Abundance Estimation With a Transient Model Under the Robust Design

Abstract: A common situation in capture-mark-recapture (CMR) studies on birds and other organisms is to capture individuals not belonging to the studied population only present during the short time of the capture session. Presence of such transient individuals affects demographic parameter estimation from CMR data. Methods exist to reduce biases on survival estimates in the presence of transients and have been shown to be particularly efficient within the Robust Design framework (several secondary capture sessions within a short time interval during which the studied population can be assumed closed). We present a new model to estimate population size accounting for transients. We first used simulated data to show that the method reduces positive biases due to transients. In a second step, we applied the method to a real CMR dataset on a reed warbler (Acrocephalus scirpaceus) population. Population size estimates are reduced by up to 50% when correcting for the presence of transients. Many field studies on managed animal populations use capture-recapture methodology to obtain crucial parameters of the focal population demography. The resulting data sets are used either to estimate population size ignoring the presence of transients, or to estimate vital rates, accounting for transients but overlooking abundance estimation. Our method conciliates these 2 approaches.     

Inferences about population dynamics from count data using multistate models: a comparison to capture–recapture approaches

Wildlife populations consist of individuals that contribute disproportionately to growth and viability. Understanding a population's spatial and temporal dynamics requires estimates of abundance and demographic rates that account for this heterogeneity. Estimating these quantities can be difficult, requiring years of intensive data collection. Often, this is accomplished through the capture and recapture of individual animals, which is generally only feasible at a limited number of locations. In contrast, N‐mixture models allow for the estimation of abundance, and spatial variation in abundance, from count data alone. We extend recently developed multistate, open population N‐mixture models, which can additionally estimate demographic rates based on an organism's life history characteristics. In our extension, we develop an approach to account for the case where not all individuals can be assigned to a state during sampling. Using only state‐specific count data, we show how our model can be used to estimate local population abundance, as well as density‐dependent recruitment rates and state‐specific survival. We apply our model to a population of black‐throated blue warblers (Setophaga caerulescens) that have been surveyed for 25 years on their breeding grounds at the Hubbard Brook Experimental Forest in New Hampshire, USA. The intensive data collection efforts allow us to compare our estimates to estimates derived from capture–recapture data. Our model performed well in estimating population abundance and density‐dependent rates of annual recruitment/immigration. Estimates of local carrying capacity and per capita recruitment of yearlings were consistent with those published in other studies. However, our model moderately underestimated annual survival probability of yearling and adult females and severely underestimates survival probabilities for both of these male stages. The most accurate and precise estimates will necessarily require some amount of intensive data collection efforts (such as capture–recapture). Integrated population models that combine data from both intensive and extensive sources are likely to be the most efficient approach for estimating demographic rates at large spatial and temporal scales.     

Diversity of migration strategies among great frigatebirds populations

Migratory behavior varies extensively between bird taxa, from long distance migration to purely sedentary behavior. Variability in migratory behavior also occurs within taxa, where individuals within some species, or even populations, show mixed strategies. The same variability occurs in seabird species. We examined the migratory behavior of distinct populations of great frigatebirds Fregata minor in three distant oceanographic basins. Great frigatebird populations showed extensive variation in post‐breeding migratory behavior. Birds from Europa Island (Mozambique Channel) made long‐distance migration to numerous distinct roosting sites in the Indian Ocean, New Caledonia birds made shorter distance migrations to roosting sites in the southwestern Pacific Ocean, and Galapagos birds were resident within the archipelago year round. Juvenile birds from Europa Is. and New Caledonia dispersed widely whereas Galapagos juveniles were resident year round. The migratory behavior of Europa Is. and New Caledonia resulted in complete separation of foraging grounds between breeding adults, non‐breeding adults, and juveniles, whereas in the Galapagos the overlap was complete. We suggest that population variability in migratory behavior may have arisen because of different environmental conditions at sea, and also depends on the availability of suitable roosting sites on oceanic islands. The results also highlight the capacity of frigatebirds to remain airborne most of the time even outside the breeding season when they have to molt.     

Preliminary analysis of the social structure of killer whales,Orcinus orca,at subantarctic Marion Island

Studies of social differentiation between populations of killer whales (Orcinus orca) are important due to the cosmopolitan nature of the species, both in terms of distribution and feeding habits. The following research provides preliminary findings describing the social structure of the killer whale, Orcinus orca, population at subantarctic Marion Island. We provide evidence for consistent, observable patterns of social interactions with animals associating and disassociating in nonrandom patterns. We show that the social structure of this population may follow a new pattern of association, displaying a blend of the traditional resident/transient model displayed in the Northern Hemisphere. However, we emphasize the critical need for further studies related to the sociality, biology, and life history of Southern Ocean killer whales.     

Survival and abundance of short‐finned pilot whales in the archipelago of Madeira,NE Atlantic

Estimates of population parameters for the short‐finned pilot whale, Globicephala macrorhynchus, are scarce in literature, contributing to an International Union for Conservation of Nature (IUCN) status of Data Deficient. In this study, photo‐identification data collected over 7 yr from Madeira were used to estimate for the first time survivorship, capture probability, and abundance in this species using mark‐recapture methodology. The Cormack‐Jolly‐Seber model estimated that the adult island‐associated (i.e., resident and regular visitor) whales had a constant survival rate of 0.960 (95% CI: 0.853–0.990) and an annual capture probability varying between 0.372 (CI: 0.178–0.619) and 0.843 (CI: 0.619–0.947). A parameterization of the Jolly‐Seber model estimated that 140 island‐associated whales (CI: 131–151) used the area throughout the course of the study. Based on a closed population model, the most precise (lower CV) annual estimate of the total number of pilot whales using the southern and eastern waters of Madeira (~900 km²) in a 3 mo period covering summer/autumn was 334 animals (CI: 260–437). No trend was observed. Despite including biases, the approach used in this study provided plausible estimates of population parameters, which can contribute to the regional conservation strategies.     

Challenges and opportunities for estimating abundance of a low-density moose population

Monitoring large herbivores across their core range has been readily accomplished using aerial surveys and traditional distance sampling. But for peripheral populations, where individuals may occur in patchy, low-density populations, precise estimation of population size and trend remains logistically and statistically challenging. For moose (Alces alces) along their southern range margin in northern New York, USA, we sought robust estimates of moose distribution, abundance, and population trend (2016–2019) using a combination of aerial surveys (line transect distance-sampling), repeated surveys in areas where moose were known to occur to boost the number of detections, and density surface modeling (DSM) with spatial covariates. We achieved a precise estimate of density (95% CI = 0.00–0.29 moose/km²) for this small population (656 moose, 95% CI = 501–859), which was patchily distributed across a large and heavily forested region (the 24,280-km² Adirondack Park). Local moose abundance was positively related to active timber management, elevation, and snow cover, and negatively related to large bodies of water. As expected, moose abundance in this peripheral population was low relative to its core range in other northern forest states. Yet, in areas where abundance was greatest, moose densities in New York approached those where epizootics of winter tick (Dermacentor albipictus) have been reported, underscoring the need for effective and efficient monitoring. By incorporating autocorrelation in observations and landscape covariates, DSM provided spatially explicit estimates of moose density with greater precision and no additional field effort over traditional distance sampling. Combined with repeated surveys of areas with known moose occurrence to achieve viable sample sizes, DSM is a useful tool for effectively monitoring low density and patchy populations.     

Do ship strikes threaten the recovery of endangered eastern North Pacific blue whales?

Blue whales were targeted in the North Pacific from 1905–1971 and are listed as endangered by the IUCN. Despite decades without whaling, abundance estimates for eastern North Pacific (ENP) blue whales (Balaenoptera musculus) suggest little evidence for a recent increase. One possible reason is fatal strikes by large ships, which have affected populations of other cetaceans and resulted in mitigation. We used a population dynamics model to assess the trends and status of ENP blue whales, and the effects of ship strikes. We estimate the population likely never dropped below 460 individuals, and is at 97% of carrying capacity (95% interval 62%–99%). These results suggest density dependence, not ship strikes, is the key reason for the observed lack of increase. We also estimate future strikes will likely have a minimal impact; for example, an 11‐fold increase in vessels would lead to a 50% chance the long‐term population would be considered depleted. Although we estimate ship strike mitigation would have minimal impacts on population trends and status, current levels of ship strikes are likely above legal limits set by the U.S. The recovery of ENP blue whales from whaling demonstrates the ability of blue whale populations to rebuild under careful management.     

Possible indirect interactions between transient and resident killer whales: implications for the evolution of foraging specializations in the genus Orcinus

Summary Two distinct forms of killer whale (Orcinus orca) occur off the coast of British Columbia, Alaska and Washington State. These have different diets, and may be reproductively isolated. Because the primary food of transient whales (pinnipeds) is a potential competitor for the primary food of resident whales (salmon), or for the smaller fishes on which salmon feed, there should be an indirect interaction between the two forms of killer whale. We use simple mathematical models to show that this interaction will be either of a plus-minus type, or a plus-plus type (indirect mutualism), depending on whether or not pinnipeds and residents are on the same trophic level. In the case of the plus-minus interaction, increasing the population density or improving the environmental conditions of transients will increase the population density of residents, while increasing resident populations will reduce the equilibrium population size of transients. In the case of the plus-plus interaction, increasing the population density or improving the environmental conditions of transients will increase the population density of residents, while increasing resident populations will reduce the equilibrium population size of transients. In the case of the plus-plus interaction, increasing the population density or improving the environmental conditions of transients will increase the population density of residents, and vice versa. Such effects may not be currently manifest due to reduced populations at most levels in the food web. Regardless, considering such indirect interactions may be important for the management of many of the species involved, and can also provide a valuable framework for examining the evolution of the two forms of killer whales. Frequency-dependent indirect interactions, acting in concert with density-dependence within populations and disruptive selection on prey-type specific foraging characteristics, may have favoured reproductive isolation of the two forms of killer whales. We suggest that these two forms of whale are in the process of speciating, i.e., the two forms are incipient species.     

Scaling Occupancy Estimates up to Abundance for Wolves

Management of wildlife populations often requires reliable estimates of population size or distribution. Estimating abundance can be logistically difficult, and occupancy models have been used as a less expensive proxy for abundance estimation. Another alternative is to use independent estimates of home-range size and mean group size to directly scale occupancy estimates up to abundance. We used simulations to explore when scaling occupancy up to abundance is reliable, and as an example we applied an occupancy approach to estimate abundance of wolves (Canis lupus) from roadside snow-tracking surveys in northern Wisconsin, USA, in 2016 and 2018. Estimates of wolf abundance were plausible and compared favorably with independent estimates produced by territory mapping, and snow-tracking data requirements were lower than for territory mapping. Simulation results suggested that reasonable abundance estimates could be obtained under some conditions but also that severe positive bias could result under other conditions, especially when populations were small and dispersed, home range size was small, and areal sampling units were large. Positive bias in abundance estimates occurs because of closure assumption violations when tracks from a single wolf or pack are detected in >1 sample unit, and the sum of the sample unit areas where tracks were detected exceed the sum of the home range areas. Bias was minimized when sampling units were small relative to home range size or when sampling units were route segments that approximate point sample units, and when home ranges were highly aggregated. We conclude that, although caution is warranted when scaling occupancy estimates up to abundance, scaled occupancy models can provide feasible and reliable estimates of abundance, assuming home range size and mean group size are accurately known or estimated, sampling units are appropriately chosen, and covariates that aggregate home ranges can be used to accurately predict occupancy probability. © 2021 The Authors. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society.     

Population Growth and Demography of Common Loons in the Northern United States

ABSTRACT We used recent developments in theoretical population ecology to construct basic models of common loon (Gavia immer) demography and population dynamics. We parameterized these models using existing survival estimates and data from long-term monitoring of loon productivity and abundance. Our models include deterministic, 2-stage, density-independent matrix models, yielding population growth-rate estimates (λ) of 0.99 and 1.01 for intensively studied populations in our Wisconsin, USA, and New Hampshire, USA, study areas, respectively. Perturbation analysis of these models indicated that estimated growth rate is extremely sensitive to adult survival, as expected for this long-lived species. Also, we examined 20 years of count data for the 2 areas and evaluated support for a set of count-based models of population growth. We detected no temporal trend in Wisconsin, which would be consistent with fluctuation around an average equilibrium state but could also result from data limitations. For New Hampshire, the model set included varying formulations of density dependence and partitioning of stochasticity that were enabled by the annual sampling resolution. The best model for New Hampshire included density regulation of population growth and, along with the demographic analyses for both areas, provided insight into the possible importance of breeding habitat availability and the abundance of nonbreeding adults. Based on these results, we recommend that conservation organizations include nonbreeder abundance in common loon monitoring efforts and that additional emphasis be placed on identifying and managing human influences on adult loon survival.     

How few whales were there after whaling? Inference from contemporary mtDNA diversity

Reconstructing the history of exploited populations of whales requires fitting a trajectory through at least three points in time: (i) prior to exploitation, when abundance is assumed to be at the maximum allowed by environmental carrying capacity; (ii) the point of minimum abundance or 'bottleneck', usually near the time of protection or the abandonment of the hunt; and (iii) near the present, when protected populations are assumed to have undergone some recovery. As historical abundance is usually unknown, this trajectory must be extrapolated according to a population dynamic model using catch records, an assumed rate of increase and an estimate of current abundance, all of which have received considerable attention by the International Whaling Commission (IWC). Relatively little attention has been given to estimating minimum abundance (N(min)), although it is clear that genetic and demographic forces at this point are critical to the potential for recovery or extinction of a local population. We present a general analytical framework to improve estimates of N(min) using the number of mtDNA haplotypes (maternal lineages) surviving in a contemporary population of whales or other exploited species. We demonstrate the informative potential of this parameter as an a posteriori constraint on Bayesian logistic population dynamic models based on the IWC Comprehensive Assessment of the intensively exploited southern right whales (Eubalaena australis) and published surveys of mtDNA diversity for this species. Estimated historical trajectories from all demographic scenarios suggested a substantial loss of mtDNA haplotype richness as a result of 19th century commercial whaling and 20th century illegal whaling by the Soviet Union. However, the relatively high rates of population increase used by the IWC assessment predicted a bottleneck that was implausibly narrow (median, 67 mature females), given our corrected estimates of N(min). Further, high levels of remnant sequence diversity (theta) suggested that pre-exploitation abundance was larger than predicted by the logistic model given the catch record, which is known to be incomplete. Our results point to a need to better integrate evolutionary processes into population dynamic models to account for uncertainty in catch records, the influence of maternal fidelity on metapopulation dynamics, and the potential for inverse density dependence (an 'Allee effect') in severely depleted populations.     

Modelling heterogeneity in detection probabilities in land and aerial abundance surveys in humpback whales (<Emphasis Type="Italic">Megaptera novaeangliae</Emphasis>)

The effective management and conservation of animal populations relies on statistically-sound and replicable surveys to obtain estimates of abundance and assess trends. Surveys of cetaceans, such as humpback whales Megaptera novaeangliae, are difficult to conduct and are particularly affected by bias in detection probability. For example, the probability of detection of whales from land decreases substantially with increased distance from the platform. This distance effect is also true for aerial surveys, combined with the problem that animals are unavailable for detection (underwater) whilst in the field of view. We present a novel approach that combines corrected double-platform land surveys with corrected aerial surveys to obtain a robust estimate of g(0), the probability of detection on the survey line, for aerial surveys of migrating humpback whales. Several sources of heterogeneity in detection probabilities were identified within the land and aerial surveys (including group composition, bearing of first sighting, number of groups being tracked simultaneously and cloud cover). After including these into our estimate of ĝ(0), we found that only 29% of available whales are being detected on the survey line (ĝ(0) = 0.288), which is a considerably smaller estimate than many available for humpback whales using other methods. Incorporating heterogeneity into the population surveys shows that we are likely to be underestimating the population size of whales on the east coast of Australia. The implications of this result for their conservation and management in light of increased whale-human conflict is discussed.     

FORAGING STRATEGIES OF SYMPATRIC KILLER WHALE (ORCINUS ORCA) POPULATIONS IN PRINCE WILLIAM SOUND, ALASKA

Killer whales ( Orcinus orca ) feed on a wide variety of fish, cephalopods, and marine mammals throughout their cosmopolitan range; however, the dietary breadth that characterizes the species is not reflected in all populations. Here, we present the findings of a 14-yr study of the diet and feeding habits of killer whales in Prince William Sound, Alaska. Two non-associating forms of killer whale, termed resident and transient (Bigg et al. 1987), were identified. All prey seen taken by transients were marine mammals, including harbor seals ( Phoca vitulina ), Dall's porpoises ( Phocoenoides dalli ), Steller sea lions ( Eumetopias jubatus ), and harbor porpoises ( Phocoena phocoena ). Resident killer whales appeared to prey principally on salmon ( Oncorhynchus spp.), preferring coho salmon ( O. kisutch ) over other, more abundant salmon species. Pacific herring ( Clupea pallasi ) and Pacific halibut ( Hippocampus stenolepis ) were also taken. Resident killer whales frequently were seen to interact in non-predatory ways with Steller sea lions and Dall's porpoises, while transients were not. Differences in the social organization and behavior of the resident and transient killer whales in Prince William Sound are discussed in the light of the dietary differences documented here.     

Gray whale Eschrichtius robustus population and stock identity

• 1 In response to conservation and management concerns about gray whale Eschrichtius robustus population and stock structure, we provide an overview of the life history and ecology of gray whales as a context for discussion of population and stock structure within the species. Historically eastern and western North Pacific gray whales were managed separately because: (i) their ranges do not overlap; (ii) genetic analyses indicate that the two populations are significantly different; and (iii) eastern gray whales have increased in abundance over the past century while western gray whales have not.
• 2 Here, we review gray whale migration timing and segregation, feeding and prey species, and reproduction and reproductive behaviour. For the eastern and western gray whale, we review their distribution, history of exploitation, abundance and current status, although most of what is known is founded on the better studied eastern gray whale and only implied for the lesser known western gray whale. Methods to investigate population and stock identity are reviewed including genetics, morphology, chemical signatures, carbon isotopes, parasites, photographic identification and trends in abundance.
• 3 While the evidence indicates that there is at least some degree of mixing within each of the gray whale populations, no stocks or sub‐stocks can be defined. Population structure is not evident in nuclear data, and because selection occurs primarily on the nuclear genome, it is unlikely that there is structuring within each population that could result in evolutionary differences. For western gray whales, there are insufficient data to assess the plausibility of stock structure within the population, owing to its extremely depleted state. Research on eastern gray whales has focused mostly on documenting changes in abundance, feeding biology and behaviour, and suggests separate breeding groups to be unlikely. Both males and females are promiscuous breeders lending little opportunity for the nuclear genome to be anything other than well mixed as is suggested by the high haplotypic diversity of the eastern population.
• 4 The available data strongly indicate that western gray whales represent a population geographically isolated from eastern gray whales and therefore that the western and eastern populations should be treated as separate management units.

     

Insights from a general,full-likelihood Bayesian approach to inferring shared evolutionary events from genomic data: Inferring shared demographic events is challenging*

Factors that influence the distribution, abundance, and diversification of species can simultaneously affect multiple evolutionary lineages within or across communities. These include changes to the environment or inter-specific ecological interactions that cause ranges of multiple species to contract, expand, or fragment. Such processes predict temporally clustered evolutionary events across species, such as synchronous population divergences and/or changes in population size. There have been a number of methods developed to infer shared divergences or changes in population size, but not both, and the latter has been limited to approximate methods. We introduce a full-likelihood Bayesian method that uses genomic data to estimate temporal clustering of an arbitrary mix of population divergences and population-size changes across taxa. Using simulated data, we find that estimating the timing and sharing of demographic changes tends to be inaccurate and sensitive to prior assumptions, which is in contrast to accurate, precise, and robust estimates of shared divergence times. We also show that previous estimates of co-expansion among five Alaskan populations of three-spine sticklebacks (Gasterosteus aculeatus) were likely driven by prior assumptions and ignoring invariant characters. We conclude by discussing potential avenues to improve the estimation of synchronous demographic changes across populations.     

Allee effects and population regulation: a test for biotic resistance against an invasive leafroller by resident parasitoids

Resident natural enemies can impact invasive species by causing Allee effects, leading to a reduction in establishment success of small founder populations, or by regulating or merely suppressing the abundance of established populations. Epiphyas postvittana, the Light Brown Apple Moth, an invasive leafroller in California, has been found to be attacked by a large assemblage of resident parasitoids that cause relatively high rates of parasitism. Over a 4-year period, we measured the abundance and per capita growth rates of four E. postvittana populations in California and determined parasitism rates. We found that at two of the sites, parasitism caused a component Allee effect, a reduction in individual survivorship at lower E. postvittana population densities, although it did not translate into a demographic Allee effect, an impact on per capita population growth rates at low densities. Instead, E. postvittana populations at all four sites exhibited strong compensatory density feedback throughout the entire range of densities observed at each site. As we found no evidence for a negative relationship between per capita population growth rates and parasitism rates, we concluded that resident parasitoids were unable to regulate E. postvittana populations in California. Despite a lack of evidence for regulation or a demographic Allee effect, the impact of resident parasitoids on E. postvittana populations is substantial and demonstrates significant biotic resistance against this new invader.     

Accounting for transients when estimating abundance of bottlenose dolphins in Choctawhatchee Bay,Florida

We investigated the potential for using mark–recapture models to estimate abundance of bottlenose dolphin populations in open systems (e.g., bays, estuaries). A major challenge in these systems is that immigration and emigration occur during sampling, thus violating one of the most basic assumptions of mark–recapture models. We assumed that dolphins using our study site were composed of both residents (those that used the study area almost exclusively during our study), and transients (those that passed through our study area but did not remain long), and examined several mark–recapture estimators for their ability to accurately and precisely estimate the abundance of residents and the superpopulation (i.e., residents + transients). Using simulated data, we found that a novel approach accounting for transients resulted in estimators with less bias, smaller absolute relative error, and confidence interval coverage closer to nominal than other approaches, but this novel approach required intensive sampling and that the “correct” transient pattern be specified. In contrast, classical mark–recapture estimators for closed populations often overestimated the number of residents and underestimated the superpopulation. Using photo-identification records, a model-averaged estimate of the superpopulation of bottlenose dolphins in and around Choctawhatchee Bay, Florida was 232 (SE = 13) animals. We estimated resident abundance at 179 (SE = 8), which was lower than the number of unique animals we encountered (188). Our results appear promising for developing monitoring programs for bottlenose dolphins and other taxa in open systems. Our estimators should prove useful to wildlife managers who wish to base conservation decisions on estimates of the number of animals that reside primarily in their study or management area. © 2011 The Wildlife Society.     

State–space mark–recapture estimates reveal a recent decline in abundance of North Atlantic right whales

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.     

DISSOLVING STOCK DISCRETENESS WITH SATELLITE TRACKING: BOWHEAD WHALES IN BAFFIN BAY

Nine bowhead whales (Balaena mysticetus) were instrumented with satellite transmitters in West Greenland in May 2002 and 2003. Transmitters were either encased in steel cans or imbedded in floats attached to wires. Transmitters mounted in steel cans had a high initial failure rate, yet those that were successful provided tracking durations up to seven months. Float tags had a low initial failure rate and initially provided large numbers of positions; however, they had deployment durations of only 2–33 d. All tracked whales departed from West Greenland and headed northwest towards Lancaster Sound in the end of May. Three tags with long tracking durations (197–217 d) recorded movements of whales (1 ♂, 2 ♀) into December in 2002 and 2003. All of these individuals remained within the Canadian High Arctic or along the east coast of Baffin Island in summer and early fall. By the end of October, all three whales moved rapidly south along the east coast of Baffin Island and entered Hudson Strait, an apparent wintering ground for the population. One of the whales did not visit Isabella Bay on east Baffin Island, the locality used for abundance estimation from photographic reidentification of individuals. The movements of whales tagged in this study raise critical questions about the assumed stock discreteness of bowhead whales in Foxe Basin, Hudson Strait, and Davis Strait and indicate current estimates of abundance are negatively biased.     

Lincoln estimates of mallard (Anas platyrhynchos) abundance in North America

Estimates of range‐wide abundance, harvest, and harvest rate are fundamental for sound inferences about the role of exploitation in the dynamics of free‐ranging wildlife populations, but reliability of existing survey methods for abundance estimation is rarely assessed using alternative approaches. North American mallard populations have been surveyed each spring since 1955 using internationally coordinated aerial surveys, but population size can also be estimated with Lincoln's method using banding and harvest data. We estimated late summer population size of adult and juvenile male and female mallards in western, midcontinent, and eastern North America using Lincoln's method of dividing (i) total estimated harvest, , by estimated harvest rate, , calculated as (ii) direct band recovery rate, , divided by the (iii) band reporting rate, . Our goal was to compare estimates based on Lincoln's method with traditional estimates based on aerial surveys. Lincoln estimates of adult males and females alive in the period June–September were 4.0 (range: 2.5–5.9), 1.8 (range: 0.6–3.0), and 1.8 (range: 1.3–2.7) times larger than respective aerial survey estimates for the western, midcontinent, and eastern mallard populations, and the two population estimates were only modestly correlated with each other (western: r = 0.70, 1993–2011; midcontinent: r = 0.54, 1961–2011; eastern: r = 0.50, 1993–2011). Higher Lincoln estimates are predictable given that the geographic scope of inference from Lincoln estimates is the entire population range, whereas sampling frames for aerial surveys are incomplete. Although each estimation method has a number of important potential biases, our review suggests that underestimation of total population size by aerial surveys is the most likely explanation. In addition to providing measures of total abundance, Lincoln's method provides estimates of fecundity and population sex ratio and could be used in integrated population models to provide greater insights about population dynamics and management of North American mallards and most other harvested species.