1054 words • 4~7 min read

Seals use signals from acoustic tags to find fish

michelleMichelle Jewell is a Zoologist specialized in predator/prey behaivour and the Scientific Communicator for EDNA Interactive.  She has spent the past 4 years studying the behaviour of white sharks and Cape fur seals at Geyser Rock, ‘Shark Alley’, South Africa.  

Anyone who has worked with seals knows they are crafty critters that will always find the easiest way to eat fish.  Take the rise and fall of acoustic deterrent devices in aquaculture farms that were designed to scare away seals and other predators.  They had limited success and resident predators habituated to the sound when they realized there was no immediate danger.  These devices have been shown to actually attract more predators over time, especially passing pinnipeds.

Scientists have used acoustic tags to monitor fish movements since the 1950s, and hundreds of species have been implanted with these tags (a ‘few’ studies listed here) throughout rivers, lakes, estuaries, and the ocean.  Could marine mammals associate tag signals with food and do they do this in the wild?  A recent laboratory study from St. Andrews (free to download here) answers the first half of this question, showing that grey seals Halichoerus grypus  were able to use the signals transmitted from Vemco V9–2H tags to identify boxes that contained fish.

Young of the year seals, naïve to foraging or the ocean, were placed in a pool and tasked with finding boxes with fish, and when successful, would consume fish.  Lead author Amanda Stansbury explains, “We had three tests.  The first test consisted of 20 boxes, 18 empty, one tagged fish, and one untagged fish. Then on top of that we had two controls; the tag only control where 19 boxes were empty and one had just a tag, no fish, and then the all fish control where 18 boxes had fish the seals couldn’t access, and one with an accessible fish with tag, and one with an accessible fish without tag.”

Seals during the tag-only control found the tag box just as quickly as they found any randomly selected box, suggesting that at this stage of the study, the tag signal did not hold any significance.  In the all fish control (to make chemosensory information unreliable), seals found the tagged fish in much less time than they found the untagged fish.  In this controlled example, grey seals were able to associate acoustic pings with food and use the signals to forage more successfully.  However, small prey species aren’t the only fish carrying acoustic tags in the sea, many megafauna predators – like white sharks – are also tagged with acoustic pingers.  Surely a seal in the wild that has learned to associate pings with fish would eventually come face to face with a foe rather than food?

This topic is near and dear to my heart, as dozens of white sharks have been tracked with acoustic tags close to Cape fur seal colonies in South Africa (i.e. our work at Geyser Rock).  We did consider that the seals may be able to hear the tags and associate them with danger, but seals were oblivious to the nearby tagged sharks and these sharks executed several successful near-colony predations.   However, seal colonies are noisy places and the signals could be washed out by the ambience of the island.  Cape fur seals also have a wide range of effective habitat specific anti-predator tactics to avoid white sharks in general, so it is unlikely that shark-savvy seals would have the chance to associate acoustic signals with risk.  We’ll need an area with high overlap of predator naïve marine mammals where the majority of predators carry acoustic tags to see if marine mammals could benefit from using pings to identify threats.  As to whether or not marine mammals are using acoustic tags as dinner bells, we’ll need fine-scale tools to assess where predators and prey are overlapping and whether or not predations are actually occurring, which is where a new tag in development is picking up the slack.

The PDAT (Predation Detection Acoustic Tag) being developed by HTI Sonar detects and reports when your tagged fish has been consumed.  Additionally, HTI tags emit signals at/above 307kHz – outside the hearing range of many marine mammals.  A study combining these tags with traditional r-codes and high-resolution marine mammal movements may reveal more about acoustic signals being used by marine mammals in the wild to forage.  Orcas and salmon anyone?  Hypothetical study ideas are welcomed in the comments!

Bottom line: can seals detect the pings of acoustic tags and associate that with food?  Yes.  Does this actually happen in the wild?  More research needed!