Feeding habits of a new Arctic predator: insight from full-depth blubber fatty acid signatures of Greenland, Faroe Islands, Denmark, and managed-care killer whales Orcinus orca

Abstract

Killer whales Orcinus orca (center) that are undergoing a northward range shift are feeding on marine mammals in Greenlandic waters.Photo: Richard Hebhardt data suggest that superficial blubber sampling through biopsy darting may not capture fine-scale and/or short-term variation in diet, and therefore the sampling approach should be carefully considered in research using fatty acids to evaluate feeding ecology of killer whales and other cetaceans.

Factors affecting the degree of vertical stratification of fatty acids in grey seal blubber

“…The increasing use of chemical tissue markers as a proxy of the diet of aquatic top predators (Budge et al 2006;Iverson et al 2007;Haug et al 2017) and the increasing awareness of the uneven vertical stratification of these markers in the adipose tissue of the animals necessitates special attention to be given to the technique of tissue sampling for dietary studies (Strandberg et al 2008(Strandberg et al , 2011Guerrero et al 2016, Bourque et al 2018. Assuming that the composition of the outer blubber layer is endogenously controlled in pinnipeds, and that the inner and middle blubber layers are metabolically more active (Thiemann et al 2004;Strandberg et al 2008Strandberg et al , 2011Liwanag et al 2012;Guerrero et al 2016), it is valid to question whether the outer blubber layer should be included in the FA analyses of the tissue.…”

“…Studying blubber fatty acid (FA) composition is a frequently used method to monitor the diet of marine mammals (e.g. Bradshaw et al 2003;Walton and Pomeroy 2003;Budge et al 2006;Guerrero et al 2016;Meier et al 2016, Bourque et al 2018Tverin et al 2019). The tissue biochemistry of a top predator is a proxy of its diet, and at the same time reflects the structure and biochemistry of the whole food web that can be influenced by global climate change, local human impact and natural environmental changes (Iverson et al 2007;Engelhard et al 2014;Lind et al 2017).…”

“…Outer blubber usually contains higher relative concentrations of 14-18 carbon monounsaturated FAs (MUFAs) and the inner layers are enriched with saturated FAs (SFAs), 20-22 carbon MUFAs and polyunsaturated FAs (PUFAs) (e.g. Käkelä and Hyvärinen 1996;Strandberg et al 2008;Waugh et al 2014;Guerrero et al 2016;Meier et al 2016;Guerrero and Rogers 2017;Bourque et al 2018). As can be learned from poikilotherms, sufficiently low melting points for animal tissue in cold water can be obtained either by enriching the tissue lipids with PUFAs or MUFAs (Radnaeva et al 2017).…”

“…the middle and inner blubber layers, are thermoneutral (Irving and Hart 1957), and thus the middle blubber integrates dietary FAs without restrictions due to FA melting points or variations in tissue temperature. Thereby, the middle blubber has a better capacity than the outer blubber to incorporate all types of dietary FAs, and the innermost layer closest to muscle is likely the metabolic interphase where recent dietary deposition or mobilization of all the dietary FA types occurs most actively (Budge et al 2004;Strandberg et al 2008;Guerrero et al 2016;Bourque et al 2018).…”

“…Since published research of a variety of seals and whales consistently regards dietary influence on inner blubber FA composition to be strong (e.g. Koopman et al 1996;Skoglund et al 2010, Strandberg et al 2011Guerrero and Rogers 2017;Bourque et al 2018;Tverin et al 2019), the inner blubber FAs were used as a possible explanatory factor for the outer blubber becoming distinct from the underlying thermoneutral blubber. The implicit prerequisite for such vertical FA stratification would be that the outer layer, with its endogenously high degree of FA ∆9-desaturation, does not reflect diet in the same degree as the dietary FA-receiving inner blubber, and consequently the vertical differences in ∆9-desaturation were also considered as a stratification-inducing factor.…”