Pup Vibrissae Stable Isotopes Reveal Geographic Differences in Adult Female Southern Sea Lion Habitat Use during Gestation

Abstract

Individuals within populations often differ substantially in habitat use, the ecological consequences of which can be far reaching. Stable isotope analysis provides a convenient and often cost effective means of indirectly assessing the habitat use of individuals that can yield valuable insights into the spatiotemporal distribution of foraging specialisations within a population. Here we use the stable isotope ratios of southern sea lion (Otaria flavescens) pup vibrissae at the Falkland Islands, in the South Atlantic, as a proxy for adult female habitat use during gestation. A previous study found that adult females from one breeding colony (Big Shag Island) foraged in two discrete habitats, inshore (coastal) or offshore (outer Patagonian Shelf). However, as this species breeds at over 70 sites around the Falkland Islands, it is unclear if this pattern is representative of the Falkland Islands as a whole. In order to characterize habitat use, we therefore assayed carbon (δ13C) and nitrogen (δ15N) ratios from 65 southern sea lion pup vibrissae, sampled across 19 breeding colonies at the Falkland Islands. Model-based clustering of pup isotope ratios identified three distinct clusters, representing adult females that foraged inshore, offshore, and a cluster best described as intermediate. A significant difference was found in the use of inshore and offshore habitats between West and East Falkland and between the two colonies with the largest sample sizes, both of which are located in East Falkland. However, habitat use was unrelated to the proximity of breeding colonies to the Patagonian Shelf, a region associated with enhanced biological productivity. Our study thus points towards other factors, such as local oceanography and its influence on resource distribution, playing a prominent role in inshore and offshore habitat use.

Fig 1. We collected southern sea lion pup vibrissae from 19 different breeding colonies across the Falkland Islands.

Samples from Big Shag Island and Turn Island were collected in 2013, while the remainder of breeding colonies were sampled in 2014. Circle sizes are proportional to annual pup production, determined from direct counts in 2014 (see also Table 1) (source of 2014 Falkland Islands census data [15]). The 200 m contour depicts the edge of the Patagonian Shelf.

Fig 2. We used the vibrissae stable isotope values of 65 southern sea lion pups as proxies for adult female habitat use.

Model-based clustering identified three distinct clusters (represented by ellipses, the multivariate analogs of standard deviations). These represent adult females that foraged in inshore habitats (black dots), offshore habitats (blue squares) or had δ¹³C and δ¹⁵N values that were intermediate to inshore/offshore values (grey triangles).