Underwater acoustic telemetry positioning is widely used to track the fine-scale movements of aquatic animals. In study areas near acoustically reflective surfaces, reflected transmissions may cause large detection outliers that can severely reduce the accuracy of positioning models. A novel time-of-arrival model for telemetry positioning is presented that utilizes a population Monte Carlo algorithm to solve positions (termed PMC-TOA). Telemetry detection error is modelled as a mixture distribution, allowing reflected detections to be identified and positions to be estimated despite their presence. Importantly, the PMC-TOA model provides good measures of positioning uncertainty, facilitating the use of post-processing state-space models to further refine position estimates. A simulated telemetry study is used to validate the PMC-TOA model and compare its performance to a conventional time-difference-of-arrival positioning model. A real case study on Atlantic salmon (Salmo salar) smolt passage behaviour is further used to demonstrate how PMC-TOA can be combined with post-processing models to produce high-resolution tracks. The resulting tracks are compared against those resulting from YAPS and TDOA positioning. The PMC-TOA model was shown to work well as either (i) a pre-processing step to remove reflected transmissions from time-of-arrival datasets, or (ii) a fast and accurate positioning method when paired with a post-processing state-space model. Positions returned by the model can be further used for animal movement statistics, allowing researchers to test the effects of experimental or environmental factors on the fine-scaled movement behaviours of aquatic animals in acoustically challenging environments. 

Migration is critical for life-cycle completion in diadromous fish species. River connectivity is vital in facilitating these large-scale movement events, but the extent of present-day river fragmentation can interfere with these migrations. Fish passage solutions (FPSs) are commonly implemented with the aim of improving river connectivity. In our study, we investigated the performance of two types of FPSs, spill regimes and complete dam removal, on Atlantic salmon (Salmo salar) smolt migrations. We used acoustic telemetry to monitor migration behavior and passage success of 120 wild smolts released in three different groups/sites: one group with two dams to pass to reach the river mouth, a second group with one dam to pass, and a control group without any barriers to pass (upstream of a recently removed hydroelectric dam). Smolt passage probabilities were similar for the two studied dams (87% and 86%) but showed variation in path choice, delay times, and loss rates. Passage success was influenced by several factors, such as body size, diel period, and water temperature, but not flow. Cumulative passage success to the river mouth was 61%, with most individuals being lost within lentic river stretches, either in the forebays of hydroelectric power stations or in naturally wide river stretches. Within the recently rehabilitated river sections (post dam removal), passage speeds were significantly faster than all other sections of the river (post-rehabilitation x<overline> = 56.1 km/day) with significantly faster speeds compared to pre-rehabilitation (pre-x<overline> = 28.0 km/day). Our findings provide valuable information on the benefits of dam removal and highlight the need for further rehabilitation measures in upriver reaches where barriers still affect downstream passage.

Despite the growing threat of pharmaceutical pollution, we lack an understanding of whether and how such pollutants influence animal behavior in the wild. Using laboratory- and field-based experiments across multiple years in Atlantic salmon (Salmo salar; n = 730), we show that the globally detected anxiolytic pollutant clobazam accumulates in the brain of exposed fish and influences river-to-sea migration success. Clobazam exposure increased the speed with which fish passed through two hydropower dams along their migration route, resulting in more clobazam-exposed fish reaching the sea compared with controls. We argue that such effects may arise from altered shoaling behavior in fish exposed to clobazam. Drug-induced behavioral changes are expected to have wide-ranging consequences for the ecology and evolution of wild populations.

Over the past decade, acoustic telemetry has become commonplace in studies on fish movement and behaviour. Over small spatial scales, arrays of acoustic receivers can be used to estimate movement paths in 2- or 3-dimensions with high temporal resolutions. Despite the growing prevalence of acoustic telemetry arrays, guidelines on how to generate robust position estimates—and further utilize this data in animal movement models such as hidden Markov models or step selection functions—are sparse. As animal movement models generally require either true positions or accurately specified spatial error distributions, understanding positioning error is crucial for behavioural inference. Here, current methods of telemetry positioning are reviewed. Simulated case studies are used to highlight the effect of state space model parameter selection on positioning accuracy, and in turn, the fitting of animal movement models.

The negative eects of hydroelectric power (HEP) on salmonid populations has longbeen recognized and studied. Downstream passage through turbines may potentially constitute asignificant source of mortality for both juvenile and adult fish in regulated rivers. Numerical modelshave been developed to calculate turbine passage mortality based on the probability of collision withthe turbine blades, but although widely used in management and conservation, their performanceis rarely validated in terms of the accuracy and bias of the mortality estimates. In this study,we evaluated commonly used blade strike models for Kaplan and Francis turbines by comparingmodel predictions with observed passage mortalities for juvenile 13–27 cm and adult 52–94 cmAtlantic salmon (Salmo salar, L.) and anadromous brown trout (Salmo trutta, L.) acquired by acoustictelemetry. Predictions made for juveniles aligned closer with observed mortality for both Kaplan andFrancis turbines (within 1–3% percentage points). However, the model severely underestimated themortality of adult fish passing through Francis turbines, with up to 50% percentage points dierencebetween predicted and observed mortalities. Furthermore, the model did not capture a clear negativecorrelation between mortality and discharge observed for salmon between 50–60 cm (grilse). Weconcluded that blade strike models are a useful tool for quantifying passage mortality for salmonidsmolts passing large, high-head turbines, but that the same models should be used with care whentrying to estimate the passage mortality of kelts in iteroparous populations. We also concluded thatthe major cause of passage mortality for juveniles is injury by collision with the turbine blade, butthat other factors seem to contribute substantially to the passage mortality of kelts. Our study reportslow mortality for smolts up to 27 cm passing through Kaplan and Francis turbines (0–12%), but highmortality for salmon over 50 cm passing though Francis turbines (56–81%).