Quantifying uncertainty in anthropogenic causes of injury and mortality for an endangered baleen whale

Understanding the causes of mortality for a declining species is essential
for developing effective conservation and management strategies,
particularly when anthropogenic activities are the primary threat. Using a
competing hazards framework allows for robust estimation of the
cause-specific variation that may exist across multiple dimensions, such
as time and individual. Here, we estimated cause-specific rates of severe
injury and mortality for North Atlantic right whales (Eubalaena
glacialis), a critically endangered species that is currently in peril due
to human-caused interactions. We developed a multistate capture-recapture
model that leveraged 30 years of intensive survey effort yielding
sightings of individuals with injury assessments and necropsies of carcass
recoveries. We examined variation in the hazard rates of severe injury and
mortality due to entanglements in fishing gear and vessel strikes as
explained by year and the age and reproductive status of the individual.
We found strong evidence for increased rates of severe entanglement
injuries after 2013 and for females with calves, with consequently higher
marginal mortality. The model results also suggested that despite vessel
strikes causing a lower average rate of severe injuries, the higher
mortality rate conditional on injury results in significant total
mortality risk, particularly for females resting from a recent calving
event. Large uncertainty in the estimation of carcass recovery rate for
vessel strike deaths permeated into the apportionment of mortality causes.
The increased rates of North Atlantic right whale mortality in the last
decade, particularly for reproducing females, puts the species at risk of
severe decline. By apportioning the human-caused threats using a
quantitative approach with estimation of relevant uncertainty, this work
can guide development of conservation and management strategies to
facilitate species recovery.