Abstract
North Atlantic right whales (Eubalaena glacialis Müller 1776) present an interesting problem for abundance and trend estimation in marine wildlife conservation. They are long lived, individually identifiable, highly mobile, and one of the rarest of cetaceans. Individuals are annually resighted at different rates, primarily due to varying stay durations among several principal habitats within a large geographic range. To date, characterizations of abundance have been produced that use simple accounting procedures with differing assumptions about mortality. To better characterize changing abundance of North Atlantic right whales between 1990 and 2015, we adapted a state–space formulation with Jolly‐Seber assumptions about population entry (birth and immigration) to individual resighting histories and fit it using empirical Bayes methodology. This hierarchical model included accommodation for the effect of the substantial individual capture heterogeneity. Estimates from this approach were only slightly higher than published accounting procedures, except for the most recent years (when recapture rates had declined substantially). North Atlantic right whales' abundance increased at about 2.8% per annum from median point estimates of 270 individuals in 1990 to 483 in 2010, and then declined to 2015, when the final estimate was 458 individuals (95% credible intervals 444–471). The probability that the population's trajectory post‐2010 was a decline was estimated at 99.99%. Of special concern was the finding that reduced survival rates of adult females relative to adult males have produced diverging abundance trends between sexes. Despite constraints in recent years, both biological (whales' distribution changing) and logistical (fewer resources available to collect individual photo‐identifications), it is still possible to detect this relatively recent, small change in the population's trajectory. This is thanks to the massive dataset of individual North Atlantic right whale identifications accrued over the past three decades. Photo‐identification data provide biological information that allows more informed inference on the status of this species.
Highlights
Measures of abundance are often deemed critical to development of wildlife conservation strategies, detecting trends in the abundance of populations of marine wildlife is a long-recognized problem (Gerrodette, 1987)
North Atlantic right whales are designated as an endangered species under U.S law, and most of the population spends a substantial amount of time in U.S waters
We developed a Bayesian implementation of an open population mark– resight/recapture (MRR) model to produce estimates of abundance and survival rates of North Atlantic right whales
Summary
Measures of abundance are often deemed critical to development of wildlife conservation strategies, detecting trends in the abundance of populations of marine wildlife is a long-recognized problem (Gerrodette, 1987). Broad-scale surveys of oceanic species are especially problematic in this regard (Taylor, Martinez, Gerrodette, Barlow, & Hrovat, 2007), but even trends in the abundance of delphinids inhabiting small home ranges in inshore coastal waters can be difficult to determine (Parra, Corkeron, & Marsh, 2006; Wilson, Hammond, & Thompson, 1999) An exception to this general rule has been North Atlantic right whales (Eubalaena glacialis Müller 1776, Figure 1), for which an annual count, based on a near complete photographic census of the population, has been available for at least the past 25 years. Together with observed annual calving rates, to assess the status of this population With these results, we assess the value of the past, and ongoing, photo-identification survey effort for North Atlantic right whales for assessing trends in abundance of this species
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