Spawning habitat and daily egg production of sardine (Sardina pilchardus) in the eastern Mediterranean

Spawning habitats of two eastern Mediterranean sardine, Sardina pilchardus (Walbaum, 1792), stocks ( coastal waters of central Aegean and Ionian Seas) are characterized from daily egg production method (DEPM) surveys conducted during the peak of the spawning period. The latter occurs earlier in the Aegean Sea ( December) than in the less-productive Ionian Sea ( February). Single-parameter quotient analysis showed that the preferred bottom depth for spawning was 40-90 m in both areas but sardine selected sites of increased zooplankton in the Aegean Sea during December and increased fluorescence in the Ionian Sea during February. Estimates of daily egg production (P) and spawning stock biomass ( B) were about four times lower for the Ionian Sea (P 7.81 eggs m(-2), B = 3652 tonnes) than the Aegean Sea (P=27.52 eggs m(-2), B = 16 174 tonnes). We suggest that zooplankton biomass might not be sufficient to support sardine reproduction in the highly oligotrophic Ionian Sea where the very small sardine stock may rely on the late-winter phytoplankton bloom. Actively selecting sites with increased zooplankton or phytoplankton and feeding plasticity (the well-known switching from selective particle feeding to non-selective filter feeding in sardines) are interpreted as adaptations to grow and reproduce optimally at varying prey conditions. Despite differences in temperature and productivity regimes, reproductive performance of sardine in the Ionian Sea was very similar to that in the Aegean Sea during the peak of the spawning period. In comparing adult parameters from DEPM applications to Sardina and Sardinops stocks around the world, a highly significant linear relation emerged between mean batch fecundity ( F) and mean weight of mature female ( W, g) (F = 0.364W, r(2) = 0.98). The latter implies that, during the peak of the spawning period, mean relative batch fecundity (eggs g(-1)) of sardine is fairly constant in contrasting ecosystems around the world.