Suchergebnisse

Mark–recapture and line-transect sampling procedures both provide estimators for visibility bias in aerial surveys, and have coexisted in the literature for decades. Mark–recapture estimators of abundance tend to be negatively biased in this context as a result of unmodelled heterogeneity. Line-transect sampling can also be negatively biased if detection probability on the line is less than 1.0. Numerous papers have described hybrid approaches using mark–recapture and line transect methods but there have been some subtle but important differences that may not be apparent to the practitioner. We have used wild horse survey data collected in south-eastern Australia and some imaginary data to highlight these subtle differences. We demonstrate the advantage of using the hybrid approach, which uses the strengths of both mark–recapture and line-transect procedures by fitting a detection function (with p(0) = 1) to the line-transect data to estimate the shape of the detection function, and uses a separate detection function for the mark–recapture data to estimate the intercept (p(0)).

Aerial mark–recapture population estimates utilising the natural markings of wild horses to identify individuals was applied in the Bogong High Plains, Alpine National Park, Victoria. A discrete population of wild horses occupying an area of 180 km2 was sampled over two days in 2005. This study explored the feasibility of a technique that aimed to enable managers to estimate the size of the horse population and monitor it over time. Four observers (including the pilot) searched for horses from a helicopter. Once horses were sighted, photographic and written observations were used to 'mark' each animal. The survey was repeated the following day with observations 'recapturing' individuals. Data were analysed using several mark–recapture estimators, and the derived population estimates ranged from 89 (±5.3, s.e.) horses to 94.7 (±7.9, s.e.) horses. We found that the method gave a level of precision relevant to management, but needs refinement. The technique and its assumptions should be tested further by increasing the number of samples and video should be used to improve identification of individuals. We believe that this is a novel application for aerial surveys, which are typically unsuitable for estimating the size of small populations. This technique was developed for horses but may be used on other conspicuous species with unique natural markings.

The cane toad (Bufo marinus) has received considerable attention because of its rapid spread in Australia and
the potential threat it may represent to native species. Although the introduction of pathogens from native
populations is now being considered to control this species, population estimates based on comparable
methods that demonstrate that native populations are in fact less dense than the introduced ones are not
available. Accurate population estimates are necessary to evaluate potential techniques for the control of the
cane toad. We estimated population densities of cane toads over a wide range of habitat types and climate
conditions by means of mark-recapture data. The capture history and location of toads each night were
analysed to explore the validity of some of the assumptions of mark-recapture models. Because migrations,
deaths and recruitment over three nights appear to be unimportant, populations may be legitimately regarded
as closed for that period. However, cane toads seem highly sensitive to disturbance effects due to trapping
andlor handling. Consequently, density estimates based on removal methods seem the most reliable because
they are not sensitive to handling and trapping effects. Similarly, analyses of residuals of regressions between
1-night counts and density estimates suggested that toad nightly activity is affected by the air temperature
during sampling. However, only 60% of the variation in estimated densities can be predicted by 1-night
counts and air temperature. Estimates of population density over a wide range of habitats in South America
were one order of magnitude lower than estimates in Australia. We speculate on the possible factors that may
account for the lower densities in populations in the native range of the species.

The key difficulty in assessing animal numbers from the air is that not all animals are seen by the observers. Methods for estimating detection probabilities, or accounting for imperfect detection, are reviewed including double surveys, use of sightability models, mark–resight, and mark–recapture. The assumptions needed for each method are considered as well as issues concerning survey design. For closed-population mark–recapture modelling particular attention is given to multiple observer studies. An emphasis is that an assumption of complete independence in double-observer studies is rarely justifiable and that independent observers will generally only satisfy an assumption of conditional independence and not complete independence.

Context
Optimal management of invasive species should determine the interval between lethal-control operations that will sustain a desired population suppression at minimum cost. This requires an understanding of the species' rate of recruitment following control. These data are difficult to acquire for vertebrate carnivores such as the red fox (Vulpes vulpes), which are not readily trapped or observed.

Aims
To provide a long-term evaluation of the effects of 1080 poison baiting on the abundance and extent of movement of red foxes in a semiarid environment.

Methods
We used non‐invasive DNA sampling of fox hairs in semi-arid Western Australia where the population was subject to two episodes of aerially delivered sodium fluoroacetate (1080) poison baits within 12 months. Sampling took place at ~45-day intervals and individual foxes were identified by genotyping eight microsatellite DNA markers and a gender-specific marker. Open-population and spatially explicit mark–recapture models were used to estimate the density, apparent survival and movements of foxes before and following baiting.

Key results
Following a severe reduction in density after baiting, fox density during the ensuing 12 months increased slowly (0.01 foxes km–2 month–1), such that density had only reached 22% of pre-baiting levels ~10 months after the initial baiting. Moreover, recovery was non‐linear as population growth was negligible for 6 months, then exhibited a nine-fold increase 7–9 months after control, coincident with the dispersal of juveniles in autumn. Fox movements between recaptures were on average 470% greater after baiting than before, in line with expectations for low-density populations, suggesting that the probability of encountering baits during this period would be higher than before baiting.

Conclusions
Baiting with 1080 poison significantly reduced the density of foxes, and the low density was sustained for more than 6 months. Foxes moved significantly further between recaptures after baiting when at low densities.

Implications
Control programs in this region may be carried out at low frequency to suppress fox density to a fraction of unbaited levels. The intensity of follow-up baiting may also be adjusted downwards, to take account of an increased probability of bait encounter in more mobile foxes.

Dissertação de Mestrado apresentada no ISPA – Instituto Universitário para obtenção de grau de Mestre na especialidade de Biologia Marinha e Conservação ; A identificação de flutuações demográficas é um dos aspetos centrais em estudos de dinâmica populacional. A evolução do tamanho da população e a obtenção de estimativas da probabilidade de sobrevivência e recrutamento são informações cruciais no delineamento de estratégias de conservação. Neste trabalho foram aplicadas duas abordagens de modelos de captura-recaptura a dados de foto-identificação, recolhidos entre 2009 e 2015, do odontocete Peponocephala electra, numa área de testes militares (AUTEC), nas Bahamas. Foram implementados modelos para populações fechadas e abertas para estimar a abundância, sobrevivência, recrutamento e probabilidade de captura. Os modelos foram implementados com recurso à metodologia Markov Chain Monte Carlo (MCMC), numa perspetiva Bayesiana. Foram avistados e fotografados 410 indivíduos distintos. e O número de vezes que o mesmo indivíduo foi avistado ao longo dos 6 anos de estudo variou entre 1 a 6. Os resultados sugerem um declínio no tamanho da população, associado a uma baixa taxa de recrutamento ao longo do tempo. Com o modelo para populações abertas estimou-se que um total de 558 (95% CRI = 547.00 – 561.00) indivíduos utilizaram a área de estudo pelo menos uma vez entre 2009 e 2015. Os resultados apontam para uma população aberta, sazonal, sugerindo uma distribuição que se estende para além da área de estudo. Porém, os dados suportam o uso regular da área durante o período primavera/verão. Este estudo estabelece as primeiras estimativas de abundância, sobrevivência e recrutamento para esta espécie, nas Bahamas, e destaca a importância de estudos de longa duração na estimação de parâmetros demográficos. ; Identifying demographic changes is fundamental for understanding population dynamics. Population trends, reliable estimates of population sizes and survival and recruitment are among the most important demographic parameters ...

Closed-population capture–mark–recapture (CMR) methods can produce biased density estimates for species with low or heterogeneous detection probabilities. In an attempt to address such biases, we developed a density-estimation method based on the change in ratio (CIR) of survival between two populations where survival, calculated using an open-population CMR model, is known to differ. We used our method to estimate density for a feral pig (Sus scrofa) population on Fort Benning, Georgia, USA. To assess its validity, we compared it to an estimate of the minimum density of pigs known to be alive and two estimates based on closed-population CMR models. Comparison of the density estimates revealed that the CIR estimator produced a density estimate with low precision that was reasonable with respect to minimum known density. By contrast, density point estimates using the closed-population CMR models were less than the minimum known density, consistent with biases created by low and heterogeneous capture probabilities for species like feral pigs that may occur in low density or are difficult to capture. Our CIR density estimator may be useful for tracking broad-scale, long-term changes in species, such as large cats, for which closed CMR models are unlikely to work.

Aerial surveys of kangaroos (Macropus spp.) in Queensland are used to make economically important judgements on the levels of viable commercial harvest. Previous analysis methods for aerial kangaroo surveys have used both mark–recapture methodologies and conventional distance-sampling analyses. Conventional distance sampling has the disadvantage that detection is assumed to be perfect on the transect line, while mark–recapture methods are notoriously sensitive to problems with unmodelled heterogeneity in capture probabilities. We introduce three methodologies for combining together mark–recapture and distance-sampling data, aimed at exploiting the strengths of both methodologies and overcoming the weaknesses. Of these methods, two are based on the assumption of full independence between observers in the mark–recapture component, and this appears to introduce more bias in density estimation than it resolves through allowing uncertain trackline detection. Both of these methods give lower density estimates than conventional distance sampling, indicating a clear failure of the independence assumption. The third method, termed point independence, appears to perform very well, giving credible density estimates and good properties in terms of goodness-of-fit and percentage coefficient of variation. Estimated densities of eastern grey kangaroos range from 21 to 36 individuals km–2, with estimated coefficients of variation between 11% and 14% and estimated trackline detection probabilities primarily between 0.7 and 0.9.

Recent developments in the application of line-transect models to aerial surveys have used double-observer sampling to account for undercounting on the transect line, a crucial step in obtaining correct population estimates. This method is commonly called the mark–recapture line-transect sampling method and estimates the detection probability at zero distance to correct line-transect estimates of abundance. An alternative approach, which uses the same methodology during data collection, is to use a range of covariates, including distance from the transect, in a mark–recapture model. This approach overcomes the implicit assumption of uniform distribution of distances in line-transect estimators. In this paper, we use three alternative approaches (a multiple-covariates distance method, a distance method incorporating adjustment for incomplete detection on the transect line using mark–recapture sampling, and a mark–recapture method with distance as a covariate) to estimate the abundance of several medium-sized mammals in semiarid ecosystems. Densities determined with the three estimators varied considerably within species and sites. In some cases distance estimates were larger than mark–recapture estimates and vice versa. Despite large numbers of observations, distance uniformity was not observed for any species at any site, nor for any species where sites were combined. Possible reasons, which include sampling variability, movement in response to the aircraft and failure of the mark–recapture independence assumption, are discussed in detail.

The abundance of Australian fur seal pups was determined at Seal Rocks,
Westernport, Victoria in late December 1997 using a mark–recapture
procedure with repeated recapture sessions. Pups (n
= 1291) were marked by clipping the black guard hair on the head to
reveal lighter underfur. Recaptures from the whole colony were made on eight
occasions 1–3 days later. In the recapture sessions, a mean of
32% of sighted pups had been marked. Estimates of pup numbers over the
eight recapture sessions were calculated using the Petersen estimate and then
combined by taking their arithmetic mean. The combined estimate was 4024
(95% confidence range 3908–4141). In 1991–92, a similar
procedure led to an estimate of pup numbers of 2817 (95% confidence
range 2703–2930). For both breeding seasons, estimates of pup numbers
from each recapture session were also combined assuming a joint hypergeometric
distribution; there was little difference in the results from the two
procedures, although the confidence intervals for the hypergeometric mean were
smaller than those for the arithmetic mean.
From 1991–92 to 1997–98, pup numbers increased by 43%, at
an exponential rate of 0.059 (95% confidence range
0.0526–0.0664), equivalent to 6.1% per annum
(5.4–6.9%). This is greater than the rate of increase of pups at
the colony between 1968–69 and 1991–92, which was 0.023
(95% confidence range 0.0198–0.0268), equivalent to 2.4%
per annum (2.0–2.7%).
Because of the rapid rate of increase of the Australian fur seal colony at
Seal Rocks and the importance of the locality for tourism, we recommend that
the abundance of pups there be determined every 3–5 years using a
mark–recapture technique.

Estimating the density of large, feral species such as wild horses at landscape scales can present a logistical hurdle for wildlife managers attempting to set density-based management targets. We undertook aerial surveys of wild horses by using a helicopter in Guy Fawkes River National Park in north-eastern New South Wales across 3 years to determine whether meaningful density estimates could be obtained efficiently by a mark–recapture technique based on recognition of individual horses. Horse groups photographed from the air on the first of two surveys conducted each year were 'marked' on the basis of a unique combination of colours and natural markings, and 'recaptured' if they were photographed and identified on the second survey. Population size was estimated with the program MARK using a range of population estimators; however, because horses appeared to be evading detection on the second survey of each year, we chose a final estimation model that accounted for detection shyness in the study species. In 2005, the density estimate was 3.8 horses per km2 (upper and lower 95% CL = 3.5–5.7 horses per km2). Following horse control in these catchments, the estimate in 2007 was 2.3 horses per km2 (upper and lower 95% CL = 2.1–3.4 horses per km2), and this change in density can be accounted for by the known number of horses removed from the survey area between survey periods. Overall, the technique proved useful for estimating densities of wild horses in deeply dissected gorge country where other estimation techniques (such as line transects) are not practical; however, low recapture rates in one of the years of the study shows that the technique may not always be applicable. Our technique should also be suitable for surveying other large mammals with broad ranges in open environments, provided recognition of individuals from unique marks is possible.

Background: Parasite evolution is hypothesized to select for levels of parasite virulence that maximise transmission success. When host population densities fluctuate, low levels of virulence with limited impact on the host are expected, as this should increase the likelihood of surviving periods of low host density. We examined the effects of Morogoro arenavirus on the survival and recapture probability of multimammate mice (Mastomys natalensis) using a seven-year capture-mark-recapture time series. Mastomys natalensis is the natural host of Morogoro virus and is known for its strong seasonal density fluctuations. Results: Antibody presence was negatively correlated with survival probability (effect size: 5-8% per month depending on season) but positively with recapture probability (effect size: 8%). Conclusions: The small negative correlation between host survival probability and antibody presence suggests that either the virus has a negative effect on host condition, or that hosts with lower survival probability are more likely to obtain Morogoro virus infection, for example due to particular behavioural or immunological traits. The latter hypothesis is supported by the positive correlation between antibody status and recapture probability which suggests that risky behaviour might increase the probability of becoming infected. ; University of Antwerp [GOA BOF FFB3567]; Antwerp study centre for disease (ASCID) [GOA BOF FFB3567]; INCO-DEV grant [ICA4-CT2002-10050]; German Research Foundation (Focus Programs from Deutsche Forschungsgemeinschaft) [GU 883/3-1, GU 883/3-2]; European Union's Horizon research and innovation programme under the Marie Sklodowska-Curie grant [707840] ; This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu.

This paper assesses the potential of mark‐recapture methods as a relatively powerful innovative research method for estimating the prevalence of "hard‐to‐reach"human populations in the social welfare field. We outline the development of mark‐recapture methods, illustrating some recent applications. The body of the paper reviews the main methodological and practical questions raised by the method. We suggest, by way of illustration, how it might be applied to estimating the prevalence of rough sleepers in a given geographical region.

Contact-recontact (aka 'mark-recapture') methods offer sociologists a potentially economical and effective means of estimating the size of a range of different populations, but also face a number of difficulties - ethical identification of matches, sample independence, population movement, population definition, and sample heterogeneity. Drawing on three recent studies for illustration (the prevalence of injecting drug use across Wales, the local prevalence of problem drinking, and the prevalence of 'rooflessness' in Scotland), it is argued that the first three of these problems can be readily addressed, while the last two are only open to partial resolution. Population definition may be partly addressed by careful selection of datasets or by combining contact-recontact methods with a screening instrument. Sample heterogeneity may be partly addressed by separate modelling of different constituent sub-populations. However, some of these 'solutions' may themselves generate further problems: the value of such methods is great in potential but uncertain in practice.

Context
Penguin Island supports the largest colony of little penguins in Western Australia. It is subjected to a suite of anthropogenic threats because of its proximity to an increasing urban population. For effective management of the colony, it is necessary to not only have knowledge of the size of the colony, but also the population trend of the colony.

Aims
To demonstrate a new cost-effective method of estimating the island-wide population of penguins on Penguin Island.

Methods
We estimated the island-wide population by combining mark–recapture sampling over 2 years on part of the island and beach counts of penguins arriving at night around the entire island. We estimated the abundance using closed population models, allowing for sex and time effects in capture probabilities. We had four capture occasions in 2008 only, and so considered heterogeneity of capture probabilities (Mh), using the Chao heterogeneity moment estimator. The proportion of all penguins counted that arrived at the four mark–recapture sites was then used to inflate the population estimate for the whole island.

Key results
In all, 62% of all penguins counted used the four mark–recapture sites. In 2007, there were an estimated 2369 ± 198 penguins, and 1543 ± 82 in 2008. When capture heterogeneity was allowed for in 2008, this estimate increased to 2069 ± 172.

Conclusions
Fewer eggs were laid and all measures of breeding performance were lower in 2008 than in 2007. Hence, the lower population estimate is most likely to represent fewer birds attempting to breed. However, further work on population estimates is required to determine whether capture heterogeneity occurs in both good and poor breeding years. Capture rates were affected by the presence of a full moon and high tides.

Implications
The estimate of the population can be used as part of the basis of a long-term monitoring program needed for effective management of the penguin colony. However, such studies must be coincident with the monitoring of a suite of reproductive and foraging parameters if short-term impacts of threats are to be recognised and well managed.