A warming Southern Ocean may compromise Antarctic blue whale foetus growth

After declining in abundance due to commercial whaling during the 20 th Century, populations of the Antarctic blue whale ( Balaenoptera musculus intermedia ) have failed to recover to pre-exploitation levels. Using historical whaling data from 1926-1954, in combination with temperature data for the Southern Ocean, a gamma GLMM with temporal dependency was fitted to 20,144 records of B. m. intermedia foetus size using Bayesian inference. There was a negative relationship between antecedent winter sea surface temperature (SST) in the Southern Ocean on foetus size. This relationship is proposed as being mediated by a positive effect of the extent of winter sea ice on Antarctic krill ( Euphasia superba ) abundance on which B. m. intermedia feed. There was also a positive density-dependent effect of a ‘krill surplus’ at low whale population sizes. However, the positive effect of a ‘krill surplus’ at low B. m. intermedia population size on foetus growth was reversed at elevated winter SST due to a negative impact on E. superba recruitment. Projected increases in temperature in the Southern Ocean are predicted to compromise the growth rates of B. m. intermedia foetuses, with implications for the capacity of the subspecies to recover from overexploitation.