Frontiers in Marine Science (Nov 2015)
Exploring fidelity of Sardines (Sardina pilchardus) to feeding grounds in the Atlantic Moroccan waters using stable isotope analyses (C, N) and otolith microchemistry
Abstract
The Atlantic Moroccan waters (21°-29°N) are part of the Canary Current Large Marine Ecosystem and are enriched by highly productive upwelling systems. Sardine (Sardina pilchardus) is both the most abundant forage fish in this area and the most heavely fish species [1]. It is described as a migratory small pelagic species that feeds on phyto and zooplankton. From population genetics studies (allozyme and micostellite DNA), it is assumed that our area of interest houses a single population [2] with at least two spawning areas [3]. In order to study trophic position and fidelity to feeding grounds, we investigated sardines from two distant sites: 23-24°N and 28-29°N, using stable isotope analysis (d15N and d13C) and otolith microchemistry. Isotopic analyses were performed on delipidated muscle subsamples, and plankton samples (150µm mesh net), using a continuous flow isotope-ratio mass spectrometry. The trophic position of sardine is compared to those of major forage fish of the same area. Otholith microelements concentrations (Li, B, Mg, Mn, Zn, Sr, Ba, Pb and U) were measured by ICP-MS laser on transects from nucleus to the edge of the otoliths of sardines. Chemical analyses were carried out on five individuals per site. Sardine individuals from the different sites showed significantly different trophic position for both C and N isotopes. The northern individuals were depleted in d15N (1‰ difference), suggesting that phytoplankton is more important in their diet. The d13C values showed greater differences (~3‰) for both Sardines and zooplankton between the two sites, pointing high fidelity for sardine individuals towards the feeding grounds. While Anchovy and Horse Mackerel showed a similar pattern with sardine for δ13C, chub mackerel showed no significant differences between the two areas suggesting a higher seasonal migratory behavior for this species. Sardine Microelements profiles reinforced the sedentary behavior hypothesis for sardine. The Barium (Ba) profiles were the most discriminant: the concentration range was 7× higher for the southern area where upwelling intensity is higher [4].
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