An individual-based model named OSMOSE (Object-oriented Simulator of Marine Ecosystems Exploitation) is used to investigate the dynamics of exploited marine fish communities. It allows the representation of age- and size-structured populations comprised of groups of individuals that interact within a spatialized food web. Within each group, which constitutes the basic interaction entity (the ‘super-individual’ in individual-based modelling terminology), fish belong to the same species, have similar biological parameters and behaviour rules. Somatic growth, reproduction, predation and starvation processes are modelled. Two rules apply for the predation process: for a given fish group, prey selection depends both on the spatial and temporal co-occurrence of the predator and its prey, and on the respective lengths of the prey versus the predator. Thus, fish feed regardless of the taxonomy of their prey. The strength of both predation and competition relationships therefore vary according to changes in relative species abundance. Preliminary investigations are conducted on a theoretical community comprising seven interacting species. The simulation results show how community stability can emerge from variability in species biomass. It is thus suggested that size-based trophic interactions, along with the existence of multiple weak links and species redundancy, favour community persistence and stability.

Fisheries exploitation systems are characterized by reciprocal interactions between fisheries activity and the harvested resource. Variability and changes affecting ‘natural’ and ‘socio-economic’ environments generate non stationarity of the variables used to describe the fishing activity, the resource and the yields. This characteristic needs to be taken into account in describing the evolution of the relationships between those variables, and in describing fisheries adaptability and sustainability. We present a dynamical system model with an application to data from surveys on activity and results of the Senegalese artisanal fishery from 1974 to 1992. A fitting procedure is proposed, giving estimates of the parameters of the model. A simulation is then made with the fitted model in order to apraise possible impacts of various events of different kind.

Long term changes in catches of two sardinella species off equatorial West Africa seem inconsistent with their thermal preferences. Sardinella maderensis is usually fished during the warm season in low salinity waters, while S. aurita is associated with cold upwelled waters. However, in the Congo fisheries, S. maderensis dominated the catches from 1964 to 1983, when temperatures were moderate or cool; subsequently S. aurita became the most important from 1984 onward, as there was pronounced warming. The movement of water masses related to Atlantic Niño-like episodes explains this paradox. These warm events are associated with enhancement of the equatorial counter currents. When the eastward transport strengthens south of the equator (Benguela Niño), it drives an influx of warm water towards Angola, which repels Sardinella aurita northwards. Then, trapped near the coast by the subsuperficial warm water intrusion and the deviation of the Congo River plume, S. aurita becomes more concentrated and vulnerable to fishing. This is the most frequent situation during warm events. However, when the anomaly involves the north equatorial counter current (Guinea Niño), we assume that this surge of water in the Bight of Biafra leads to a water leakage toward the south, along the Gabon coast. This warm and low salinity water tongue, homologous to the Peruvian El Niño current, would drive Sardinella maderensis towards the Congo fisheries, but would force Sardinella aurita offshore making it less accessible to the fisheries. This situation is likely to have occurred in 1987–1988 when there were large catches of several species linked to brackish waters (bongas, Pseudotolithus, soles, catfish, sharks and spiny lobsters). On the contrary, decadal variations of catches of the two species are consistent with regime shifts of the Congo River: Sardinella maderensis is dominant when the outflow is above average (1960s and 1970s) and is replaced by Sardinella aurita when the runoff is weak (1980s and 1990s). In Angola, the catches of horse mackerel shift southwards during the warm years preceding 1976, then northwards during a cool period ending in 1983. From 1984, the abundance of horse mackerels over the shelf increases after warm events.

Is the seasonal growth of Strangomera bentincki (Clupeidae) and Engraulis ringens (Engraulidae) a consequence of their reproductive strategy to adapt to the seasonal upwelling ecosystem they inhabit? This question is addressed by analysing monthly length-frequency data, gonadosomatic index and condition factor of the species in relation with the seasonal patterns of environmental variables. Modal progression analysis of mean length-at-age of cohorts along the time axis was used to study the growth in the period 1990–1997. A seasonally oscillating growth curve was estimated for both species, with the slowest growth rate occurring between April and May, a few months before the higher reproductive activity occurring in August–September. The reproductive strategy is to spawn when environmental conditions are related with onshore transport in winter (August), favouring the concentration and retention of eggs and larvae. One month later, a moderate upwelling determines an enrichment in food particles and the spawning area is transformed in a nursery area for juveniles. The reproductive strategy is combined with an ‘energy storage strategy’ during the period of upwelling. The energy stored is used for reproduction several months later, affecting the growth process of the species. It is concluded that the regularity in the seasonal growth in both species is a response, from an evolutionary point of view, of a long-term reproductive adaptation to the seasonal upwelling ecosystem of the central southern area off Chile.

The Eastern Channel (ICES division 7d) is considered a unit for stock management of the common sole Solea solea and the plaice Pleuronectes platessa. At this scale, the aim of this work is to model the juvenile distribution of these flatfish. The database used, based on different surveys undertaken during the two last decades, includes 4500 coastal beam trawl hauls throughout the Eastern Channel coasts. Multivariate analyses are used to study the interannual fluctuations in flatfish recruitment of the region. This approach allows us to separate the Eastern Channel into homogeneous sectors with regard to these fluctuations. The physical parameters that contribute to the observed juvenile sole and plaice distributions are identified and their distributions are modelled using generalized linear models. The results of these models, and the areas of the different populations derived from a geographic information system, are used to identify the main nursery grounds and to compare their respective importance. This quantitative approach identifies the lack of estuarine dependence of the nursery grounds, and highlights those areas where anthropogenic disturbance could influence these populations.

A total of ten Heptranchias perlo, eight Deania histricosa, two D. calcea, and two D. profundorum from the Great Meteor Bank, central East Atlantic, were studied for cestodes. The elasmobranchs were caught using bottom trawl or long-lines during September 1998 in 298–1075 m water depth. Seven different cestode species were found: Tentacularia coryphaenae, Deanicola minor, D. protentus, Grillotia meteori sp. nov., Grillotia cf. microthrix (Trypanorhyncha), Crossobothrium dohrnii, and Crossobothrium sp. (Tetraphyllidea). T. coryphaenae occurred encapsulated in the stomach wall and all other species were found within the host intestine. G. meteori sp. nov. and G. cf. microthrix additionally infested the musculature as well as the stomach and intestinal wall. D. profundorum harboured five different cestode species and D. histricosa, H. perlo, and D. calcea were infested with four, three and two species, respectively. With 332 and 181 specimens, D. minor and G. cf. microthrix were the most abundant cestodes. Grillotia meteori sp. nov., Crossobothrium sp., and C. dohrnii were host specific for Deania profundorum, D. histricosa and H. perlo, respectively. The other species were found in more than one elasmobranch, with Deanicola minor and D. protentus restricted to Deania sp. Most parasites are recorded for the first time from the central East Atlantic around the Great Meteor Bank.

The reproductive characteristics of Oreochromis niloticus, a multiple spawner, have been compared between populations submitted experimentally to three types of spatial structure. Fish from a same original stock were divided and reared for 2 months in: 1) concrete tanks with a bare bottom, 2) concrete tanks with a sandy bottom, or 3) concrete tanks equipped with an artificial reef. The females were then killed and their gonads examined for sexual stage and fecundity determination. Fecundity was inversely correlated to the spatial complexity of the environment whereas oocyte size tented to increase together with spatial complexity. Females placed in tanks with artificial reefs presented a lower fecundity and reproductive investment and bigger oocytes than those maintained in the bare bottom tanks. Fish reared in tanks with a sandy bottom had intermediate reproductive traits. The response of females to changes in the spatial structure of their environment was observed after only one or two reproductive cycles, providing the first experimental evidence of such rapid phenotypic adaptation to environmental complexity. The results also indicate that the perception of living space, arising from the availability of refuges and spawning sites, is more likely to influence the reproduction of tilapias than substrate quality.