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

Tracing aquatic food webs using fatty acids: from qualitative indicators to quantitative determination

“…Asterisks indicate examples of FA with large deviations from 1:1 but which occur at only minor or trace amounts in marine lipids, with contributions from biosynthesis, and routinely not used in QFASA modeling. Data taken from Iverson et al (2004) ;Tollit et al (2006) , Iverson et al (2007) ;and Nordstrom et al (2008) often not possible. Understanding the detection limits of prey is important, especially in cases where there is interest in prey species rather than predators.…”

Evaluating quantitative fatty acid signature analysis (QFASA) using harbour seals Phoca vitulina richardsi in captive feeding studies

“…The analysis requires a library of FA profiles and lipid contents from potential prey to compare with the predators' blubber FA profile as well as calibration coefficients (CCs) to correct for effects of biosynthesis or modification of specific FA in the predator. To date, efforts to establish CCs and test the efficacy and interpretation of QFASA have been confined to a few species , 2006, in press, Hoberecht 2006, Tollit et al 2006. Diet estimates generated from these studies have varied, sometimes significantly, depending on the CCs and/or the FA subset employed, even when only prey actually fed was used as input.…”

Harbor seal diet in northern Puget Sound: implications for the recovery of depressed fish stocks

“…Genetic and molecular techniques, including the use of DNA, stable isotopes, and fatty acid signatures, are increasingly being used to assess the diets of a variety of marine predators, including harbor seals, and can yield different answers than traditional techniques, such as scat and stomach content analyses (Smith et al 1996, Tollit et al 2006, Nordstrom et al 2008, Deagle et al 2009). A parallel study in the San Juan Islands using fatty acids confirms that salmon and herring were prevalent in seal diet but that some individual seals consume more rockfish than indicated by scat (J. Bromaghin pers.…”