||Bifrequency acoustic data, hydrological measurements and satellite data were used to study the vertical distribution of
macrozooplankton in the Bay of Biscay in relation to the hydrological conditions and fish distribution during spring 2009.
The most noticeable result was the observation of a &8216;biocline&8217; during the day i.e., the interface where zooplankton biomass
changes more rapidly with depth than it does in the layers above or below. The biocline separated the surface layer, almost
devoid of macrozooplankton, from the macrozooplankton-rich deeper layers. It is a specific vertical feature which ties in
with the classic diel vertical migration pattern. Spatiotemporal correlations between macrozooplankton and environmental
variables (photic depth, thermohaline vertical structure, stratification index and chlorophyll-a) indicate that no single factor
explains the macrozooplankton vertical distribution. Rather a set of factors, the respective influence of which varies from
region to region depending on the habitat characteristics and the progress of the spring stratification, jointly influence the
distribution. In this context, the macrozooplankton biocline is potentially a biophysical response to the search for a
particular depth range where light attenuation, thermohaline vertical structure and stratification conditions together
provide a suitable alternative to the need for expending energy in reaching deeper water without the risk of being eaten.