Declining reproductive success of humpback and fin whales in the Gulf of St. Lawrence from 2005 to 2016

Rachel Sullivan-Lord, Christian Ramp, Julien Delarue, Richard Sears, Martine Berube, Per J. Palsbøll

Abstract:

Female humpback and fin whales show strong site fidelity to North Atlantic feeding grounds, returning with their calves each summer. We have conducted long-term photo identification and biopsy sampling for species in the Gulf of St. Lawrence (GSL), Canada since 1980. Since 2010 we have noticed a sharp decline in observed calves and are quantifying here the reproductive success for these sympatric species for 2005-2016. Females were considered reproductively active for a given year if they had been sighted for more than 10 years prior to, or had previously been sighted with, a calf (78 humpback whales). Average birth rate for humpback whales for 2005-2010 was 0.38, compared to 0.16 for 2011-2016. We calculated annual crude birth rate to compare humpbacks (n=373 total, 86 calves) to fin whales (n=337 total, 76 calves), for which individual sighting histories too short to identify reproductively active females. For 2005-2010 the crude birth rate was 0.12 and 0.09 for humpback and fin whales respectively, and for 2011-2016 was 0.05 and 0.01 respectively. The average calving interval for humpbacks for 2005-2010 was 2.62 years, and 4.66 years for 2011-2015. With so few fin whale calves after 2010, calving intervals could not be estimated. Since re-sighting rates of reproductively-active female humpbacks in this feeding aggregation are high, the decrease in birth rate and crude birth rate over the last decade indicates either increased post-natal mortality or lower pregnancy rates. We suggest prey abundance and/or quality could be a major influence on reproductive success, as shown for other baleen whales in the North Atlantic. The contribution of GSL prey to humpback and fin whale annual nutrition remains to be quantified. Our results demonstrate the value of long-term population monitoring and call for an assessment of forage fish abundance and quality in the GSL.