Identifying blue whiting (Micromesistius poutassou) stock structure in the Northeast Atlantic by otolith shape analysis

Information on stock identification and spatial stock structure provide a basis for understanding fish population dynamics and improving fisheries management. In this study otolith shape analysis was used to study the stock structure of blue whiting (Micromesistius poutassou) in the northeast Atlantic using 1693 samples from mature fish collected between 37°N and 75°N and 20°W and 25°E. The results indicated two stocks located north and south of ICES Divisions VIa and VIb (54°5 to 60°5 N, 4° to 11°W). The central area corresponds to the spawning area west of Scotland. Sampling year effects and misclassification in the linear discriminant analysis suggested exchanges between the northern and southern stocks. The results corroborate previous studies indicating a structuring of the blue whiting stock into two stocks, with some degree of mixing in the central overlap area.L’identification et la connaissance de la structuration spatiale de stocks sont essentielles pour étudier la dynamique des populations de poissons et ainsi gérer les pêcheries. Dans cette étude, la forme des otolithes a été employée pour comprendre la structuration des stocks des merlans bleus (Micromesistius poutassou) dans le nord-est de l’Océan Atlantique a` partir de 1693 poissons matures échantillonnés entre 37°N–75°N et 20°O–25°E. Les résultats indiquent la présence de deux stocks situés au nord et au sud, respectivement, des divisions VIa et VIb du CIEM (de 54°5N a` 60°5N, de 4°O a` 11°O). La zone centrale correspond a` la zone de frai a` l’ouest de l’Écosse. Les effets du facteur année d’échantillonnage et la mauvaise classification dans l’analyse linéaire discriminante suggère des échanges entre les composants nordique et méridionale. Ces résultats corroborent des études antérieures indiquant une structuration des merlans bleus en deux stocks avec un certain niveau de mélange dans la zone de chevauchement

Overlap Between Pelagic and Demersal Fishes at the Mid-Atlantic Ridge

The deep-pelagic zone (\u3e 200 m) is essentially boundless in 3 dimensions for most of its extent, structured only by fluid features of the seawater itself. However, near certain topographic features the deep-pelagic zone intersects the seafloor. The mid-ocean ridge system is by far the largest of these features. Unlike the ecosystems of the continental margins, the mid-ocean ridge systems do not receive terrigenous nutrient inputs. Thus, the deep-water fauna associated with mid-ocean ridges ultimately depend on the generally very limited local surface production. Despite this limited surface production, there is evidence that near-ridge demersal fish biomass is increased above the mid-Atlantic Ridge (MAR). To understand the dynamics of the MAR an international study, MAR-ECO, of the animals inhabiting the northern Mid-Atlantic was undertaken. Utilizing multiple technologies the meso-, bathy- and benthopelagic realms were sampled using pelagic and bottom trawls, ROV video and bottom long-lines. Taxonomic analysis to date has revealed at least 294 species (74 families, 22 orders), with ongoing analysis expected to reveal more species, some new to science. Pelagic sampling collected ca. 206 species, with typical orders dominating (e.g., Myctophiformes, Stomiiformes, Osmeriformes). Bottom trawling collected ca. 175 species, with typical demersal families (Alepocephalidae, Macrouridae, Ophidiidae, Moridae), but also pelagic families occurring in numbers higher than would be expected by contamination during deployment and retrieval alone. Discrete, near-bottom pelagic trawls confirmed this observation. In all, 84 species were caught in both pelagic and bottom trawls, with some species showing enhanced abundances in the near-bottom boundary layer, suggesting that overlap of deep-pelagic and demersal faunas is likely a key process regulating mid-ocean ridge community structure