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International audience; Background Oxygen minimum zones (OMZs) are expanding in the World Ocean as a result of climate change and direct anthropogenic influence. OMZ expansion greatly affects biogeochemical processes and marine life, especially by constraining the vertical habitat of most marine organisms. Currently, monitoring the variability of the upper limit of the OMZs relies on time intensive sampling protocols, causing poor spatial resolution. Methodology/Principal Findings Using routine underwater acoustic observations of the vertical distribution of marine organisms, we propose a new method that allows determination of the upper limit of the OMZ with a high precision. Applied in the eastern South-Pacific, this original sampling technique provides high-resolution information on the depth of the upper OMZ allowing documentation of mesoscale and submesoscale features (e.g., eddies and filaments) that structure the upper ocean and the marine ecosystems. We also use this information to estimate the habitable volume for the world's most exploited fish, the Peruvian anchovy (Engraulis ringens). Conclusions/Significance This opportunistic method could be implemented on any vessel geared with multi-frequency echosounders to perform comprehensive high-resolution monitoring of the upper limit of the OMZ. Our approach is a novel way of studying the impact of physical processes on marine life and extracting valid information about the pelagic habitat and its spatial structure, a crucial aspect of Ecosystem-based Fisheries Management in the current context of climate change.

In ice-covered southeastern Hudson Bay (northern Quebec, Canada) the production of suitable zooplankton prey for manne fish larvae was simdar withln and outside the area covered by the Great Whale h v e r plume Within the plume however, hght attenuahon by the turbld surface layer reduced the foraging efficiency of first-feeding Arctic cod Boreogadus s a ~ d a nd sand lance Ammodytes sp larvae In daytime, first-feeding larvae accumulated at the pycnocline where food availability (l e light X prey density) was maximum Below the pycnocline, the average number of prey lngested by indiv~dual arvae (foraging gain) decllned with depth At night, fish larvae and their prey redistnbuted more uniformly over the water column, suggesting a similar passive response to the turbulence field In the absence of a light gradient The observed Ideal free distributions (IFDs) were better explained by unequal foraging abilities of the larvae than by density-dependent interactions among the assemblage of planktonic predators

Les variations spatio-temporelles d'abondance du stock de bobo de Guinée sont analysées sur la période 1985/98 par trois catégories d'approches complémentaires. Premièrement, une évaluation du stock est conduite par les méthodes d'analyse de cohorte et de rendement par recrue, en s'appuyant sur les statistiques de captures disponsibles pour la période récente (96/98). Deuxièmement, des indices annuels de densité sont calculés par modélisation linéaire des données des campagnes de chalutages scientifiques, réalisées depuis 1985 dans les strates bathymétriques de 5 à 30 m. Enfin, l'estimation des surfaces des strates permet de mettre en cohérence ces deux types de résultats et d'estimer un modèle de répartition des biomasses en début et fin de période, dans l'ensemble des strates de 0 à 30 m. Dans la période récente, la biomasse du stock est estimée à environ 9000 tonnes ; l'exploitation, très majoritairement liée à la pêche artisanale, conduirait à une situation globale de pleine exploitation. Les indices de densité montrent une forte diminution de l'abondance du bobo, entre début et fin de période. Cette diminution est particulièrement marquée dans les strates de forte abondance, entre 5 et 10 M. Le modèle de répartition des biomasses, montre qu'une part essentielle du stock (probablement supérieure à 75%) se situe dans la strate la plus côtière (0 à 5m) qui échappe au navire océanographique. La biomasse de début de période, qui correspondait à une situation proche de l'état vierge, est estimée à environ 18 000 tonnes. Le développement de l'exploitation s'est ainsi traduit par une baisse d'abondance, avec un impact qui s'avère être différent selon les strates bathymétiques. On met ainsi en évidence une rétraction du stock sur la zone la plus côtière. (Résumé d'auteur)

L'étude concerne les déplacements verticaux des thonidés face aux contraintes du milieu profond dans l'océan Pacifique Sud tropical dans la ZEE de la Polynésie française. Les thons constituent une ressource très importante au niveau de la pêche, et la connaissance de leurs stratégies et mécanismes adaptatifs en environnement naturel extrême permettrait de mieux comprendre leur comportement de manière indirecte. Les résultats confirment l'hypothèse de l'auteur quant au rôle très important joué par la vessie gazeuse et les branchies pour la gestion des gaz dissous et de ce fait, pour la colonisation de niches écologiques plus profondes malgré les contraintes environnementales fortes (pression, température...)

Two acoustical surveys took place in October 1993 and February 1994, covering the main part of the Java Sea, below a 50 m depth. The echo-integration process was performed through a BioSonics 120 kHz Dual Beam echo-sounder. Fish shoal characteristics and their behavioural aspects were observed. The shoals' configurations were extracted from the echograms and quantified. This information allows the spatial and bathymetric distribution to be presented. An analysis on the abundance and the distribution of shoals is proposed, as well as their contribution to the global clusters and a description of their behaviour during the day and night periods. (Résumé d'auteur)

Analyses of the acoustical data collected during several cruises made in the Java Sea from 1991 to 1994, in the context of the Pelfish project, have facilitated the elaboration of a biological model based on three groups of fish, each having particular distribution characteristics. This paper shows how the analysis of bathymetric distribution on the same database gives valuable information on fish behaviour, and how this method could be used first to define different groups of fish (in our case, to increase the types up to five groups) and secondly, to raise several questions on the determinants of the fish distribution. (Résumé d'auteur)

The coastal borders of northern Java and eastern Sumatra have been prospected during the acoustic surveys, in the depth of about 115 to 50 m, in wet season. The measures of relative density per nautical mile, the vertical distribution of the abundance and shoals have been analyzed. The general characteristics can be drawn-up : they are the extension of an abundant semipelagic fauna occupying the bathymetric strata from 25 to 45 m, and the existence of a dominant pelagic fauna are more coastal. Shoals, low reverberating, are present in all areas but are dominant in the coastal border ; the more reverberating aggregations are more off shore. From the East to the West, their bathymetric localization increases in the sectors near oceanic influences, they are more dense and near the surface. From the East to the West, the abundance of schools and total density decrease. This fact seemingly has to do with the extension of the relatively shallow zone. Two sectors show a particularity : the Bay of Semarang with densities sometimes big, which could be related to the seasonal topography, and the surroundings of the Sunda Strait, zone of passage and exchange for the deep pelagic population. These observations globally confirm the stratification tentative which was carried out during the Workshop of 1994, in specifying the geographical extension of the coastal stratum, very narrow in front of Java, and in proposing that one part of the semipelagic population which constitutes the wealth of the East of Java in dry season, should migrate in wet season towards the slopes of the coastal border. (Résumé d'auteur)

An acoustic survey, from Semarang towards the South China Sea, allows the haline situation and the distribution of abundance in pelagic fishes at the end of the wet season to be described. At that moment, the fleet of Javanese seiners operates in the south-west of the Natuna Islands, in depths of 50 meters. The analysis of haline vertical profiles shows the presence of three zones : the Java Sea with vertical gradients due to the coastal desalinization, the Karimata Strait with isohaline water at 33.5% and the South China Sea with deep waters at 34%. The last sector where the fleet was operating, was three times richer than the Javanese waters at that time. Without direct correlation with that, the zonation of the abundance (scattered or aggregated fishes) follows the zonation of salinity. The water of the strait is poorer, and the water of the Java Sea is richer while the desalinization is stronger. It seems that the richness of water is particularly related to the bathymetry by the presence of an semipelagic fauna when the bottom exceeds 30 meters. This semipelagic fauna commutes twice migration towards the surface during the night, a behaviour which is more accentuated in this fishing zone than in the Java Sea. In the Karimata Strait, where the depth is less than 30 meters, the pelagic and benthic faunas are dominant. In the Java Sea, a great aggregation of larva and juveniles has been detected particularly in the South of Belitung Island, that confirms the role of the basin in the reproduction of pelagic species. At that period of the year, it is the slopes of the basin which seem to be richer. These populations, thus, avoid the strong coastal desalinization and also its deeper part. All these observations do not seem to confirm the theory of Hardenberg, which assumes the existence of a migration between the South China Sea and the Java Sea. (Résumé d'auteur)