Ontogenetic and seasonal analysis of the diet and isotopic niche of humpback whales in the Magellan Strait, Chile

Abstract

The ecological niche is dynamic, since the position and width of the niche can vary at different spatial and temporal scales. We examined the trophic ecology of southeast Pacific humpback whales Megaptera novaeangliae in the Magellan Strait feeding area in 2011, 2012 and 2017, analyzing changes in diet and the width of the isotopic niche in relation to ontogeny and seasonality. The isotopic composition of carbon (δ13C) and nitrogen (δ15N) in whale skin and in putative prey species was analyzed. Bayesian mixing models were used to determine the diet, and the isotopic niche was estimated using the standard ellipse area. Differences were found between the diets of juveniles and adults; Fuegian sprat Sprattus fuegensis was consumed mostly by adult individuals. We found no differences in the diet or in the isotopic niche of humpback whales throughout the feeding season in all years. This study suggests that the differences in diet between age classes are influenced by the ability to find and capture prey, whereas seasonality does not influence the niche width of humpback whales in the Magellan Strait. Our results suggested that fluctuations in the abundance of prey populations could influence in the trophic niche dynamics of humpback whales in the Magellan Strait. In particular, a reduction in the availability of Fuegian sprat would mainly affect adult individuals, leading to a dietary switch and/or to an expansion of their feeding area.