Stable carbon isotope ratios for the gray whale (Eschrichtius robustus) in the breeding grounds of Baja California Sur, Mexico

Abstract

This work investigated the feeding ecology and behaviour of gray whales in Bahı´a Magdalena. Underwater observations of bottom feeding were made (n ¼ 4). Skin biopsies of the gray whale had a carbon isotope value of )16.5 ± 0.1& (range from )16.4 to )16.7, n ¼ 7). Prey in Bahı´a Magdalena had a carbon isotope value of )18.4&. Dietary enrichment from prey in Bahı´a Magdalena would correspond to 2 ± 0.1&, whereas previously published results for prey in Alaska would result in an enrichment of 3&, which suggests that whales were more likely feeding on prey from Bahı´a Magdalena. Carbon isotopic oscillation along the baleen plate of a stranded 1-year-old whale showed a variation in diet during the year, which suggests continual feeding during this time and corresponding to dietary sample measurements from Bahı´a Magdalena in winter and Alaska in summer.

Feeding ecology of gray whales inferred from stable-carbon and nitrogen isotopic analysis of baleen plates

“…Stable-carbon isotope values of the diet calculated for point C (i.e. close to the death date) showed that most of the whales had values close to -18.4&, which matches that of the amphipod prey for the gray whale (mostly Ericthonius brasiliensis) in Bahía Magdalena (Caraveo-Patiñ o & Soto, 2005). Quite possibly then, most of these whales fed on prey from areas close to the wintering lagoon prior to their death.…”

“…From the plates collected in Bahía Magdalena, GW51 was extracted from an individual probably death during January of 2000, with a body length of~8 m; GW31 and GW32 were extracted from adult gray whales, but their body lengths and their probable death date were unknown; GW00 was the same plate of a yearling animal previously analyzed for d 13 C only by Caraveo-Patiñ o & Soto (2005), which was collected during the wintering season in 2000, its probable death date was January also. The other two plates (GW11and GW12) found in Laguna Ojo de Liebre, these were collected from adult whales (>9.5 m) during the wintering season in 2002, their probably death data was between March and April.…”

“…Plate GW00 was sampled at~1 cm intervals (n = 17), in order to investigate if the d 15 N data show a same trend detected by Caraveo-Patiñ o & Soto (2005) with d 13 C values. The baleen plate GW51 was analyzed at~0.6 cm intervals (n = 30) in order to investigate if a larger data set along the baleen plate of a young whale modified the isotopic pattern detected previously by Caraveo-…”

“…We calculated d 13 C values of dietary sources corresponding to the period of formation of baleen at various points using the same trophic enrichment factor (D dt = 2&) established previously by Caraveo-Patiñ o & Soto (2005) for gray whales.…”

“…Recently, Caraveo-Patiñ o & Soto (2005) applied stable-carbon isotope techniques to investigate if the baleen of a yearling gray whale, provided evidence of feeding on the wintering grounds. They found evidence for feeding based on the fact that the difference in baleen d 13 C values between summer and winter periods corresponded to reported differences between amphipod prey on the breeding and wintering grounds.…”