STABLE ISOTOPES IN SOUTHERN RIGHT WHALE (EUBALAENA-AUSTRALIS) BALEEN AS INDICATORS OF SEASONAL MOVEMENTS, FEEDING AND GROWTH

Ratios of stable carbon and nitrogen isotopes were examined in the bal een of 11 southern right whales originating from South Africa, includi ng one neonate, six juveniles and four adults. Oscillations in carbon isotope ratios were marked, and indicated feeding north of or at the S ubtropical Convergence (STC) alternating with feeding south of the STC . There was an inverse relationship in juveniles between the periodici ty of the oscillations and the length of the baleen plate, indicating a reduction in baleen growth with age. The size of the periodicity pre dicted for the smallest juvenile plate was equivalent to the length of the baleen at 1 yr of age as estimated from the rate of baleen growth in calves, suggesting that the oscillations in carbon isotope ratios were annual events. On this assumption: the timing of the formation of the most recent carbon enrichment peak could be calculated for each i ndividual, given the date of death and the rate of baleen growth in th e preceding year; formation occurred over a period of 100 d from Janua ry to April (mean in February). A similar analysis indicated that vall eys in the nitrogen isotope ratios were formed between January and Jun e (mean in April), and enrichment peaks between August and May (mean i n December). These patterns were not inconsistent with previous scenar ios of southern right whale migration, if the enrichment peaks in carb on isotopes were taken to represent feeding just north of the STC, the subsequent decline in enrichment levels to represent feeding south of the STC in autumn, and the persistence of an isotopic signature chara cteristic of high latitude plankton throughout the winter and early sp ring to indicate that feeding essentially ceased when the northern mig ration began, and did not resume until the southern migration was unde r way. The oscillations in nitrogen isotope ratios would support this interpretation, if they were assumed to represent cycles of starvation and recovery. A comparison with baleen growth rates for bowhead whale s allowed the ages of the six juveniles to be assessed. Their size at age, when compared to the lengths and growth rates of calves measured photogrammetrically, suggested that growth in body length of southern right whales slows markedly between weaning and 1 yr, and may be almos t negligible from 1 to 4 yr of age.