Evaluating receiver contributions to acoustic positional telemetry: a case study on Atlantic cod around wind turbines in the North Sea

Abstract

Background The effect of individual acoustic receiver contributions to animal positioning is a crucial aspect for the correct interpretation of acoustic positional telemetry (APT). Here, we evaluated the contribution of each receiver within two APT designs to the number of tag signals detected and the position accuracy of free-ranging Atlantic cod, through data exclusion of single receivers from the analysis. The two APTs were deployed around offshore (ca 50 km) wind turbines at which 27 individual cod were tagged. Results We found that the exclusion of data from an APT receiver that was positioned within the movement area of the individual fish reduced the number of tag signals detected and the position accuracy of the set-up the most. Excluding the data from a single receiver caused a maximum of 34% positions lost per fish and a maximum increase in core area of 97.8%. Single-receiver data exclusion also caused a potentially large bias in the reconstruction of swimming tracks. By contrast, exclusion of a receiver that was deployed within 50 m from a turbine actually improved fish position accuracy, probably because the turbine can cause signal interference as a reflective barrier. Conclusions We recommend that an exploratory small-scale study like the one presented here be conducted before embarking on a larger-scale APT study. By excluding the data of single receivers from the positioning analysis, we were able to explore the suitability of a receiver set-up for the movement patterns of our target species. Furthermore, when a receiver is lost from an APT during deployment, the data should be treated with care as our results show that changes in triangulation outcome can lead to considerable differences in swimming tracks and home range estimates.

North Sea soundscapes from a fish perspective: Directional patterns in particle motion and masking potential from anthropogenic noise

“…Van der Knaap et al (2021) also found behavioural effects for the fish that stayed during the seismic survey as these individuals spent significantly less time being locally active (likely including foraging behaviour) and switched to being more inactive. Although Van der Knaap et al (2021) did not find a clear dose-response pattern, they did reveal that the daily rhythm of the cod in activity fluctuations as observed during days without survey was disrupted during the sound exposure.…”

“…We collected our acoustic recordings in Belgian territorial waters in the southern part of the North Sea (51.670 N, 2.802 E). The area is a shallow continental shelf sea with depths of less than 45 m. We deployed the recording equipment in the southwest corner of Belwind wind farm, situated on the Bligh Bank at the marine border between Belgium and the Netherlands (Brabant et al, 2013;Van der Knaap et al, 2021), about 50 km offshore (see Fig. 1).…”

“…The average transect line length was 21 km for the series of repeated loops. Scheduled interruptions were restricted to the northern loop ends in the current design to maximize local exposure time for the tagged fish of the experiment (Van der Knaap et al, 2021). There were some additional, short interruptions for operational airgun configuration and necessary performance tests.…”

“…1). The placement was central to the turbines at which cod were caught, tagged, and released (Van der Knaap et al, 2021). Acoustic recordings of the sound pressure and particle motion were made using a standard omnidirectional hydrophone (M36-V0-100, Geospectrum Technologies Inc., Dartmouth, NS, Canada) along with a three-axis PCB 356B18 MEMS accelerometer (PCB Piezotronics, Depew, NY).…”

“…We received permission to conduct the experimental seismic survey from both the Belgian authorities [Belgian Department of Bestrijding van de Sociale Fraude, Privacy en Noordzee (BPN) after positive advice from Royal Belgian Institute of Natural Sciences (RBINS)] and the Dutch authorities (MINEZ after positive advice from RWS). The ethical approval for working with live fish, associated with the current study but reported elsewhere (Van der Knaap et al, 2021) The seismic survey transect area was adapted to have the lowest possible impact to Belgian and Dutch Natura 2000 sites. A dedicated and independent marine mammal observer (MMO) was on board throughout the operation and on duty throughout the survey during daylight hours.…”