For the last 50 years, scientists have used ultrasonic transmitters, known as pingers, to study the behavior of marine organisms. They’ve been used on many species including sharks and rays, bony fishes and invertebrates – and have become an important tool in the discovery of the movements and life history of these species. But the big mystery has always been, can marine mammals hear these signals?
After pingers and fish bones were found on the bottom of the sea, a difficult question was asked. Could it be that marine mammals use these ultrasonic transmitters as a make-shift “dinner bell” to target prey fish? In response, Hubbs-SeaWorld Research Institute researchers created a series of tests to calculate a potential detection range for various marine mammals with the help of SeaWorld San Diego and the U.S. Navy's TRANSDEC calibration facility. Marine mammals have varying acoustic abilities and the data collected confirms that some could use pingers as cues to the presence of prey species or predators. But, it is not clear whether animals have adapted a feeding strategy based on searching for prey with pingers attached. More data are being collected to see how and when pingers are targeted by marine mammals. The finding is important because ultrasonic coded transmitters are now used to estimate prey mortality rates for management purposes.
“Ultrasonic” frequencies are defined as sounds exceeding the top level of human hearing, which is normally around 20 KHz. Although modern-day pingers use a frequency that cannot be heard by most fish, it appears that there may be some exceptions to this rule.
To test this theory, several brands of pingers were measured under varied conditions – in a pool at SeaWorld San Diego, in a nearshore environment off of Mission Beach, California and at a U.S. Navy facility. Care was taken to keep the integrity of experiments by placing test pingers into a thin nylon mesh bag to isolate them from vibration and prevent rotation – and consider factors such as wavelength, absorption and bottom composition.
What the researchers found was surprising. The data collected supports the notion that marine mammals could use pingers as cues to the presence of prey species or predators – and that those Harbor Seals may very well have targeted the White Sea Bass discovered earlier.
However, the results also suggest that the effects of audibly detected signals may work in favor of prey species as well. Large predators, such as sharks are often instrumented with lower level pingers that could be heard by sea lions, offering them a chance to steer clear of danger.
At 69 KHz, the ranges of detection for the species evaluated were as follows: California Sea Lions were unlikely to detect pings except with a few centimeters of the device; Harbor Seals were estimated to detect pings at 21-25 meters; Bottlenose Dolphin and Harbor Porpoise were predicted to detect pingers at ranges of many hundreds of meters. And although the pingers tested only have a detection range of about 500 meters in nearshore waters, it’s suspected that some marine mammals, such as certain types of whales, have an auditory sensitivity that might well exceed that distance.
It’s still not clear whether marine mammals have begun searching for instrumented prey species as a feeding strategy. Data must be developed to determine whether this is indeed becoming a source of mortality. While there’s still much work to be done – it seems that the best strategy for preventing the predation of these fish will be to ensure that marine mammals never learn to associate pings with food.