Effects of prey concentration on ingestion rates of European sardine Sardina pilchardus larvae in the laboratory

The quantification of prey requirements for larval fish is essential to understand how environmental factors act to restrict suitable habitats and recruitment success in marine fish. The effect of prey concentration on ingestion rates of the European sardine Sardina pilchardus was estimated for larvae through 50 d post-hatch (dph) under controlled laboratory conditions at 15 degrees C. Prey were nauplii and copepodites of the calanoid copepod Acartia grani, which were provided to larvae at 3 concentrations (0.5, 2 and 6 nauplii ml(-1) and 0.1, 0.5 and 1 copepodites ml(-1)). Larvae were not able to capture copepod nauplii at the beginning of exogenous feeding, suggesting that early larvae depend on smaller prey types and/or less mobile prey than copepods. The mean size of prey found in the guts of sardine larvae increased from 145 to 348 mu m for larvae of total length increasing from 6 to 18 mm, respectively. Maximum ingestion rates (232 +/- 8.0 mu g C larva(-1) h(-1)) were reached at the highest prey concentration diet for individuals >40 dph (1500 to 2500 mu g C dry weight). These feeding rates are higher than values previously reported for the larvae of small pelagic fish. The inability of sardine larvae to feed at low prey concentrations, particularly during the first weeks of life, suggests that this species relies on and is adapted to forage within dense prey patches. Given this feeding strategy, bottom-up processes causing food limitation may strongly impact the survival and growth of sardine larvae.