Reed warblers are widespread throughout Eurasia, Africa, and Australasia, and many species undertake long seasonal migrations. By contrast, other species of the genus Acrocephalus are sedentary and endemic to single oceanic islands. The Pitcairn Reed Warbler Acrocephalus vaughani is confined to the small volcanic island of Pitcairn in the South Pacific Ocean, and no population assessments have ever been conducted for this species. Due to its restricted range, the presence of invasive species, and the loss of natural habitat, the species is considered as globally “Endangered”, but its actual conservation status is entirely speculative. We conducted transect surveys and nest monitoring in the austral summer of 2022/23 and present abundance estimates for the species. We counted between 51 and 158 reed warblers along 54 transects that were each 100 m long and covered all habitats and roughly 13% of the vegetated island area. Using binomial mixture models accounting for imperfect detection and habitat variation in abundance, we estimated that Pitcairn may hold 1,568 (95% confidence interval 812–3,237) Pitcairn Reed Warblers, and that the species appeared to be most abundant in introduced Rose Apple Syzygium jambos stands. Based on the monitoring of 49 nests, of which only four failed to fledge any young, we estimated that Mayfield nest survival rate was 0.69 and estimated productivity was 1.07 (± 0.39 standard deviation) fledglings per nest. Assuming that Pitcairn Reed Warblers have similar annual survival probabilities as other island reed warblers, the productivity appears sufficient to maintain the population and there is no indication that the species has decreased significantly over the past three generations. Given the limited extent of occurrence, and the stable current population size between 442 and 2,774 mature individuals, we recommend that the global conservation status of the Pitcairn Reed Warbler be classified as “Vulnerable”.

Methods that accurately estimate animal abundance or density are crucial for wildlife management. Although numerous techniques are available, there have been few comparisons of the precision and cost-effectiveness of different approaches. We assess the precision and cost of three methods for estimating densities of the Endangered Grevy's zebra Equus grevyi. We compare distance sampling and photographic capture–recapture, and a new technique, the random encounter model (REM) that uses camera-trap encounter rates to estimate density. All three methods provide comparable density estimates for Grevy's zebra and are preferable to the common practice of raw counts. Photographic capture–recapture is the most precise and line-transect distance sampling the least precise. Line transects and photographic capture–recapture surveys are cost-effective in the first year and REM is most cost-effective in the long-term. The methods used here for Grevy's zebra may be applied to other rangeland ungulates. We suggest that for single species monitoring programmes in which individuals can be identified, photographic capture–recapture surveys may be the preferred method for estimating wildlife abundances. When encounter rates are low, distance sampling lacks the precision of the other methods but its cost advantage may make it appropriate for long-term or multi-species monitoring programmes. The REM is an efficient and precise method of estimating densities but has high initial equipment costs. We believe REM has the potential to work well for many species but it requires independent estimates of animal movements and group size.

Grey seals (Halichoerus grypus) are a species of conservation concern that require Marine Protected Area management and population status assessment under the EU Habitats Directive. Aerial surveys are commonly used to monitor grey seal pup production at their globally important UK colonies. However, in Wales more than half of pups are born in cryptic breeding habitats such as sea caves. These cryptic habitats preclude the use of aerial monitoring methods and necessitate ground-based counts, which are costly in resources. In this study, we compare a ground-based pup production census with a reduced effort plot-sampling survey to estimate pup production, derive a total population size and assess cost effectiveness. Pup production in North Wales was estimated at 91 (95% confidence interval: 70–112) by the plot-sampling design and was a good approximation of the ‘true’ value of 96 derived from the census. The total population size in North Wales was estimated at between 242 and 307 grey seals. The plot-sampling design reduced survey effort by 46% and saved 30% on logistical costs compared to the full census. We outline the suitability of this method as part of a monitoring programme for grey seal pup production and suggest our approach may be applicable to other regions where grey seals use cryptic breeding habitat.

Monitoring the abundance of green turtles (Chelonia mydas) is necessary to assess population trends and risks of collapse. This note presents a study aimed at comparing three techniques for the direct estimation of green turtle numbers in their foraging habitats (seagrass beds and reef flats). The experiment was carried out at Mayotte Island, Western Indian Ocean. The techniques involved were surveys by snorkel, and aerial surveys using a microlight aircraft and a paramotor. Each technique had shortcomings and advantages. While each technique provided estimations of turtle numbers only surveys by snorkel permitted identification of species and sex, whenever visibility and turtle behaviour permitted. Along the shorelines, and over foraging areas, the paramotor was found to be most suitable for direct estimations of turtle numbers. The major advantage of this technique lied in its capability to obtain a synoptic snapshot of turtle distribution over foraging areas. Linear surveys from a microlight aircraft are better suited to monitor foraging areas located further away from the shore.

Acoustic surveys for determining fish abundance in shallow water present many challenges. One constraint to down-looking (or vertical) surveys is that the sampling volume is limited due to the short range and typically narrow beams of acoustic transducers. By utilizing more of the acoustic beam, into the side-lobes for example, the sampling volume may be increased. Although the echo energy contribution from the side-lobes is usually small, these echoes may influence target strength estimation and echo counting, and by extension affect abundance estimation either directly or indirectly. With the advent of wide dynamic range echosounders, it is possible to assess both small and large acoustic-size targets. In this case, it is possible to collect signals from large targets in the beam side-lobes, and therefore an algorithm to use side-lobe data is proposed. The smoothed Expectation Maximization method (EMS) is used to estimate densities and target strength of fish from only main-lobe and main-lobe with side-lobe observations in simulation, and suggestions are made for real data from surveys.

Schooling fish may aggregate in very high densities covering very small areas. Thus the probabilityof hitting such high-density spots during a large-scale sampling is very low. The sampling fluctuations of the tail of the histogram are thought to be very significant. The stock biomass estimate and its precision rely greatly on how precisely the tail of the histogram can be sampled. In order to acquire elements for improving survey designs and abundance estimators wc study here the relation in space that the high values have with the other values. A disjunctive kriging approach is used. Different quantiles of the histogram are coded by indicators. The spatial structure of each indicator and its spatial covariation with the others are studied by computing experimental indicator variograms and cross-variograms. Such analysis is applied to dissect finely the spatial structure of a Norwegian herring stock sampled acoustically. It is shown that when going from low-density areas to high-density ones, intermediate values are not necessarily crossed. Thus a particular disjunctive kriging model with no transition in space is well adapted to the herring data. The model is based on the regressions of each indicator on the one immediately below it. In the model one can estimate the probability for the fish density to trespass a given cut-off at a given location when knowing that the density trespasses lower cut-offs at surrounding points. It is shown on the data that the high densities are structured and show small aggregations. Then it is shown that having trespassed a certain cut-off, i.e. inside the corresponding areas in space, the high-density aggregations can be considered to be positioned independently from the other values. These areas, where the structuring of the high values is not correlated to the structuring of the other values, are mapped using the fitted disjunctive kriging model. The implications for survey designing of the existence of such areas and of their geometry are discussed. Also discussed is the possibility of stratifying the data in spatially uncorrelated boxes when deriving the biomass estimate and its precision, on the basis of an observed spatial non-correlation property between the spatial distribution of different quantiles.