Stansbury, A.L., T. Gotz, V.B. Deecke, and V.M. Janik. 2014. Grey seals use anthropogenic signals from acoustic tags to locate fish: Evidence from a simulated foraging task. Proceedings from the Royal Society B: Biological Sciences, 282: 20141595. DOI: 10.1098/rspb.2014.1595
Grey seals are a species of seal that are found only in the North Atlantic. Because of their cute and playful nature, these marine mammals don’t always get the credit they deserve for being ferocious predators. A grey seal’s diet consists entirely of fish and other oceanic critters, with a single seal consuming as much as 10 pounds of fish per day! Unlike fish which are rarely able to detect sounds greater than 3,000 Hertz (Hz) in the water, many marine mammals including seals, dolphins, and whales are able to detect sounds greater than 160,000 Hz. These higher frequency sounds are often used by marine mammals for communication and location of prey items.
Acoustic telemetry is a method of tracking animals that has recently been heavily utilized by fisheries scientists to track the movement of many species of fish. Acoustic telemetry works by capturing a fish, then implanting them with a small tag. This tag is programmed to emit a sound (usually undetectable by the tagged animal) over a regular a time period, which can be detected by acoustic receivers deployed at the study locations. These acoustic receivers are essentially microphones placed in the water which continuously listen for “pings” emitted by the tags. The receivers are able to differentiate between tags allowing scientists to record the presence of individual fish every time they come within range of a receiver. Acoustic telemetry has become such a powerful tool that fisheries scientists are now able to track fish movements in 3D by triangulating the position of a fish relative to multiple acoustic receivers, just like your GPS device triangulates your position relative to multiple satellites! Although acoustic telemetry has proven a very powerful tool in studying the movements of fish, the frequency of the sound emitted by acoustic tags (~69,000 Hz), though usually undetectable by the fish itself, is within the hearing range of many marine mammals, including seals. The researchers in this study attempted to determine whether or not grey seals would learn to recognize the sound emitted by acoustic tags as a source of food and use it as a homing beacon to locate prey in the future.
To test whether or not grey seals would learn to home in on acoustic tag signals for locating prey, the researchers used 10 juvenile grey seals. The seals were brought into captivity before ever entering the sea which guaranteed that the seals had never learned to associate sound with food. The researchers designed a pool that contained 20 boxes along the bottom which seals could poke their heads into and potentially find a fish inside (Figure 2). In each trial, a single seal was released into the testing pool and allowed to freely forage. In the original experiment, one box contained an untagged fish, and one box containing a fish tagged with an acoustic tag. For each new repeated trial, the box containing the tagged and untagged fish would change, so that seals wouldn’t learn to associate geographic location with the presence of the fish. Two control experiments were also conducted to control for the seals potentially using other cues, like scent, to locate the fish. During the first control experiment, no fish were place in any of the 20 boxes. However, acoustic tags (without a fish) were placed in two of the boxes to see if seals would visit the boxes with acoustic tags more quickly and more frequently than untagged boxes. In the second control experiment, every box contained a fish. However, in eighteen of the twenty boxes, the fish was blocked by a plate, preventing the seals from grabbing it. This second control experiment prevented the seals from being able to locate which box contained an accessible fish using scent. Of the two fish that were accessible to the seals, one of them had an acoustic tag while the other did not.
All seals in the study became better at locating fish in the boxes as they were given more trials. The time required to find both tagged and untagged fish in the original experiment decreased as the seals gained more experience in the test pool. This suggests that the seals were able learn how to utilize a number of environmental cues to locate their food more quickly. However, the interesting findings of this study came from the two control experiments. The first control experiment demonstrated that in the absence of scent, seals would move more quickly to acoustic tag boxes in an attempt to find food (Figure 3).
The second control experiment demonstrated that when the scent of fish could not lead the seals to an accessible fish, the seals would utilize the acoustic sound to find accessible prey (Figure 4). Note that in the second trial, even the untagged fish was discovered very quickly in this experiment. The researchers attributed this to the fact that inaccessible fish were reused for each trial, while accessible fish were eaten by the seals and had to be replaced with fresh fish for a subsequent trial. Therefore, seals were likely able to differentiate between the smell of fresh fish vs. reused fish.
This study demonstrated that young grey seals have the capacity to associate audible pings emitted by acoustic tags frequently used by fisheries scientists with potential prey. This discovery means that predators in the wild may learn to disproportionately search for and prey on tagged animals. However, it is important to recognize that this study was conducted under controlled lab settings when seals were exposed to a static environment and the presence of an acoustic signal in every trial. It is unclear whether or not marine mammals in the wild would be able to make the connection between acoustic signals and prey item as quickly and efficiently as the test subjects in this study, or if wild animals would give any preference to tagged fish.
Whether or not marine mammals in the wild are able to locate prey that have been tagged by fisheries scientists may be unclear, but the possibility is undeniable. With a steep price tag of usually over $200 per tag, some fisheries scientists have reason to be concerned about marine mammals wreaking havoc on the sample sizes of their acoustic tracking studies; not to mention unforeseen population and behavioral consequences.