Methods of estimating marine mammal diets: A review of validation experiments and sources of bias and uncertainty

Abstract

Diet estimation in marine mammals relies on indirect methods including recovery of prey hard parts from stomachs and feces, quantitative fatty acid signature analysis (QFASA), stable isotope mixing models, and identification of prey DNA in stomach contents and feces. Experimental evidence (9 species/13 studies) shows that digestion strongly influences the proportion and size of otoliths that can be recovered in feces. Number correction factors (NCF) and digestion coefficients have been experimentally determined to reduce the biases in fecal analysis. Correction factors and coefficients have not been determined for diet estimated from stomach contents. QFASA estimates which prey species and amounts must have been eaten to account for the fatty acid composition of the predator. Experimental studies on mammals and seabirds (9 species/10 studies) indicate that accurate estimates of diet can be determined using QFASA. Stable isotope mixing models provide rather coarse taxonomic resolution of diet composition. Prey DNA analysis shows promise as a method to estimate the species composition of diet, but further development and testing is needed to validate its use. Obtaining a representative sample from marine mammal populations is a significant challenge. Therefore, the use of complementary methods is recommended to obtain the most informative results.

Reliability of qPCR for quantitative gut content estimation in the circumglobally abundant pelagic tunicate Dolioletta gegenbauri (Tunicata, Thaliacea)

“…Although initially we hypothesized that the digestion of prey DNA would be less than that of algal pigments because of the greater stability of DNA compared to photosynthetic pigments (chlorophyll a), these studies indicated that, at least in copepods, prey DNA digestion was significant. Similar results have been reported in other studies with copepods (Durbin et al, 2012), other marine crustaceans (Tobe et al, 2010) and a wide variety of other marine and terrestrial predators (Bowen and Iverson, 2013;Hunter et al, 2012;Weber and Lundgren, 2009). However, it is difficult to predict based on first principles whether prey digestion will be a significant impediment to the quantification for specific predators.…”

Diet of beluga whales, Delphinapterus leucas, in Alaska from stomach contents, March-November

“…Diet studies using stomach contents have well known biases (Tollit et al, 2010;Bowen and Iverson, 2012). Stomach contents represent some portion of an individual's most recent feeding activity.…”

Multi-approach analysis to assess diet of harbour porpoises Phocoena phocoena in the southern North Sea

“…quantitative versus qualitative and species versus general trophic level) and they also differ in time scale (i.e. more recent diet versus longer-term diet) (reviewed in Bowen & Iverson 2013). To our knowledge, only a few studies have combined the 3 methods to assess the foraging ecology of marine mammals (Hooker et al 2001, Jansen 2013.…”