23 October 2019 #MOSAiC Expedition Update & Fishy, Fishy, Swim Into My Net

#MOSAiC Expedition Update 23 October 2019

We are headed to a small bay on the coast of Russia. There we will say goodbye to the MI-8 helicopters and their crew before transiting back along the coast to Tromso, Norway.

We couldn’t quite spot land before it got dark out, but I anticipate we’ll wake up to the Archangelsk coast in the morning.

Fishy, Fishy, Swim Into My Net

How do you catch a fish you don't know is there?

This is the question members of the ecology team are facing as part of the MOSAiC project. They are tasked with identifying which species of fish are present in the Central Arctic Ocean. Ideally, they will also figure out some rough estimate of how many fish are there.

The MOSAiC ecologists use four main methods to figure out whether fish are present in the Central Arctic Ocean (and if so how many and which ones). They use echo-sounders. With an echo-sounder, sound waves are aimed into the ocean. If they hit something, these waves are bounced back to the a receiver on the Polarstern. The echo-sounder shows you the depth of the objects as well as the frequency bounced back by the objects. They have been seeing a layer already at about 300-500 meters depth (about 1/4 of a mile). So something is there! But what are they? The frequency information, the size of the objects, and the density of the objects can be used to figure out what type of 'things' are in that layer. The information from the echo-sounder indicates that the layer is living organisms and that it is not any hard-bodied zooplankton like krill or copepods. It is likely fish, but it also could be a gelatinous type of zooplankton called siphonophores. Siphonophores have a gas bubble inside of them that looks similar through an echo-sounder to the swim bladder on many pelagic Arctic fish.

An EK 60 echogram used to locate krill before setting the bongo nets and tucker trawl. Dr. Kristina Kvile points to the location of a suspected krill swarm. Aboard the R/V Sikuliaq in the Beaufort Sea. Photo by Lisa Seff (PolarTREC 2017), Courtesy of ARCUS.

Once they know where the organisms are, the ecologists use two other methods to try to figure out what the organisms are. The first technique is deploying an underwater camera to take photos at that depth. Hopefully the camera will be able to photograph any organisms that are present, and then the researchers can identify them. The other technique they will use is actually fishing. They will put a long line under the sea ice. The long line will extend and run horizontally at about 300 meters under the ice. Many hooks will be attached along the long line, and the hooks will be baited with shrimp or octopus. Nearby, a net with different web sizes (holes) will be set up about 300 meters below the sea ice. And to increase their chances, they are also offering fishing poles and other gear to members of the Expedition to use for ice fishing during their down time. There is a string attached to this loaner program: If they catch anything, it has to be turned over to the ecologists. The scientists are very hopeful that at least one of these techniques will successfully catch some of the fish that are there. But it is hard to know the best technique for catching fish when you don't know what species are present, their life stages, how they behave, or what they eat. The scientists also have to figure out how long to leave the net or long line in the water. If the fish are few and scattered, they improve their chance of catching something by leaving the gear in the water longer. But if there are predators around like seals, their fish might get snatched out of the net or off the hooks. It is a tricky balance, and one that is particularly hard when there is no commercial or local fishery to base your techniques off of. This is the only place in the world so far where a stock assessment (analysis of how big a fish stock is and if/how it can be sustainably harvested) will come before any commercial fishery starts.

The final technique they will use is collecting water samples from different depths. They will analyze these water samples for eDNA. eDNA or Environmental DNA describes DNA that can be found floating free in the environment. It includes DNA from cells that have sloughed off of organisms as well as DNA that has been excreted in waste. In the ocean, DNA from a fish could float quite far. So if they find a few bits of genetic material from a clownfish, it doesn't meant that clownfish live in the Central Arctic Ocean. However, if they find a whole lot of DNA from a species over and over in different samples, it is a good indication that this species lives in the Central Arctic Ocean.

Going into this research, they aren't quite sure what they will find. It seems likely that these waters are home to a fish called Boreogadus saida. In this part of the world, this fish is known as Polar cod. Back in Alaska, it is known more commonly as Arctic cod. This species makes up more than 90% of the pelagic fish community in the Canadian Arctic. But scientists don't really know for sure if this fish is present in large numbers in the Central Arctic Ocean. The last research of fish in this part of the Arctic took place more than 50 years ago! And it sounds like the research was sort of a side project, with the primary objective of the fishing being food for the scientists doing sea ice research.

Arctic cod that were caught in the mid-water trawl net. Aboard the R/V Sikuliaq in the Beaufort Sea. Photo by Lisa Seff (PolarTREC 2017), Courtesy of ARCUS.

Fish biologists know quite a bit about Arctic Cod in the shelf regions, but there are still many mysteries about their life history. They are pretty incredible fish! They feed throughout the Polar Night. Arctic cod have extremely sensitive eyes to find their prey in low light conditions. They also probably use chemical cues or mechanically sense disturbances in the water to eat in the winter. A study in the Canadian Arctic found that they are mostly eating copepods, krill, and amphipods in the winter months. These are all small, shrimp-like zooplankton. Copepods tend to be the smallest of these organisms and krill tend to be the largest. Arctic cod will also feed quite a bit on other young fish during the Polar Night. When food is abundant, probably in the summer months, they are able to store lipids (fats). Later when food is scarce, they can rely on these lipid stores for energy when they need it.

In the winter, the temperature of Arctic seawater is regularly below 0 degrees Celsius (32 degrees Fahrenheit). The salt content of the sea water keeps it from freezing solid at these 'below freezing' temperatures. This is a problem for any organisms that contain freshwater. The freshwater in their bodies will freeze at temperatures below 0 degrees Celsius even if the surrounding saltwater is still liquid. If the water inside the fish blood or cells freezes, the ice crystals can cause lots of damage. Arctic cod prevent this from happening by producing antifreeze compounds.

In many regions of the Arctic, these fish are believed to migrate to deeper parts of the ocean during the winter. They probably go down to about 300-1000 meters (1/5-3/5 of a mile). Many species of copepods perform a similar migration; they go down deep in the water for diapause (sort of like hibernation) each fall. It makes sense that the Arctic cod would follow the copepods to deeper waters and feed on these animals. However, scientists don't know for sure that this is why the Arctic cod migrate vertically with the seasons. And whether or not Arctic cod do this in the Central Arctic Ocean remains a mystery!

In the Beaufort Sea near Alaska, Arctic Cod reproduce in deep water during the winter. The eggs are released in the early spring and float towards the surface. The eggs get stuck under the sea ice and hatch there, or hatch near the surface. This cycle is timed to match when the spring light returns. This is a good time to be a young Arctic cod as there is lots of ice algae growing under the sea ice and phytoplankton is blooming (growing and reproducing quickly). It is unclear how much of the algae/phytoplankton they eat directly, but the cod definitely feed on zooplankton that feed on the blooms. This phenology (timing of life history events) is really important for the success of the Arctic cod. If retreating sea ice leads to earlier blooms of algae and phytoplankton, it is possible that the eggs of Arctic cod won't hatch in time to take advantage of this rich food source. This phenology mismatch may be one way that climate change negatively impacts Arctic cod.

What happens to those juvenile cod throughout the summer and into the next fall? It is believed that in most places they migrate to deeper waters in the fall just like the adults do, but this pattern isn't completely confirmed. And it isn't known at all for the Central Arctic Ocean! Perhaps the eggs that hatch in the Beaufort Sea under the sea ice become juveniles that 'ride' under the sea ice as it drifts. Some of this sea ice will stay near the Beaufort Sea, but other floes of sea ice drift across the Central Arctic Ocean and into the Barents Sea, Fram Strait, and Canadian Arctic. Do the juvenile cod go with the ice across the Arctic? If so, the Polar Drift generally only goes in one direction: from the Alaskan/Canadian/Siberian Arctic to the Atlantic. Given this, do the Arctic cod just get flushed out into the North Atlantic? Or do they perform some sort of counter-migration back to the other side of the Arctic Ocean? There are a lot of questions! So this part of the MOSAiC study is really important. It is also a fun activity for the scientists and crew to participate during the days off. They just have to remember not to eat the fish they are trying to study!

Your Questions and Curiosities

Rebecca T., from Homer, Alaska, asked about whether orca whales are making their way farther north into the Arctic Ocean and what the impact of this is. There are not marine mammal ecologists involved in this part of MOSAiC, but I did talk to someone who studies the borealization of the Arctic. This refers to changes in the Arctic that make it more favorable to species that usually live in the North Atlantic or North Pacific Oceans. He studies how fish like sand lance, capelin, Atlantic cod, Atlantic mackerel, and beaked redfish are creeping northward. The concern is that if these boreal fish become established in the Arctic, they may shift the food web by competing with or eating key Arctic species. The same would likely apply to possible shifts in the range of orca whales. If less extreme ice and temperature conditions allow orca whales to push farther north, they could prey upon animals like ice seals, narwhals, and beluga whales that have otherwise been sheltered from them by the sea ice. In this, they also might compete with the highest predators in the Arctic. Both polar bears and people that depend on marine mammals for sustenance could find themselves in a negative competition with orcas.

Education Extension

The push for understanding fish in the Central Arctic is largely driven by a need to understand the ecology of these fish stocks before the area becomes ice-free enough for commercial fishing to take place. This is sort of stock assessment is at the heart of the MOSAiC fish research. The pressure of commercial fishing on fish stocks is often increased by changes in technology that makes fishing more intensive. Overall, use of resources and environmental impacts are affected largely by changes in technology. In this Community Interviews activity from the Center for Alaskan Coastal Studies, students interview community members about changes they’ve witnessed in their lives, particularly related to changing technology and the impacts of that on the environment and community.