Limited Survey Effort Research Articles

Distribution models of baleen whale species in the Irish Exclusive Economic Zone to inform management and conservation

Irish waters are under increasing pressure from anthropogenic sources including the development of offshore renewable energy, vessel traffic and fishing activity. Spatial planning requires robust datasets on species distribution and the identification of important habitats to inform the planning process. Despite limited survey effort, long-term citizen science data on whale presence are available and provide an opportunity to fill information gaps. Using presence-only data as well as a variety of environmental variables, we constructed seasonal ensemble species distribution models based on five different algorithms for minke whales, fin whales, humpback whales, sei whales, and blue whales. The models predicted that the coastal waters off the south and west of Ireland are particularly suitable for minke, fin and humpback whales. Offshore waters in the Porcupine Seabight area were identified as a relevant habitat for fin whales, sei whales and blue whales. We combined model outputs with data on maritime traffic, fishing activity and offshore wind farms to measure the exposure of all the species to these pressures, identifying areas of concern. This study serves as a baseline for the species presence in Irish waters over the last two decades to help develop appropriate marine spatial plans in the future.

Hidden Afrotropical Bat Diversity in Nigeria: Ten New Country Records from a Biodiversity Hotspot

Knowledge of Afrotropical bat diversity is hindered by hidden diversity because of inefficient sampling techniques, limited survey effort, and taxonomic description. With 90 bat species recorded, Nigeria supports almost a third of the bat diversity known to occur in Africa, yet the country remains relatively under-sampled. Southeastern Nigeria and southwestern Cameroon are predicted to be the largest of six bat diversity hotspots in Africa, with the Cameroonian side receiving more sampling effort. This area is also recognized to support hyperdiverse mammalian assemblages and endemic species. Notwithstanding, two major protected areas on the Nigerian part of this bat diversity hotspot were hitherto not sampled, raising the likelihood of discovering new country records in these conservation priority forests. We conducted an intensive survey using harp traps and mist nets in forests of Afi Mountain Wildlife Sanctuary and Cross River National Park (Okwangwo Division). We also recorded echolocation calls of captured individuals to create the first in-country call library. We captured 36 bat species of which 10 are new country records raising the number of bat species known from Nigeria to 100. Harp trap captures represented 99% of forest understory bats sampled with 15 out of 18 species caught exclusively in traps. Rarefaction curves of captures suggest near-complete sampling with harp traps but incomplete sampling with mist nets. Eight of the 10 new country records were captured exclusively in harp traps, demonstrating the value of contemporary techniques for frequently missed species — here referred to as methodological blind-spot diversity. We report the re-identification of two museum specimens that were previously identified as Rhinolophus hillorum (R. cf. hillorum) and Miniopterus inflatus (M. villiersi) thus, our captures of the true forms confirm the presence of both species in Nigeria for the first time. We also provide noteworthy taxonomic comments on other species. To improve the efficiency of future Afrotropical bat surveys we recommend the deployment of complementary trapping techniques in identified diversity hotspots, especially sites that have been under-sampled.

Seasonal distribution, abundance, habitat use and population identity of humpback whales in Oman

Previously published data on the occurrence of humpback whales (Megaptera novaeangliae) in the Arabian Sea suggests that the region hosts anon-migratory population that adheres to a Northern Hemisphere breeding cycle. In order to investigate the distribution and abundance of thispopulation, twelve small boat surveys were conducted in three main locations off the coast of Oman between February 2000 and November 2004.Humpback whales were observed during surveys in Dhofar and Gulf of Masirah on Oman’s Arabian Sea coast, but not during surveys in the Muscatregion in the Gulf of Oman. An even ratio of males to females was observed and sampled during surveys in the Gulf of Masirah, which was surveyedin October and November (n = 38), while almost all whales sampled in Dhofar in February/March were male (n = 28). Song was detected frequentlyin the bay surrounding the Halaniyat Islands (formerly known as the Kuria Muria Bay) in February/March, but observations of mother-calf pairswere sparse, and competitive groups were absent. Feeding was observed in both October/November and February/March, but behavioural andenvironmental observations indicate that the Gulf of Masirah is primarily an important feeding ground, while the Dhofar region, particularly theHalaniyat Bay, may be a breeding area. However, limited survey effort and a lack of recent observations of mother-calf pairs or competitive groupsraises the possibility that the primary mating, calving and nursing areas are yet to be identified. Sixty-four individual whales were identified usingphotographs of dorsal fins or tail flukes. A high rate of re-sightings between years and between survey areas at different times of the year indicatesyear-round residence off the coast of Oman. A Chapman’s modified Petersen estimator was applied to various data pairings to calculate abundance.All pairings yielded estimates of less than 100 individuals, but sample sizes were small and there were various sources of possible bias. Analysisof scarring on the caudal peduncle region of identified individuals in Oman indicates that between 30 and 40% are likely to have been involved inentanglements with fishing gear. Comparison of the Oman photo-identification catalogue with those from Zanzibar, Antongil Bay (Madagascar)and Mayotte and the Geyser Atoll (Comoros Archipelago), yielded no photographic matches. These data are consistent with the hypothesis of adiscrete population. The distribution of fluke pigmentation rankings from the Oman catalogue, which varied significantly from those of Madagascarand Mayotte, provides further evidence for this theory. The evidence presented here provides a strong underpinning for the recent IUCN Red Listclassification of the Arabian Sea sub-population of humpback whales as Endangered. In light of ongoing coastal development and other threats tothis population’s habitat and future survival, urgent research and conservation measures are recommended.

Oversummering in the southern hemisphere by long-distance migratory shorebirds calls for reappraisal of wetland conservation policies

In many migratory bird populations, individuals forgo migrating to the breeding grounds for one or more years and instead ‘oversummer’ in non-breeding areas. This behaviour occurs among long-distance migratory shorebirds, typically species exhibiting delayed maturity. As the oversummering areas remain poorly described, conservation strategies will be incomplete even in species experiencing ongoing global declines. Oversummering areas could be a weak link in the chain if they are spatially distinct from non-breeding grounds, and used during critical stages throughout individual’s lifetime. Here, we present a combining study based on (1) GPS-tracking adult Hudsonian godwits Limosa haemastica oversummering in inland South America, (2) two consecutive seasons of surveys to estimate minimum godwit abundance in that area, and (3) analyses of 13 years of shorebird counts based on the Neotropical Waterbird Census (NWC) in Argentina. Godwits arrived at pampa plain wetlands in early May and spent 5–6 months there. We observed godwits in six out of 44 wetlands surveyed, with a minimum of 366 and 746 godwits counted each survey. Based on frequency of occurrence in NWC, oversummering propensity in Argentina was high for three out of nine Nearctic shorebird species, all of which are of international conservation concern. Despite our limited survey effort, we found several hundred godwits that will likely represent only a small fraction of the population oversummering in this region. Given their delayed maturity, many other long-distance migratory shorebirds may spend large portions of their lives in previously undocumented wetlands while deferring migration. These unrecognized oversummering habitats fall outside the scope of today’s conservation efforts for Hudsonian godwits, because they are not spatially nested within the non-breeding grounds, an issue to be studied for other shorebirds. We therefore propose including specific criteria for oversummering areas when designating wetlands of international concern for the conservation of long-distance migratory shorebirds.

Evidence of Newell’s Shearwaters and Hawaiian Petrels on Oahu, Hawaii

Abstract Hawaii’s only 2 endemic seabirds, Newell’s Shearwater (Puffinus auricularis newelli) and Hawaiian Petrel (Pterodroma sandwichensis), are listed under the United States Endangered Species Act. Threats to both species include light attraction and fallout, collisions with power lines and other structures, predation by invasive animals, and habitat degradation. Both species were assumed to be extirpated from the island of Oahu despite limited survey effort. We used survey data from Kauai (both species) and Maui (Hawaiian Petrel only) to model suitable habitat and light conditions. We then projected this model onto Oahu to identify potential survey sites. From April to September of 2016–2017, we deployed automated acoustic recording units at 13 potentially suitable sites across Oahu. We detected Newell’s Shearwaters at 2 sites; one on the leeward slopes of Mount Kaala in the Waianae Mountains and another at Poamoho in the Koolau Mountains. We detected Hawaiian Petrels at one location on the windward slope of Mount Kaala. All 3 sites were in nearly intact native forest with steep slopes. The frequency of detections at these sites suggests that both species are regularly prospecting on Oahu and potentially could be breeding there. If they are breeding, these individuals could represent missing links in the population connectivity of both species among islands. Protecting any remnant breeding populations would be of high conservation value given their recent population declines.

Bowhead and Beluga Whale Distributions, Sighting Rates, and Habitat Associations in the Western Beaufort Sea in Summer and Fall 2009–16, with Comparison to 1982–91

We analyzed data from line-transect aerial surveys for marine mammals conducted in the western Beaufort Sea (shore to 72˚ N, 140˚–157˚ W) from July to October of 2009–16 to investigate the distribution, behaviors, sighting rates, and habitat use preferences of bowhead and beluga whales. The habitat use data allowed for direct comparison with data collected in the same area from 1982 to 1991. Both species are ice-adapted, migrating through leads in sea ice in spring, and are seasonal inhabitants of the western Beaufort Sea during summer and fall. From 2009 to 2016, bowheads were seen in all survey months, with the highest overall sighting rate (whales per km) in August. Bowhead sighting rates were highest in the whales’ preferred habitats: outer shelf habitat (51–200 m depth) in July and inner shelf-shallow habitat (≤ 20 m depth) in August, September, and October. Beluga whales were also seen in all survey months, with highest overall sighting rate in July. Beluga whales were overwhelmingly associated with continental slope habitat (201–2000 m depth) in all months. Bowhead distribution and depth preferences in summer months of 2009–16 differed from those observed in 1982–91, when bowheads were not seen during limited survey effort in July and preferred outer continental shelf habitat in August. These differences indicate that bowhead whale preference for shallow shelf habitat now occurs earlier in summer than it used to. Beluga distribution and depth preference remained similar between 1982–91 and 2009–16, with strong preference for continental slope during both periods. Differences in sea ice cover habitat association for both species are likely due more to the relative lack of sea ice in recent years compared to the earlier period than to shifts in habitat preference. Habitat partitioning between bowhead and beluga whales in the western Beaufort Sea remained evident except in July, when both species used continental slope habitat. In July – October 2009–16, the distribution, sighting rates, and behavior of both bowheads and belugas in the western Beaufort showed considerable interannual variation, which underscores the importance of annual sampling to accurate records of the complex western Beaufort Sea ecosystem.

Applicability of a Bayesian state-space model for evaluating the effects of localized culling on subsequent density changes: sika deer as a case study

At the landscape scale, localised culling is often conducted to achieve various deer management aims. However, few studies have assessed the effects of localised culling on deer population dynamics, owing to the spatially and temporally insufficient datasets of deer abundance that are derived from limited survey efforts. In this study, we estimated the population dynamics of a sika deer (Cervus nippon) population in the Tanzawa Mountains, central Japan, by Bayesian state-space modelling using spatiotemporally insufficient abundance indices and evaluated the effects of unit-specific culls on subsequent density changes in 56 units. The responses of deer density to unit-specific culls differed greatly among units, and a very weak correlation was observed between the intensities of unit-specific culls and the reduction in density. Deer populations in some units tended to resist density decreases despite high culling pressure, whereas those in other units were susceptible to density decreases with little to no culling pressure. Because the spatial scales of each unit were relatively small, annual density changes in each unit were largely influenced by deer movement in this estimation. The obscured effects of unit-specific culls, which were probably derived from deer movement among units in this case study, re-emphasized that deer migration should be incorporated into the planning of localised culling and that deer management should be coordinated over a wide area beyond landscape components and landownerships. Thus, we conclude that Bayesian state-space modelling is valuable for practical deer management programs at a large spatial scale even if different abundance indices are used.

Presence of the snow leopard Panthera uncia confirmed at four sites in the Chinese Tianshan Mountains

AbstractThe Endangered snow leopard Panthera uncia is a flagship species of mountainous central Asia, and a conservation concern. China has the largest extent of potential snow leopard habitat and is thus crucial for snow leopard conservation. There are many challenges to snow leopard conservation in China, however, and there is still little information on the species for many geographical locations, including the Tianshan Mountains of Xinjiang province, which are important because they potentially connect snow leopard populations in Krygyzstan and Kazakhstan with those in Mongolia. We used camera traps in four areas across eastern, central and western Tianshan, with a total survey effort of 3,216 camera-trapping days. We confirmed the presence of snow leopards and an abundance of potential snow leopard prey, including the Siberian ibex Capra sibirica, in all areas. We found 2–3 individual adult snow leopards at each site, with relatively limited survey effort, and more study is needed to fully investigate the importance of the Tianshan Mountains for the species. Establishing more protected areas is essential for snow leopard conservation, and we have used data from this study to apply for protected area status for several areas.

Untangling taxonomic confusion and diversification patterns of the Streak-breasted Scimitar Babblers (Timaliidae: Pomatorhinus ruficollis complex) in southern Asia

Biodiversity patterns in Asia are poorly understood due to inferences drawn from incorrect taxonomy and limited survey effort. The Streak-breasted Scimitar Babblers (Pomatorhinus ruficollis complex) have a wide distribution across southern Asia and exhibit a high degree of plumage variability within and among populations. Continued use of traditional subspecies designations over revised species-limits based on plumage and DNA sequence data furthers confusion in this group and obscures complex biogeographic patterns. In this study, we combined previously published DNA sequences with newly sampled populations to produce a comprehensive dataset for the ruficollis complex. Phylogenetic analysis of these data confirms that traditional subspecies based on plumage alone are paraphyletic and therefore not good descriptors of evolutionary history. With increased sampling, our study supported previous delimitations of phylogenetic species as distinct units, refined the range limits of two taxa – P. reconditus (throughout central China) and P. nigrostellatus (Hainan, Guangxi, N Vietnam), showed two additional clades that may be distinct species, and uncovered a ‘suture’ zone where populations of multiple species occur in the same localities. Diversification within the ruficollis complex indicates a clade of Sino-Himalayan and SE Asian species sister to a clade distributed in central and southern China species. The ‘suture’ zone where different ruficollis species are in contact coincides with the meeting of these four major geographic areas in a highly geomorphologically complex region.

The North West Cape, Western Australia: A Potential Hotspot for Indo-Pacific Humpback Dolphins Sousa chinensis?

Indo-Pacific Humpback Dolphins Sousa chinensis (Humpback Dolphins hereafter) are listed as ‘near threatened’ on an international level and ‘migratory’ in Australian waters. There is limited information on Humpback Dolphins in Western Australian State waters, where the species remains unlisted. This lack of knowledge hinders the management and conservation of the species in a region of rapidly increasing coastal development. We conducted opportunistic boat-based surveys in April 2010 and present data on the location, size and composition of Humpback Dolphin groups encountered in the near-shore waters around the North West Cape, Western Australia. A total of 42 groups were encountered in a variety of habitats during 145 h on the water over ca. 80 km coastline. Group size ranged from 1 to 15, with a mean (± SE) of 5.3 (± 0.48) individuals. A total of 54 Humpback Dolphins were identified from photographs of the unique markings on their dorsal fins. The lack of a plateau in the cumulative discovery curve of identified individuals over the duration of the study suggests that only a subset of dolphins in the area was identified. This region is close to the south-western limit of the species’ Australian distribution and appears to represent an important location for Western Australian Humpback Dolphins. In light of increasing anthropogenic activity around the North West Cape and Exmouth Gulf, these preliminary findings from a limited survey effort indicate that further research into this population is required.

Species distribution models that do not incorporate global data misrepresent potential distributions: a case study using Iberian diving beetles

Aim In this study, we (1) determine whether simple species distribution models based on regional data provide incomplete descriptions of potential distributions; (2) investigate whether underrepresented areas where potential distributions are estimated using only regional data are spatially and environmentally structured; and (3) examine why regional data may not adequately describe potential distributions. Location Iberian Peninsula. Methods We used a multidimensional envelope procedure to estimate the potential distributional areas of 73 species of Iberian diving beetles (Dytiscidae) using two data sets (Iberian data and data from the entire range). We used a Mann–Whitney U-test to compare the features (climate, number of database records and proportion of human-transformed land uses) of these underrepresented areas with those of the remaining Iberian territory. Results By comparing species-richness estimates obtained by overlaying predicted species distributions modelled using either global or regional data, we found that some areas of species’ potential distributions are underrepresented when only regional data are used. Incomplete estimates of potential distributions when using only Iberian data may be partly attributable to limited survey efforts combined with unique local climates, but none of the considered factors by itself seems to fully explain this underrepresentation. Main conclusions Our results show that species data from regional inventories may provide an incomplete description of the environmental limits of most species, resulting in a biased description of species’ niches. The results of distribution models based on partial information about the environmental niche of a species may be inaccurate. To minimize this error, we highlight the importance of considering all known populations of a given species or at least a sample of populations distributed across the whole range, to include environmental extremes of the distribution. We highlight some methodological and conceptual concerns that should be considered when attempting to infer potential distributions from occurrence data.

Status of the black-legged kittiwake (Rissa tridactyla) breeding population in Greenland, 2008

Based on the intensified survey efforts (since 2003) of Greenlandic breeding colonies of black-legged kittiwake (Rissa tridactyla), the total Greenland breeding population was estimated at roughly 110 000 breeding pairs, constituting about 4% of the total North Atlantic breeding population. This population estimate of black-legged kittiwake is the most reliable and updated estimate hitherto reported for Greenland. The results confirm considerable population declines in many areas of West Greenland. The breeding population of blacklegged kittiwakes in the Qaanaaq area appears healthy, whereas the rest of the west coast has experienced declines, especially the north-western region (in the area from Upernavik to Kangaatsiaq). Exactly when these reductions have occurred is uncertain because of the limited survey effort in the past, but some colonies declined as far back as the mid-1900s, whereas declines of other colonies have occurred since the 1970–80s. East Greenland data from the past are few, but recent aerial surveys confirm that the abundance of breeding kittiwakes on this inaccessible coast is low. The reasons for the West Greenland declines are not documented. Poor feeding conditions and a high hunting pressure, particularly prior to 2002 when the open season was shortened considerably, are possible explanations.

A new species of Malagasy blind snake of the genus Typhlops Oppel (Serpentes: Typhlopidae)

Madagascar is the third hottest biodiversity hotspot in the world with regard to the reptile fauna, after the Caribbean and middle-America (Myers et al. 2000). Moreover, 92% of the 365 described species are endemic to the island (Glaw & Vences 2007). Malagasy snakes of the family Typhlopidae are no exception to this pattern of endemism: except for the cosmopolitan, widespread and introduced Ramphotyphlops braminus (Daudin, 1803), all the other species of the family encountered in Madagascar are endemic to the island. The species are allocated into two genera: Xenotyphlops (endemic to Madagascar) and Typhlops (with a worldwide distribution). However, when contrasted with other tropical forests worldwide, and given the extraordinarily high rates of micro-endemism characterizing several reptile lineages on Madagascar (Yoder et al. 1995), the current species richness in Malagasy blind snakes is relatively low: only eight species are recognized in the genus Typhlops and two in Xenotyphlops (Glaw & Vences 2007). The opportunistic discovery of a Typhlops, that could not be assigned to any named taxon, close to a widely frequented research station of the eastern humid forest highlights the limited survey efforts in this region. The purpose of this paper is to describe this new taxon and to stress the need to increase field investigations in the eastern rain forest.

The stripe‐backed weasel Mustela strigidorsa: taxonomy, ecology, distribution and status

ABSTRACT The stripe‐backed weasel Mustela strigidorsa is one of the rarest and least‐known mustelids in the world. Its phylogenetic relationships with other Mustela species remain controversial, though several unique morphological features distinguish it from congeners. It probably lives mainly in evergreen forests in hills and mountains, but has also been recorded from plains forest, dense scrub, secondary forest, grassland and farmland. Known sites range in altitude from 90 m to 2500 m. Data are insufficient to distinguish between habitat and altitudes which support populations, and those where only dispersing animals may occur. It has been confirmed from many localities in north‐east India, north and central Myanmar, south China, north Thailand, north and central Laos, and north and central Vietnam. Given the limited survey effort, the number of recent records shows that the species is not as rare as hitherto believed. Neither specific nor urgent conservation needs are apparent.

Diversity and Range of Amphibians of the Yukon Territory

Four amphibian species occur in the Yukon: Western Toad (Bufo boreas; first verified record in 1961), the Boreal Chorus Frog (Pseudacris maculata, first record in 1995), the Columbia Spotted Frog (Rana luteiventris, first record in 1993), and the Wood Frog (Rana sylvatica; first record in 1933). The Western Toad is restricted to the Liard Basin in five geographically separated areas. Breeding sites have been located on the lower Coal River and vicinity. The Boreal Chorus Frog has been documented from a small area of the lower La Biche River valley near the Yukon-British Columbia-Northwest Territories border. The Columbia Spotted Frog occurs in two widely separated areas, at Bennett Lake in the southwest and in the Liard Basin in the southeast. The Wood Frog is widespread below treeline. Limited survey efforts continue to hinder our knowledge of amphibian distribution. Erratum included.

Estimates of forest elephant abundance: projecting the relationship between precision and effort

Summary A key element of African elephant conservation is monitoring how different forms of human activity influence elephant distribution and abundance. Information on human–elephant interaction is critical for guiding conservation action at all levels, from very local management interventions, through national protected area planning, to international policy initiatives such as the ban on ivory trade. However, methods for monitoring elephant numbers often produce rather poor estimate precision. Therefore, a major challenge for elephant monitoring programmes at all levels is to allocate survey effort efficiently: the greatest possible statistical power to detect the effects of particular forms of human activity must be obtained from limited survey effort.This study examined an approach to survey design based on the assumption that the precision of an abundance estimate is directly proportional to sampling effort. Field data from three sites in Central Africa were used to evaluate the potential of this approach for projecting how much effort will be necessary to survey and monitor forest elephants.Variance in elephant dung pile encounter rate was proportional to sampling effort, but variance was also influenced by mean encounter rate. The relationship between variance and mean encounter rate followed a power law more closely than the linear model suggested by many previous authors. The linear model tended to substantially underestimate the effort level required for areas with high encounter rates. A bootstrapping exercise suggested that modelling the precision–effort relationship across strata should produce more accurate projections than estimating a distinct precision–effort relationship for each stratum.Our results suggest that the survey design framework examined could substantially improve the statistical power of forest elephant monitoring programmes, although the use of a power law (rather than linear) model seems preferable. A similar approach should be appropriate for designing savanna elephant monitoring programmes and for addressing other issues, including rates of hunting and human elephant conflict. Furthermore, the parameter estimates reported in this study should be useful in designing elephant survey and monitoring programmes at other forested sites in Central Africa.

The avian community structure of a bolivian savanna on the edge of the Cerrado system

The avian community structure of a Bolivian savanna on the edge of the Cerrado ecosystem was studied during August and September 1994, using line-transects. The avifauna of two habitats, wet campo and dense campo, is described in detail. A third habitat, wooded savanna, is identified, but only briefly described due to limited survey effort. The diversity of these habitats is compared to data from other tropical studies. Areas of wet campo contained the greatest density of birds, but were characterized by a relatively simple bird cornmunity, formed primarily of granivorous, ground-gleaning species. Dense campo habitats, characterized by greater vegetation cover, supported a higher diversity of foraging guilds, including frugivores, insectivores and omnivores, exhibiting a variety of foraging strata and behaviours. Consequently, this habitat contained the greatest number of species. Bird species diversity was therefore related to vegetation structure and niche availability.

Evaluation of an Aerial Survey of Pacific Walruses (Odobenus rosmarus divergens)

An aerial survey of Pacific walruses (Odobenus rosmarus divergens) was evaluated to determine the reliability of estimates of population abundance. The probability of detecting groups of walruses on the pack ice remained uniform to at least 0.93 km from the flight line, whereas the probability of detection decreased significantly beyond 0.23 km for walruses in the water. Walruses were more abundant along the ice-edge zone between 162 and 165°W than in other areas of the Chukchi Sea during September 1975. Few walruses were observed in consolidated pack ice north of the ice-edge zone or in ice-free water to the south. More walrus groups and larger mean group size were observed on September 8 than on other days. We estimated abundance for each day and all days combined using methods based on sample area and numbers of strip samples. Estimates varied among days by over an order of magnitude; this variation is attributed to the combined effect of chance sampling of an aggregated population and variation in the fraction of walruses hauled out. The coefficient of variation of the estimates ranged between 0.25 and 0.99. This imprecision was due to the aggregated distribution of walruses and the large variation in group size. Using the survey data as a basis for stratification, we calculated that, due to the high variability within strata, a sample size of 40% of the total area or 56% of the total available strips would be required to obtain 95% confidence limits within 10% of the estimate of total abundance. Variation contributed by observer error in estimating group size also is relatively unimportant to the precision of abundance estimates. Studies of natural history, particularly those oriented toward activity and habitat selection, would help investigators estimate bias due to the variable fraction hauled out and design surveys based on meaningful strata. Estimates of total abundance based on limited survey efforts will provide information of little reliability. Key words: aerial survey, aggregation, Chukchi Sea, estimation, population, stratification, walrus