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© D. Guyomard À la suite des démarches statistiques exploratoires, qui ont permis de dégager notamment les principales relations pouvant exister entre les paramètres de l’environnement océanique et les résultats des pêches, une approche de modélisation des comportements des grands pélagiques dans le paysage océanique tropical du sud-ouest de l’océan Indien est proposée. Deux domaines nous ont semblé particulièrement porteurs de concepts et de méthodes originales : l’écologie du paysage et la ...

Simmonds, E.J., Gutiérrez, M., Chipollini, A., Gerlotto, F., Woillez, M., and Bertrand, A. 2009. Optimizing the design of acoustic surveys of Peruvian anchoveta. – ICES Journal of Marine Science, 66: 1341–1348. Acoustic surveys of stocks of small pelagic fish, in particular the Peruvian anchovy or anchoveta (Engraulis ringens), have been carried out off the coast of Peru since the late 1970s. In all, 51 of these have been carefully archived since 1983. The surveys provide a wealth of data on the distribution and abundance of pelagic fish in the most productive of the world's upwelling ecosystems. The data comprise integrated acoustic data at a resolution of 1 or 2 nautical miles along the cruise tracks, and trawl data, which include catch-by-species and catch-by-length information for anchoveta. Data since 1992 are sufficiently complete to allow a full re-evaluation of the surveys to determine their precision, taking account of the spatial variability of the catch and acoustic data. The methods used include bootstrap by trawl sample and transect segment, geostatistical simulation, and simulated surveys. The results reveal consistent spatial patterns of abundance with a more variable distribution of variance and a strong relationship between the local mean abundance and the variance. The temporal and spatial variabilities are considered in an evaluation of alternative survey designs, including pre-stratified and adaptive designs.

Round scad mackerel sampled in 1995–1998 were analysed for genetic variation using mitochondrial-DNA and nuclear-DNA markers. Sequence variation for a fragment of the cytochromeb gene (mitochondrial), amplified by polymerase chain reaction, was screened across individuals using single strand conformation polymorphism (SSCP). Sequence analysis of all SSCP haplotypes indicated two mitochondrial clades separated by, on average, 2.3% nucleotide divergence. The geographic distribution of haplotypes was homogeneous (Weir and Cockerham's \( \hat \theta \) =–0.002). Also, no geographic heterogeneity was detected for length polymorphism of Intron 1 of the gene encoding aldolase B (\( \hat \theta \) =0.005). Although homogeneity in allele frequencies throughout the Indo-Malay archipelago conformed to the expectations for a widely distributed pelagic fish in a highly connected habitat, this was at variance with the sharp geographic structure previously uncovered in Indian scad mackerel, Decapterus russelli, a fish with life-history characteristics similar to D. macrosoma. A remarkable similarity, however, was the occurrence of two similarly distinct clades within each species, suggesting a common history of geographic isolation. Low sea levels in the Pleistocene might have caused the separation and vicariance of populations within both D. macrosoma and D. russelli. Subsequent genetic exchange between populations would then have erased allele-frequency differences at the cytochrome b and aldolase B loci in D. macrosoma while some barrier to gene flow was maintained in D. russelli.

The Northwestern Mediterranean Sea (NWMS) is one of the most productive areas of the Mediterranean Sea. The NWMS pelagic planktonic ecosystem is strongly influenced by hydrodynamics, in particular winter deep convection. Here, we investigate the response of this ecosystem and associated carbon cycle to oceanic and atmospheric winter conditions interannual variability. For that we developed a tridimensional coupled physical-biogeochemical model, ran annual simulations forced by XXth climate conditions and performed statistical and budget analysis.Our coupled model reproduces correctly the seasonal evolution of the NWMS pelagic planktonic ecosystem. It however overestimates the contribution of nanophytoplankton to the total phytoplanktonic biomass and GPP, underestimates the bacteria biomass and represents the spring bloom with 1 month delay. Our results confirm that the control of phytoplanktonic development and bacteria growth by the phosphorus availability is a marked specificity of the NWMS, that is, temporally reduced by deep convection. They confirm the relevance of the Behrenfeld (2010) hypothesis in explaining the bloom dynamics. The variability of the winter atmospheric conditions induces differences of vertical mixing and water temperature that propagate into the whole NWMS ecosystem through a chain of relationships. The high frequency filtering associated with averaging diagnostics explains that this variability seems weak at the NWMS scale. However for most of the variables and processes, differences induced by the winter atmospheric variability are significant at the annual scale. Net metabolism and deep carbon export are systematically positive and show larger variabilities related, respectively, to the water temperature and convection intensity.

Tuna distribution may be directly linked to food availability, the density of which can be assessed by echo-sounding. A study was performed in the French Polynesian EEZ (Economic Exclusive Zone) between 1995 and 1997 with a 38 kHz echo sounder working down to 500 m depth. With the settings used, acoustic back-scattered energy may be considered as representative of micronektonic fish biomass and provided information on horizontal and vertical micronekton variabilities and the structure of the echograms. Three different zones were defined by multivariate analysis. The classical biomass decreasing gradient from the equator to higher latitudes did not appear. Instead, the maximum micronekton abundance was found between Marquesas Archipelagos and a west-northwest/east-southeast oriented line stretching between 11 and 14°S, i.e. in a weak convergence, favourable to micronekton development due to the concentration of lower trophic levels with no oxygen limitation in the deep layers. Two zones with very different hydrological features, but with comparable micronektonic abundances, surround the richest micronekton zone. To the north, waters are enriched by the equatorial upwelling, but intense organic matter remineralisation limits oxygen availability under the thermocline. To the south, waters are influenced by the great southern gyre and present oligotrophic characteristics less favourable to micronekton development. The present results suggest that prediction of tuna forage distribution should take not only trophic parameters into account, but also environmental ones. They also suggest that echo-sounding data should be used more extensively in the validation of models predicting tuna forage.

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) were investigated in European anchovy (Engraulis encrasicolus) collected in the Gulf of Lions. The contaminants were determined in different organs and tissues of male and female adult fish sampled during the spawning and resting seasons. The influence of sex, age and reproduction on contaminant concentrations was evaluated.

The News Focus story "Giant marine reserves pose vast challenges" (C. Pala, 8 February, p. [640][1]) discusses the potential role of large marine reserves in conserving pelagic species. We highlight three points that are misrepresented in the article. First, the story describes tropical tuna stocks

The main scientific goal of COSTAS project is to bring useful knowledge to the understanding of the conditions, which favor the accumulation and transfer of organic and metallic contaminants at the primary trophic levels (autotrophes and hetetrophes), and then within the food web of small pelagic fish (anchovy and sardine) in the Gulf of Lion, Western Mediterranean

Several pelagic fish species are known to associate with floating objects. However, quantitative information on the main factors that drive this associative behaviour is still lacking. Small pelagic fish offer a particularly interesting case study for this phenomenon, since the small spatial scales involved in their association with floating objects allow experimental data to be collected at a fine scale. Here, we monitored twelve acoustically-tagged bigeye scads, Selar crumenophthalmus (Carangidae), (mean fork length 16.4 cm, SD 2.1 cm) around a floating object moored in shallow water (15 m depth) playing the role of a coastal fish aggregating device (FAD). To quantify the role played by variations in current and daylight, we calculated the speed distribution, pair correlation function and group polarization for the tagged fish hourly, from midday to dusk (13:00–18:00), for varying current strengths and daylight conditions. We found that the current induced a shift in the position of the aggregation upstream of the FAD, at distances that increased with the current strength. We found evidence of an expansion and a higher coordination in the aggregation at dusk, with increasing speed, distance among conspecifics and alignment. We discuss possible scenarios in which group polarization increases at dusk and suggest complementary measurements for future experiments that could confirm our findings.

For 15 years, there has been a seasonal tuna fishery in the open equatorial Atlantic Ocean. There are no good reasons for explaining this concentration of fish, except the possible abundance of micronekton. However, micronekton are only found during the night in the upper layers, whereas tuna feed during the day, according to current assumptions. An acoustic cruise carried out in November 1992 clearly showed the presence in this area of a large schooling biomass of a small mesopelagic fish, Vinciguerria nimbaria (Photichtyidae). Instead of diving to a great depth during the day as usual, they remained in the upper layers and tuna (Katsuwonus pelamis and Thunnus albacares) were found to feed on them. The atypical behaviour of Vinciguerria involved two processes: reverse phototaxis and a break in the diel vertical migrations. A review of the literature shows that this fish: (1) is a common item in tuna stomach contents; and (2) is never caught in the upper layers during the day. This apparent discrepancy has been resolved by our results: schooling behaviour during the day prevents Vinciguerria from being sampled adequately by the usual systematic net tows. The need to use acoustics for successful sampling is obvious. It is likely that this behaviour occurs in other areas and could explain the local concentration of tuna.

In upwelling ecosystems, fish tend to avoid spawning in areas dominated by strong offshore transport and strong wind-mixing. Spawning grounds are usually located downstream of the main upwelling centres and spawning is mostly tuned to avoid the peak of the upwelling season. However, in areas such as Peru, Senegal and Cote d’Ivoire-Ghana, spawning and upwelling occur simultaneously. What are the mechanisms that allow such reproductive strategies to be successful? To attempt to answer this question, some environmental characteristics of the spawning ground of Sardinella aurita in the Senegalese upwelling system are investigated. S. aurita adults migrate from Mauritania to Senegal in early winter, following the southward extension of upwelling along the coast. Intense spawning in late spring is concentrated south of Cap Vert. There, the surface distribution of several environmental parameters is quite unusual for an upwelling area. Sea surface temperature is minimum over the shelf and increases towards and away from the coast. Chlorophyll distribution peaks nearshore. This unique surface structure is interpreted as the result of a “double cell” structure in the upwelling vertical circulation: a first cell located at the shelf break, the main upwelling cell that brings cold and nutrient-rich subsurface water to the surface, and a second cell located inshore of the shelf break. A nearshore convergence tends to trap and retain phytoplankton and other biological components. In an upwelling system with a double-cell circulation structure, the coastal cell represents a very favourable environment for fish to reproduce: eggs and larvae do not spread offshore, but are retained in the productive and relatively stable coastal waters. Clearly, therefore, upwelling is not always detrimental for larval survival. Rather, upwelling and retention can occur simultaneously and together provide a favourable reproductive habitat.

26 pages, 6 figures, 4 tables, A suite of ecological indicators was selected for communicating, in a comparable way, how fishing affects the state of several upwelling ecosystems and others in which small pelagic fish play key ecological roles. Detailed background and understanding of system-specific processes and changes is needed for proper interpretation of results. In particular, environmental forcing is important in driving dynamics in upwelling systems; fishing impacts cannot be understood without understanding the corresponding dynamics of the environment. The Saharan Coastal (Morocco) and southern Benguela, both having experienced upsurges in low-trophic-level species, differed from other ecosystems when considering indicator trends. The ecosystem off Portugal emerged as showing reduced signs of fishing impacts in recent years, although the change may also be reflecting climate change favouring recruitment and abundance of demersal stocks. The indicator suite confirmed general understanding that the Mediterranean ecosystems have been notably degraded for several decades. Results and conclusions from this descriptive synthesis are compared with other comparisons of more complex, model-derived indicators. Even in upwelling and comparable systems, the simple data-based indicators are useful in synthesizing information on the status of an ecosystem, in particular on the ecosystem effects of fishing, to provide an ecological diagnosis at the ecosystem level, to be used in decision-making. Indicators of recent ecosystem state and trends over time are needed to assess the effects of fishing, but more indicators measuring biodiversity attributes and environmental change would complement the suite, providing fuller assessment of the status of upwelling and comparable ecosystems, The IndiSeas Working Group was funded by the European Network of Excellence EUR-OCEANS (FP6, contract no. 511106), the European collaborative project MEECE - Marine Ecosystem Evolution in a Changing Environment (FP7, contract no. 212085), and IRD (Institut de Recherche pour le Développement, France). Thiao Djiga (CRODT/ISRA (Senegal, Dakar) and IRD (France) are thanked for providing helpful insights for Senegal, Louize Hill (IPIMAR, Portugal) for collating and updating the Portuguese indicator series, and Maria de Fátima Borges (IPIMAR) for providing explanations linked to the increased production of demersal fish off Portugal in recent years. We are grateful to an anonymous reviewer, and especially to Astrid Jarre, for their insightful, challenging and thought-provoking comments that greatly improved the paper. Edward Hill (University of Cape Town) is thanked for his help revising the figures. LJS was financially supported by the MEECE project and Astrid Jarre's South African Research Chair in Marine Ecology and Fisheries. MC was supported financially by a post-doctoral fellowship from the Spanish Ministry of Science and Technology, by the European Community Marie-Curie Post-doctoral Fellowship through the International Outgoing Fellowships (IOF) (Call: FP7-PEOPLE-2007-4-1-IOF), and by Dalhousie University (Halifax, Canada)

Cette étude analyse le rôle de la seconde et petite saison d'upwelling dans les changements récents de l'écosystème pélagique côtier ivoiro-ghanéen. Depuis le début des années 1980, les captures de #Sardinella aurita$ ont nettement augmenté au Ghana et en Côte d'Ivoire où les modifications de l'abondance etde la dynamique de l'espèce ont été les plus surprenantes. La taille des poissons a augmenté et l'activité reproductrice de l'espèce est maintenant d'une intensité équivalente pendant les deux saisons d'upwelling. On observe, entre 1970 et 1990, une tendance à la diminution de la différence entre l'intensité des deux saisons d'upwelling. Le rôle croissant de la petite saison d'upwelling dans l'écosystème permet de donner une interprétation aux changements observés. Cette saison, plus intense en Côte d'Ivoire qu'au Ghana, intervient en effet pendant la période de l'année défavorable pour la productivité de l'écosystème pélagique et peut être comparée à un "goulot d'étranglement" pour cette productivité. (Résumé d'auteur)

Paleontological and historical stock abundance estimates indicate that pelagic fish populations inhabiting upwelling ecosystems undergo large interdecadal variations in abundance with amplitudes equal to, if not larger than, the interannual variability. The interdecadal variability is characterized by periods of high and low abundance, termed “pseudo-cycles”, because of their irregular periodicity. Fisheries targeting small pelagic fish suffer from overall overcapitalization, like many other fisheries, but also from an additional overcapitalization problem: a phase displacement between rapid fish population decreases and a slower disinvestment which follows. This lag produces economic hardship. Here we document the fish:fishery relationship for the Peruvian anchoveta. Anchoveta pseudo-cycles of 20 to >100 years are observed, with the present stock abundance most likely located near upper part of the cycle. Fleet overcapacity expressed as the proportion of unused present capacity is estimated at 72% and processing overcapacity at 89%. A simple bio-economic model demonstrates the risks associated with the pseudo-periodicity in fish stock abundance in conjunction with fishery investment, open access, and overcapacity: a timing bomb for the fishing sector. The lag between disinvestment and decrease in fish abundance is quantified. A reduction of the fishing and processing capacity and measures to decrease the investment lag are recommended to limit the social, economical and political tensions that will result from the expected decrease in stock abundance. Finally, some management options to reduce these risks are discussed.

Conspecific predation among longnose lancetfish Alepisaurus ferox was investigated in four spatio-temporal strate of the western Indian Ocean. The cannibalism level varied from 0 to 45.5% by frequency of occurrence and was negatively related with abundance of non-evasive prey (such as crustaceans Charybdis smithii and Natosquilla investigatoris) and foraging success. Predation by lancetfish is often described as a non-selective process, constrained by local prey availability and by its feeding speed during an attack of prey. Our results show that lancetfish may adapt its opportunistic foraging behavior, feeding on non-conspecific abundant prey such as crustaceans when available, and switching to a high level of conspecific predation in poor waters.

Marine oxygen-deficient environments with high sedimentation rates and high primary productivity can provide relevant information regarding variations of ocean–climatic conditions in the past. In the Humboldt current ecosystem, which now hosts huge populations of pelagic fishes (mainly anchovy and sardine), fish scale abundance in the sedimentary record may be useful indicators of environmental change. Here we assess such a proxy record in a 42 cm-long sedimentary core collected from 80 m in Mejillones Bay (23°S, northern Chile). We also analyse fish remains in surface sediment sampled along a bathymetric transect (from 10 to 110 m water depth) in the same bay. In the core-top record, the fluctuations of sardine and anchovy scale deposition rates (SDR) agreed with those of industrial catches for these two species in northern Chile, tending to validate the SDR as a proxy of local fish biomass when bottom anoxic conditions prevail. However, apparent SDR for records prior to 1820 have probably been influenced by dissolution processes linked to the oxygenation of the bottom environment of Mejillones Bay, as suggested by other proxy records. After 1820, the fluctuations in the relative abundance of sardine and anchovy scales point to alternating warm and cold conditions during about 30 years and then a progressively cooler period. Since ca. 1870, marked fluctuations of SDR of both species are observed, probably as a consequence of the onset of a different oceanographic regime characterized by intensified upwelling, stronger subsurface oxygen deficiency, higher primary productivity, and enhanced “ENSO-like” interdecadal variability. While anchovy SDR fluctuated in periods of 25–40 years, only two peak periods of sardine SDR occurred (late 19th century and late 20th century), suggesting that sardine abundance depends on other ocean–climatic factors.

Correspondance: michael.fontaine@ulg.ac.be; International audience; Background: Understanding the role of seascape in shaping genetic and demographic population structure is highly challenging for marine pelagic species such as cetaceans for which there is generally little evidence of what could effectively restrict their dispersal. In the present work, we applied a combination of recent individual-based landscape genetic approaches to investigate the population genetic structure of a highly mobile extensive range cetacean, the harbour porpoise in the eastern North Atlantic, with regards to oceanographic characteristics that could constrain its dispersal.Results: Analyses of 10 microsatellite loci for 752 individuals revealed that most of the sampled range in the eastern North Atlantic behaves as a 'continuous' population that widely extends over thousands of kilometres with significant isolation by distance (IBD). However, strong barriers to gene flow were detected in the south-eastern part of the range. These barriers coincided with profound changes in environmental characteristics and isolated, on a relatively small scale, porpoises from Iberian waters and on a larger scale porpoises from the Black Sea.Conclusion: The presence of these barriers to gene flow that coincide with profound changes in oceanographic features, together with the spatial variation in IBD strength, provide for the first time strong evidence that physical processes have a major impact on the demographic and genetic structure of a cetacean. This genetic pattern further suggests habitat-related fragmentation of the porpoise range that is likely to intensify with predicted surface ocean warming

Cape anchovy Engraulis encrasicolus spawners in the southern Benguela showed an eastward shift in their distribution on the Agulhas Bank that occurred abruptly in 1996 and has since persisted. We assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf of the Agulhas Bank to the east of Cape Agulhas abruptly became 0.5&#176C colder than in previous years and has since remained that way. In addition, signals, coherent with the 1996 shift recorded in sea surface temperatures, were also found in atmospheric surface pressure and zonal wind data for that region; interannual coastal SST variability is also shown to be correlated with zonal wind-stress forcing. As a result, increased wind-induced coastal upwelling east of Cape Agulhas is proposed as the main driver of the observed cooling in the coastal region. The synchrony between the environmental and biological signals suggests that the eastward shift in anchovy spawner distribution was environmentally mediated and arose from a change in environmental forcing that altered the relative favourability for spawning between regions to the west and east of Cape Agulhas. The results highlight how a relatively minor change in environmental conditions can lead to a drastic spatial reorganisation of the life history of one species in an ecosystem. Keywords: Agulhas Bank; anchovy; Cape Agulhas; distributional shifts; Engraulis encrasicolus; environmental variability; southern BenguelaAfrican Journal of Marine Science 2007, 29(3): 309–319

The schooling behaviour of sardine Sardinops sagax in False Bay on the south coast of South Africa was studied in October 1995 using high-resolution sonar and a hull-mounted, echo-integration unit. School formation and disintegration were studied, and school shape, packing density, swimming behaviour and inter- and intra-school event rates were quantified. Mean fish density was 29.5 fish m-3 (SD 46 fish m-3), but it varied between schools by a factor of about 100 (from 2 to 233 fish m-3). Tracked schools moved at average speeds of 0.67–1.59 m s-3. Schools changed shape on average every 2.08 minutes, and underwent either splits or merges with other schools on average every 5 minutes. Relationships between the geometric dimensions and biomass of the schools were established.Afr. J. mar. Sci. 25: 185–193

Cette thèse s'intéresse au comportement d'orientation d'espèces pélagiques tropicales vis-à-vis d'attracteurs. Le chapitre 1 présente les concepts d'attracteurs et d'orientation, décrit le milieu marin et propose une revue méthodologique. Le chapitre 2 concerne le comportement d'orientation du thon à nageoires jaunes (Thunnus albacares) et de la dorade coryphène (Coryphaena hippurus) vers des dispositifs de concentration de poissons (DCP). Des trajets naturels de thons et des déplacements expérimentaux de coryphènes montrent que les thons s'orientent vers les DCP à une distance d'au moins 7 km et les coryphènes à une distance maximale de 815 m. Le chapitre 3 concerne l'orientation à grande distance chez la tortue verte (Chelonia mydas). Des déplacements expérimentaux révèlent qu'elles ne compensent pas la dérive due aux courants. Des indices géomagnétiques interviennent dans leur orientation. Les tortues ne nécessitent pas d'informations olfactives issues de leur site de ponte pour y revenir, no abstract

A mass-balanced trophic model was developed for the coral reef lagoon of Uvea atoll (New Caledonia) using the Ecopath software. The model accounts for both pelagic and soft-bottom communities to describe the whole trophic structure and biomass flows in the shallowest part of the atoll lagoon. Phytoplankton production approximately equals the benthic primary production. Benthic biomass accounts for more than 80% of the total living biomass in the shallow lagoon. The benthic domain requires input of food from the pelagic system (mainly zooplankton) and from adjacent areas to sustain the biomass of predatory fishes. Predation pressure was found to be a major force structuring the food web, but it is also suggested that water circulation within the lagoon influences the amount of primary resources, such as plankton, benthic microphytes and detritus.