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

Abstract

Quantitative fatty acid (FA) signature analysis (QFASA) has recently been developed to estimate the species composition of predator diets by statistically comparing FA signatures of predator adipose tissue with that of their potential prey. Captive feeding trials were used to further test the technique with newly weaned harbour seals Phoca vitulina richardsi (N = 21). Two groups of seals were fed monotypic diets of either Pacific herring Clupea pallasii or surf smelt Hypomesus pretiosus for 42 d while a third group was fed smelt for 21 d followed by herring for 21 d. Blubber biopsies were taken dorsally at Days 0, 21 and 42. Specific calibration coefficients (CC) used by QFASA were developed from 4 juvenile harbour seals and in some cases differed by 2-fold with previously reported phocid CC values. The QFASA diet estimates were evaluated using 2 CC sets, 15 FA subsets and a library of 3 to 11 potential prey species. Diet switches were best tracked using the harbour seal CC and the new FA subset. Overall prey misclassifications were apparent (mean = 12%, range = 4 to 25%) when modeled with 8 additional prey not fed, a trend consistent with overlapping prey FA signatures. Blubber FA turnover rates were not strictly linear and in the order of 1.5 to 3 mo in newly weaned seals. Following parameter optimization of the model, QFASA estimates reflected major diet trends in the feeding study, but were sensitive to the CC and FA subsets used as well as to prey species with similar FA signatures. Our results have important implications in the application of QFASA to the study of pinniped diets with more complex feeding histories and wider prey fields.

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.…”

“…Despite issues concerning its further advancement, QFASA has now been successfully validated for a variety of predators including pinnipeds, mink, and seabirds Cooper et al 2005 ;Nordstrom et al 2008) . Since most validation studies are performed within the constraints of captive animal experiments, tested diets are usually composed of only one or a few prey species, potentially limiting conclusions drawn.…”

“…Finally, while it has been demonstrated that QFASA estimates correctly reflect major components of diet, the ability to identify trace components may be limited. Care is also clearly required in interpreting results from prey species with similar FA signatures, which may cause false positives in estimates (e.g., Iverson et al 2004 ;Budge et al 2006 ;Nordstrom et al 2008) .…”

Blubber fatty acid profiles indicate dietary resource partitioning between adult and juvenile southern elephant seals

“…Fatty acid (FA) signatures derived from the blubber of marine mammals have been used in conjunction with stomach contents and genetic analyses to provide more details on dietary composition and variation and to avoid the previous biases associated with stomach content analysis alone (Hooker et al 2001, Herman et al 2005, Nordstrom et al 2008. FAs are the main constituent of most lipids, and, unlike other components such as proteins that are readily broken down during digestion, FAs are released from ingested lipid molecules (e.g.…”

Fatty acid profiles in the gonads of the sea urchin Strongylocentrotus droebachiensis on natural algal diets

“…the fatty acid signature; Iverson 1993) in fat stores indicates specific prey consumption (Kirsch et al 1998, Iverson et al 2004, Nordstrom et al 2008. However, the same taxonomic resolution may not be attainable with certain invertebrate herbivores, which are capable of greatly modifying or biosynthesizing FAs (Dalsgaard et al 2003, Iverson 2009).…”