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. 2017 Sep 18;7(21):8730-8741.
doi: 10.1002/ece3.3406. eCollection 2017 Nov.

State-space Mark-Recapture Estimates Reveal a Recent Decline in Abundance of North Atlantic Right Whales

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Free PMC article

State-space Mark-Recapture Estimates Reveal a Recent Decline in Abundance of North Atlantic Right Whales

Richard M Pace 3rd et al. Ecol Evol. .
Free PMC article

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.

Keywords: Bayesian mark–recapture; Eubalaena glacialis; open population abundance; recovery; survival.

Figures

Figure 1
Figure 1
Overhead view of a feeding North Atlantic right whale, Eubalaena glacialis. Image collected under U.S. Marine Mammal Protection Act research permit number 17355. Photograph credit: National Oceanic and Atmospheric Administration/Northeast Fisheries Science Center/Christin Khan
Figure 2
Figure 2
Abundance of North Atlantic right whales 1990–2015 as estimated from mark–resight data as calculated from two procedures. Black diamonds are counts of the minimum number of individuals seen alive (MNA) that year plus those seen before and after that year. Circles with error bars are posterior medians and associated 95% credible intervals from a Bayesian mark–recapture model allowing random fluctuation among years, age effects and adult female effects on survival, as well as sex and time effects and random effects of individual catchability on capture probabilities together with their 95% critical regions. Inset shows the posterior distribution of estimated growth of the North Atlantic right whale population between 2015 and 2010 measured as a ratio (N2015/N2010) of abundance estimates from Bayesian implementation of J‐S model. Almost all (99.99%) of the estimates of growth are below 1.0, indicating a population decline
Figure 3
Figure 3
Estimated survival rates and associated 95% credible intervals of three classes of North Atlantic right whales 1990–2015 based on a Bayesian implementation of J‐S model allowing random fluctuation among years and using known states as data
Figure 4
Figure 4
Median abundance and associated 95% credible intervals by sex of North Atlantic right whales 1990–2015 based on a Bayesian MRR model allowing random fluctuation among years for survival rates, treating capture rates as fixed effects over time, and using both observed and known states as data
Figure 5
Figure 5
Estimated recapture probability and associated 95% credible intervals of North Atlantic right whales 1990–2015 based on a Bayesian MRR model allowing random fluctuation among years for survival rates, treating capture rates as fixed effects over time, and using both observed and known states as data
Figure 6
Figure 6
Annual productivity index (a) for North Atlantic right whales calculated as the number of detected calves/(median of posterior distribution of Estimated population size) and (b) crude growth (N t+1/N t) rate based on model medians. Note that the last two points in plot (a) assume the 2015 population size for the calculation of API

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