Today scientists are tracking many marine fish and invertebrate species using acoustic coded radio transmitters (‘pingers’). From 2002 through 2004, Aquaculture Program Field Supervisor Mike Shane implanted pingers in white seabass to find out where they went after release and how well they survived. However, he and his crew sometimes found themselves tracking harbor seals instead, suggesting that the instrumented fish had been eaten. Concerned that their instruments had attracted predators, the Aquaculture Program and Bioacoustics Laboratory teamed up to find out if pinnipeds (seals, sea lions) could hear instrumented fish.
The team measured the acoustic characteristics of tracking pingers in the ocean near Mission Bay and at the U.S. Navy’s TRANSDEC transducer calibration facility. Using the results, they calculated that the pings would be audible to seals, but only at the upper limit of their hearing, and sea lions weren’t expected to hear them at all. However, because little is known about pinniped perception of sounds at the upper limits of hearing, in 2008, under NOAA funding, the team tested spontaneous reactions of seals and sea lions at SeaWorld. They expected either no reaction or curiosity. What they found instead was an intriguing story, perhaps explaining why pinniped interactions with instrumented fish are only now being reported after many years of use.
Seals weren’t expected to hear sounds with frequencies greater than 70 kHz. So, the team tested pingers just below and above the limits of hearing (69 kHz and 83 kHz). Although test pings should have been faint rather than loud and annoying, three of four harbor seal subjects jumped out of the water when exposed to 69 kHz pings! In retrospect, the team should have expected that the seals might react that way. Both seals and sea lions had avoided or exhibited aggression to 10 kHz pingers used as net alarms in earlier research under NOAA funding. Because the only natural source of this type of high-frequency tonal signal in the ocean is other marine mammals, the most reasonable hypothesis the team has come up with to explain the seals’ odd behavior is that it is a defensive mechanism against cetacean predators, particularly the killer whale.
The results of the project have some important implications. First, they show that pingers should be deployed with care. A number of marine mammal species have learned to exploit aquaculture fish even when protected by intense pings from harassment devices, so small tracking pingers won’t deter them for long. Small cetaceans will be able to hear tracking pingers even better than pinnipeds. If predators learn to target pingers, tracking studies will be compromised and there are no good alternative instruments. In addition, instrumented predators such as sharks may have difficulty catching pinnipeds once they learn to associate pings with attacks.
Additional laboratory and field research is needed to show the range at which wild marine mammals detect and react to instrumented fish. It will also be important to determine how they learn the association between the sound and consequences, such as access to food or an oncoming predator.