Foraging behaviour of southern right whales (Eubalaena australis) inferred from variation of carbon stable isotope ratios in their baleen

ROWNTREE, VICTORIA; VALENZUELA, LUCIANO O; FRANCO FRAGUAS, PAULA; SEGER, JON

Encuentro; International Whaling Commission Scientific Committeee; 2008

The isotopic composition of a whale s diet is recorded in its tissues including its baleen. The longest baleen plates of an adult right whale contain a 6- to 7-year record of the isotopic composition of its prey (copepods and krill). The stable carbon isotope ratios of prey change with water temperature (latitude), depth, freshwater inputs and current systems, so the isotopic signals in baleen provide information about the annual foraging paths of individuals. Baleen from five adult southern right whales (Eubalaena australis) that died and stranded on their nursery ground at Península Valdés, Argentina, were sampled at 2 cm intervals from the gum line to the tip of the baleen plate. The stable carbon isotope ratios (δ13C) in the baleen oscillate between high and low values on annual cycles. Higher values of δ13C appear to indicate feeding on warmer-water prey (Patagonian Shelf, 18 to 23 ) while lower values indicate colder-water prey (South Georgia, 24 to 29 ). Each annual cycle was analyzed to determine the distances (in centimetres of baleen growth) between peaks, the highest and lowest values of δ13C, the rates of change in δ13C between peaks and troughs, and the δ13C values at plateaus where signals remained approximately constant. Baleen grew an average of 31cm/yr with large interannual variation (sd 5.8cm) within individuals but no significant differences between individuals. Baleen growth in a given year was positively correlated with the lowest (most southerly) δ13C value in that year. Individuals differed significantly in their mean isotopic ratios and in the ranges of their annual oscillations (latitudinal foraging ranges). Two whales had narrow ranges (spanning 3.0 and 2.7 δ13C), but one fed in warm waters at higher latitudes (mean δ13C = 18.2 ) while the other fed in cool waters at lower latitudes (mean δ13C = 23.8 ). The other three whales had significantly broader ranges (spanning 5.1 to 6.8 δ13C). δ13C values change more rapidly in spring (~0.3 /wk) than in fall (~0.2 /wk). We conclude that carbon isotope ratios in baleen record individual differences in foraging and distribution, and that they also record each individual s yearly responses to changing environmental conditions, possibly including fluctuations in the abundances of copepods and krill.