Fin whales on the decrease in the Gulf of St. Lawrence: A mark-recapture study from 1990 to 2016


Anna Schleimer, Julien Delarue, Christian Ramp, Martine Berube, Richard Sears, Per J. Palsbøll, Philip Hammond



Following the exploitation of North Atlantic stocks by commercial whaling in the 20thcentury, fin whales (Balaenoptera physalus) received protection through national and international legislations, aiming to promote the recovery of depleted stocks. However, owing to the ongoing uncertainty about population structure and movement patterns, monitoring through large-scale surveys could fail to detect regional demographic changes. Previous work found indications of a decline in apparent survival rates from 2007 to 2010 in fin whales in the Gulf of St Lawrence, which have been hypothesised to have limited exchange with adjacent populations. Here, we apply mark-recapture models to extended photo-identification data from 1990 to 2016, collected in the northern Gulf of St Lawrence from June to October, to investigate recent evidence for declines in apparent survival and abundance. The data comprised 5000 identifications of 490 individuals. Based on QAIC model selection, the most parsimonious Cormack-Jolly-Seber model included a linear temporal trend in non-calf apparent survival rates, declining from 0.977 (95% CI: 0.962-0.986) in 1990 to 0.903 (95% CI: 0.850-0.938) in 2016. The available super-population from 2010 to 2016 was estimated at 305 individuals (95% CI: 281-329) by POPAN models, with a decreasing temporal trend in recruitment (“probability of entry”). These results, together with the previous abundance estimate of 328 (95% CI: 306 to 350) for the 2004 to 2010 period, provide compelling evidence for a negative trend in fin whale apparent survival and abundance in the Gulf of St Lawrence. The underlying drivers for these trends are likely a combination of higher rates of mortality and permanent emigration. Anthropogenic noise, changes in prey availability, ship strikes, toxic pollutants, and entanglement have been identified as potential threats and their cumulative impacts on fin whales in the Gulf of St Lawrence should be investigated in light of these new results.