Diet of sardine Sardina pilchardus:an 'end-to-end' field study

摘要

The diet of sardines was analyzed from samples collected at a coastal site in the North Aegean Sea (eastern Mediterranean) in July 2007 and July 2008 (stratification periods), and in December 2007 (early phase of the mixing period) and February 2009 (late phase of the mixing period). Concurrent measurements of environmental and planktonic community variables (from bacteria to mesozooplankton) were carried out to infer major trophic pathways in the pelagic food web and determine how these pathways are related to sardine prey selection. The bulk of the dietary carbon in adults was derived from calanoid copepods, > 1 mm total length, although the diet was numerically dominated by phytoplankton. In juveniles, phytoplankton consumption was negligible. The diet composition and prey selection seemed to be driven by the availability of large prey. During summer, microbial processes prevailed (the ratio of autotrophs <20 um:>20 μm was 13 to 15, and the ratio of autotrophic to heterotrophic pico- and nanoplankton biomass was <0.5), the mean size of mesozooplankton was smaller, and filter-feeding cladocerans and appen-dicularians were very abundant. In February 2009, autotrophs >20 μm dominated the carbon budget, and the abundance of larger copepods (e.g. Centropages) was high. In December 2007, when waters were mixed but still relatively warm, both the 'microbial' and 'classical' (herbivorous) trophic pathways seemed to be important. The mean size of mesozooplankton (copepods and cladocerans) in sardine stomachs was highly correlated with their mean size in the field, and the latter was in turn highly positively correlated with the concentration of diatoms. Finally, a strong negative relationship between the Shannon-Wiener diversity index and average size of mesozooplankton prey in the stomachs was found, which could be explained in terms of the interplay of feeding modes, i.e. filter (non-selective) and particulate (selective) feeding.