Sexual segregation and stable pregnancy rates in the Gulf of St Lawrence’s minke whales (Balaenoptera acutorostrata) amidst environmental changes

Evaluating the health of baleen whale populations is crucial for understanding how environmental changes impact these top predators. Methodological advances, particularly in endocrine profiling, have enabled us to measure reproductive rates of populations as a proxy for population health. The Gulf of St. Lawrence (GSL), Canada, is an important summer feeding ground for various North Atlantic baleen whale species and has undergone major ecosystem changes in recent decades. To explore the potential impacts on population health of minke whales Balaenoptera acutorostrata, we combined genetic analyses, endocrine profiling, and environmental data on prey availability to investigate population demographics and possible drivers of pregnancy rates between 2007 and 2015. Biopsy samples collected between May and October were sexed (n = 187) using PCR, revealing a strong female bias (88.2%). Pregnancy status was determined through blubber progesterone quantification, with progesterone concentrations of 0.061-8.04 ng g-1 for non-pregnant individuals and 10.02-359.73 ng g-1 for pregnant individuals. High annual pregnancy rates were observed, ranging from 60 to 89% (mean: 74 ± 10%), with no consistent trend detected over the study period. Generalised linear model results suggested species-specific prey availability in the year prior to pregnancy did not explain annual variation in pregnancy rates. We posit that this is due to the generalist feeding behaviour of minke whales. The results presented here indicate minke whales in the GSL exhibit sex-specific and reproductive spatial segregation. These pregnant females are likely using the area as a feeding ground prior to giving birth, with sufficient behavioural plasticity to withstand fluctuating food availability.