Foraging behaviour of the South American sea lion (Otaria byronia) in two disparate ecosystems assessed through blubber fatty acid analysis

Abstract

Fatty acids have been widely used as trophic biomarkers in marine mammals. However, for the South American sea lion, the most abundant otariid in the eastern South Pacific, there is no information about blubber fatty acids and their link to diet. Here, we compare fatty acid profiles of sea lions from two distinct oceanographic regions in northern and southern chile. their fatty acids vary greatly between regions, suggesting dietary differences at a spatial scale. The fatty acid C22:6ω3 was more abundant in sea lions from the northern region, likely associated with consumption of anchovy, cephalopods, and crustaceans, which are rich in that fatty acid, and have been reported as their main prey items. Sea lions from the southern region were richer in C22:1 and C20:1, characteristic of teleost fish, suggesting a piscivorous diet. Males displayed a more diverse fatty acid composition than females, suggesting a wider trophic niche. few individual sea lions within the southern region had unusually high levels of C18:2ω6, commonly found in terrestrial environments. This suggests consumption of farmed salmon, whose diet is usually based on terrestrial sources. this demonstrates how human intervention is being reflected in the tissues of a top predator in a natural environment. Deciphering trophic interactions of top predators is important to understand the structure and functioning of whole ecosystems. The South American sea lion (Otaria byronia) is a top predator with a wide distribution range along the eastern Pacific as well as the western Atlantic coasts of South America 1,2. They are an abundant species, with population estimates of up to 250,000 individuals, and some populations increasing steadily 1,3. South American sea lions are considered to be opportunistic predators, therefore, their diet is expected to vary in space and time depending on food availability 4. In fact, decreases in their population in some areas have been linked to declines in the availability of certain prey species 5. Unsurprisingly Muñoz et al. 4 reported a great variation in diet composition between northern and southern populations of South American sea lions in Chile. Similarly, Sepúlveda et al. 6 demonstrated temporal differences in their diet composition in a population from southern Chile. Most of our knowledge on the trophic ecology of South American sea lions relies on traditional methods such as stomach and scat content analyses 7,8 , or on observations of feeding events, which are relatively rare and limited in aquatic environments 9-11. Through these methods, a broad-spectrum diet has been described 12,13. These methods provide valuable information about recently consumed diets but are not ideal to determine long-term dietary trends, since this would require of repeated sampling throughout long time periods 14,15. Furthermore, identifying inter-individual variation in diet preferences is very challenging due to, for example, the difficulty of assigning scats to particular individuals in crowded rookeries 16 .

Complementary use of stable isotopes and fatty acids for quantitative diet estimation of sympatric predators, the Antarctic pack-ice seals

“…Outer layer FAs, for example, maintain their composition during lactation in Weddell seals (Wheatley et al 2007 ), and northern elephant seals (Fowler et al 2014 ). However, although the proportion of FAs may differ between layers, the overall FA pattern is similar (Hooker et al 2001 ); thus, the outer layer is thought to be an indicator of longer-term diet (Moller et al 2003 ; Budge et al 2006 ; Guerrero et al 2020 ) compared to the inner layer that reflects from hours to months (Kirsch et al 2000 ; Budge et al 2004 , 2006 ). Here, we used only the outer layer for FA analysis since it can be obtained remotely, hence, requires comparatively less effort and lower economic costs than the collection of whole blubber samples.…”

“…Biochemical methods, such as fatty acid (FA) and stable isotope (SI) analyses, can reconstruct diets by overcoming the caveats of the traditional techniques. These two biochemical methods are based on the principle that an animal’s diet is reflected in the patterns of FAs and SIs of their tissues (Hooker et al 2001 ; Guerrero et al 2020 ). Many FAs, particularly polyunsaturated FAs, can only be synthesized at low trophic levels and are transferred to higher trophic levels with minimal modification (Iverson 1993 ) and thus, can be used to distinguish dietary preferences (Dalsgaard et al 2003 ; Guerrero et al 2020 ).…”

“…These two biochemical methods are based on the principle that an animal’s diet is reflected in the patterns of FAs and SIs of their tissues (Hooker et al 2001 ; Guerrero et al 2020 ). Many FAs, particularly polyunsaturated FAs, can only be synthesized at low trophic levels and are transferred to higher trophic levels with minimal modification (Iverson 1993 ) and thus, can be used to distinguish dietary preferences (Dalsgaard et al 2003 ; Guerrero et al 2020 ). Similarly, the isotopic composition of an animal’s tissues is correlated with those of its prey items and change in a predictable way between trophic levels (Gannes et al 1997 ; Hückstädt et al 2012 ).…”

Evaluating the performance of the Bayesian mixing tool MixSIAR with fatty acid data for quantitative estimation of diet

“…The transverse variation in FAs across the blubber has been attributed to the different roles that the outer and inner blubber layers play [55][56][57] . The FAs of the inner blubber layer have an active turnover rate as they are more directly influenced by the consumer's diet 56,58,59 while the FAs in the outer blubber play an active role in thermoregulation 55 , they are more stable and less affected by short-term dietary changes 55,60,61 . For example, Struntz et al 61 observed that the outer layer of bottlenose dolphins, Tursiops truncatus, remained unchanged after a period of undernourishment.…”

Blubber fatty acid compositions in different geographic populations of finless porpoise in Chinese waters: implications for thermal adaptation

“…Previous studies have shown that there are geographic differences in blubber FA composition of various marine mammal species, for example, bearded seals [Erignathus barbatus (Erxleben, 1777)], beluga whales [Delphinapterus leucas (Pallas, 1776)], ringed seals [Pusa hispida (Schreber, 1775)], walruses [Odobenus rosmarus (Linnaeus, 1758)], harbor seals (Phoca vitulina Linnaeus, 1758) and sea lions [Otaria byronia (de Blainville, 1820)], particularly in the MUFA and PUFA compositions and distributions within their blubbers (Thiemann et al 2008;Guerrero et al 2020). Most studies attributed the geographic differences in blubber FA composition to the diet, but a strong impact from FA metabolism was also found among common minke whales (Balaenoptera acutorostrata Lacépède, 1804) from different regions (Meier et al 2016).…”