Acoustic Properties of Ultrasonic Coded Transmitters and the Behavioral Responses of Pinnipeds Exposed to Them

Ultrasonic coded transmitters (UCTs) in the 60-80 kHz range are used increasingly to track fish and invertebrates in coastal and estuarine waters.  UCT frequencies lie within the hearing range of some marine mammals, so it is possible that they could detect and depredate instrumented animals.  Author Shane tracked two fish instrumented with 69 kHz UCTs in the vicinity of harbor seals (Phoca vitulina) over several consecutive days.  The tracking locations coincided with movements of the harbor seals, suggesting they had eaten the fish.  To determine the potential for detection, acoustic properties of UCTs were measured in coastal waters and at the U.S. Navy TRANSDEC facility.  Results revealed that nominal frequencies matched the manufacturer’s specifications, while source level varied substantially among UCTs and with orientation in the horizontal plane.  Based on these measurements, simple empirical propagation models, and published data on marine mammal psychophysical capabilities, California sea lions (Zalophus californianus) were unlikely to hear the pingers, but harbor seals might hear the highest-amplitude 69 kHz UCTs at useful ranges.  To determine whether seals would detect and approach pinging UCTs, four harbor seals and a ringed seal (Pusa hispida) at SeaWorld San Diego were exposed to pings from 69 and 83 kHz UCTs and their spontaneous responses observed.  The test UCT was randomly placed in one of several housings that the subjects could investigate freely.  A baseline-exposure protocol was used, with observations for 15 minutes before and after ping onset.  Three of four harbor seals reacted with aversion to the VEMCO 69 kHz pinger, swimming into a refuge pool or jumping out of the water.  None responded to the lower-amplitude Sonotronics transmitters at either frequency.  One harbor seal, the ringed seal, and six California sea lions did not respond to either UCT.  Although the present experiments were limited, they indicate that some phocids could detect 69 kHz UCTs and that interactions between instrumented fish and pinnipeds could be prevented by raising the frequency of UCT pings above 80 kHz.  They also suggest that motivation to respond to pingers is complex. This work was funded by a grant from the San Diego County Fish and Wildlife Advisory Commission (2005/06-05).  Measurements at TRANSDEC and analysis were supported by NOAA (Contract NA06NMF4720233).  Additional support and access to animals and facilities were provided by SeaWorld San Diego and the Hubbs-SeaWorld Society.