“Ecología trófica de los pinnípedos de las Islas Galápagos: análisis espacial y temporal”

Abstract

ABSTRACT This study is focused on the comparative analysis of variables related to the trophic niche plasticity in the Galapagos sea lion (Zalophus wollebaeki) and the Galapagos fur seal (Arctocephalus galapagoensis), which are endemic species of this Archipelago. There is a great concern regarding the future of these otariids because of their high trophic vulnerability so that the study not only the diet but also the way these animals obtain their food will be useful to understand and predict their survival possibilities in the long term. There exist several techniques to determine the trophic ecology in pinnipeds as: 1) scats analysis, 2) Stable isotopes (δ13C y δ15N) and 3) Satellite telemetry; that complement each other to determine foraging habits, with the aim of obtaining higher resolution. The objectives of this study are: A) To know the foraging habits of Z. wollebaeki and its space-time variation, B) To establish if there exist differences in terms of foraging strategies, between Z. wollebaeki and A. galapagoensis from rookeries where they coexist. The scat analysis (n=330) in Z. wollebaeki gave as a result space-time changes in the foraging spectrum composition and also an intra-population diversification in diet during the 2006 and 2009 breeding seasons, while its trophic breadth corresponded to a specialist predator (Levin = 0.07). A differential behavior was observed in Z. wollebaeki foraging trips (n=10), a result that was given by different diving behavior patterns (epipelagic and benthic). In spatial terms, isotopic values (n=321) showed differences regarding foraging grounds (δ13C: Kruskal-Wallis test, p=0.015); however an equilibrium was maintained in relation to the trophic level of their diet (δ15N: Kruskal-Wallis test, p=0. 152). δ13C and δ15N for both species xxii (n=91) were associated to differences in foraging grounds and trophic level (t-test, p≤0.001 and p=0.001 respectively), where Z. wollebaeki showed a coastal strategy, focused on the consumption of small epipelagic fishes, in contrast to A. galapagoensis that turned out to be a more oceanic predator, feeding on small size squids. These results were consistent with the location of animals at sea; geopositioned registers indicated that these species were found in the environments previously mentioned. The results of this study showed a relevant finding in a scenario where species that manage to survive in a highly demanding ecosystem in terms of availability of resources, have developed a high degree of plasticity in their foraging habits that allows them to facilitate their survival.