Blue whale survival and abundance in the Gulf of St. Lawrence - no encouraging signs for the population
Miranda Unger, Christian Ramp, Richard Sears, Per J. Palsbøll, Philip Hammond
The Northwest Atlantic blue whale (Balaenoptera musculus) population is endangered and low recruitment, declining numbers of sightings and ongoing anthropogenic pressures, including entanglements and ship strikes, have raised doubts about the recovery of this depleted population. Improved knowledge of survival rates and abundance is critical to assist assessment efforts and conservation planning. Blue whales in the Gulf of St. Lawrence have been studied since 1979 and previous analyses of data through 2002 found little evidence of sex-specific survival. Here, we analyse 12 additional years of sighting histories of 185 photo-identified and biopsied blue whales from 1990 to 2014. Applying Cormack-Jolly-Seber mark recapture models, a sex-specific survival model performed best. Model averaging resulted in apparent survival estimates of 0.966 (95% CI=0.944-0.970) for males (n=99) and 0.941 (95% CI=0.917-0.959) for females (n=86), but the effect size was not significant (0.0244, 95% CI= -0.0031-0.0519). Results of the previous analysis found little difference between the sexes and an overall survival probability of 0.975 (95% CI=0.960-0.985); the recent lower survival estimates are a concern, especially for females. A 7-year period with high and very consistent effort in one specific area (St. Lawrence Estuary) in which 127 blue whales were identified was used to estimate abundance. The open POPAN model estimated a super population of 151 animals (95% CI=138-165) but is biased low because of heterogeneity in capture probabilities and we regard this as a lower bound. The closed Pledger mixture model that models capture heterogeneity estimated abundance at 190 individuals (95% CI=155-271). To what extent the results from the St. Lawrence are representative of the Northwest Atlantic population remains unknown. But with an apparently declining survival, low recruitment and frequent natural mortalities (ice-entrapment) the future does not bode well for the recovery of the population.