Trophic interaction between zooplankton and anchovy larvae (Engraulis encrasicolus) with daily growth variability implications

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Zooplankton samples were collected at night time by means of vertical tows simultaneously with oblique tows for icthyoplanktonic sampling during July 2009. Engraulis encrasicolus larvae were sorted onboard for a combined study of daily growth and isotopic signature. Anchovy larvae were collected from two contrasting environmental scenarios, particularly larvae originating in the Ebro River spawning area on the Catalonian coast and larvae retained in the Almeria Bay (Alborán Sea). A greater microzooplankton biomass (p < 0.0001; 55–200 μm) fraction was observed in Alborán waters, together with higher carbon (C, p < 0.0001) and nitrogen (N, p < 0.003) content. Different daily growth patterns for anchovy larvae were observed according to their size–weight relationship, within the same standard length (SL) range. Better somatic growth covariated by age was observed (p < 0.005) in Alborán larvae, whereas heavier (p < 0.0001) and older (p < 0.008) larvae were found in Ebro‐spawned larvae. The C/N analysis revealed a linear decrease with age in both larval groups, with higher ratios (p < 0.005) in the Ebro‐spawned larvae. Although δ15N did not show significant differences for microzooplankton, it was higher (p < 0.0001) in Ebro‐spawned larvae, in line with its higher nitrogen isotopic enrichment (p < 0.0001), which indicates a greater trophic specialization. Higher δ13C found in Alborán Sea microzooplankton and larvae could indicate its pelagic energy origin in contrast to the Ebro River ecosystem. The positive correlation (r = 0.4; p < 0.0001) observed between microzooplankton and larval δ15N indicate its foraging preference. This study shows trophic pathways variations in relation to larval growth differences from combining otolith microstructure analysis and early life foodweb dynamics.