Hydrographic mesoscale structures and Poleward Current as a determinant of hake (Merluccius merluccius) recruitment in southern Bay of Biscay

Abstract

The relationship between the recruitment of European hake (Merluccius merluccius) and environmental conditions in southern Bay of Biscay is examined. The historical series of autumn bottom trawl surveys carried out in Galicia and Cantabrian Sea waters from 1983 show that the processes of hake recruitment lead to well-defined patches of juveniles, found in localized areas of the continental shelf. These concentrations vary in density according to the strength of the year-class, although they remain generally stable in size and spatial location. The size of the patches, estimated using basic geostatistical techniques, is found to be from 20 to 35 km in diameter. The existence of spatial correlation is assumed by computing variograms, and the year-on-year repetition of the spatial patterns is shown to be a way of linking them to environmental conditions. In the eastern, progressively narrowing, shelf of the Cantabrian Sea, years of massive inflow of the eastward shelf-edge current produce low recruitment indices, due to larvae and pre-recruits being transported away from spawning areas to the open ocean. Under these conditions, high mortality is expected because of the difficulties juveniles have in finding the shelf grounds. On the other hand, the transport of larvae within anticyclonic mesoscale structures moving towards the recruitment areas will be an aid to recruitment. These eddies displace westward according to the condition of potential vorticity conservation. When orographic features, such as big capes, occur in their drift path their eastern edges are held back. This situation causes patches of recruits to be located east of the main capes of the western Cantabrian Sea. The pattern of feeding of juvenile hake includes vertical migrations searching for small pelagic fish. Upward motions of nutrient-rich deeper water masses were found close to the recruitment areas, stemming from variations of the vorticity field by mesoscale eddies. The resulting enhanced primary production seems to affect the distribution and size of hake recruit concentrations.