Strong thermal stratification reduces detection efficiency and range of acoustic telemetry in a large freshwater lake

Abstract

Background The successful use of acoustic telemetry to detect fish hinges on understanding the factors that control the acoustic range. The speed-of-sound in water is primarily a function of density, and in freshwater lakes density is primarily driven by temperature. The strong seasonal thermal stratification in the Great Lakes represent some of the steepest sound speed gradients in any aquatic system. Such speed-of-sound gradients can refract sound waves leading to greater divergence of acoustic signal, and hence more rapid attenuation. The changes in sound attenuation change the detection range of a telemetry array and hence influence the ability to monitor fish. We use 3 months of data from a sentinel array of V9 and V16 Vemco acoustic fish tags, and a record of temperature profiles to determine how changes in stratification influence acoustic range in eastern Lake Ontario. Result We interpret data from an acoustic telemetry array in Lake Ontario to show that changes in acoustic detection efficiency and range correlate strongly with changes in sound speed gradients due to thermal stratification. The steepest sound speed gradients of 10.38 m s−1/m crossing the thermocline occurred in late summer, which caused the sound speed difference between the top and bottom of the water column to be greater than 60 m/s. V9 tags transmitting across the thermocline could have their acoustic range reduced from > 650 m to 350 m, while the more powerful V16 tags had their range reduced from > 650 m to 450 m. In contrast we found that when the acoustic source and receiver were both transmitting below thermocline there was no change in range, even as the strength of sound speed gradient varied. Conclusion Changes in thermal stratification occur routinely in the Great Lakes, on timescales between months and days. The acoustic range can be reduced by as much as 50% compared to unstratified conditions when fish move across the thermocline. We recommend that researchers consider the influences of thermal stratification to acoustic telemetry when configuring receiver position.

Telemetry-based spatial–temporal fish habitat models for fishes in an urban freshwater harbour

“…Another key consideration is variation in acoustic receiver detection ranges over space and time, as variation in system performance may confound inferred patterns of fish habitat associations if not accounted for in analyses (Kessel et al, 2014 ; Kuai et al, 2021 ). Recently, a relatively simple and tractable approach has been developed (Brownscombe et al, 2020a ), but was not integrated in the study design here.…”

The Ecological Consequences of Hypoxia in a Temperate, Freshwater Ecosystem.

“…Acoustic telemetry relies on ultrasonic sound waves emanating from fish and being detected on stationary microphones (called hydrophones), in a dynamic, urbanized system with heavy recreational and commercial shipping traffic generating a noisy environment. Many environmental factors can interfere with the efficiency of the equipment, including noise, (Brownscombe et al, 2020;Kessel et al, 2014;Klinard et al, 2019;Kuai et al, 2021;Wells et al, 2021), and I attempted to explore this and incorporate into my analyses and interpretations. This is particularly important for the environmental changes that occur seasonally, like thermal stratification, and when trying to interpret seasonal changes on behaviour (Mathies et al, 2014;Payne et al, 2010).…”