Abstract
BackgroundThe 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.ResultWe 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.ConclusionChanges 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.
Highlights
The successful use of acoustic telemetry to detect fish hinges on understanding the factors that control the acoustic range
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
Summary
The successful use of acoustic telemetry to detect fish hinges on understanding the factors that control the acoustic range. It is well known that many lakes have seasonal thermal stratification, with warm surface waters and cooler waters at depth This thermal stratification strongly influences fish movement and behaviour (e.g., [14, 21]). The thermal stratification of these 10–30 m deep waters was characterized by a strong summer thermocline capping remnant winter water, so that there could be as much as a 15 °C temperature difference between surface and bottom waters Their findings suggest that in most cases, thermal stratification reduced efficiency and range, when the acoustic signal traversed the thermocline. They present new marine data from the mid-Atlantic Bight, where detection range and efficiency increased for signal transmissions within the hypolimnion during strong thermal stratifications. Understanding how thermal stratification influences the transmission of acoustic signals, in freshwater lakes is important, especially as these are the focus of intense research activities [9, 12, 20]
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