An integrated approach to historical population assessment of the great whales: case of the New Zealand southern right whale

Abstract

Accurate estimation of historical abundance provides an essential baseline for judging the recovery of the great whales. This is particularly challenging for whales hunted prior to twentieth century modern whaling, as population-level catch records are often incomplete. Assessments of whale recovery using pre-modern exploitation indices are therefore rare, despite the intensive, global nature of nineteenth century whaling. Right whales (Eubalaena spp.) were particularly exploited: slow swimmers with strong fidelity to sheltered calving bays, the species made predictable and easy targets. Here, we present the first integrated population-level assessment of the whaling impact and pre-exploitation abundance of a right whale, the New Zealand southern right whale (E. australis). In this assessment, we use a Bayesian population dynamics model integrating multiple data sources: nineteenth century catches, genetic constraints on bottleneck size and individual sightings histories informing abundance and trend. Different catch allocation scenarios are explored to account for uncertainty in the population's offshore distribution. From a pre-exploitation abundance of 28 800–47 100 whales, nineteenth century hunting reduced the population to approximately 30–40 mature females between 1914 and 1926. Today, it stands at less than 12% of pre-exploitation abundance. Despite the challenges of reconstructing historical catches and population boundaries, conservation efforts of historically exploited species benefit from targets for ecological restoration.

Demography and ecology of southern right whales Eubalaena australis wintering at sub-Antarctic Campbell Island, New Zealand

“…It is estimated that prior to whaling there were 27,000 (95% CL 22,000, 38,000) southern right 27 whales Eubalaena australis in New Zealand waters (Jackson et al 2011). This large, long-lived 28 species aggregates during winter in coastal waters to breed and calve and then migrates to 29 offshore foraging grounds during summer months (Richards 2002).…”

“…A key wintering ground was 30 once mainland New Zealand (North and South Islands), where sheltered inshore waters were 31 used for calving in winter (Dawbin 1986). However, between 1830 and 1970 up to 40,000 E. 32 australis were killed in New Zealand waters (Carroll et al 2014a) and the species was 33 commercially extinct by 1851 (Jackson et al 2011;Richards 2002). …”

Managing direct and indirect threats to marine ecosystems to balance multiple objectives

“…Finally, other southern right whale preexploitation abundance estimates for New Zealand waters (28,000 -47,000, Jackson et al 2016) are higher compared to our best-fit model estimates for the Pacific (total 24,848 including males).…”

“…Geneva Convention for the Regulation of Whaling prohibited the taking or killing of right whales by all nations) (Jackson et al 2016). These illegal harvests from already depleted populations reduced southern right numbers in the Pacific to an estimated <130 mature females (Fig.…”

“…We then estimated an additional scaling parameter ϑ A for the carrying capacity of right whales, which accounts for right whale already having been depleted to low numbers prior to the start of our model (1890), and hence assumes the starting numbers are the carrying capacity multiplied by ϑ A (Appendix Table A5.5). Model convergence (in terms of obtaining a reliable fit of the model to the data) for right whales was also difficult to achieve initially when estimating the carrying capacity multiplier ϑ A due to large variation in estimates of pre-exploitation numbers (between 50,000 and 100,000) and thus large variation in our estimates of depletion (IWC 2001;Jackson et al 2016), and limited understanding of their spatial distribution between areas. Because of this, we bounded the 121 parameter ϑ A (0.2 -0.5) and estimated it during the last phase of the estimation process.…”