Trophodynamics of anchovy in a non-upwelling system: direct comparison with sardine

摘要

Anchovies and sardines hold a very important position in pelagic ecosystems and are usually found in species pairs, exhibiting fluctuations in abundance. In upwelling systems, alternations in small pelagic fish have been hypothesized to be, at least partly, trophically mediated. In non-upwelling areas, direct comparisons of sardine and anchovy trophodynamics are lacking, which represents an impediment to our understanding of forage fish interactions. In the present study, we analyzed the diel feeding periodicity, daily ration and diet composition of anchovy Engraulis encrasicolus in a coastal area of the eastern Mediterranean. The anchovy feeding parameters and diet compositions were then compared with published data on sardine Sardina pilchardus, captured in the same set of pelagic trawl hauls and analyzed using identical methods. Both anchovy and sardine foraged in daytime in summer, and mainly at night in winter. Their feeding intensities per daily sampling time were highly correlated. In both species, daily consumption was significantly and similarly related to fish size and per capita food availability. In terms of dietary carbon, intraspecific dietary differences between juveniles and adults were insignificant. Copepods (especially large ones) were the main energy source for both species. In contrast to adult sardines, anchovies did not consume phytoplankton, and ingested large prey (including decapod larvae) more frequently. Both species broadened their trophic niche in summer; dietary differences between the stratification and mixing periods mostly reflected corresponding changes in prey availability in the field. The strong size-based partitioning of plankton resources described for anchovy and sardine in upwelling systems was not corroborated in the Mediterranean Sea.