceux du golfe du Lion. Il n'y apas non plus de différence significative entre les années. L'ensemble dugolfe du Lion est caractérisé par des valeurs N2 et des indicesS plus élevés que dans le golfe de Gascogne. Selon lescourbes K-dominance, le peuplement du golfe de Gascogne est caractérisé parun plus petit nombre d'espèces dominantes que dans le golfe du Lion. Enfin, lesespèces dominantes du golfe du Lion sont caractérisées par unelongévité et/ou une taille maximale plus grande que celles du golfe deGascogne. Le golfe de Gascogne correspondrait à un système adaptéà l'existence de perturbations (mortalités imprévisibles), tandis quele golfe du Lion correspondrait à un système adapté à unenvironnement moins perturbé. Un impact de l'exploitation n'a pu êtreinterprété à partir des indices choisis, utilisés à cetteéchelle. Toutefois, lorsque les indices de diversité sont mis en relationavec la stratégie démographique des espèces, des éléments dedynamique des peuplements sont mis en évidence.

Brown trout (Salmo trutta) were selected for growth for 4 generations. We tested the effects of selection on voluntary feed intake measured by self-feeders, feed efficiency and size variability. The specific effects of a slight feed restriction and of food deprivation were also investigated. Fish were issued from groups of eggs of selected females fertilised with sperm of selected (S group) or control males (S1/2 group). According to the growth rates expected for the selected and control lines, the S1/2 group was fertilised 12 days before the S group, so that all the fish reached 8 g at the same time. At 8 g, they were allotted to 8 tanks (500 fish per group) and 3 experimental periods followed. Fish were accustomed to self-feeders during a 28 days pre-experimental period. Then, half of the groups were fed ad libitum, and half were restricted (80% of the expected ad libitum level) for 171 days; growth and feed intake were recorded regularly and any uneaten food was weighed. Then followed a 56 days starvation period. At the end of the pre-experimental, feeding and starvation periods, individual weights and lengths were measured on 50 trout per tank. The response to selection at the end of the feeding period varied with the feeding level. In ad libitum fed groups, the mean final body weight of S was +6.1% higher than that of S1/2 and feed efficiency was similar (1.10 for S and S1/2). The higher growth of S compared to S1/2 was related to a higher feed intake of the S groups (+5.3%). When fish were restricted, the final body weight was lower in S (117.1 ± 2.1 g) than in S1/2 groups (123.8 ± 1.7 g). This was mainly related to a slightly lower feed efficiency of S compared to S1/2 at the beginning of the feeding period. Neither the group nor the feeding level affected the size variability of the fish. At the end of the starvation period, the relative loss of weight was equivalent for all the groups, and the variability of the weight was higher for S than for S1/2. The results highlight the fact that genetic gain can only be expressed when brown trout are fed ad libitum.

L'affinage traditionnel des huîtres creuses, pratiqué dans lesclaires ostréicoles de la façade Atlantique française, est soumis auxfluctuations des facteurs environnementaux. Un procédé d'affinagecontrôlé en terme de matières organique et inorganique particulaires(MOP et MIP) et de température a étédéveloppé afin de réduire la variabilité de croissance de lachair des huîtres. Cet élevage intensif repose sur la production de ladiatomée Skeletonema costatum, distribuée aux huîtres avecune concentration moyenne de 4–5 mg deMOP·L–1. Un modèle écophysiologique del'huître Crassostrea gigas, simulant les croissances du soma et desréserves–gonades, a été appliqué aux conditions d'affinagecontrôlé dans le but d'analyser les réponses du bivalve. Sonélaboration a nécessité un retour à l'expérimentation. Deuxfonctions d'alimentation ont été étudiées au laboratoire : lafiltration et la production de pseudofèces, intervenant dans la régulationde l'ingestion. Les résultats, pour une température de 14 °C, etpour une gamme de MOP et de MIP variant respectivement de 4 à18 mg·L–1 et de 15 à55 mg·L–1, montrent que l'ingestion est réguléepar la production de pseudofèces, la filtration ne présentant pas devariation significative (moyenne de2,09 ± 0,11 L·h–1·g–1, avec un taux d'activité de 59%). Cette production de pseudofèces, quiengendre une augmentation de la fraction organique ingérée par lemécanisme de sélection pré-ingestive, permet de compenser lavariabilité des concentrations en MIP. Des simulations ont permisd’analyser les effets négatifs de ce facteur environnemental sur lacroissance du bivalve. Il apparaît alors que la ration utilisée en affinagecontrôlé (4–5 mg de MOP.L–1) permet unecroissance en chair sèche malgré des concentrations simuléesatteignant 50 mg·L–1 de MIP. Lors del'élaboration du modèle pour les conditions spécifiques àl'affinage contrôlé, la gamétogenèse est apparue comme unprocessus déterminant pour la répartition de l'énergie chez lebivalve.