Environmental influences on the recruitment process inferred from otolith stable isotopes in Merluccius merluccius off the Balearic Islands

Oxygen (δ18O) and carbon (δ13C) isotope ratios in sagittal otoliths were analysed in recruits and juveniles of European hake Merluccius merluccius L. caught in the northwest (Sóller, SO) and south (Cabrera, CA) of the Island of Mallorca (western Mediterranean) over 2 consecutive years (2003 and 2004). The analytical method used allowed data to be gathered on both environmental and trophic conditions experienced by fish during the pelagic early life stages (registered in the composition of the inner part of the otolith, core area) and the demersal aggregations of recruits (at the otolith edge). Results on the seasonal variation of oxygen isotope signatures at the edge of fish otoliths captured in SO and CA indicated different vertical migration behaviours of hake between the bottom and the thermocline. δ13C showed a clear ontogenetic pattern between the core area and the edge, confirming the differences in diet from the early stages to post-recruits. The seasonal analysis of δ13C data from the otolith edge indicated that fish from the early stages in June 2003 encountered poorer trophic resources, which resulted in poor fish condition and subsequent poorer recruitment. The estimated temperature from the core area of otoliths showed lower temperature regimes in the 2002 hatching season compared to the 2003 hatching season, which could be a possible explanation for the observed differences in success of subsequent recruitment.