Natural abundance of stable nitrogen isotopes reflect changes in pelagic food webs and mobility of size classes of the north Iberian sardine (Sardina pilchardus)

Abstract

The natural abundance of stable nitrogen isotopes (δ15N) in sardine (Sardina pilchardus) and plankton was determined from samples collected between 1998 and 2003 in the northwestern shelf of the Iberian Peninsula with the purpose of studying the variability in the trophic position of sardine. Sardines were grouped in three size-length classes (small: < 16 cm; medium: 16-20 cm; large: > 20 cm). Plankton samples were size-fractionated between ca. 0.7 and 2000 μm of individual length. The plankton at the base of the pelagic food web in the western shelf region (Galicia) was more enriched in δ15N than plankton in the northern and eastern region (Mar Cantábrico), likely because of upwelling. Variability in δ15N in sardine was related both to changes in feeding strategies during growth and to the mobility of size classes between adjacent shelf regions. Large sardines showed regional differences in δ15N similar to those of small plankton, suggesting that old specimens remained for long periods in the same region. In contrast, small and medium-sized sardines have similar δ15N values in all regions. We hypothesize that interannual differences in δ15N of sardine are caused mainly by variations in the mobility of large size classes, as the isotopic signature of the small sardines was relatively less variable and suggests low variability in the plankton food web. In turn, such variations would be related to changes in the size of the sardine population in the north Iberian shelf.