Acoustic characteristics of ultrasonic coded transmitters for fishery applications: Could marine mammals hear them?

Abstract

Ultrasonic coded transmitters (UCTs) producing frequencies of 69-83 kHz are used increasingly to track fish and invertebrates in coastal and estuarine waters. To address concerns that they might be audible to marine mammals, acoustic properties of UCTs were measured off Mission Beach, San Diego, and at the U.S. Navy TRANSDEC facility. A regression model fitted to VEMCO UCT data yielded an estimated source level of 147 dB re 1 μPa SPL @ 1 m and spreading constant of 14.0. Based on TRANSDEC measurements, five VEMCO 69 kHz UCTs had source levels ranging from 146 to 149 dB re 1 μPa SPL @ 1 m. Five Sonotronics UCTs (69 kHz and 83 kHz) had source levels ranging from 129 to 137 dB re 1 μPa SPL @ 1 m. Transmitter directionality ranged from 3.9 to 18.2 dB. Based on propagation models and published data on marine mammal auditory psychophysics, harbor seals potentially could detect the VEMCO 69 kHz UCTs at ranges between 19 and >200 m, while odontocetes potentially could detect them at much greater ranges. California sea lions were not expected to detect any of the tested UCTs at useful ranges.

Auditory detection of ultrasonic coded transmitters by seals and sea lions

“…The value of a is based on the Francois-Garrison model of acoustic absorption and is a function of frequency, temperature, depth, salinity and pH (Francois and Garrision, 1982a,b). For any given distance, the transmission loss estimated by this model will be higher than that estimated by the model of Bowles et al (2010), which was developed based on direct transmission loss measurements of a 69 kHz Vemco tag. Consequently, the detection ranges generated by Eq.…”

“…Measured thresholds for both subjects and all three treatments are summarized in Table I, along with threshold estimations based on Bowles et al (2010) for comparison. False alarm rates for each of the three treatments were between 10 and 15 % for both subjects.…”

“…Considering these hearing thresholds relative to typical UCT frequencies (50-400 kHz) and source levels (140-165 dB re 1 lPa), it becomes apparent that harbor seals should detect all but the highest frequency, lowest SPL UCTs at considerable distances, while California sea lions should detect only the lowest frequency, highest SPL UCTs at close range, if at all. Bowles et al (2010) estimated detection ranges for pinnipeds and small cetaceans to certain UCTs. In order to accomplish this, transmission loss for a tag output was directly measured as a function of distance in a nearshore marine environment.…”

“…To ensure adequate understanding of the influence of signal frequency and duration on signal detectability, hearing thresholds were measured for a trained individual from each species to an extended (500 ms) narrowband 69 kHz stimulus, a brief (10 ms) narrowband 69 kHz stimulus, and a series of eight 10 ms, 69 kHz puretone pulses recorded directly from a Vemco V16-3H UCT. These auditory data were used to empirically refine the earlier predictions by Bowles et al (2010), and to test the viability of the "dinner bell" hypothesis with direct hearing measurements.…”

Potential Uses of Anthropogenic Noise as a Source of Information in Animal Sensory and Communication Systems

“…Many of these studies use acoustic coded radio transmitters (also known as pingers) which typically emit an ultrasonic acoustic signal that is inaudible to fish, but is audible to some marine mammal predators (Bowles et al, 2010). If the signal is detectable, the sound could be associated with the presence of prey and cause increased predation through a learned 'dinner bell' effect.…”