Just over a year ago, the Institute of Marine Research (IMR) tagged ten large saithe off the Lofoten archipelago. Now the researchers have received the data from 9 of the 10 satellite tags and can tell us a bit about the fates of these fish.
“We have received most of the satellite tags, which give us access to a lot of data for our research. And we also got some good stories about the fate of the saithe we tagged”, says marine scientist Keno Ferter.
Eaten by a whale
One of the tags reported back from off North Cape at the end of April. When Ferter received an e-mail containing the tag’s data, he was slightly confused.
“The data showed that the saithe had stayed somewhere with a temperature of plus 36 degrees Celsius, which is, to put it mildly, unusual both for saithe and for the area in which the satellite tag appeared” says Ferter.
Satellite tags work like a data logger, recording information about light, depth, time and temperature. There were several things about the data from this tag that didn’t quite match up with the expected behaviour of a saithe.
“There was also something about the saithe’s movement in the water that wasn’t quite right. When we put two and two together, or rather combined temperature, light and depth data, we realised that the saithe must have been eaten by a whale. So, the tag was simply excreted just off North Cape”, explains Ferter.
Floated to Russia
Unfortunately, the tag that already had made it through the whale’s digestive system was lost after a battle against time and weather. It floated from North Cape towards Eastern Finnmark and ended up floating across the Russian border.
“I was at home following the signal, ready to jump on the next plane. For a while we were hopeful it would drift ashore near Vardø, but it carried on straight towards the Russian border. Eventually we lost the signal.”
Another big saithe took the fishing lure at the same location it was caught and tagged a few months earlier.
“It was even in the same boat, with the same fishing guide on board”, says Ferter.
Another saithe was caught off Røst in a gillnet, and the fisherman who caught it was quick to report he found the satellite tag. If the tag is switched off immediately after being found, it can be reused. But in order to do this, you need a magnet.
“Since the fisherman didn’t have a magnet on board, he had to improvise. He managed to switch off the tag by putting his hand into an electric motor.”
“I also received a phone call from the military base at Andenes, where an employee out harvesting mussels had found a tag in the intertidal zone, which really confused them. Another tag was transmitting signals from shore by Nappstraumen in Lofoten. It was, to say the least, hard work tracking it down. I had to search high and low before I finally found the tag on a steep hillside. There must have been a bird messing with me”, laughs Ferter.
The satellite tags are programmed to release themselves from the saithe after a year. Data is then sent via satellite to the researchers, who can reconstruct the route taken by the fish.
If the tag itself is found and returned, the scientists get more detailed information to work with.
“That’s why there is a finder’s reward for our tags. If you find one of our tags – in the sea, on the shore or on a fish you catch – we want to get the actual tag back. The reward is NOK 1,000 for satellite tags and NOK 500 for our DST tags”, says Keno Ferter.
9 of 10 satellite tags have reported back over the course of the past year, and eight of them have been found. That means the researchers have plenty of data to work with.
What does the scientists know now - that they didn’t last year?
Previously, tagging experiments were carried out on smaller saithe measuring under 60 cm. In addition, the saithe stock is monitored during the IMR’s annual coastal surveys. However, few large saithe have been observed during these surveys.
“We therefore wanted to know more about where large saithe go between spawning seasons, and where their feeding grounds are, so that we can adjust our surveys to cover a larger proportion of the population”, explains Arved Staby. He is stock coordinator at the IMR responsible for the Northeast Arctic saithe stock.
Staby has started analysing the data, and he has made some preliminary discoveries that are interesting. However, he does not want to draw any firm conclusions yet.
“What we can say with some confidence is that the behaviour of most saithe that we tagged was quite similar. A preliminary estimate of their migration routes shows that in the autumn the saithe probably swims further out into the Norwegian Sea to depths of up to 200–300 metres, before returning to the coast and the upper waters in early spring. The presence of the saithe in the upper water layers in summer and late autumn can partially explain why we don’t observe large saithe during our coastal survey in the late autumn”, says Staby.
The project is part of the coastal survey project.
10 large saithes were satellite tagged in June 2021 with the help of experienced fishing guides from Nordic Sea Angling. The average length of the fish was 108 cm, which very likely makes them older than 14 years.
Satellite tags work like a data logger, recording information about light, pressure, time and temperature.
Tags release themselves after a year and transmit information to the researchers by satellite.
A statistical model using temperature, depth and light data allows the scientists to reconstruct the swimming route of the fish.
The system used for attaching the satellite tags was originally developed and produced by Finn Økland of the Norwegian Institute for Nature Research (NINA) for tagging eels.
In 2021, 30 saithes were also tagged with DSTs that record depth and temperature.
Quick facts about the Northeast Arctic saithe
Latin name: Pollachius virens Other English names: Pollock, coalfish, coley, Boston blue Family: Cod family Maximum size: 20 kg and 130 cm Lifespan: Up to 30 years Distribution: Along the Norwegian coast from Stad to the Kola Peninsula Main spawning ground: On coastal banks from Lofoten to the North Sea Spawning season: Winter, with a peak in February Diet: Calanus finmarchicus, krill and other pelagic crustaceans, herring, sprat, blue whiting, Norway pout and juvenile haddock Predators: Seals and whales Characteristics: Found in dense concentrations, often in the pelagic zone where currents concentrate their prey