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.
Highlights
Population assessments of the recovery of the great whales from exploitation have a long history [1], and over 50 years analysis methods have improved, along with the underlying population dynamics modelling framework used ([2] and references therein)
The offshore summer distribution of this population is largely unknown. Since both whaling and recovery levels will be impacted by the level of connectivity with neighbouring stocks and offshore mixing, this uncertainty has been encompassed through the development of two catch scenarios to evaluate recovery: (i) if New Zealand whales are only distributed in waters close to New Zealand and (ii) if New Zealand whales migrated further offshore, and could be hunted in waters off southeast Australia, termed the southwest Pacific catch series
Our Bayesian population dynamic model provides the first estimates of the pre-exploitation abundance (K) of New Zealand right whales
Summary
Population assessments of the recovery of the great whales from exploitation have a long history [1], and over 50 years analysis methods have improved, along with the underlying population dynamics modelling framework used ([2] and references therein). Population assessments place current abundance and trends in the context of past exploitation, measuring current recovery relative to pre-exploitation baselines, and providing a view on the state of ecological restoration and rate of recovery of a population. Such reviews are critically useful for conservation management decision-making regarding the protection of species and their habitats. Integrations include the use of current genetic diversity to put a constraint on the minimum number of whales surviving exploitation (i.e. minimum number of maternal lineages surviving the exploitation bottleneck [6]), the inclusion of catch record uncertainty [9] and the direct incorporation of mark recapture models into the population dynamic framework, to inform population abundance and trend [10]
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