The underwater sound of R/V "Johan Hjort" as a function of aspect angle has been mapped by cruising along parallel transects at various distances from the recording unit. At frequencies over 125 Hz there was a minimum in front of the vessel, and lobes of higher intensity on both sides. About 20% of the herring schools recorded during a survey in the North Sea seemed to react to the survey vessel. The distance at which the schools reacted varied from about 25 m and up to about 1000 m in front of the vessel. Within a sector of 20° on each side of the path of the vessel, the fraction of schools reacting increased to about 50%. Most of the schools that reacted seem to be herded in front of the vessel. This reaction pattern may indicate either that the schools reacted to the higher sound intensity to the side of the vessel or that the pattern occurred because of a "pursuit effect". Despite the apparent herding, more than 40% of the schools in front of the vessel were not recorded by the echo sounder. It is therefore argued that horizontal avoidance close to the vessel may have caused an underestimation of the biomass of herring of about 20%.

European hake Merluccius merluccius is the main commercial demersal species in the Gulf of Lions (Northwestern mediterranean). This fish is exploited by French and Spanish trawler fleets and by small-scale fisheries using gillnets and longlines. Data on landings and the demography of catches by type of gear are now available for the period 1988–1991. The assessment of the hake stock was carried out using methods of virtual population analysis (VPA), or cohort analysis, based respectively on mean pseudocohorts and cohorts by age. The results of these VPA were then analysed and compared from a methodological point of view. The general results have confirmed previous studies with regard to the level of exploitation of the species. The two methods gave quite similar results for fishing mortality rates and for mean biomass. The maximal fishing mortality rate was observed at age 4, but it was already high at age 1, due to the exploitation of juveniles by trawlers. The mean biomass was about 7 000 tons. The high values for turnover (about 80% or more) and for the maximal biomass/mean biomass ratio (23% or more) indicated a high level of exploitation but did not seem high enough to be dangerous for the stock. Yield-per-recruit analysis showed also that the present level of exploitation is far above the optimal sustainable level.

Neural networks provide a "black box" model for explaining and predicting trout abundance with 8 environmental variables. This work investigates the specific effect of each variable, by inputting fictitious configurations of explanatory variables and by checking the responses of the model. The comparison between this response of the model to environmental variables on one hand, and results from field observations on the other hand, shows similarities and indicates neural network modelling can be trusted. The elevation appears to be the major explanatory factor. The influence of shelters, bottom velocity and Froude number also play an important role. When considered separately, depth does not have a notable influence on the density of trout. Such confirmations of field observations suggest that these models can be used to obtain a clear identification and hierarchization of the factors influencing the abundance of trout and the mode of action of the factors. This approach can be extended to other applications in quantitative ecology in which non-linear relationships are usually observed.

Atlantic salmon, Salmo salar, and several species of whitefish, Coregonus nasus, C. peled, C. Lavaretusbaeri, males were treated with a long-acting medical androgen preparation - "Testoenatum". This preparation contains 24 mg of testosterone enanthate and 111 mg of testosterone propionate in 1 ml peach oil solution. Multiple (2–5) or single intraperitoneal injections of a standard dose (0.1 ml/kg fish body weight) were used. Treatment performed shortly before the beginning or within the breeding season resulted in a significant stimulation of spermiation, improvement of sperm parameters and extention of the gonad functional maturity period for 1–4 months. Low water temperatures (0.5–l.0 °C) seem to be essential to obtain a long-lasting effect after one injection. The histology of the pituitary gland suggests that the gonadotropic cells were activated through a positive feedback mechanism. Testoenatum suppressed Sertoli cells' phagocytary activity.

Successful rearing of hatchery-reared sea bass, Lates calcarifer, fry in illuminated floating cages was demonstrated in a 42-day experiment. Three feeding regimes, i.e. natural zooplankton (NZ) + minced fish flesh (MFF), NZ alone, or MFF alone and two stocking densities (600 and 1 200 individuals m2) were tested in a 3 × 2 factorial experiment. Fish reared in unlit cages and fed MFF alone during daytime served as the control. Results showed that no interaction existed between stocking density and feeding regime and that the two stocking densities used did not influence fish growth in terms of mean final body size. In general, sea bass reared in lit cages (NZ + MFF and NZ) grew and survived better than the control fish (MFF). However, fish reared under NZ + MFF feeding regime had the highest final mean total length (TL, 42.1 mm) and body weight (BW, 1 311.8 mg) followed by fish reared under NZ feeding regime (mean TL = 26 mm, BW = 415 mg). Fish in the unlit control cages exhibited the poorest growth (final mean TL and BW: 26 mm and 277.6 mg BW). Furthermore, specific growth rates (range: 5.7–8.5% day−1) of fish in lit cages were significantly better than those of fish in the unlit control cages (mean: 3% day−1). Percentage survival (38%) of fish stocked at 600 m−2 density and fed NZ was not significantly different from fish in the NZ + MFF feeding regime. However, increasing the density to 1 200 ind. m−2 tended to significantly decrease percentage survival (20%) of fish with NZ feeding. Fish reared in the unlit control cages had the poorest survival of 13–14%. The high percentage composition by number (CN, 88%) of copepods in the stomachs of sea bass fry fed on NZ alone and the equally high percentage feeding incidence (94%) indicated that fish fed sufficiently on natural zooplankton. Supplemental feed using minced fish flesh contributed about 43–59% of the fish diet in addition to natural zooplankton.

Tilapia cameronensis fry were reared in plastic tanks and earthen ponds at the Fishculture Research Station, Foumban, Cameroon. Their growth rate was compared to that of the Nile tilapia (Oreochromisniloticus) fry, reared under identical conditions. Different treatments were applied (food or organic manure only; food combined with organic manure); physical-chemical variables were quantified. The best growth rate was recorded for fry reared in tanks receiving both food and organic manure: the daily weight gain of T. cameronensis fry in tanks was nearly half of that of the Nile tilapia, while in ponds T. cameronensis showed a higher growth rate than O. niloticus (0.19 g.d−1versus 0.13 g.d−1). The growth rate decreased with increasing stocking density. In another experiment Tilapia cameronensis fry were reared in aquaria at different stocking densities in a recirculated water system. The highest growth rate was recorded for fry stocked at 0.5 fish.l−1 and receiving daily a feeding rate of 15% of the total biomass. This growth rate was comparable to that of fry stocked at 0.25 fish.l−1 receiving the 5% feeding rate. The latter had the lowest FCR. Mortality due to aggressive interactions increased with decreasing stocking density.

Manila clams, Tapes philippinarum (Adams and Reeve) are cultivated beneath plastic netting, to protect them from excessive predation, and harvested after approximately two years. Both the on-growing and harvesting process have the potential to alter benthic communities. In order to study these effects, we surveyed a clam lay and uncultivated areas at a site of commercial clam cultivation in south-east England. Surveys were undertaken at the end of the growing stage, immediately after harvesting by suction dredge and seven months later. Infaunal abundance was greatest within a net covered clam lay than in proximate and distant control areas, but the total number of species encountered was similar in all areas (20–22). These differences were not attributable to variation in sediment structure or environmental variables between the areas sampled. Tube-building polychaetes, such as Lanice conchilega and Euclymene lumbricoides, were particularly abundant within the cultivated area as was the errant polychaete, Syllis gracilis. Harvesting by suction dredge altered sediment composition by removing the larger sand fractions down to the underlying clay substratum, consequently there was a large reduction in the density of all individuals and the total number of species. Seven months later, no significant difference was found between the infaunal community in the harvested clam lay or either of the control areas and sedimentation had nearly restored the sediment structure. These observations indicate that the practice of clam cultivation does not have long-term effects on the environment or benthic community at this site.

Cette étude d'écophysiologie a été réalisée sur l'huître japonaise (Crassostrea gigas) cultivée en baie de Marennes-Oléron (France) en mai 1992. Les mesures sont effectuées dans des conditions alimentaires naturelles au cours de trois cycles de mesure (a, b, c) à des coefficients de marée de 49,57 et 88. La matière particulaire totale est mesurée en continu durant les cycles de marée. La fonction de consommation est calculée selon une nouvelle formulation prenant en compte un "échappement particulaire" précédant le tri pré-ingestif et représentant une perte passive de particules de petites tailles entraînées par la masse d'eau à travers le filtre branchial. La comparaison des fonctions de consommation et d'absorption de la matière organique totale et de la chlorophylle a, montre des différences significativement supérieures pour la charge particulaire de 55 mg.l−1 (c) comparée aux autres conditions (a, b) de charges particulaires inférieures à 30 mg.l−1. La comparaison des coefficients de sélectivité et des taux d'absorption de la chlorophylle a, (43,3 et 81,6 %), respectivement, supérieurs à ceux de la matière organique totale (33,4 et 68,7 %), respectivement, montre le tri de la matière organique effectué par l'huître. Le bilan énergétique calculé prend en compte le matériel d'origine phytoplanctonique et le matériel organique total. Les potentiels de croissance ainsi obtenus de 0,88-11,60 et 47,60 J.h−1.g−1 respectivement pour les conditions à 13,5–28,9 et 54,6 mg.l−1 de charge particulaire totale, traduisent une forte variabilité journalière d'énergie disponible pour la production organique dans le milieu naturel. Ces résultats sont cohérents avec les résultats de croissance d'huîtres en élevage dans la baie de Marennes-Oléron.