Estimating diet composition in sea lions: Which technique to choose?

Abstract

Reliable estimates of diets are vital to monitor impacts of sea lion populations on their ecosystems and their interactions with fisheries, and to understand the role of food to animal nutrition and health. Approaches include using (1) prey remnants in stomach contents, spews and scats; (2) prey DNA in scats; (3) fatty acid signatures in blubber; and (4) stable isotope ratios in predator’s tissue. Each methodology has particular advantages and limitations, many of which can be assessed and improved through controlled captive feeding trials. Analysis of prey remnants from captive sea lion scats have shown significant variability in digestion between and within prey species, which, coupled with preferential regurgitation and enumeration biases, can confound accurate diet quantification, but does not prevent spatial or temporal comparisons. Correction for partial digestion and use of additional structures besides otoliths can provide reliable prey size estimates. Prey DNA can be consistently isolated from soft remains in scats from captive sea lions, and with further development this approach may allow quantification of diet. Genetic methods can be expensive and representative of only one to two Sea Lions of the World 293 Alaska Sea Grant College Program • AK-SG-06-01, 2006 Tollit et al.—Estimating Diet Composition in Sea Lions days foraging (like prey remnant analysis), but may be less affected by differential digestion and can identify prey in scats that could not be identified through structural remnants. Validation of fatty acid signature analysis to quantify diet at longer temporal scales in sea lions is ongoing. This new technique promises to be particularly useful to assess biases in traditional methods, identify the onset of weaning, and highlight the prey that most contribute to lipid reserves. Stable isotope analysis of predator tissues gives only trophic level data, but can provide data on diet changes on many temporal scales. Remote video monitoring of foraging events and lavage/enema techniques can provide valuable diet information, but, like many newer techniques, animal capture is required. Ideally a suite of techniques should be used to study diet. While methods and correction factors developed for Steller sea lions can likely be applied to the other five sea lion species, they should be verified experimentally. Introduction Reliable estimates of diets are vital to monitor impacts of sea lion populations on their ecosystems and their interactions with fisheries, and to understand the role of food to animal nutrition and health. The traditional approach was to examine stomach contents. More recent methods include using hard remains in scats and spews, isolating prey DNA from scats, fatty acid signatures in blubber, stable isotope ratios in predators’ tissues, and direct video observation. All methods of estimating diet have pros and cons (Table 1), many of which can be assessed and improved through controlled captive feeding trials. Methods Captive feeding studies with 2-5 year old female Steller sea lions at the Vancouver Aquarium Marine Science Centre have assessed three different methods to quantify diet. In this review, we summarize the findings of these studies, complement them with results from additional captive studies with sea lions, and discuss the benefits and drawbacks of alternate techniques presently being used to study sea lion diet (Table 1). 1. Prey remains found in scats The analysis of prey skeletal structures found in scats (feces) is now the most widely used technique for estimating the diet of pinnipeds, with sagittal otoliths being the most commonly used identifying structure (Frost and Lowry 1980, Olesiuk et al. 1990, Bowen et al. 1993, Tollit and Thompson 1996). However, there remain a number of well recognized problems related to identifying prey without hard remains, differential rates of digestion (hence, recovery), and choice of skeletal structures used to identify prey (see reviews by Pierce and Boyle 1991, Bowen 2000). 294 Sea Lions of the World T a b le 1 . P ro s a n d c o n s o f d iff e re n t m e th o d s u s e d t o e s ti m a te d ie t in o ta ri id s .