Abstract
Antarctic minke whales (Balaenoptera bonaerensis) are migratory capital breeders that experience intensive summer feeding on Antarctic krill (Euphausia superba) in the Southern Ocean and winter breeding at lower latitudes, but their prey outside of the Antarctic is unknown. Stable isotope analyses were conducted on δ13C and δ15N from the baleen plates of ten pregnant Antarctic minke whales to understand the growth rate of the baleen plate and their diet in lower latitudes. Two to three oscillations along the length of the edge of the baleen plate were observed in δ15N, and the annual growth rate was estimated to be 75.2 ± 20.4 mm, with a small amplitude (0.97 ± 0.21 ‰). Bayesian stable isotope mixing models were used to understand the dominant prey that contributed to the isotopic component of the baleen plate using Antarctic krill from the stomach contents and reported values of Antarctic coastal krill (Euphausia crystallorophias), Antarctic silver fish (Pleuragramma antarcticum), Australian krill spp., and Australian pelagic fish spp.. The models showed that the diet composition of the most recent three records from the base of the baleen plates (model 1) and the highest δ15N values in each baleen plate (model 2) were predominantly Antarctic krill, with a contribution rate of approximately 80%. The rates were approximately 10% for Antarctic coastal krill and less than 2.0% for the two Australian prey groups in both models. These results suggest that pregnant Antarctic minke whales did not feed on enough prey outside of the Antarctic to change the stable isotope values in their baleen plates.
Highlights
The Antarctic minke whale (Balaenoptera bonaerensis) is a small rorqual that migrates to its feeding ground in the Southern Ocean during the summer
The average stable isotope values at the base of baleen plates (0 mm) were higher than those of Antarctic krill and Antarctic coastal krill, but lower than those of other prey types (Fig. 1). This indicates that the synthesis of the baleen plates in this study can be made by either Antarctic krill and/or Antarctic coastal krill
The Bayesian stable isotope mixing model showed that Antarctic krill had the highest contribution rate of more than 80%, with the second highest rates for Antarctic coastal krill in all the three recent sample points (Model 1: 0, 10, 20 mm) (Table 1)
Summary
The Antarctic minke whale (Balaenoptera bonaerensis) is a small rorqual that migrates to its feeding ground in the Southern Ocean during the summer. According to an ear plug analysis, the gestation period in Antarctic minke whales is approximately 10 months (Ohsumi and Masaki 1975; Lockyer 1984; Kato 1990), and Best (1982) estimated a yearly reproductive rate of 78.4% Both the numbers of animals recruited from the young generation to the reproductive population and those leaving the reproductive population due to either death or over aging are unknown or limited. The inter-reproductive cycle of this species is not yet understood comprehensively, which means that the number of pregnant individuals migrating lower latitudes every year remains unclear, though pregnant females do need to migrate to breeding areas. This is likely important for newborns without adequate adipose tissue to survive in the cold Antarctic environment. Knowledge of Antarctic minke whale feeding behavior outside of the Southern Ocean is limited, as most research on this species has been conducted in the Southern Ocean during summer
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