Distance Sampling Surveys Research Articles

Density, Abundance, and Habitat Associations of the Inland Swamp Sparrow (Melospiza georgiana georgiana) in Iowa

Wetlands continue to decline throughout North America and the Prairie Pothole Region, thus emphasizing the importance of understanding population trends and habitat associations of wetland species to ensure effective conservation and habitat management of those species. We estimated density and abundance and evaluated habitat associations of the Inland Swamp Sparrow (Melospiza georgiana georgiana) in Iowa. We conducted standardized distance sampling surveys for Swamp Sparrows and measured habitat characteristics at 307 wetlands in two regions of Iowa in 2009 and 2010. We used Program Distance to model detection probability and estimate region-specific breeding densities of Swamp Sparrows at Iowa wetlands. We then extrapolated density estimates to the total area of wetlands in each region to obtain estimates of breeding abundance. We correlated Swamp Sparrow counts to nine habitat variables using Poisson regression in Program R. Swamp Sparrow counts were positively correlated with percent cover of cattail (Typha spp.) and water depth (cm) and negatively correlated with percent cover of woody vegetation, vegetation size (m), and wetland size (ha). We estimated breeding densities of Swamp Sparrows to be 1.488 birds/ha (95% CI 5 1.308 � 1.692) in region 1 (Des Moines Lobe landform) and 0.041 birds/ha (95% CI 5 0.006 � 0.275) in region 2 (remainder of the state). Our results, in comparison to those of other studies, indicate that Swamp Sparrows associate with a variety of wetland characteristics depending upon what is available. Swamp Sparrows are relatively uncommon breeders in Iowa, and our work confirms that most occur in the Des Moines Lobe landform in north-central and northwestern Iowa. Biologists and land managers should incorporate our findings on this species' habitat associations into management activities to ensure that Swamp Sparrow populations persist into the future. Received 5 January 2015. Accepted 30 April 2015.

Current Land Bird Distribution and Trends in Population Abundance Between 1982 and 2012 on Rota, Mariana Islands

Abstract The western Pacific island of Rota is the fourth largest human-inhabited island in the Mariana archipelago and designated an Endemic Bird Area. Between 1982 and 2012, 12 point-transect distance-sampling surveys were conducted to assess bird population status. Surveys did not consistently sample the entire island; thus, we used a ratio estimator to estimate bird abundances in strata not sampled during every survey. Trends in population size were reliably estimated for 11 of 13 bird species, and 7 species declined over the 30-y time series, including the island collared-dove Streptopelia bitorquata, white-throated ground-dove Gallicolumba xanthonura, Mariana fruit-dove Ptilinopus roseicapilla, collared kingfisher Todiramphus chloris orii, Micronesian myzomela Myzomela rubratra, black drongo Dicrurus macrocercus, and Mariana crow Corvus kubaryi. The endangered Mariana crow (x̄ = 81 birds, 95% CI 30–202) declined sharply to fewer than 200 individuals in 2012, down from 1,491 birds in 1982 (95% CI = 815–3,115). Trends increased for white tern Gygis alba, rufous fantail Rhipidura rufifrons mariae, and Micronesian starling Aplonis opaca. Numbers of the endangered Rota white-eye Zosterops rotensis declined from 1982 to the late 1990s but returned to 1980s levels by 2012, resulting in an overall stable trend. Trends for the yellow bittern Ixobrychus sinensis were inconclusive. Eurasian tree sparrow Passer montanus trends were not assessed; however, their numbers in 1982 and 2012 were similar. Occupancy models of the 2012 survey data revealed general patterns of land cover use and detectability among 12 species that could be reliably modeled. Occupancy was not assessed for the Eurasian tree sparrow because of insufficient detections. Based on the 2012 survey, bird distribution and abundance across Rota revealed three general patterns: 1) range restriction, including Mariana crow, Rota white-eye, and Eurasian tree sparrow; 2) widespread distribution, low abundance, including collared kingfisher, island collared-dove, white-throated ground-dove, Mariana fruit-dove, white tern, yellow bittern, black drongo, and Micronesian myzomela; and 3) widespread distribution, high abundance, including rufous fantail and Micronesian starling. The Mariana crow was dispersed around the periphery of the island in steep forested land-cover types. In contrast, the Rota white-eye was restricted to the high-elevation mesa. Only for the white-throated ground-dove was there a significant difference among cover types, with lower occupancy in open field than in forested areas. Vegetation was included in the best-fit occupancy models for yellow bittern, black drongo, Micronesian myzomela, and Micronesian starling, but vegetation type was not a significant variable nor included in the top models for the remaining five species: white tern, island collared-dove, Mariana fruit-dove, collared kingfisher, and rufous fantail. Given declining population trends, the Rota bird-monitoring program could benefit from establishing threshold and alert limits and identifying alternative research and management actions. Continued monitoring and demographic sampling, in conjunction with ecological studies, are needed to understand why most bird species on Rota are declining, identify the causative agents, and assess effectiveness of conservation actions, especially for the Mariana crow.

Trends of harbour porpoise (Phocoena phocoena) density in the southern North Sea

In the southern North Sea, harbour porpoise occurrence increased in recent years after a phase of low abundances during earlier decades. Only very few studies on porpoise presence in the southern German North Sea exist so far. As anthropogenic activities will strongly increase in this part of the North Sea during the next years it is most important to assess population level effects. This study focuses on the analysis of temporal and spatial trends in porpoise density in this area of recent change. Dedicated aerial line-transect distance sampling surveys were conducted in the southern German North Sea between May 2002 and June 2013 to assess porpoise density and distribution. Statistical inferences on porpoise population trends were made using a Markov Chain Monte Carlo (MCMC) technique. Two approaches were chosen to test for a trend in porpoise density and an additional model focused on the change in density of calves. During 55,820km of survey effort 4377 porpoises including 140 calves were recorded. A significant effect of increasing spatial aggregation from the lower density areas in the south-eastern German Bight to hot spot areas in the western parts was detected. For the western part of the study area a significant increase in porpoise density between 2002 and 2013 was detected. Seasons were significantly different with highest porpoise density in spring and successively decreasing densities in summer and autumn. From 2008 onwards high densities were also observed in summer. Calf density increased during the study period and was significantly higher in the west. On the basis of this extensive and unique data set on porpoise occurrence in the southern German North Sea the findings clearly show that especially the south-western German North Sea serves as habitat of increasing importance for porpoises throughout the last decade. Definite reasons still remain unresolved. Changes in prey abundance or less favourable conditions in other areas could be important factors, which may also have caused a southward shift from high density areas in northern waters. On this baseline, further integrative approaches might lead to a sound understanding of the effect of anthropogenic activities on the future development of porpoise populations.

Mixture models for distance sampling detection functions.

We present a new class of models for the detection function in distance sampling surveys of wildlife populations, based on finite mixtures of simple parametric key functions such as the half-normal. The models share many of the features of the widely-used “key function plus series adjustment” (K+A) formulation: they are flexible, produce plausible shapes with a small number of parameters, allow incorporation of covariates in addition to distance and can be fitted using maximum likelihood. One important advantage over the K+A approach is that the mixtures are automatically monotonic non-increasing and non-negative, so constrained optimization is not required to ensure distance sampling assumptions are honoured. We compare the mixture formulation to the K+A approach using simulations to evaluate its applicability in a wide set of challenging situations. We also re-analyze four previously problematic real-world case studies. We find mixtures outperform K+A methods in many cases, particularly spiked line transect data (i.e., where detectability drops rapidly at small distances) and larger sample sizes. We recommend that current standard model selection methods for distance sampling detection functions are extended to include mixture models in the candidate set.

An open-population hierarchical distance sampling model.

Modeling population dynamics while accounting for imperfect detection is essential to monitoring programs. Distance sampling allows estimating population size while accounting for imperfect detection, but existing methods do not allow for estimation of demographic parameters. We develop a model that uses temporal correlation in abundance arising from underlying population dynamics to estimate demographic parameters from repeated distance sampling surveys. Using a simulation study motivated by designing a monitoring program for Island Scrub-Jays (Aphelocoma insularis), we investigated the power of this model to detect population trends. We generated temporally autocorrelated abundance and distance sampling data over six surveys, using population rates of change of 0.95 and 0.90. We fit the data generating Markovian model and a mis-specified model with a log-linear time effect on abundance, and derived post hoc trend estimates from a model estimating abundance for each survey separately. We performed these analyses for varying numbers of survey points. Power to detect population changes was consistently greater under the Markov model than under the alternatives, particularly for reduced numbers of survey points. The model can readily be extended to more complex demographic processes than considered in our simulations. This novel framework can be widely adopted for wildlife population monitoring.

A Unifying Model for Capture–Recapture and Distance Sampling Surveys of Wildlife Populations

A fundamental problem in wildlife ecology and management is estimation of population size or density. The two dominant methods in this area are capture–recapture (CR) and distance sampling (DS), each with its own largely separate literature. We develop a class of models that synthesizes them. It accommodates a spectrum of models ranging from nonspatial CR models (with no information on animal locations) through to DS and mark-recapture distance sampling (MRDS) models, in which animal locations are observed without error. Between these lie spatially explicit capture–recapture (SECR) models that include only capture locations, and a variety of models with less location data than are typical of DS surveys but more than are normally used on SECR surveys. In addition to unifying CR and DS models, the class provides a means of improving inference from SECR models by adding supplementary location data, and a means of incorporating measurement error into DS and MRDS models. We illustrate their utility by comparing inference on acoustic surveys of gibbons and frogs using only capture locations, using estimated angles (gibbons) and combinations of received signal strength and time-of-arrival data (frogs), and on a visual MRDS survey of whales, comparing estimates with exact and estimated distances. Supplementary materials for this article are available online.

Rainforest disturbance affects population density of the northern cassowary Casuarius unappendiculatus in Papua, Indonesia

AbstractNominally protected areas in Papua are under threat from encroachment, logging and hunting. The northern cassowary Casuarius unappendiculatus is the largest frugivore of the lowland rainforest of New Guinea and is endemic to this region, and therefore it is an important conservation target and a potential flagship species. We investigated effects of habitat degradation on the species by means of distance sampling surveys of 58 line transects across five distinct habitats, from primary forest to forest gardens. Estimated cassowary densities ranged from 14.1 (95% CI 9.2–21.4) birds km−2 in primary forest to 1.4 (95% CI 0.4–5.6) birds km−2 in forest garden. Density estimates were intermediate in unlogged but hunted natural forest and in > 30 year-old secondary forest, and considerably lower in recently logged (< 3 years) forest. Cassowary abundance was positively correlated with canopy cover and with tree height and diameter, and negatively correlated with hunting traps and human trails. In generalized linear models cassowary abundance was best explained by the number of fruiting trees and potential water sources. The results suggest that although the northern cassowary is moderately tolerant of intermediate disturbance it is relatively intolerant of heavy disturbance such as intensive logging. To secure the populations of northern cassowary and of other large animal species in Papua, forest degradation needs to be addressed by enforcing regulations in existing protection forest and/or establishing new protected areas, such as wildlife reserves.

New density estimates of a threatened sifaka species (Propithecus coquereli) in Ankarafantsika National Park

Propithecus coquereli is one of the last sifaka species for which no reliable and extensive density estimates are yet available. Despite its endangered conservation status [IUCN, 2012] and recognition as a flagship species of the northwestern dry forests of Madagascar, its population in its last main refugium, the Ankarafantsika National Park (ANP), is still poorly known. Using line transect distance sampling surveys we estimated population density and abundance in the ANP. Furthermore, we investigated the effects of road, forest edge, river proximity and group size on sighting frequencies, and density estimates. We provide here the first population density estimates throughout the ANP. We found that density varied greatly among surveyed sites (from 5 to ∼100 ind/km2) which could result from significant (negative) effects of road, and forest edge, and/or a (positive) effect of river proximity. Our results also suggest that the population size may be ∼47,000 individuals in the ANP, hinting that the population likely underwent a strong decline in some parts of the Park in recent decades, possibly caused by habitat loss from fires and charcoal production and by poaching. We suggest community-based conservation actions for the largest remaining population of Coquerel's sifaka which will (i) maintain forest connectivity; (ii) implement alternatives to deforestation through charcoal production, logging, and grass fires; (iii) reduce poaching; and (iv) enable long-term monitoring of the population in collaboration with local authorities and researchers.

Reducing effort while improving inference: Estimating Dall's sheep abundance and composition in small areas

Recent work has demonstrated that aerial distance sampling surveys are more efficient and effective than unadjusted minimum count surveys for estimating Dall's sheep (Ovis dalli) abundance, although large sample size requirements (e.g., 150-200 detections) may discourage implementation of these methods in small (<2,500 km 2 ) or low density areas. However, a Bayesian analytical approach using informed priors and borrowing detection information across surveys can increase precision and decrease required sample sizes. Using these methods, we conducted distance sampling surveys across a majority of the Dall's sheep habitat within National Park Service units in Alaska during 2010-2011. We compared 4 analytical scenarios using increasing amounts of detection information to demonstrate the increases in efficiency that can be gained over time through the use of this approach. Based on our analysis using all available survey information in the estimation of the detection function, we estimated that 2,252 (1,871- 2,765), 2,809 (2,361-3,379), 1,669 (1,339-2,120), and 12,428 (10,780-14,470) sheep occurred in Denali National Park and Preserve (DENA), the Western Arctic National Parklands (WEAR), the Itkillik preserve subarea of Gates of the Arctic National Park and Preserve, and Wrangell-St. Elias National Park and Preserve (WRST), respectively. These estimates were achieved with relatively small numbers of group detections in DENA (n ¼ 57), the Itkillik preserve area (n ¼ 48), and WEAR (n ¼ 100), suggesting that sample size requirements for Dall's sheep distance sampling surveys can be reduced by an additional 50-75% over previously recommended levels when adequate prior information is available. In addition, we describe a formal approach for estimating the size of individual composition classes (i.e., lambs, ewe-like sheep, <full- curl rams, � full-curl rams) and sex and age ratios, corrected for incomplete detection. We implemented the composition analysis within the distance sampling analytical framework as part of the abundance estimation process.This approach to theestimation ofpopulation composition could replace commonly usedindices and provides more detailed and rigorous estimates that are directly comparable among survey areas and years. We found that ratios offull curl rams:100 ewe-like sheep in WEAR and Itkillik preserve subarea populations were less than in the DENA population, whereas lamb:ewe-like ratios were similar across all 4 survey areas, suggesting relatively consistent productivity. We recommend that aerial distance sampling survey methods using prior information, combined with direct estimation of population composition, be used to increase the effectiveness of Dall's sheep population monitoring and management throughout their range. 2013 The Wildlife Society.

Effects of pile-driving on harbour porpoises (Phocoena phocoena) at the first offshore wind farm in Germany

The first offshore wind farm ‘alpha ventus’ in the German North Sea was constructed north east of Borkum Reef Ground approximately 45 km north off the German coast in 2008 and 2009 using percussive piling for the foundations of 12 wind turbines. Visual monitoring of harbour porpoises was conducted prior to as well as during construction and operation by means of 15 aerial line transect distance sampling surveys, from 2008 to 2010. Static acoustic monitoring (SAM) with echolocation click loggers at 12 positions was performed additionally from 2008 to 2011. SAM devices were deployed between 1 and 50 km from the centre of the wind farm. During aerial surveys, 18 600 km of transect lines were covered in two survey areas (10 934 and 11 824 km2) and 1392 harbour porpoise sightings were recorded. Lowest densities were documented during the construction period in 2009. The spatial distribution pattern recorded on two aerial surveys three weeks before and exactly during pile-driving points towards a strong avoidance response within 20 km distance of the noise source. Generalized additive modelling of SAM data showed a negative impact of pile-driving on relative porpoise detection rates at eight positions at distances less than 10.8 km. Increased detection rates were found at two positions at 25 and 50 km distance suggesting that porpoises were displaced towards these positions. A pile-driving related behavioural reaction could thus be detected using SAM at a much larger distance than a pure avoidance radius would suggest. The first waiting time (interval between porpoise detections of at least 10 min), after piling started, increased with longer piling durations. A gradient in avoidance, a gradual fading of the avoidance reaction with increasing distance from the piling site, is hence most probably a product of an incomplete displacement during shorter piling events.

Accounting for animal density gradients using independent information in distance sampling surveys

Distance sampling is extensively used for estimating animal density or abundance. Conventional methods assume that location of line or point transects is random with respect to the animal population, yet transects are often placed along linear features such as roads, rivers or shorelines that do not randomly sample the study region, resulting in biased estimates of abundance. If it is possible to collect additional data that allow an animal density gradient with respect to the transects to be modelled, we show how to extend the conventional distance sampling likelihood to give asymptotically unbiased estimates of density for the covered area. We illustrate the proposed methods using data for a kangaroo population surveyed by line transects laid along tracks, for which the true density is known from an independent source, and the density gradient with respect to the tracks is estimated from a sample of GPS collared animals. For this example, density of animals increases with distance from the tracks, so that detection probability is overestimated and density underestimated if the non-random location of transects is ignored. When we account for the density gradient, there is no evidence of bias in the abundance estimate. We end with a list of practical recommendations to investigators conducting distance sampling surveys where density gradients could be an issue.

Identifying conservation successes, failures and future opportunities; assessing recovery potential of wild ungulates and tigers in Eastern Cambodia.

Conservation investment, particularly for charismatic and wide-ranging large mammal species, needs to be evidence-based. Despite the prevalence of this theme within the literature, examples of robust data being generated to guide conservation policy and funding decisions are rare. We present the first published case-study of tiger conservation in Indochina, from a site where an evidence-based approach has been implemented for this iconic predator and its prey. Despite the persistence of extensive areas of habitat, Indochina's tiger and ungulate prey populations are widely supposed to have precipitously declined in recent decades. The Seima Protection Forest (SPF), and broader Eastern Plains Landscape, was identified in 2000 as representing Cambodia's best hope for tiger recovery; reflected in its designation as a Global Priority Tiger Conservation Landscape. Since 2005 distance sampling, camera-trapping and detection-dog surveys have been employed to assess the recovery potential of ungulate and tiger populations in SPF. Our results show that while conservation efforts have ensured that small but regionally significant populations of larger ungulates persist, and density trends in smaller ungulates are stable, overall ungulate populations remain well below theoretical carrying capacity. Extensive field surveys failed to yield any evidence of tiger, and we contend that there is no longer a resident population within the SPF. This local extirpation is believed to be primarily attributable to two decades of intensive hunting; but importantly, prey densities are also currently below the level necessary to support a viable tiger population. Based on these results and similar findings from neighbouring sites, Eastern Cambodia does not currently constitute a Tiger Source Site nor meet the criteria of a Global Priority Tiger Landscape. However, SPF retains global importance for many other elements of biodiversity. It retains high regional importance for ungulate populations and potentially in the future for Indochinese tigers, given adequate prey and protection.

MAXIMIZING NONHUMAN PRIMATE FECAL SAMPLING IN THE REPUBLIC OF CONGO

Techniques for detection of pathogens in wildlife feces allow disease surveillance of species that are difficult to locate and capture (e.g., great apes). However, optimal strategies for detection of feces in logistically challenging environments, such as the forests of Central Africa, have not been developed. We modeled fecal gorilla sampling in the Republic of Congo with computer simulations to explore the performance of different fecal sampling designs in large tropical landscapes. We simulated directed reconnaissance walk (recce) and line-transect distance-sampling survey designs and combinations thereof to maximize the number of fecal samples collected, while also estimating relative ape density on a virtual landscape. We analyzed the performance of different sampling designs across different densities and distributions of ape populations, assessing each for accuracy as well as cost and time efficiencies. Past ape density surveys and fecal deposition rates were used to parameterize the simulated fecal sampling designs. Our results showed that a mixed sampling design that combines traditional transect and a directed reconnaissance sampling design maximized the number of fecal samples collected and estimates of species density. Targeted sampling produced strongly biased estimates of population abundance but maximized efficiency. This research will help design the fecal sampling component of a larger study relating great ape density to Ebola fecal antibody prevalence.

Investigating the potential use of aerial line transect surveys for estimating polar bear abundance in sea ice habitats: A case study for the Chukchi Sea

AbstractThe expense of traditional capture‐recapture methods, interest in less invasive survey methods, and the circumpolar decline of polar bear (Ursus maritimus) habitat require evaluation of alternative methods for monitoring polar bear populations. Aerial line transect distance sampling (DS) surveys are thought to be a promising monitoring tool. However, low densities and few observations during a survey can result in low precision, and logistical constraints such as heavy ice and fuel and safety limitations may restrict survey coverage. We used simulations to investigate the accuracy and precision of, DS for estimating polar bear abundance in sea ice habitats, using the Chukchi Sea subpopulation as an example. Simulation parameters were informed from a recent pilot survey. Predictions from a resource selection model were used for stratification, and we compared two ratio estimators to account for areas that cannot be sampled. The ratio estimator using predictions of resource selection by polar bears allowed for extrapolation beyond sampled areas and provided results with low bias and CVs ranging from 21% to 36% when abundance was &gt;1,000. These techniques could be applied to other DS surveys to allocate effort and potentially extrapolate estimates to include portions of the landscape that are logistically impossible to survey.

Standardising distance sampling surveys of parrots in New Caledonia

Standardised surveys are essential for monitoring populations and identifying areas that are critical for conservation. With the aim of developing a standardised method of surveying parrots in the rainforests of New Caledonia, we used distance sampling to estimate densities of New Caledonian Parakeets (Cyanoramphus saisseti), Horned Parakeets (Eunymphicus cornutus), Ouvea Parakeets (E. uvaeensis), and New Caledonian Rainbow Lorikeets (Trichoglossus haematodus deplanchii). We carried out surveys in the early morning and late afternoon, when parrots were easiest to detect. To minimise errors associated with estimating distances and flock sizes by ear, we conducted brief searches to locate parrots, then measured their distance from the transect line. We recorded birds in flight and consider these records to be important when estimating parakeet populations. In agreement with existing knowledge on distance sampling, we found line transects to be more efficient than point transects for estimating the density of parakeets. Our results indicate that parrots located beyond 50–70 m from the transect line have little influence upon density estimates. In addition, surveys on roads are likely to underestimate densities if not corrected for road width. We generated relatively stable and precise density estimates (CV < 0.25) with approximately 40–50 detections, yet additional effort may be warranted under different study conditions. Although we aimed to improve parrot surveys in New Caledonia, our suggestions may be useful to other researchers studying rainforest birds, and can be adapted to suit different species or environments.

Status of the relict population of the Critically Endangered Madagascar spider tortoise Pyxis arachnoides

AbstractThe Madagascar spider tortoise Pyxis arachnoides is endemic to the coastal dry forests of south-west Madagascar. In recent years its range has been reduced by c. 71%, with the species now confined to eight fragmented populations occupying a total of 2,464 km2. These remaining populations are facing a significant threat of extinction because of habitat destruction and other anthropogenic pressures. We developed methodology for a line transect distance sampling survey and applied it systematically across the species' range. The resulting distance model estimated a mean density of 226.9 tortoises km−2 (95% confidence interval, CI, 168.1–306.3) and a total population of 664,980 (95% CI 492,680–897,550). Fragmentation of the species’ range suggests the current population could be &lt;30% of the historical population. Of the remaining population 73.5% falls within protected areas. However, nine of these 12 protected areas are designated as IUCN category III, V or VI parks, allowing some extractive activities to be undertaken. The most effective strategy for the conservation of P. arachnoides would be to reduce the threats to this species and its habitat, and to develop and expand the current community-based conservation and poverty alleviation programmes in the region.

Ecological and Environmental Modelling Workshops

The Centre for Research into Ecological and Environmental Modelling (CREEM) is hosting three linked workshops in the summer of 2012 in our purpose-built facilities at the University of St Andrews, Scotland. The aim of these workshops is to train participants in the latest methods for design and analysis of distance sampling surveys, including line and point transects. The workshops are taught by leading researchers in the field, using industry-standard software. The first workshop (21–24 August) will run at an introductory level, and will focus on “conventional” distance sampling methods, as described in the standard reference book Introduction to Distance Sampling. The workshop will be a blend of theory and practice, and participants will learn how to use the program “Distance.” Participants will gain a solid grounding in both survey design and methods of analysis for distance sampling surveys. Note that we have moved the “automated survey design” topic out of the advanced workshop into the introductory workshop. The advanced distance sampling workshop (27–30 August) will include advanced treatment of: incorporating covariates in detection function modeling, analyses in which detectability on the transect line is not assumed to be perfect (the so-called g(0) problem) and spatial (or density surface) modeling. The aim of this workshop is to bring participants up to date with the latest developments in distance sampling methods and software. It is also an opportunity for those actively engaged in the design, analysis, and execution of distance sampling surveys to discuss common issues and problems, and set future research directions. The workshop will be a combination of lectures and computer sessions, with considerable time for discussion. In addition, this year we are running a one-day introduction to density estimation from passive acoustic data (25 August). Passive acoustic monitoring is often used to obtain indices of relative abundance, such as number of detections per unit time. However, for many applications, the quantity we really want to estimate is absolute population abundance (i.e., the number of animals in the population) or density (number of animals per unit area). This day-long tutorial will give an insight into methods to achieve this. For all workshops, participants are encouraged to bring their own data sets, and can expect to do some preliminary analyses with their data. Computer sessions take place in our modern computer classroom (attached to the seminar room); participants can use our computers or bring their own laptops. Additional details regarding the workshop can be found at our web site 〈http://www.ruwpa.st-and.ac.uk/distance.workshops/distance2012/workshop_overview.html〉

Population and prey of the Bengal Tiger Panthera tigris tigris (Linnaeus, 1758) (Carnivora: Felidae) in the Sundarbans, Bangladesh

The results from intensive small scale surveys are often difficult to extrapolate to wider spatial scales, yet an understanding at such scales is critical for assessing the minimum densities and populations of rare and wide ranging species. In this paper, the minimum size of population and minimum density estimates of Bengal Tigers Panthera tigris tigris and its prey were conducted from 2005 to 2007 using camera traps for 90 days and using distance sampling surveys for over 200 days, respectively. The results were extrapolated from the core study area in Katka-Kochikhali, southeastern Sundarbans, to five additional sites using indices of abundance. With the use of 10 camera-traps at 15 trap-points, field data provided a total of 829 photos, including seven photos of five individual tigers. A total of 5.0 (SE = 0.98) tigers (adults and sub-adults) are thus estimated in the core area with an estimated density of 4.8 tigers/100km2. Distance sampling surveys conducted on large mammalian prey species obtained an overall density estimate of 27.9 individuals/km2 and a biomass density of 1,037kg/km2. Indices of abundance were obtained by using tiger track sighting rates (number of tracks/km of riverbank) and the sighting rates of the prey species (number of prey/km of riverbank) in the core area and in five additional sites across the region. The densities of tiger tracks and sighting rates of prey were strongly correlated suggesting a wide scale relationship between predator and prey in the region. By combining the estimates of absolute density with indices of abundance, an average of 3.7 tigers/100km2 across the region is estimated, which given an area of 5,770km2, predicts a minimum of approximately 200 tigers in the Bangladesh Sundarbans.

Gentle Giants in Dark Waters: Using Side-Scan Sonar for Manatee Research

Abstract: Manatees are tropical marine mammals that live in a wide variety of aquatic habitats ranging from coastal marine areas to freshwater lakes and rivers located hundreds of kilometers inland. All manatee species are currently characterized as Vulnerable by the International Union for Conservation of Nature (IUCN), primarily due to anthropogenic causes (i.e. poaching, habitat destruction, pollution, collisions with boats) and all of the environments they live in present challenges to biologists and wildlife managers. Tropical freshwater systems are especially difficult to work in because they are generally tannin-stained or turbid and water clarity is very restricted. Locating manatees and observing their behavior in these environments is extremely difficult and is a major limiting factor in determining population parameters. We summarize the use of sonar technology as an aid to detect and study manatees in these freshwater systems. First, by a summary of what has been attempted before our efforts, followed by presenting the various ways we have used this technology in the past 6 years to 1) detect manatees, 2) characterize manatee habitat in ways that would not be possible otherwise, 3) identify mother-calf pairs, and 4) assist in manatee captures. Finally, we discuss the advantages and limitations of using this technology for manatee conservation and research and present directions in which we believe future work can be directed, such as the determination of manatee abundance via distance sampling surveys using side-scan sonar and the use of one of the latest developments in acoustic technology, dual-frequency identification sonar.

Small Population Size of the Pribilof Rock Sandpiper Confirmed through Distance-Sampling Surveys in Alaska

Abstract. The Rock Sandpiper (Calidris ptilocnemis) is endemic to the Bering Sea region and unique among shorebirds in the North Pacific for wintering at high latitudes. The nominate subspecies, the Pribilof Rock Sandpiper (C. p. ptilocnemis), breeds on four isolated islands in the Bering Sea and appears to spend the winter primarily in Cook Inlet, Alaska. We used a stratified systematic sampling design and line-transect method to survey the entire breeding range of this population during springs 2001–2003. Densities were up to four times higher on the uninhabited and more northerly St. Matthew and Hall islands than on St. Paul and St. George islands, which both have small human settlements and introduced reindeer herds. Differences in density, however, appeared to be more related to differences in vegetation than to anthropogenic factors, raising some concern for prospective effects of climate change. We estimated the total population at 19 832 birds (95% CI 17 853–21 930), ranking it among the smallest ...