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Ocean warming will undoubtedly affect the migratory patterns of many marine species, but specific changes can be predicted only where behavioural mechanisms guiding migration are understood. Southern right whales show maternally inherited site fidelity to near-shore winter nursery grounds, but exactly where they feed in summer (collectively and individually) remains mysterious. They consume huge quantities of copepods and krill, and their reproductive rates respond to fluctuations in krill abundance linked to El Niño Southern Oscillation (ENSO). Here we show that genetic and isotopic signatures, analysed together, indicate maternally directed site fidelity to diverse summer feeding grounds for female right whales calving at Península Valdés, Argentina. Isotopic values from 131 skin samples span a broad range (–23.1 to –17.2‰δ13C, 6.0 to 13.8‰δ15N) and are more similar than expected among individuals sharing the same mitochondrial haplotype. This pattern indicates that calves learn summer feeding locations from their mothers, and that the timescale of culturally inherited site fidelity to feeding grounds is at least several generations. Such conservatism would be expected to limit the exploration of new feeding opportunities, and may explain why this population shows increased rates of reproductive failure in years following elevated sea-surface temperature anomalies off South Georgia, the richest known feeding ground for baleen whales in the South Atlantic. [ABSTRACT FROM AUTHOR]

Lactation is the most energetically expensive aspect of mammalian reproduction. As capital breeders, lactating southern right whales (Eubalaena australis) are completely dependent on their stored nutrients. The relative proportion of different endogenous nutrient pools used during lactation could be assessed using stable isotopes. We determined the stable carbon and nitrogen isotope difference between skin samples of 42 southern right whale mothers and their calves. The mean δ15N value of calves was 0.51‰ higher than that of their mothers, but their δ13C values were identical. However, when analyzed by year, the mother-calf pairs showed no isotope differences in 2004, but calves had higher δ15N (0.85‰) and δ13C (0.63‰) in 2003 and 2005. We hypothesize that the interannual variability was a consequence of different levels of nutritional stress. A decline in food abundance prior to the nursing seasons could result in mothers with relatively poorer physical condition that would not be able to meet the high energetic demands of their offspring. Thus, the calves would be forced to utilize proteins as well as lipids to meet this demand, increasing their nitrogen and carbon isotope ratios. This hypothesis is supported by an independent assessment of the proportion of stranded whales over the same time period. [ABSTRACT FROM AUTHOR]