Differences in foraging location of Mexican and California elephant seals: Evidence from stable isotopes in pups

Female northern elephant seals, Mirounga angustirostris, from Ano Nuevo (AN) in central California feed offshore in mid-latitude waters (40 degrees-55 degrees N). Migratory patterns and foraging locations of seals from Mexico are unknown. Rookeries on San Benitos (SB) islands in Baja California Sur, Mexico, are similar to 1,170 km south of AN. Although the colonies are similar in size, seals from SB begin breeding earlier and have an earlier breeding birthing peak than seals from AN. To determine if the foraging location of seals from Mexico was similar to that of seals from California, we measured delta(13)C and delta(15)N values in the hair of 48 suckling pups at SB and 37 from AN, assuming that their isotopic signatures reflected those of mothers' milk, their exclusive diet. The mean delta(13)C and delta(15)N values for SB pups (-16.1 parts per thousand +/- 0.9 parts per thousand and 17.7 parts per thousand +/- 0.9 parts per thousand, respectively) were significantly higher than those for AN pups (-17.6 parts per thousand +/- 0.4 parts per thousand and 15.6 parts per thousand +/- 1.0 parts per thousand, respectively). From data on environmental isotope gradients and known behavior of SB and AN populations, we hypothesize that the isotope differences are due to females in the SB colony foraging similar to 8 degrees south of seals from AN. This hypothesis can be tested by deployment of satellite tags on adult females from the SB colony.