Fish tags attract seal predators

Sneaky seals have learned to listen out for acoustic tags on fish to find their next meal.
24 November 2014

Interview with 

Amanda Stansbury, University of St Andrews


Marine biologists often use electronic tags to keep track of fish populations and Sealmovements. A small transponder is attached to the fish and emits a series of high frequency 'pings', alerting researchers to a shoal's location. But a team at St Andrews University have now shown that grey seals have learned to associate these 'pings' with food - the tags are basically acting as a giant dinner bell! Graihagh Jackson spoke to Amanda Stansbury about what this means for our fishy friends...

Amanda -   Fish tags are used on several different fish species to track populations and measure the survivability rate so we get an idea of how fish stocks are doing.

Graihagh -   So, how do these tags actually work?

Amanda -   So, what these tags do is we put them on the animals and the tags emit a noise that's high pitched in frequency.  What they do is when these fish are swimming along and they emit these sounds, we can then pick up the sounds with receivers or listening stations.  By that way, we can track where the fish are and how many tagged fish go past a certain receiving station.

Graihagh -   So, could you picture a bit like sonar on submarines, you get this poop, poop, poop.

Amanda -   Yes, though the tags that we use, it has a high frequency tone with a little bit more like hearing an actual sound and it's a little bit longer.  Though it's really interesting because with seals, the frequency of these tags, it's very high frequency and it's just on the edge of the seal's hearing limits.

Graihagh -   Indeed, it's way, way, way beyond anything we can hear.  These fish tags are emitting sounds at 69,000 hertz and you and I, we can only hear up to 20,000 hertz.  So, if I played you this and this, you can't hear the beeps of the fish tag.  But given the extremely high frequency of the sounds that these fish tags are emitting, it's surprising that seals can detect them at all.  So, what's going on?  How do they know where these tagged fish are?

Amanda -   Instead of maybe hearing this high frequency sound, they might actually hear a click that's created from the onset of the tag sound.  Each time the tag produces a sound, it's in a sequence of 8 pulses.  These pulses are very short and quick and it results in what sounds like a click when it starts and stops.

Graihagh -   With a bit of clever digital manipulation, we changed the frequency of the fish tag beeps, so it would be within the human hearing range.  These clicks essentially act as a giant dinner bell.  Fish anyone?

Amanda -   Past research had known that the seals might be able to hear these sounds but our study is the first to show that they learn to use the tags to their benefit and find their prey.

Graihagh -   Has this behaviour been seen in the wild before?  Could it not just be a case of these seals learning to know what these sounds meant within your tests?

Amanda -   Currently, there aren't any known studies that have tracked these in wild animals.  However, we do think this might be occurring for example in wild salmon that have been tagged.  Some salmons have been tagged with acoustic tags, just like the ones we use and other salmon have been tagged with a silent tag.  It doesn't make any noise.  When we look at the survivability of these animals, the acoustically tagged salmon are less likely to survive.  Presumably, we're thinking that predators might be finding them and that's why the tags might be having an effect in wild populations.

Graihagh -   So, this is presumably great news for predators, but not so great for their prey because I'm guessing, we're tagging these fish stocks because they're in decline.  Given that there's also a lot of research on how these human made noises like boat engines have, have negative impacts on marine life.  Should we therefore be thinking, re-thinking even how we track and monitor not just fish stocks, but also birds and other mammals as well?

Amanda -   Yes and so, what's really important with this is when we tag animals, we need to consider the entire ecosystem that they're in and the other animals that are in the environment, whether they can perceive the tag.  It's important if we put tags on prey species.  If they're around predators, we don't want to inadvertently decrease their numbers.  But also vice versa, if we were to put tags on predators then the prey might hear predators approaching and evade them.  And so, we need to be really careful when we introduce these sound systems into an environment and what kind of impact they'll have.


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