Hydro-plankton characteristics and their relationship with sardine and anchovy distributions on the French shelf of the Bay of Biscay

The spatial pattern in hydro-plankton and fish distributions and their relationship were analysed based on the spring 2000 fisheries acoustic survey. The importance of this survey was that it was a multi-disciplinary platform which collected an extensive set of parameters in the hydro-plankton leading to a potentially finer description of hydro-plankton conditions and fish habitats. More than 50 variables were measured on a grid of stations, in four compartments of the ecosystem: hydrology, nutrients, primary producers and meso-zooplankton. First, a joint analysis of all hydro-plankton compartments was performed using multiple factor analysis (MFA). The method was used to estimate a compromise factorial space common to all compartments in which the stations were grouped by hierarchical clustering. The groups were represented spatially and a strong spatial pattern was evidenced. The fish and their spawned eggs were sampled along transect lines using acoustics and CUFES (continuous underway fish egg samplers). The distribution of the fish and their eggs was analysed in relation to the hydro-plankton groups of stations and difference in fish density across hydro-plankton conditions was tested by a pair-wise multiple comparison procedure. Anchovy was associated with a lesser number of hydro-plankton conditions than sardine. Eggs of both species were also associated with a lesser number of conditions than the fish. Finally, the gain provided by using the extensive set of hydro-plankton parameters for mapping large-scale hydro-plankton conditions was analysed in comparison with the situation in which a small set of parameters was available. The extensive set of parameters allowed more hydro-plankton conditions to be identified but only in the coastal area and not on the shelf. Size fractionated chlorophyll was determinant for tracking river plume hydro-plankton condition. However, the fish did not respond to the variety of the coastal hydro-plankton conditions.