Stable isotope assessment of a mass stranding of short-beaked common dolphins (Delphinus delphis delphis) reveals their provenance: Integrating knowledge of a little-known odontocete in the Gulf of California

The short-beaked common dolphin (Delphinus delphis delphis) (SBCD) is an odontocete inhabiting the Gulf of California (GC), Mexico; distributed widely in temperate to tropical waters, it is sometimes observed close to shore or in shallower areas. On 13 February 2018, a mass stranding of 54 SBCDs occurred in La Paz Bay in the southern GC; 21 individuals died, but the rest were released. Skin samples were removed from dead individuals for stable isotope analysis in order to infer their probable geographical provenance. Their mean ± S.D. were 17.0 ± 0.3‰ (δ¹⁵N) and -18.3 ± 0.2 (δ¹³C), with no significant differences between sexes or length. These isotope ratios do not correspond to the GC. Their low δ¹³C and δ¹⁵N values reflect offshore foraging grounds not under the ¹⁵N-enriching effects of the intense denitrification in the GC. Rather, these values were within the range of other piscivorous odontocetes that have been sampled in the GC and classified as outsiders (Pacific Ocean). Mean total length of these stranded SBCDs was similar to that reported for SBCDs off the Pacific coast of Baja California and southern California, but widely smaller than that reported for both eastern tropical Pacific SBCDs and long-beaked common dolphins (Delphinus delphis bairdii) in the northern and southern GC. These differences in isotopic niche and size between SBCDs stranded in La Paz Bay and long-beaked common dolphins from other regions of the GC highlight the need to be able to distinguish between the common dolphins (Delphinus) that inhabit the GC from those that visit the GC from off the Pacific coast of Baja California. Our findings confirm the ecological importance of the southern GC, where resident and visitor marine mammals congregate, leading to events like mass strandings that provide a unique source of information useful for research and conservation.