Combined study of daily growth variability and nitrogen–carbon isotopic signature analysis of schooling
Abstract:A combined study of Alborán Sea Sardina pilchardus larval daily growth and isotopic signature was carried out to elucidate the cause of morphometric differences observed in a single larval school whereby the standard length (L S) v. dry mass (M D) relationship differentiated a heavier‐by‐size larval group from a lighter larval group. The daily growth analysis revealed that this difference originated from two larval growth patterns, where a fast growing population (F) in contrast to a slow‐growing larval population (S) was distinguished. The S‐growing larval cohort had a significantly higher nitrogen (N) content as a result of greater somatic mass build up with time in the form of structural proteins. Alternatively, the F‐growing population showed a significantly greater amount of carbon (C) content with age, indicating faster metabolic rates of C accretion compared to the S‐growing group. In consequence, the C:N ratios of the F‐growing larvae were significantly higher than the S‐group. C:N ratios of both larval populations showed significant linear decrease with age (and size), while K showed an inverse relationship. The stable isotopes of N did not show significant differences between the S and F‐growing larvae. In F‐growing larvae, however, a significant linear increase in δ 15N (by age class) was observed, indicating that as larvae undergo ontogenetic development, trophic level tends to increase. This was also made evident by the significant decrease in age of δ 15N coefficients of variation (by age class). The higher δ 13C values in the S‐growing larvae were related to the lower growth rates observed in this group. These results suggest a broader trophic flexibility in younger larvae, but moving towards a trophic specialization and more selective diets with age. This trophic specialization shows a tendency of a greater prey size with age. These findings suggest that S‐growing larvae have a more omnivorous diet than the F‐growing ones.
Document Type: Regular Paper
Publication date: October 1, 2011