How compromised is reproductive performance in the endangered North Atlantic right whale? A proposed method for quantification and monitoring

The endangered North Atlantic right whale (Eubalaena glacialis) showed limited recovery from the cessation of industrial whaling until 2011, and has since been in decline. Research is therefore focused on identifying what factors are limiting recovery and what conservation actions will be most effective. A compromised reproductive rate is one of the primary reasons for this lack of recovery, yet there is no consensus on how to quantify reproductive performance. As one potential solution, we propose a relatively simple approach where we calculate the theoretical maximum number of calves each year. Comparing this expected number to those observed provides a means to quantify the degree to which reproduction is being compromised and trends thereof over time. Implementing this approach shows that, between 1990 and 2017, the number of calves born never came close to the theoretical maximum, resulting in overall reproductive performance being only about 27% of that expected. In addition to quantifying the magnitude of the reproductive problem, this approach should also be useful for quantifying the role of reduced reproductive performance in limiting species recovery, and for aiding research programs focused on identifying what factors are compromising reproduction.