MEPS 568:231-247 (2017)  -  DOI: https://doi.org/10.3354/meps12063

ABSTRACT: A recent stable isotope analysis of striped dolphins Stenella coeruleoalba stranded along the Spanish Mediterranean coast suggested a shift from a diet dominated by sardine Sardina pilchardus in 1990 towards a diet dominated by juvenile hake Merluccius merluccius in 2007-2008. This shift, apparently related to trophic cascade effects caused by overfishing, could entail long-term problems for the energetic balance of dolphins. In this study, we re-examined these conclusions using stomach content analysis of 140 striped dolphins stranded in the same area during the period 1990-2012. Remains of 13769 individual preys of 37 fish and 24 cephalopod taxa were detected. Oceanic and neritic prey were regularly found throughout the whole period; however, in the last decade, there was a strong relative decrease of oceanic cephalopods and a significant increase of 2 lower-shelf prey, i.e. juvenile hake and southern shortfin squid Illex coindetii, which together accounted for over 50% of the index of relative importance. Sardine was found only in 1 and 10% of dolphins in the 1990s and 2000s, respectively. Surveys in the study area indicate that striped dolphins favour waters 900 to 1900 m in depth, which suggests that dolphins increased predatory incursions into neritic waters in the 2000s. This habitat shift was most likely related to an increase of some neritic prey, especially juvenile hake which, according to trophodynamic models, was linked to the decline of demersal top predators. At present, stomach contents and demographic data do not support a putative impact of overfishing on the striped dolphin population.

KEY WORDS: Striped dolphin · Stenella coeruleoalba · Hake · Overfishing · Habitat shift · Dietary plasticity · Long-term data · Stomach content analysis