Using tagging data and aerial surveys to incorporate availability bias in the abundance estimation of blue sharks (Prionace glauca)

Repeatability of wildlife surveys for estimating abundance: A method to assess the consistency of detection probability and animal availability

A primary consideration of abundance studies that use unmarked animals is whether survey counts accurately reflect the population size or if unknown variation in animal movement or detection probability biases counts irrespective of population size. We posited that high repeatability in counts among temporally replicated surveys would indicate that counts are a good index of abundance. We temporally replicated 49 nocturnal spotlight surveys of white-tailed deer (Odocoileus virginianus) up to three times each (n = 128 total samples) to test the repeatability of this commonly used wildlife monitoring technique. Repeatability was high (R = 0.86), suggesting spring spotlight surveys provide a reliable index of deer population size in Iowa, USA. Fourteen percent of the variation among replicated counts was explained by day of year and, to a lesser degree, a vegetation green-up index. Detection probability was high (~0.70) early in the sampling season and declined considerably during the following 6 weeks. Deer abundance was greater at sites with higher percent landcovers of forest and hay/pasture and was lower at sites with higher landcover in crops. Our findings suggest deer managers should sample prior to green-up in the spring to maximize the proportion of the population that is detectable, and that accounting for seasonality on detection estimation is important for reliable abundance estimates if sampling occurs over a range of phenological progression. Finally, we show that temporal replication of surveys is a logistically feasible method to assess the reliability of abundance estimates from study designs that are normally conducted with single visits.

Combining passive acoustic data from a towed hydrophone array with visual line transect data to estimate abundance and availability bias of sperm whales (Physeter macrocephalus)

Visual line transect (VLT) surveys are central to the monitoring and study of marine mammals. However, for cryptic species such as deep diving cetaceans VLT surveys alone suffer from problems of low sample sizes and availability bias where animals below the surface are not available to be detected. The advent of passive acoustic monitoring (PAM) technology offers important opportunities to observe deep diving cetaceans but statistical challenges remain particularly when trying to integrate VLT and PAM data. Herein, we present a general framework to combine these data streams to estimate abundance when both surveys are conducted simultaneously. Secondarily, our approach can also be used to derive an estimate of availability bias. We outline three methods that vary in complexity and data requirements which are (1) a simple distance sampling (DS) method that treats the two datasets independently (DS-DS Method), (2) a fully integrated approach that applies a capture-mark recapture (CMR) analysis to the PAM data (CMR-DS Method) and (3) a hybrid approach that requires only a subset of the PAM CMR data (Hybrid Method). To evaluate their performance, we use simulations based on known diving and vocalizing behavior of sperm whales (Physeter macrocephalus). As a case study, we applied the Hybrid Method to data from a shipboard survey of sperm whales and compared estimates to a VLT only analysis. Simulation results demonstrated that the CMR-DS Method and Hybrid Method reduced bias by >90% for both abundance and availability bias in comparison to the simpler DS -DS Method. Overall, the CMR-DS Method was the least biased and most precise. For the case study, our application of the Hybrid Method to the sperm whale dataset produced estimates of abundance and availability bias that were comparable to estimates from the VLT only analysis but with considerably higher precision. Integrating multiple sources of data is an important goal with clear benefits. As a step towards that goal we have developed a novel framework. Results from this study are promising although challenges still remain. Future work may focus on applying this method to other deep-diving species and comparing the proposed method to other statistical approaches that aim to combine information from multiple data sources.

Fine-scale vertical habitat use of white sharks at an emerging aggregation site and implications for public safety

Abstract ContextOver the past decade, the coastal waters off Cape Cod, Massachusetts, have emerged as the only known aggregation site for the white shark (Carcharodon carcharias) in the western North Atlantic. During periods of seasonal residency, white sharks patrol the shoreline in search of pinniped prey, bringing them in close proximity to popular beaches where people recreate. AimTo examine whether white sharks off Cape Cod are more likely to occupy shallow depths (and consequently more likely to overlap with recreational water users) under certain conditions. MethodsWe deployed short-term, pop-up satellite archival transmitting (PSAT) tags and acoustic transmitters on 14 subadult and adult white sharks off the coast of Cape Cod during the summer and fall of 2017. PSAT tags provided fine-scale depth and temperature data, which were combined with high-resolution location data obtained from an acoustic telemetry array, to identify the depth and temperature preferences of white sharks when resident in the area. Key resultsSharks spent the majority (95%) of tracked time at depths of 0–31m and at temperatures from 8.9°C to 20.7°C. During resident periods along Cape Cod, individuals spent almost half (47%) of their time at depths of less than 4.5m, but made frequent excursions to mid-shelf depths, alternating between the surf zone and deeper offshore waters. Sharks were slightly more likely to occupy shallow depths at night during the new moon. The relationship between shark depth and lunar phase varied over the course of the day, suggesting the mechanism underlying lunar effects differs among diel periods. ConclusionsAlthough the overall risk posed to humans by white sharks is low, there is a high potential for overlap between white sharks and recreational water users off Cape Cod. The risk of interaction may be slightly higher during periods when local environmental conditions favour the species’ predatory stealth by influencing prey behaviour or detectability. ImplicationsThis study provides the first glimpse into the fine-scale vertical habitat use of white sharks off Cape Cod, which can be used to better understand the risk to recreational water users and to inform public safety practices.

Hierarchical distance sampling to estimate population sizes of common lizards across a desert ecoregion

Multispecies wildlife monitoring across large geographical regions is important for effective conservation planning in response to expected impacts from climate change and land use. Unlike many species of birds, mammals, and amphibians which can be efficiently sampled using automated sensors including cameras and sound recorders, reptiles are often much more challenging to detect, in part because of their typically cryptic behavior and generally small body sizes. Although many lizard species are more active during the day which makes them easier to detect using visual encounter surveys, they may be unavailable for sampling during certain periods of the day or year due to their sensitivity to temperature. In recognition of these sampling challenges, we demonstrate application of a recent innovation in distance sampling that adjusts for temporary emigration between repeat survey visits. We used transect surveys to survey lizards at 229 sites throughout the Mojave Desert in California, USA, 2016. We estimated a total population size of 82 million (90% CI: 65-99 million) for the three most common species of lizards across this 66,830 km2 ecoregion. We mapped how density at the 1-km2 scale was predicted to vary with vegetation cover and human development. We validated these results against independent surveys from the southern portion of our study area. Our methods and results demonstrate how multispecies monitoring programs spanning arid ecoregions can better incorporate information about reptiles.