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The Guadalupe fur seal (Arctocephalus philippii townsendi or A. townsendi) was at the brink of extinction due to overhunting during the eighteenth and nineteenth centuries. Today, the Guadalupe fur seal remains classified as an Endangered species by Mexican law and as Threatened under the US Endangered Species Act of 1973. Currently, the entire population is estimated at around 40,000 individuals, with a single well-established breeding site on Guadalupe Island. Around 20 years ago, it was discovered that Guadalupe fur seals had recolonized San Benito Archipelago, which is 260 km southeast of Guadalupe Island. However, breeding activity at this site continues to be negligible with

Guadalupe fur seals (Arctocephalus townsendi) and California sea lions (Zalophus californianus) are the only two otariids that inhabit Mexican islands. California sea lions are widespread along the west coast of the Baja California Peninsula and within the Gulf of California, whereas Guadalupe fur seals are only found on Guadalupe Island and San Benito Archipelago, in the Pacific. At San Benito Archipelago, the two species are sympatric. By 2014, there were approximately 7000 California sea lions and 3700 Guadalupe fur seals in the archipelago. Stable isotope analysis from juveniles and adults of both species show that Guadalupe fur seals forage offshore with widely distributed foraging grounds, which we attribute to their squid dominated diet. California sea lions are more coastal and feed primarily on fish from higher trophic levels than the fur seals’ diet. There is also a latitudinal effect in the isotopic signature with a greater proportion of Guadalupe fur seals presumed to forage at higher latitudes with lower δ15N baseline values than California sea lions. Guadalupe fur seals were considered extinct 100 years ago, and the population is still recovering. Understanding its trophic ecology in relation to potential competitors, such as the California sea lion, is therefore of great importance especially in a region where there have been recent large-scale impacts from oceanographic warming anomalies.

Research using stable isotopes analysis (SIA) of carbon (δ13 C) and nitrogen (δ15 N) in blood components is lacking, because of the challenge of sample collection, processing, and storage in remote areas. There also is a paucity of information regarding the effect of tissue biochemical composition on isotopic ratios with few comparisons among taxa. We collected blood samples from shortfin mako sharks (n = 70; 2016) and Guadalupe fur seals (n = 25; 2017). All samples were centrifuged to obtain plasma from sharks and serum from the Guadalupe fur seals, and all the samples were prepared for SIA and analyzed using a Costech 4010 elemental analyzer interfaced with a Delta V Plus isotope ratio mass spectrometer. We found significant differences between plasma δ13 C values of shortfin mako sharks (-17.6 ± 0.9‰) and serum of Guadalupe fur seals (-20.3 ± 1.2‰), but we did not find any differences for δ15 N values between the two species. The differences in δ13 C values between species are probably due to the specific blood composition and to the different biochemical characteristics and different adaptations within taxa. These findings highlight the importance of further research on the influence of biochemistry features on isotopic results, in this way a more accurate assessment will be possible for this factor, separating it from the dietary influences on stable isotopic values.

Intra-population resource partitioning is a foraging strategy that could minimize intra-specific competition. This behavior may be ecologically relevant for species in recovery like the Guadalupe fur seal Arctocephalus philippii townsendi (GFS), which was considered extinct 70–80 years ago. The present study provides, via scat and stable isotope (δ15N and δ13C) analyses, trophic knowledge around GFSs from Guadalupe Island just prior to and the 2013 breeding season, with emphasis on inter-individual variability. A total of 107 scat samples were analyzed, and 98 neonate fur samples (proxies for adult female foraging) were isotopically assessed. The overall trophic spectrum included 12 items, with a slight increase in diversity among main prey during the second half of the breeding season. The isotopic analysis revealed three distinct groups, reflecting apparent variations in trophic position and coastal/oceanic habitat use. As in previous studies, a mostly teutophagous diet was clear; however, our work is the first to identify inter-individual prey and habitat partitioning. Based on our results and those of other studies, GFSs exhibited an opportunistic foraging strategy across the breeding season. Both analyses were complementary in terms of diversification and useful for the understanding GFS foraging strategies, which are relevant to evaluating population recovery as factors such as prey availability is suggested to be critical to this species recovery.