About cod, the climate, polar cod and benthic animals

About cod, the climate, polar cod and benthic animals

Top image: Paul Renaud (right) and Jørgen Berge (left) with the catch from a trawl in Isfjorden on 21 August 2019. A few large cod (13) accounted for about 20% of the total weight of the catch, while polar cod accounted for 70% (estimated at over 12,000 individuals). Photo: Fredrik Broms.

In July 1873 some nets were put out by whalers in Raudfjorden in Svalbard, to obtain food. They had expected to catch Arctic char, but ended up filling the nets with cod.

6 September 2019
Text:  Professor Jørgen Berge (UiT Norway’s Arctic University, UNIS and NTNU) and Professor Paul Renaud (Akvaplan-niva and UNIS.)

The same year, Greenland shark fishermen reported large quantities of cod at Grønfjorden. The following year (1874), three vessels were equipped and sent from Tromsø to fish cod. They returned home with about 37,500 cod in the cargo. In the years that followed there was a large cod fishery in Svalbard, mainly from Bellsund in the south to Raudfjorden in the north. Fishing was conducted exclusively by leash, and in the peak year 1879 nearly 600,000 cod were caught and brought down to the mainland. The hopes of a good fishing and prospects of a good income for the families at home were probably high when 18 ships started fishing in the summer of 1883. Only three cod were caught that year. Throughout the 1900s there have been periods of much cod in Svalbard, especially during some periods around 1939, and from 1960 to the end of the 1970s.

Cod – a ferocious creature
In the last 15 years we have again seen a lot of cod in the fjords along the west coast of Svalbard, although it has not been commercially fished. Alongside cod, we have documented occurrences of capelin, haddock, herring, saithe and mackerel in the Isfjorden and Kongsfjorden, to name a few other fish species. While these species have obviously spread northward, the population of polar cod on the west coast of Svalbard has apparently declined sharply. Small free-living osprey of polar cod often compete for the same food as osprey of both cod and haddock, while polar cod is also a much sought after treat for big cods. A cod of 3-4 kg can have up to 10 grown polar cod in the stomach. Now, it is not only polar cod that is on the cod menu – it is a very ferocious creature that eats mostly what it comes across of benthic animals.

If we look in isolation at what has happened in the last 15 years, the picture is reasonably clear: mussels, crown jellyfish, redfish and cod are all southern species we do not initially associate with a cold Arctic climate, but which we have seen constantly increasingly in Svalbard. This also coincides with significantly increased temperatures and reduced sea ice cover. It is therefore easy to draw the conclusion that the occurrence of these more southern species is a direct consequence of a warmer climate. While such a conclusion may not be so completely wrong for mussels, crown jellyfish and Calanus finmarchicus, the truth about cod is much more complex. And the story of what happened in 1883 may contain some important clues!

Reduced level
Cod is a very ferocious fish, and it is known to eat almost anything of benthic animals and fish. Particularly shrimp, seaweed, shells, brushwood and other important bottom animals are heavily preyed upon by the cod. This is something we have studied on our annual UNIS cruise with UNIS marine biology students, where we, with the research vessel “Helmer Hanssen”, have done regular surveys of the fish and benthic fauna in both Kongsfjorden and Isfjorden. In 2003, before the large arrival of cod to the west coast of Svalbard, the catch in a bottom trawl was dominated by polar cod and shrimp, but with a strong element of other benthic animals. Common cod was more than a rare guest, accounting for between 2% and 7% of the trawl catch in these two fjords.

Thirteen years later, in 2016, cod accounted for 93% of the catch, with a marked decline (2-3% of the catch) for both shrimp and polar cod. This year, however, we have observed that the presence of cod in the outer parts of Isfjorden and Kongsfjorden is again sharply reduced, and now accounted for 20-35% of the catch (dominated by a few large individuals), while the proportion of polar cod and shrimp was on a total of 45-80%. In Isfjorden, the shrimp accounted for only 2% (somewhat more in Kongsfjorden), and in both fjords it was almost no other benthic animals. In comparison to 2003, polar cod has “returned” and become very dominant, while shrimp and other bottom animals are still greatly reduced. Further in Kongsfjorden, near Kongsbreen, we found much more cod than previous years (66% of the catch). In previous years, we have gone to the inner parts of the fjords to find especially polar cod. This year, the catch of polar cod made up only 1% of the catch.

Hypothesis
Our hypothesis is that these changes between years are a direct result of the cod simply eating the fjord empty of food, both small animals on the seabed and small fish. If we look at Kongsfjorden, which is probably the best studied fjord in Svalbard, it seems that there is still a lot of cod in the fjord, but that it has thus drawn further into areas where there is still a lot of food to find at the bottom. The polar cod, on the other hand, which for many years has fled into shallower water for fear of being eaten up by the ferocious cod, finds much less food on the bottom. On the contrary, polar cod almost always looks upwards in the water column for food, especially copepods and amphipods. It is therefore not particularly affected by the fact that the cod has eaten the bottom animals it is hunting. We believe that this is exactly what happened in 1883, when disappointed fishermen returned from Svalbard with three cod on board. Then there had been a lot of cod in the deep parts of the fjords on the west coast for several years, but then suddenly they were gone. Maybe this year’s cod also had to leave the deep areas of Isfjorden and Kongsfjorden just as disappointed (and hungry) as the fishermen, and rather seek happiness in new hunting grounds? Surely it is at least that in the year 1883 there was no cod left in the deep areas of the fjords, where in the years 1874-1882 there had been intense fishing.

Now of course we have no evidence that this is the way it is, but the trend is clear: the fjords in Svalbard are not a permanent home for the cod. It may be easier (physiologically) for cod to migrate north in a warmer climate, but perhaps it is not the climate that regulates its occurrence in the fjords in Svalbard, at least not directly. Some researchers have concluded that the sharp decline of polar cod in the 21st century is a regime change, where especially capelin has replaced polar cod in the food chain, and that this is a shift driven by a warmer climate. If this is correct, there is little reason to expect an increase in polar cod as we have observed this year. On the contrary!

Of course we should be very careful about drawing solid conclusions on the basis of the catches we have made this year, but there is reason to point out how important it is with long time series and observations over time. And if we look at the changes that have taken place in Isfjorden and Kongsfjorden all the way back to the 1870s, then an ecologically complex interaction between prey (polar cod, shrimp and other benthic animals), predators (cod) and climate appears as a probable model of explanation. We must of course keep up with developments in the coming years, but if our hypothesis is correct, for some years to come we will see (relatively) little cod in the areas on the west coast where we have observed large quantities of cod in recent years. Only when the bottom fauna here has time to come back, and if the conditions otherwise allow, is our prediction that we will again see large deposits of cod at the mouth of Isfjorden and Kongsfjorden.

 

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