Résumé: We developed delta generalised additive models (GAMs) to predict the spatial distribution of different size classes of South African hakes, Merluccius capensis and M. paradoxus, using demersal trawl survey data and geographical (latitude and longitude) and environmental features (depth, temperature, bottom dissolved oxygen and sediment type). Our approach consists of fitting, for each hake size class, two independent models, a binomial GAM and a quasi-Poisson GAM, whose predictions are then combined using the delta method. Delta GAMs were validated using an iterative cross-validation procedure, and their predictions were then employed to produce distribution maps for the southern Benguela. Delta GAM predictions confirmed existing knowledge about the spatial distribution patterns of South African hakes, and brought new insights into the factors influencing the presence/absence and abundance of these species. Our GAM approach can be used to produce distribution maps for spatially explicit ecosystem models of the southern Benguela in a rigorous and objective way. Ecosystem models are critical features of the ecosystem approach to fisheries, and distribution maps constructed using our GAM approach will enable a reliable allocation of species biomasses in spatially explicit ecosystem models, which will increase trust in the spatial overlaps and, therefore, the trophic interactions predicted by these models.

Résumé: * Mapping diversity indices, that is estimating values in all locations of a given area from some sampled locations, is central to numerous research and applied fields in ecology. * Two approaches are used to map diversity indices without including abiotic or biotic variables: (i) the indirect approach, which consists in estimating each individual species distribution over the area, then stacking the distributions of all species to estimate and map a posteriori the diversity index, (ii) the direct approach, which relies on computing a diversity index in each sampled locations and then to interpolate these values to all locations of the studied area for mapping. * For both approaches, we document drawbacks from theoretical and practical viewpoints and argue about the need for adequate interpolation methods. First, we point out that the indirect approach is problematic because of the high proportion of rare species in natural communities. This leads to zero-inflated distributions, which cannot be interpolated using standard statistical approaches. Secondly, the direct approach is inaccurate because diversity indices are not spatially additive, that is the diversity of a studied area (e.g. region) is not the sum of the local diversities. Therefore, the arithmetic variance and some of its derivatives, such as the variogram, are not appropriate to ecologically measure variation in diversity indices. For the direct approach, we propose to consider the β-diversity, which quantifies diversity variations between locations, by the mean of a β-gram within the interpolation procedure. We applied this method, as well as the traditional interpolation methods for comparison purposes on different faunistic and floristic data sets collected from scientific surveys. We considered two common diversity indices, the species richness and the Rao's quadratic entropy, knowing that the above issues are true for complementary species diversity indices as well as those dealing with other biodiversity levels such as genetic diversity. * We conclude that none of the approaches provided an accurate mapping of diversity indices and that further methodological developments are still needed. We finally discuss lines of research that may resolve this key issue, dealing with conditional simulations and models taking into account biotic and abiotic explanatory variables.

Résumé: In Senegal, a significant decrease in catches indicates that many demersal fish stocks are being overexploited. The white grouper Epinephelus aeneus, locally known as the 'thiof', is exploited by both small-scale and industrial fisheries. A 28-year database of E. aeneus catches along the Senegalese coast provided by the Centre for Oceanographic Research of Dakar-Thiaroye, and size at maturity measured in Dakar (Senegal) from monthly samples in 2010, were used to analyse changes in population structure in the area over the past 37 years. Catches from the northern fishing areas were lower than those from the southern fishing areas, and decreased steadily during the period (Kolmogorov-Smirnov test, D = 0.243, p = 0.0002). The individual mean weight of catches decreased from 1974 to 2010 (linear regression, r(2) = 0.40, n = 37) and only 60% of the individuals were mature. The calculated sizes at maturity were 49 cm total length (TL) for females and 55 cm for males, and the optimal length of capture for a sustainable fishery was 96 cm, but only 0.03% of E. aeneus caught reached this length. Most of the catch consisted of juveniles; the larger reproductive individuals had disappeared. The number of individuals caught decreased significantly between 1974 and 2010 (1974-1983, r(2) = 0.98, n = 74 674; 1984-1993, r(2) = 0.95, n = 96 696; 1994-2003, r(2) = 0.93, n = 12 619; 2004-2010, r(2) = 0.91, n = 12 887), whereas the length range remained the same (10-110 cm TL). Biological indicators clearly showed that E. aeneus stocks in Senegal are overexploited and the species is now endangered. Immediate active management of fishing pressure is needed, therefore, to maintain E. aeneus populations in the area. Our results suggest a minimum size of <50 cm should be introduced and that fishing effort should be reduced.

Résumé: Atlantic bluefin tuna Thunnus thynnus (ABFT) is a fish of high market value which has recently become strongly overexploited, notably in the Mediterranean Sea. This area is an essential habitat for ABFT reproduction and growth. We present here an approach for deriving the daily mapping of potential ABFT feeding and spawning habitats based on satellite-derived sea surface temperature (SST) and chl a concentration. The feeding habitat was mainly derived from the simultaneous occurrence of oceanic fronts of temperature and chl a content while the spawning habitat was mostly inferred from the heating of surface waters. Generally, higher chl a contents were found to be preferred for the feeding habitat and a minimum SST value was found for the spawning habitat. Both habitats were defined by the presence of relevant oceanographic features and are therefore potential and functionally-linked habitats. This approach provides, for the first time, a synoptic view of the potential ABFT habitats in the Mediterranean Sea. The model performs well in areas where both satellite data and ABFT observations are available, as 80% of presence data are in the vicinity of potential habitats. The computed monthly, seasonal and annual maps of potential feeding and spawning habitat of ABFT from 2003 to 2009 are in good agreement with current knowledge on ABFT. Overall, the habitat size of ABFT is about 6% of the Mediterranean Sea surface. The results displayed a strong seasonality in habitat size and locations as well as high year-to-year variations (30 to 60%), particularly for the potential spawning habitat, which is key information for evaluating the utility of ABFT Marine Protected Areas in the Mediterranean Sea.