A fishy tale from the changing Arctic Ocean

The Arctic is warming rapidly, up to twice as fast as the global average1. Hopefully I do not have to go into the specifics of what changes this might incur, both regionally and globally, as they will be both broad and far reaching2. But the issue in the Arctic is that this system is so poorly understood that it is hard for scientists have confidence in exactly what changes might or are occurring.

In an attempt to resolve this knowledge gap, the UK and German governments are co-funding the Changing Arctic Ocean (CAO) program which brings together scientists from across the globe to try and better understand the structure and functioning of all aspects of the Arctic ecosystems and how they are likely to be impacted by climate change. I’m fortunate enough to be a part of this program as a post-doctoral research associate based at Newcastle University, as part of the Coldfish project whose focus is on…, well, fish!

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Polar cod, Boreogadus saida (Lepechin, 1774), is a sea ice associated fish species found in the Arctic and is a focus species of the Coldfish project. It can tolerate sub-zero temperatures due to the antifreeze proteins in its blood, but it may be pushed out of the Barents Sea as the waters warm, the sea ice retreats and more competitive species expand northward.

Coincidentally, the week that I set about to write this blog post, the BBC News posted an article highlighting some of the rapid changes that are occurring in my research study area, the Barents Sea. The Barents Sea is an important gateway into the Arctic from the Atlantic Ocean, located north of Norway and Russia: a shallow shelf sea that is over twice as large as the North Sea by area. Being at the interface between warmer Atlantic and colder Arctic waters makes the region incredibly productive (biologically) and the fish community that live there support many commercial fisheries, marine mammals and sea bird populations. Therefore the Coldfish project is focussing on how the ecologically and economically important fish community in the Barents Sea is responding to two main issues associated with warming: sea ice retreat and an influx of warmer water (boreal) species. Specifically, the project aims to explore how different key fish species behave in terms of diet and metabolism, and whether these behaviours change with the environment. This information is crucial for fisheries management in order to predict climatic induced changes going into the future.

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Map of the pan-Arctic region highlighting key areas. Credit given to Dr Koen Hufkens for his online tutorial on producing polar region plots in R programming language.

The fish community in the Barents Sea is composed of 2 broad groups: and cold water arctic community (such as polar cod) that dominates in the North and East; and a warmer water boreal community (including Atlantic cod and haddock) extending into the south and west from the Norwegian Sea. Where the two groups meet results in a mixed composition, however fisheries survey data has revealed that the boreal group is currently expanding its range in the Barents Sea3. Based on stomach content data4, we know that the boreal species are more generalist in their feeding behaviour – they consume a variety of different prey from the water column and off the sea floor, whereas arctic species are more specialised – they are fussy eaters. Therefore the expectation is that the Barents Sea food web, which is currently compartmentalised, will become more connected in the future as the boreal – generalist feeding behaviour will dominate and the sea-ice specialist compartment will be lost as this habitat retreats. Changes in the food web structure will change how biomass flows through the system and may have negative consequences species of commercial or conservation interests. An example is Brünnich’s guillemot, an Arctic seabird that relies heavily on polar cod and capelin, another arctic fish species, for food (Brünnich’s guillemot is the focus of another CAO project – LOMVIA).

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The Brünnich’s guillemot, Uria lomvia (Linnaeus, 1758) is an auk that is dependent upon arctic fish, such as the polar cod, for food. Photo credit: Sebastian Gerland, Norwegian Polar Institute.

A key issue with the expectation of greater food web connectance is that it depends upon the different fish species behaving the same (in terms of diet) – the generalist feeders will remain generalist as they move into new environments and are potentially exposed to new competitors. In reality, we really don’t know whether this is true or not and this is where Coldfish comes in. Instead of using stomach contents, which really only tells you information on an animal’s last meal, our project will use stable isotopes. Stable isotopes, atoms with varying numbers of neutrons that react the same chemically but do not decay (unlike radioactive isotopes), act as biomarkers of diet. Different food sources have different proportions of stable isotopes, and these are slowly incorporated into the fish muscle as they consume food and grow, so by measuring the chemical composition of muscle tissue, we can trace what the fish has been feeding on over a long time period (weeks to months)! We will specifically looking at carbon, nitrogen and sulphur stable isotopes, but different elements are used as tracers for other information too.

So what we aim to do for key fish species such as polar cod, Atlantic cod and haddock among others is describe the dietary behaviour (the sciency term is called the trophic niche) using stable isotopes, and see whether this changes in different areas in the Barents Sea. For example, does Atlantic cod maintain the same levels of generalist feeding behaviour in the colder, northern regions of the Barents Sea compared to the south western areas or is it more restricted in what it eats? If its feeding behaviour does change then this has different consequences for the arctic fish species that it may be competing with.

That’s the diet part of our project, but I also mentioned metabolism earlier too! While the diet provides the chemical energy for our fishy friends to swim, grow, reproduce etc., metabolism is the process through which animals convert this food into usable energy in the body. As you can imagine, this is really hard to measure – scientists are often restricted to using controlled chambers in labs to estimate oxygen consumption rates of fish (a proxy for metabolism). In addition, this approach doesn’t really tell us about metabolism out in the field, where animals behave very differently compared to laboratory conditions. However, because stable isotopes are awesome (I may be marginally biased), a new approach has recently been developed where they can be used to estimate metabolism from otoliths5 – the ear bones of fishes. Further, because the otolith is grown incrementally like tree rings, it contains an isotopic record of the metabolism of the individual fish over its whole life!

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An Atlantic cod otolith (the ear bone of a fish) showing characteristic growth rings similar to those found in trees. Photo credit: Victoria Neville, CFER.

Here at Coldfish, we will be using this new technique to explore how the metabolism of polar cod and Atlantic cod varies across the Barents Sea. This will help us address uncertainties around things like competition: do individuals expend more energy (have a higher metabolic rate) in areas where they are competing with each other? Does expanding north into the Barents Sea result in a greater energetic cost for Atlantic cod which may affect their ecological fitness?

Now hopefully all this research sounds as cool to you as it does to me (pun very much intended), and I am really excited to have the next two and a half years dedicated to the often under appreciated fish. My hope is that others will come to appreciate that fish matter just as much as the whales, polar bears and feathered friends that all too often seem to hog the lime light in a changing Arctic Ocean.

This post was written by Dr Matthew Cobain. Matthew is a Post-Doctoral Fellow at the University of Newcastle, UK. @CobainMat

References:
1 – Kaplan, J. O. & New, M. 2006. Arctic climate change with a 2⁰C global warming: Timing, climate patterns and vegetation change. Climate Change, 79, pp213-241
2 – Larsen, J.N., O.A. Anisimov, A. Constable, A.B. Hollowed, N. Maynard, P. Prestrud, T.D. Prowse, and J.M.R. Stone, 2014: Polar regions. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L.White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1567-1612.
3 – Fossheim, M., Primicerio, R., Johannesen, E., Ingvaldsen, R.B., Aschan, M.M. and Dolgov, A.V., 2015. Recent warming leads to a rapid borealization of fish communities in the Arctic. Nature Climate Change, 5(7), p.673.
4 – Kortsch, S., Primicerio, R., Fossheim, M., Dolgov, A.V. and Aschan, M., 2015. Climate change alters the structure of arctic marine food webs due to poleward shifts of boreal generalists. Proc. R. Soc. B282(1814), p.20151546.
5 – Chung, M.T., Trueman, C.N., Godiksen, J.A., Holmstrup, M.E. and Grønkjær, P., 2019. Field metabolic rates of teleost fishes are recorded in otolith carbonate. Communications Biology, 2(1), p.24.
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