Bathymetric segregation of spawning stages in the Atlantic sardine Sardina pilchardus

The present study analyzed the spatio-temporal pattern of spawning aggregations of the Atlantic sardine Sardina pilchardus using a large set of reproductive data derived from 5 daily egg production method (DEPM) surveys conducted off Portugal and the Gulf of Cadiz. Similar to previous results for the Mediterranean sardine, sardines in the Atlantic were shown to form ephemeral spawning aggregations in which actively spawning females separated spatially from the remaining population, taking with them a large proportion of males. These aggregations were shown to take place at depth; specifically, bathymetric positioning of reproductively active individuals changed during the spawning cycle in such a way that imminent spawners were located at greater depths compared to recent spawners or non-spawning females. This pattern of bathymetric segregation of sardine spawning stages is in agreement with previously published evidence that sardines spawn at depth, either from indirect acoustic sampling data or from the occurrence of early embryonic stages at greater depths. A number of physiological mechanisms that may explain changes in the bathymetric positioning of actively spawning individuals are also suggested.