little fish in a big net

According to Franz, polar bears are really just repackaged arctic cod. Polar bears eat seals and seals eat arctic cod. These little fish bring the carbon that is created at the base of the food web (by algae and zooplankton) up to the higher predators (seals and polar bears). That makes them a keystone species; a crucial component of the arctic ecosystem. According to Hauke, from Germany, they are called polar cod, not arctic cod (same fish, just different names in Europe and the US).

We know surprisingly little about these polar/arctic cod, given how important they are. They live a relatively secluded life in a place rather inhospitable to humans. The GoWest team, led by Hauke Flores and Franz Mueter, are studying these important little fishes to better understand how they interact with and impact their ecosystem.

The hypothesis for this expedition was that arctic cod in the western Arctic hang out right under the sea ice as it forms in the autumn, despite the fact that this zone is basically a biological desert for how little life thrives there.

To test this hypothesis, the GoWest team went fishing under the ice with a huge rugged custom-built SUIT net (Surface and Under Ice Trawl). They wanted to see first if there were arctic cod there at all, under this new ice along the Beaufort shelf, and if there were, how they were surviving. In addition to catching fish, the SUIT can also capture the fish’s potential food sources, like zooplankton and algae, which helps to fill in the story of their icy existence.

The SUIT net is big enough to fit the whole team

Both Hauke and Franz admit their expectations were low; they would both consider it to be a successful venture if they caught even a handful of these sub-6” long juvenile fish.

When the net came back onboard after its first cast in the ice, as the last of the short daylight evaporated, Hauke peeked into the fish basin and the whole team went quiet. They were all cold and tired from a long dark day of fishing; it was the first time this team had worked together to launch something the size and weight of a small car off the back of the boat.

“We have fish!” He exclaimed, not bothering to hide the relief and excitement from his voice. The team whooped and high-fived, then got right down to work processing their catch.

It was a big moment. Because whether they caught fish or not, either way it is a result. The challenge with the negative result (not catching any fish) is that it’s hard to draw any conclusions from it. Was it because there were no fish, or did you have the wrong tools or were you looking in the wrong place, or a hundred other reasons?

The GoWest team did not have that problem. Over the following weeks, they would go on to catch over 100 arctic cod, at least a few – and sometimes dozens – coming up with every successful cast of the SUIT net.

Juvenile arctic cod among the ice captured in the SUIT

Back in the lab, the team measured and weighed each and every fish, then dissected it to examine its stomach contents and its ear bones. Ear bones? It turns out ear bones, actually called otoliths, can reveal a lot about a fish’s existence. These otoliths are hard mineral structures in the brain that help the fish with hearing and balance. They grow in increments, like a tree, so the number of rings on them indicates the fish’s age. And with some testing, these tiny little structures can tell the story of what kind of water masses that fish dwelled in during its life. This is all very helpful to the scientists trying to understand the life story of an elusive little fish.

There are still years of processing and analysis to do. But from just the single fact that every net cast pulled in fish, Hauke and Franz and their team were able to substantiate the idea that arctic cod associate with the sea ice during the autumn.

It was a big month for this little floppy fish (or at least for our understanding of them).