A multistate capture-recapture model to estimate reproduction of North Atlantic right whales

Abstract

The recent steep decline of critically endangered North Atlantic right whales (Eubalaena glacialis) can be attributed to high mortality combined with low reproduction. While the former is a clear result of anthro-pogenic activity, the latter involves more complexity. Evidence suggests that both short-term fluctuations in prey availability and long-term decline in health are responsible for depressed right whale calving rates. To facilitate an assessment of extinction risk, we developed a multistate capture-recapture model that estimated the probability of calving using extensive sightings data from 1990-2019. The model estimated sub-lethal effects of severe injury on calving probability, and modeled temporal variability in calving as related to indices of prey availability (Calanus spp. biomass) and an apparent regime shift. The average annual probability of calving for known-breeding females, given average prey conditions, decreased from 0.217 [0.162, 0.281] to 0.142 [0.067, 0.252] after the 2010 regime shift. The model indicated strong evidence of a relationship between calving probability and the prey index from the eastern Gulf of Maine, though this relationship effectively disappeared after 2010; moderate evidence for a relationship with prey from the southwest Gulf of St. Lawrence remained. Weak evidence of reduced calving probability due to severe injury resulted from low sample sizes, given increased mortality for individuals observed with severe injuries. The regime effect is hypothesized to be capturing a long-term decline in health due to a combination of decreasing habitat quality resulting from climate change and potentially chronic sublethal injuries (e.g., entanglements). Our reproduction model provides demographic parameter estimates that can be used in population projections for North Atlantic right whales, though uncertainty remains in the mechanisms responsible for recent declines in calving.