Age and size-at-maturity of Loligo vulgaris from Portuguese waters were evaluated in order to explain its complex population structure, which is strongly influenced by continuous spawning. Age was obtained by increment counting in statoliths. Maturity ogives by age group indicated that males mature one month earlier (at 277 days) than females (at 298 days). Females mature at a ML50% of 17.6 cm, while males mature at smaller sizes. The later, however, showed a high degree of complexity in the size at maturity with evidence of two size at maturity groups. In both sexes, maturity was ultimately found to be primarily dependent on size rather than age. The effect of hatching season on age-at-maturity, size-at-maturity and reproductive investment was analysed by comparing two groups of squid hatched under distinct environmental conditions, namely the temperature during the first 3 months of life, the cold cohort (CC), hatching between December and March and the warm cohort (WC), hatching between May and September. Significant differences were found between cohorts on age-at-maturity, size-at-maturity and reproductive investment, giving indication of the environmental influences on sexual maturity. CC squid mature ca. 2 months later in life than WC squid and at a significantly larger size. The reproductive investment as measured by GSI was higher in the WC squid. Much of the variability in age and size-at-maturity of females in the population was due to differences between cohorts but this was not the case for males.

The African catfish Clarias gariepinus is undoubtedly the most suitable species for aquaculture throughout its distributional range. The reasons for this. which relate principally to the biology and ecology of the species, are briefly reviewed and commented upon. the species is currently farmed on a commercial and subsistence basis in over 12 African countries, the most important of which in terms of tonnage produced are Nigeria, South Africa, Zambia and Chana. The scientific and technological foundation for the farming of African catfish is sound. and has been developed mainly in The Netherlands, SouthAfrica, Belgium and the Central African Republic. Most of the important culture parameters, inclusive of spawning, incubation, larval nutrition and rearing, production and feed formulation have received adequate attention for the successful farming of the species in Africa. Production levels. depending on the type of operation, range from < 1 to 40 t/ha per year, and exceed 800 kg/m3/year under ultra-high density commercial culture conditions. Food conversion ratios vary between 1:6 using agricultural waste, to 1:1.1 using formulated, least-cost feeds. Significant progress has also been made in the development of the technology for the farming of a related species, Heterobranchus longifilis and the hybrid of thisspecies with Clarias gariepinus. Despite the technological kow-hnow, total recorded production of clariid catfish in Africa in 1993 was a mere ca. 4500 tonnes. Despite the fact that there may be a considerable margin of error in the reported production figures the farming of catfish in Africa is a marginal activity. The reasons for this are manifold and can be primarily pinned on market forces. inadequate regional infrastructures, production costs, the socio-economics of fish farming and the philosophy upon which aquaculture development in Africa is largely based. Nevertheless the future potential for the farming of clariid catfish throughout their distributional ranges is immense.

Radio tracking has been extensively used to enhance our knowledge on the movement and home range of fish in general and salmonids in particular. However, the use of various temporal sampling protocols is likely to overlook fish movements, and produce experimental artefacts, the amplitude of which is unknown and may compromise comparison of fish behaviours revealed by different protocols. Starting from a day-by-day tracking study of brown trout in Belgian waters (Aisne stream, nine fish, minimum 39 daily locations per fish), we re-examined their home range and mobility, through a subsampling process, as if the fish had been located at longer time intervals (2–14 days). The estimates from subsamples were compared to the original data set in order to quantify the corresponding reduction of accuracy from observed data, and how far this could be predicted on the basis of locating frequency. The results clearly indicate that all intervals longer than one day generally generate substantial biases (reduction of accuracy from 0 to 82% for home range and from 5 to 92% for mobility) but these can be partly corrected through the use of predictive models. This analysis demonstrates that any comparison between studies relying on different locating frequencies can generate some ambiguity when interpreting biological phenomena or geographical differences.

Long-term effects of constant exogenous ammonia concentrations were investigated in two different batches of turbot juveniles (53 and 73 g) under controlled environmental and feeding conditions. Over the 84-day experiments, ambient un-ionized ammonia (UIA-N, NH3) concentrations were steady (coefficient of variation, 12-16%) and water pH range was 7.88-7.99. Survival was maximum up to 0.33 mg.l−1 UIA-N and at 0.73, 50% mortalities were observed on day 52 (73 g) or day 77 (53 g). No-growth concentration was 0.73 mg.l−1 UIA-N in the two groups and the 84-day lowest-observable-effect oncentration range 0.14-0.33 mg.l−1. In the most tolerant group (53 g), the 84-day no-observable-effect concentration was 0.14 mg.l−1 UIA-N. The estimated 28-day efficient concentration that gave 50% of the specific growth rate of controls ranged from 0.60 to 0.75 mg.l-1 UIA-N. Specific growth rate to ambient ammonia concentration patterns were different in the two groups for the lowest concentrations (0.14-0.34) and similar to food intake to ammonia pattern. Food efficiency was negatively correlated to ambient ammonia concentration, with major changes in food conversion ratio, and protein utilisation observed above 0.33-0.70 mg.l−1 UIA-N. The main change in body composition was an increase in water content in all ammonia-exposed fish. A significant increase in urea-N excretion rate was also observed over 0.33 mg.l−1 UIA-N. Thresholds for growth were affected by exposure duration. Turbot juveniles growth capacity may be depressed by usual ambient ammonia concentrations under intensive farming conditions.

This paper presents the simulator SHADYS (‘simulator halieutique de dynamiques spatiales’). SHADYS is a tool devoted to the representation of spatial processes involved in fisheries dynamics. It puts together three fundamental entities in an explicit spatio-temporal way: the environment, the fish populations under consideration and the fishing fleet. It is based on simple, realistic and well identified mechanisms (density-dependent habitat selection, advection and diffusion of a fish population, fishermen search strategy, etc.) implemented by interfacing numerical models with a GIS (geographic information system). As a virtual laboratory, SHADYS allows experiments to be performed in a theoretical world, such experiments being generally impossible to perform in reality. As an illustration, different scenarios of a marine refugia-based management have been explored using the SHADYS simulator. Different simulations are conducted to assess the short-term effects on yield of a marine protected area. It is shown that for diffusive or migratory species, the yield per recruit as a function of the protected surface can reach a maximum. Under this condition, the concept of ‘space overfishing’ is meaningful. Protected areas then behave like ‘sources’ and exploited areas like ‘sinks’. For resident populations, the larger the protected area is, the lower the catches per recruit are. Regarding the spatial distribution of the fishing effort, it is shown that if it is spatially distributed in order to maximize catches, then fishing boats will slowly tend to be distributed all along the boundaries of the protected area.

The Sydney rock oyster (Saccostrea glomerata) (SRO) is an oyster species that only occurs in estuaries along Australia’s east coast. The SRO industry evolved from commercial gathering of oyster in the 1790s to a high production volume aquaculture industry in the 1970s. However, since the late 1970s the SRO industry has experienced a significant and continuous decline in production quantities and the industry’s future commercial viably appears to be uncertain. The aim of this study was to review the history and the status of the SRO industry and to discuss the potential future prospects of this industry. This study summarised findings of the existing literature about the industry and defined development stages of the industry. Particular focus was put on the more recent development within the industry (1980s-present) which has not been covered adequately in the existing literature. The finding from this study revealed that major issues of the industry are linked to the management of prevailing diseases, the handling of water quality impairments from increasing coastal development, increasing competition from Australia’s Pacific oyster (Crassostrea gigas) industry and the current socio-economic profile of the industry. The study also found that policy makers are currently confronted by the dilemma of saving a “dying art”. Findings from this industry review may be vital for current and future fisheries managers and stakeholders as a basis for reviewing industry management and development strategies. This review may also be of interest for other aquaculture industries and fisheries who are dealing with similar challenges as the SRO industry.

The temperature evolution over the past 40 years in the Bay of Biscay (North-East Atlantic) is investigated from an in situ data analysis, completed with a satellite SST (Sea Surface Temperature) analysis over the last 20 years. The in situ dataset is an interannual version of the BoByClim climatology, covering the Bay of Biscay area with a 10-km horizontal resolution and a 5-m vertical step. The satellite dataset is the European COastral Sea Operational Observing and Forcast System Program (ECOOP) daily analysis, covering the IBIROOS (Iberian-Biscay-Irish Sea) area with a 4-km resolution. The study area (43°N-50°N/12°W-1°W) extends over the intersection of the domains covered by these two analyses. In the 0–200 m layer of this domain, a heat budget has been computed from an ocean circulation model over the period 1965-2004. Heat is essentially imported from the west by the North Atlantic Drift, then exported southward and northward, or transferred downward into deeper layers. The annual average of air-sea flux is weak, with a zero isoline crossing the domain from the northwest to the southeast. Ocean and atmosphere, forming a coupled system, have comparable contributions and may both result in a heat sink or source. At the interannual scale, variability is mainly controlled by the air-sea flux, with a secondary contribution from oceanic transport. A strong warming of $\sim $ 0.2 °C/decade for the period 1965-2004 is apparent from the surface down to the 200-m depth. The decadal evolution is consistent with several analyses over the North Atlantic. Meanwhile, warming is approximately twice faster than in the whole basin. This trend is not steady, as a cooling period occurred until the early 1970's, and the temperature increase has been significantly stronger during the last 20 years ( $\sim $ 0.3 °C/decade between 0 and 100 m, from 1986 to 2005). The trend and the interannual variability are maximal over the northern part of the shelf. The typical duration of interannual anomalies is two years and their penetration depth is ~100 m, although they occasionally exceed 200 m. The interseasonal anomalies are also stronger within the mixed layer (root-mean-square, RMS = 0.6 °C at the surface) than below (RMS = 0.3 °C at 100 m). Their geographic distribution is radically different between the surface and the deep levels. A seasonal dependence of the warming rate is evident in the upper layer, as the temperature trend is stronger in summer than in winter (by a factor of 2 at the surface). Some of the potential implications for the ecosystem are discussed, particularly in terms of species distribution and dynamics of species exploited by fisheries.

Enzymatic polymorphism was studied in nine enzyme systems of four species of large AfricanBarbus (B. bynni occidentalis Boulenger, 1911; B. sacrutus Daget, 1963; B. petitjeani Daget 1962; and B. wurtzi Pellegrin, 1908), two species of small African Barbus (B. guineensis Pellegrin, 1913 and B. cadenati Daget, 1962), and two species of European Barbus (B. barbus Linnaeus, 1758 and B. meridionalis). Like European Barbus, the large African Barbus were found to express more than 20 enzymatic loci for these systems (21 to 23), whereas the small African Barbus only expressed 14. This result suggests that large African Barbus are tetraploid like European Barbus and that small African Barbus are diploid like the Asian species. The two groups of African Barbus do not appear to be phylogenetically closer to each other than they are to European Barbus. Consequently, tetraploidi-zation must have occurred long ago in large African Barbus, and may correspond to the same event responsible for the origin of the European species.

Lagoon fish communities often consist of complex assemblages of numerous species, difficult to manage with conventional stock assessment models. Useful data are still lacking for evaluating the importance of man-made disturbance and reference situations are missing, especially in developing countries. As a consequence, by analysing data collected 20 years ago in two of the six sectors of the Ebrie lagoon (Ivory Coast), this study aims to evaluate the impact of fishing effort on fish assemblages. These two sectors (V and VI), located far from the inlet, have similar physical, chemical, biological and fish fauna characteristics. The major difference lies in the fishing intensity: sector VI has a low fishing intensity (fishing for personal consumption using only individual gear), whereas sector V is heavily fished (professional fishing with both individual and collective gear, particularly beach seines which result in a considerably higher fishing effort). Comparisons between the two sectors were based on two complementary scientific approaches: a 3-year commercial fisheries survey (1978-1980) and a 1-year experimental survey (1981). The impact of fishing on fish assemblages is analysed through the main characteristics of fish populations and communities. The results show that there were major changes including an increased catch yield (37.5-189 kg ha–1 y–1), a lowering of fish diversity in catches, of fish biomass (100-20 kg ha–1), of average catch length (22.6-14.6 cm) and of trophic level of catches (26-58.5% of herbivore/detritivore species in total catches). Such results are quite unusual because they occurred even in non-overfished ecosystems: the fish assemblage was deeply modified in sector V compared to the lightly fished adjacent sector VI, even though fishing effort in sector V was only high but at a reasonable level. These data must be completed by similar studies in tropical lagoons with variable levels of fishing intensity in order to understand fish assemblage re-organization when submitted to stresses of different intensity.

Meal frequency is a key parameter in fish larviculture, especially in highlycannibalistic species. Knowledge of the biological bases of cannibalism (growth capacityof cannibals, morphological constraints on cannibalism, prey size preference) can helppredicting the risks of cannibalism for different feeding schedules under cultureconditions. This study relied on the day-by-day analysis of prey size preference andbioenergetics of individual cannibals of the catfish Pseudoplatystoma punctifer(8–65 mm standard length, SL, 0.5–400 mg dry mass,DM) at 28.5 °C under 12L:12D. The results were equated with theontogenetic variations of morphological factors (head and mouth width) and feed efficiencyof larvae feeding on Artemia nauplii, in order to calculate the risks ofcannibalism among fish fed 2–7 daily meals. The predation capacities of P.punctifer were highest at 8 mm SL and decreased in larger fish(largest prey  =  86% and 70% SL in fish of 8 and  >30 mmSL, respectively). Cannibals of increasing size preferred increasinglysmaller prey relative to their own size, but also to their predation capacities. Thesemorphological and behavioural constraints were largely compensated for by bioenergeticsperformance. Cannibals consumed high daily food rations (as high as 171 and 29%DM in fish  <1 and  >300 mg DM,respectively), exhibited high gross conversion efficiencies (0.50–0.55 and about 0.70, infish  <1 and  >30 mg DM, respectively), and grew rapidly(90 and 18% DM day-1 in fish  <1 and  >300 mgDM, respectively). The growth advantage of cannibals over siblings fedArtemia nauplii was decisive, except for high meal frequencies (6–7daily meals). This study supports the view that the risk of cannibalism and adequatefeeding strategies can be largely predicted in a particular fish species if themorphological, behavioural and bioenergetics bases of cannibalism are examined altogetherin an ontogenetic perspective.

Through a review of the scientific literature and a more in-depth qualitative meta-analysis of 16 case studies distributed worldwide, this article aims to study impacts of MPAs on marine living resources, ecosystems and related fisheries and to highlight their criteria of efficiency as management tools for a sustainable exploitation. MPAs are efficient for conservation purposes and resource restoration, especially inside their borders. MPAs can also be part of fisheries management systems, but there is a lack of knowledge about their wider scale impacts on fish stocks, ecosystem and fisheries. Adjacent fisheries can increase their catches near closed areas, but such effects are delayed until after MPA establishment and are often limited over distance. Even though local specificities in ecosystems and fishing resources lead to high variability in MPA effects, four major criteria modulate the efficiency of MPAs for fisheries management: (1) the size of the closed area; (2) the level of protection of essential habitats for exploited resources; (3) MPA integration as part of wider integrated fisheries management plans; and (4) efficient monitoring and regulation systems, including participative decision making, to ensure that restrictive measures are respected.

Domoic acid (DA), the toxin responsible for the illness known as amnesic shellfish poisoning (ASP), is an algal toxin produced naturally by some species of the diatom genus Pseudo-nitzschia. The toxin has been detected in a diverse array of marine organisms from copepods to whales. Cephalopods, which are important members of the food chain and active predators of known toxin vectors such as bivalves, crabs and some fishes, have just recently been implicated in DA transfer or accumulation. Here we present data showing detectable values of DA determined by HPLC-UV (high-performance liquid chromatography and ultraviolet detection) and confirmed by HPLC-MS (mass spectrometric detection) in two octopus species collected along the Portuguese continental coast: Eledone cirrhosa and E. moschata. Domoic acid was frequently detected in the digestive gland of E. moschata and occasionally reached concentrations exceeding 100 µg g−1. In contrast, E. cirrhosa contained lower concentrations of DA on the few occasions that it was detected. This suggests that E. moschata is a potential vector for DA transfer to higher trophic levels in the coastal marine food web, not excluding humans. These data, combined with known aspects of the life history of the species, are a necessary step towards achieving an understanding of the accumulation of phycotoxins in cephalopods.

The food utilisation and partitioning between three age-groups of Atlantic salmon parr were studied in the subarctic river Tana (70° N, 27° E) by analysis of fish stomach contents and invertebrate composition in the three main lotic feeding habitats: bottom substratum, water column and surface. The salmon parr exhibited large seasonal variation in their food choice, but the different age-groups had a similar diet, dominated by mayfly and stonefly nymphs in May, flying insects and simuliid larvae and pupae in July, and caddis fly larvae in August and September. Some differences in the food resource use of the three age-groups were however also observed, mainly related to size-dependent differences in feeding abilities, but partly also to different use of feeding habitats. Small-sized stonefly and mayfly nymphs and simuliid larvae and pupae decreased in importance with increasing fish age, whereas Trichoptera larvae and flying insects increased. The low resource partitioning between the different age-groups of salmon parr suggests that intraspecific competition for food may occur. All age-groups mostly fed on prey types associated with the bottom habitat, and substantial drift feeding was only observed in July, probably due to low drift rates throughout most of the summer season in this subarctic river.

Cupped oysters (Ostreidae, genus Crassostrea) occupy nearshore marine and estuarine habitats worldwide, providing many ecosystem goods and services as well as being a commercially important group of bivalves. In this study, the species identification of an “adulterant” oyster with small body size, which is often misidentified as a “young individual” of other sympatric species, including C. sikamea and C. hongkongensis, was determined for the first time, based on molecular markers (partial mitochondrial cox1 and rrnL genes), phylogenetic analysis, and morphometric approaches. This novel species, C. zhanjiangensis, commonly known as the “cat ear oyster” in Guandu (a famous estuarine oyster farming region of Zhanjiang, Guangdong Province, China), appears to potentially influence the efficiency of C. hongkongensis spat collection due to niche competition on spat collection devices. Phylogenetic analyses confirm its status as the most basal taxon of the Indo-Pacific Crassostrea. A comparative study of the shell characteristics of C. zhanjiangensis, and other Crassostrea species revealed several distinctive morphological traits, including a generally smaller body size, a deeply cupped left valve, and a right valve that is convex in adults but flat in young individuals. Other distinctive features of the new species include life cycle traits that are unique compared with the sympatric C. hongkongensis and C. sikamea species, such as a higher growth rate in the fast growth phase after settlement, followed by a significantly slower growth rate and mass mortality during subsequent life stages. This study provides the basic information necessary for further ecological and population genetic studies on this new species.

The benefits of implementing no-take Marine Protected Areas (MPAs) for the conservationof highly migratory species are not easy to assess. They depend on several factors, suchas the fish mobility, fisher behaviour and the area covered by the MPA with respect to thedistribution area of the species to protect. In this study, we explore the simultaneouseffects of MPAs and fishing scenarios on skipjack tuna population dynamics, using thespatially-explicit APECOSM-E model. The model represents the size-structured populationdynamics of skipjack tuna in the Indian Ocean and their dependence on climatic variabilityand exploitation by fisheries. Numerical experiments were run from the beginning ofindustrial fisheries in the early 1980s to the year 2030, considering different scenariosfor the future development of fisheries. These scenarios combined different trends infishing effort and technological development, either assuming a continuous increasefollowing historical trends or a stabilization of these factors at present values. Thesimulations were designed to explore the effects of two MPAs of different size andlocation: the recently established Chagos MPA, and a hypothetical MPA covering a largepart of the Western Indian Ocean, where most of the skipjack catches are presently made.We modelled the redistribution of fishing effort around the MPAs assuming that the fishershad partial knowledge of the spatial distribution of the skipjack population. The effectsof the two MPAs on the population dynamics, catch and fishing mortality are shown. Ourresults revealed a very minor effect of the Chagos MPA on the skipjack tuna population,while the Western Indian Ocean MPA had an important impact on the fishing mortality andsucceeded in stabilizing the spawning population. The simulations also showed that theeffect of an MPA depends on the evolution of fisheries and it is therefore important toexplore different fishery scenarios to assess the future benefits of an MPA.

A biotelemetry experiment was designed to determine whether the selection of a residence area by adult barbel Barbus barbus (Cyprinidae) was dependent on the availability of physical habitat features, on the presence of fish shoals and/or on the presence of specific partners. The experiment took place in 3.0 km stretch of the River Ourthe (Belgium), where the latter features had been characterised by habitat and electric fishing surveys (1 190 fish ≥ 15  cm, at 265 locations). Six barbel (34.8–49.5 cm) belonging to a single shoal of 34 fish were tagged with 40 MHz radio transmitters, transplanted in pairs at different distances (−1 km, −30 m, +1 km) from the capture site and tracked from 3 September to 7 October 1990.

Fish showed variable mobility patterns and home range sizes (200–2 400 m). They selected 17 residence areas of which the cumulative frequency of occupation ranged from 0.1 to 79.5 day × fish. The comparison between the residence areas selected, habitat suitability and the location of fish shoals in the study area indicate that the presence of a shoal of at least 10 resident fish within a suitable habitat (normalised suitability index ≥ 1.0) is necessary but not sufficient to guarantee its long term selection and occupation. Homing movements of transplanted fish were directed towards the activity area rather than to the residence area itself. The fish that did not home, selected residence areas in a part of the river with a higher availability of potential feeding areas. These elements are discussed within the context of habitat utilisation and foraging strategies by a gregarious species. It is suggested that resident fish of the same species may ease the exploitation of a non familiar environment by a naive fish, and that this may positively trade off the benefit provided by home site fidelity, depending on the availability of food resources.

The degree to which larvae of the invasive American slipper limpet (Crepidula fornicata) and the Japanese oyster (Crassostrea gigas) may compete for food was examined during 2003 in the laboratory. Larval microalgae uptake, growth and mortality were compared for larvae fed each of six species of unicellular algae, ranging in length from 2 to 10 µm. Tested diets included the two flagellates Tetraselmis chui (Prasinophyceae) and Isochrysis affinis galbana (T-ISO, Haptophyceae), one member of the Chlorophyceae (Nannochloris atomus), and three diatom species (Chaetoceros calcitrans forma pumilum, Chaetoceros gracilis, Skeletonema marinoï). We found that the limpet larvae ingested phytoplankton over a wider range of cell sizes and ate at higher rates on each diet than did the oyster larvae. For example, oyster larvae consumed 2216 cells h−1 of N. atomus, while limpet larvae consumed the same phytoplankton cells at approximately twice that rate, 5159 cells h−1, on the same diet. Larvae of both species grew more quickly on a mixture of flagellates than on any of the diatom alone (12 versus 7 µm d−1 for oyster larvae and 41 versus 28 µm d−1 for limpet larvae). Our results suggest that in the Bay of Mount Saint-Michel (France, Western Channel), where larvae of both species co-exist in the summer, intensive grazing by limpet larvae can potentially deplete phytoplankton concentrations to cause competition with oyster larvae, particularly for smaller sized phytoplankton species.

Several virulence factors involved in the potential pathogenic capacity of Vibrio P1, the causative agent of brown ring disease (BRD) affecting cultured Manila clam adults (Ruditapes philippinarum), have been evaluated in comparison with other strains of several Vibrio species isolated from diseased clams. The importance of bacterial cell surface associated properties as virulence factors has been studied considering both non-specific and specific bacterial adhesion to clams. Vibrio P1 showed moderate hydrophobicity, but high affinity to bind to Congo Red dye and the presence of appendages, characterized as fimbriaeor pili. All the strains of Vibrio P1 secreted haemolysis and cytotoxins, and were also strong exotoxin producers. The presence of a large 49.2-MDa plasmid in all the strains of Vibrio P1 may be used as an epidemiological marker, but its involvement in pathogenic mechanisms has not yet been established. Although in some Vibrio strains, iron-acquisition systems play an essential role in their pathogenicity, they do not seem to be an important factor in Vibrio P1, since this pathogen lacks siderophore-mediated iron transport mechanisms.

Jusqu'aux dernières décennies on considérait que la limite de répartition entre sardines et sardinelles se situait aux environs du cap Blanc. Au nord de ce cap, les remontées d'eaux ont lieu toute l'année, la production primaire est forte, mais non la biomasse zooplanctonique, du fait de la rapidité de l'advection. Au sud, les résurgences sont saisonnières, la circulation au-dessus du plateau comprend deux veines opposées. La production secondaire suit rapidement les poussées de phytoplancton, notamment grâce à la superposition de courants opposés entre lesquels des migrations de zooplanctonpermettent un couplage des productions primaires et secondaires. Sardines et sardinelles se nourrissent de particules en suspension, mais les sardines peuvent s'accommoder d'un régime presqu'entièrernent végétal, à la différence des sardinelles. Ceci leur confère un avantage sur les sardinelles dans les régions d'upwellings violents. Durant les années 70's, la pêcherie de sardines s'est déplacée vers le sud, or ces mêmes années, les alizés se sont renforcés sur leur bordure méridionale, conduisant à un renforcement des upwellings le long des côtes mauritanienne et sénégalaise. Ceci a entraîné une modification du biotope, favorable aux sardines. D'autre part les courants, modifiés par diminution de leur composante nord et augmentation de celle portant au sud, pourraient être responsables d'un nouveau mode de dispersion des larves, entraînant davantage de sardines au sud et moins de sardinelles vers le nord.

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.