Conservation and management measures for exploited fish species rely on our ability to monitor variations in population abundance. In the case of the eastern stock of Atlantic bluefin tuna (ABFT), recent changes in management policies have strongly affected the reliability of fishery-dependent indicators due to drastic changes in fishing season/area, fisheries selectivity and strategy. However, fishery-independent indices of abundance are rare for large pelagic fish, and obtaining them is often costly and labor intensive. Here, we show that scientific aerial surveys are an appropriate tool for monitoring juvenile bluefin tuna abundance in the Mediterranean. We present an abundance index based on 62 aerial surveys conducted since 2000, using 2 statistical approaches to deal with the sampling strategy: line and strip transects. Both approaches showed a significant increase in juvenile ABFT abundance in recent years, resulting from the recovery plan established in 2007. Nonetheless, the estimates from the line transect method appear to be more robust and stable. This study provides essential information for fisheries management. Expanding the spatial coverage to other nursery grounds would further increase the reliability and representativeness of this index.

Different dolphin and tuna species have frequently been reported to aggregate in areas of high frontal activity, sometimes developing close multi-species associations to increase feeding success. Aerial surveys are a common tool to monitor the density and abundance of marine mammals, and have recently become a focus in the search for methods to provide fisheries-independent abundance indicators for tuna stock assessment. In this study, we present first density estimates corrected for availability bias of fin whales (Balaenoptera physalus) and striped dolphins (Stenella coeruleoalba) from the Golf of Lions (GoL), compared with uncorrected estimates of Atlantic bluefin tuna (ABFT; Thunnus thynnus) densities from 8 years of line transect aerial surveys. The raw sighting data were further used to analyze patterns of spatial co-occurrence and density of these three top marine predators in this important feeding ground in the Northwestern Mediterranean Sea. These patterns were investigated regarding known species-specific feeding preferences and environmental characteristics (i. e. mesoscale activity) of the survey zone. ABFT was by far the most abundant species during the surveys in terms of schools and individuals, followed by striped dolphins and fin whales. However, when accounted for availability bias, schools of dolphins and fin whales were of equal density. Direct interactions of the species appeared to be the exception, but results indicate that densities, presence and core sighting locations of striped dolphins and ABFT were correlated. Core sighting areas of these species were located close to an area of high mesoscale activity (oceanic fronts and eddies). Fin whales did not show such a correlation. The results further highlight the feasibility to coordinate research efforts to explore the behaviour and abundance of the investigated species, as demanded by the Marine Strategy Framework Directive (MSFD).

Abstract-Time series of depth data, obtained for 6 oceanic whitetip sharks (Carcharhinus longimanus) tagged with pop-up satellite archival tags, were analyzed in detail. The aim was to improve our knowledge of the vertical behavior of this species. Individuals were tagged in the Atlantic Ocean (n=5) and Indian Ocean (n=1) between 2011 and 2012. Deployment periods for these tags varied from 100 to 178 d. The sharks spent most of their time in the mixed layer, displaying the typical behavior of an epipelagic species. However, analyses revealed complex vertical movement patterns, including marked diel changes that reflect 3 different types of behavior. Results of the generalized additive models indicate that vertical movements were strongly correlated with variations in the depth of the mixed layer. A correlation between vertical movements and sea-surface temperature (SST) was also observed. When SST was above average, oceanic whitetip sharks increased their vertical amplitude, disregarding the reduction of the mixed layer. Thermoregulation likely allows this type of vertical movement.The oceanic whitetip shark (Carcharhinus longimanus) is a pelagic predator threatened across the tropical oceans of the world (Bonfil et al., 2008). This species is a common bycatch of pelagic fisheries that target tuna (Thunnus spp.), swordfish (Xiphias gladius), and other tuna-like species (Beerkircher et al., 2002; Hall and Roman, 2013;Gallagher et al., 2014;Frédou et al., 2015;Oliver et al., 2015). The oceanic whitetip shark acquired its status of vulnerable globally and critically endangered in the northwest and western central Atlantic Ocean on the IUCN Red List of Threatened Species because of the increasing fishing pressure throughout its range and because of a lack of information regarding its biology and ecology (Baum et al., 2015). As a precautionary approach, a series of management measures that banned the landing, storing, and selling of oceanic whitetip sharks were implemented recently by management organizations of all regional tuna fisheries (Tolotti et al., 2015a). To date, this species is the only pelagic shark protected in the Atlantic, Pacific, and Indian oceans. The Convention on International Trade in Endangered Species of Wild Fauna and Flora has also included the oceanic whitetip shark in its Appendix II (CoP16 Prop. 42), which includes species for which trade must be closely controlled.The lack of information regarding the biology and ecology of the oceanic whitetip shark is partly due to it being captured as bycatch-a situation that has historically resulted in few incentives for research and conservation (Barker and Schluessel, 2005). In light of the ocean-wide population declines observed for this species and because of the increasing interest in the conservation of bycatch species, Tolotti et al.: Vertical movements of oceanic whitetip sharks (Carcharhinus longimanus) 381 the scientific community recently has undertaken significant research to fill these wide gaps in scientific information. These studies ...