Stable Isotopes Reveal Long-Term Fidelity to Foraging Grounds in the Galapagos Sea Lion (Zalophus wollebaeki)

Abstract

Most otariids have colony-specific foraging areas during the breeding season, when they behave as central place foragers. However, they may disperse over broad areas after the breeding season and individuals from different colonies may share foraging grounds at that time. Here, stable isotope ratios in the skull bone of adult Galapagos sea lions (Zalophus wollebaeki) were used to assess the long-term fidelity of both sexes to foraging grounds across the different regions of the Galapagos archipelago. Results indicated that the stable isotope ratios (δ(13)C and δ(15)N) of sea lion bone significantly differed among regions of the archipelago, without any significant difference between sexes and with a non significant interaction between sex and region. Moreover, standard ellipses, estimated by Bayesian inference and used as a measure of the isotopic resource use area at the population level, overlapped widely for the sea lions from the southern and central regions, whereas the overlap of the ellipses for sea lions from the central and western regions was small and non-existing for those from the western and southern regions. These results suggest that males and females from the same region within the archipelago use similar foraging grounds and have similar diets. Furthermore, they indicate that the exchange of adults between regions is limited, thus revealing a certain degree of foraging philopatry at a regional scale within the archipelago. The constraints imposed on males by an expanded reproductive season (~ 6 months), resulting from the weak reproductive synchrony among females, and those imposed on females by a very long lactation period (at least one year but up to three years), may explain the limited mobility of adult Galapagos sea lions of both sexes across the archipelago.

Fig 1. Map of the Galapagos archipelago showing the islands where sea lion skulls were collected and the chlorophyll levels.

Chlorophyll values are the cumulative average values of chlorophyll-a concentration (mg/m³) from 1 September 1997 to 31 August 2001 derived from SeaWiFS Project (http://oceancolor.gsfc.nasa.gov). The hydrogeographic regions, in agreement with Ruttenberg et al.[42], are denote by the names in brackets.

Fig 2. Mean (±SD)values of δ¹³C (A) and δ¹⁵N (B) for bone from the skulls of male and female sea lions collected in three different regions of the Galapagos archipelago.

Regions with different superscript (lower-case letters) are statistically different in their mean values according to the Scheffe's post hoc test. Vertical bars show standard deviation.

Fig 3. Area of the isotopic niche of Galapagos sea lions in the three different regions of the archipelago.

Ellipse areas (solid lines)calculated with SEA_C using the bivariated isotopic values (solid circles) of males and females from each region(West, Central and South).