Conventional distance sampling versus strip transects and abundance indices for estimating abundance of greater gliders (Petauroides volans) and eastern ringtail possums (Pseudocheirus peregrinus)

ContextAccurate estimates of abundance are extremely useful for wildlife management and conservation. Estimates generated from distance sampling are typically considered superior to strip transects and abundance indices, as the latter do not account for probability of detection, thereby risking significant error. AimTo compare density estimates generated from conventional distance sampling (CDS) of arboreal marsupials with strip transect density estimates and abundance indices. MethodsOff-track CDS and strip transects were used to estimate densities of P. volans and P. peregrinus across ~2.6km2 of remnant eucalypt forest at Mt Duval in north-eastern New South Wales. Key resultsCDS density estimates for P. volans (1.36ha−1, 95% confidence interval (CI) of 1.07–1.72ha−1) and P. peregrinus (0.28ha−1, 95% CI 0.22–0.35ha−1) were consistent with densities reported in other studies conducted in open eucalypt forests. A strip transect width of 40m for P. volans resulted in a collective set of values for density (1.35ha−1), error (s.e.±0.14), precision (cv 0.10) and 95% CI (1.07–1.62ha−1) closest to those associated with the CDS-generated density estimate (1.36ha−1, s.e.±0.15, cv 0.10, 95% CI 1.07–1.72ha−1). Strip widths of 10 to 40m resulted in density estimates for P. peregrinus closest to those generated through CDS, but much less precise. ConclusionsAlthough a 40-m wide strip transect provided a robust density estimate for P. volans at Mt Duval, this is unlikely to be consistent across different study areas. Strip transects provided less precise density estimates, or underestimated P. peregrinus density at Mt Duval, when compared with CDS density estimates. CDS should be favoured over strip transects or abundance indices for estimating P. volans and P. peregrinus abundance, because it is capable of providing more meaningful and robust abundance estimates by accounting for the probability of detection from the transect line across different habitats. ImplicationsResearchers, conservation managers and decision makers should be aware that common methods for assessing arboreal marsupial abundance have serious potential weaknesses. Thus, it would be prudent to invest in studies that address imperfect detection to improve the quality of monitoring data.

Estimating distribution and abundance of feral horses (Equus caballus) in a coniferous plantation in Australia, using line-transect surveys of dung

Context Feral horses are a growing problem in Australia, despite implementation of management strategies. The incidence of horse sightings and horse-associated vehicle collisions within the Tuan and Toolara State Forest (TTSF), a coniferous plantation in south-eastern Queensland, has increased in the past decade, indicating an increase in population and the necessity to undertake an accurate appraisal of population abundance. Aim To determine the feasibility of using dung counts, defecation and dung-decay rates to determine the distribution and abundance of feral horse population in a managed coniferous plantation where dense vegetation prohibits the use of visual-based animal detection methods. Methods Population distribution was assessed by dung count incorporated into a vehicle strip-transect survey on 582 km of forest tracks. Population abundance was estimated from dung density, the mean defecation frequency of feral horses, and the mean number of days required for dung to decay. Dung density was obtained from on-ground distance line-transect survey that sampled all representative plantation habitats and consisted of 111 transects totalling 44.3 km. Key results The strip-transect survey clearly showed that although feral horses were dispersed across the plantation, the distribution was uneven, with the central region of the plantation being the most heavily populated. The combination of dung counts, defecation rate (mean ± s.d., 7.97 ± 8.74) over 24 h and dung-decay rate (444 ± 150.7 days) provided an estimation of the density of feral horses in various habitats and indicated that the plantation was occupied by 1321 (95% CI 940–1965) horses, which corresponded to an average density of 1.8 horses km–2. The method clearly identified variations in horse abundance among the various habitats within the surveyed areas. Open habitats, created following harvesting, showed higher occurrence rates, whereas the habitats of mature forest were scantily occupied. Key conclusions Dung counts are a simple, effective and practical technique that can provide information on distribution and abundance of feral horse population in densely forested habitats where visual-based techniques are not applicable. However, unbiased and precise defecation and decay rates must be estimated. Implications The study validated the use of dung counts to provide information on feral horse distribution and abundance in densely forested environments where direct methods of census may be difficult to obtain. The methods are applicable to a range of ecosystems, but defecation and dung-decay rates must be determined separately for each ecosystem.

Pilot aerial study of the marine megafauna in northern coast Alagoas, Brazil

This study aimed to investigate the feasibility of strip-transect aerial survey for the study of the marine megafauna in northern coast of Alagoas, chosen as the pilot area due to the better water clarity and the occurrence of the Antillean manatee. It was sighted 15 dolphins, 13 sea turtles and 10 manatees. From five aspects tested on the experimental flight, three adjustments were necessary on the methodology as following: 1) reduction of the transect angle to 40º to increase the sampling area; 2) constant transects of 1.5 nautical miles from shore to standardize the area of coverage; and 3) transects restricted to the mouths of estuaries due to low visibility in turbid waters. This study demonstrated the effectiveness of aerial surveys for the detection of marine megafauna. However, the adjustments proposed are necessary to minimize the characteristic limitations of each species and/or habitat. Keywords: aerial method, manatees, dolphins, sea turtles, limitations

Delphinid spatial distribution and abundance estimates over the shelf of the Bay of Biscay

Certain, G., Ridoux, V., van Canneyt, O., and Bretagnolle, V. 2008. Delphinid spatial distribution and abundance estimates over the shelf of the Bay of Biscay. – ICES Journal of Marine Science, 65: 656–666. The small delphinid community (bottlenose Tursiops truncatus, common Delphinus delphis, and striped Stenella coeruleoalba dolphins) of the Bay of Biscay (100 000 km2 of continental shelf along the French Atlantic coast) has been studied here by combining strip-transect aerial surveys conducted between 2001 and 2004 and ship-based surveys conducted between 2003 and 2006. Distribution was modelled spatially in relation to several large-scale descriptors of the environment. Highest densities of small delphinids were associated with the shelf break, in particular in two hotspots located in the north and the south of the bay. Using ship-based data, we found strong spatial segregation between common and bottlenose dolphins in spring, with common dolphins associated with coastal areas (and especially river plumes) and bottlenose dolphins on the outer shelf and the shelf break. Assuming a detection probability of 1, a strip-transect abundance estimate for the small delphinid community was obtained in August 2002 with 56 500 (95% CI 29 100–90 400), but relative abundance varied across months.