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Mystery behind collapse of large fishery now solved

Tuesday 05 Jan 16
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by Line Reeh

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Einar Eg Nielsen
Professor
DTU Aqua
+45 35 88 31 15

Cod fishery off West Greenland

Historically, cod has been the single most important fishery resource in West Greenland, but stocks have varied considerably. During the cod boom in the 1960s, the international fleet of trawlers fishing off West Greenland totalled around 250 vessels. The boom peaked in 1962 with catches exceeding more than 400,000 tonnes per year. The Greenlanders’ own cod catches were small in comparison, less than 30,000 tonnes annually. Cod catches saw a dramatic decline at the end of the 1960s, and disappeared altogether in 1992. The new genetic studies have shown that the Greenlandic cod initially constituted two-thirds of the catches, but that it had almost disappeared by 1970. Focus then shifted to Icelandic offshore cod fishing, which continued until stocks began to decline in the 70s.

DNA tool makes it possible to ‘travel back in time’ and see exactly what went wrong, when the cod disappeared

Throughout history, we have on numerous occasions witnessed the collapse of large fisheries because the fish disappeared.

“People have always discussed the cause, when large fish stocks disappeared. Was it due to environmental changes or overfishing? We now have a tool that has made it possible to ‘travel back in time’ and find out exactly what went wrong,” says Einar Eg Nielsen, Professor of Fish Genetics at DTU Aqua in Silkeborg, Denmark.

Together with colleagues from DTU Aqua, Aarhus University, and the Greenland Institute of Natural Resources in Nuuk, he has—using modern genetic analyses—examined why the West Greenland cod, Greenland’s main economic resource in the 1950s and 60s, disappeared. And the answer is both—overfishing as well as environmental changes caused the cod to disappear.

“Where we previously believed that a cod was just a cod, we now know—from genetic analyses—that the cod caught off West Greenland actually originated from several different stocks: a slowly growing West Greenlandic stock and a fast-growing stock that mainly lives near Iceland. Fishing in West Greenland first collapsed because of overfishing of the local Greenlandic stock, and as the environmental conditions changed at the same time, among other things resulting in lower temperatures, the area offering favourable living and reproduction conditions for the cod diminished,” says Einar Eg Nielsen, DTU Aqua.

DNA test provides answers

Here, 50 years later, it has been possible to find out which types of cod were caught, because the Institute of Natural Resources in Nuuk has a large collection of several hundred thousand old ear stones (otoliths) from cod caught off the west coast over the years. Ear stones are calcium carbonate structures from the cod’s head traditionally used for age determination, as they grow like rings in a tree. The ear stones have microscopic traces of blood and mucus from the cod, which have been used for DNA analyses and thus to determine which stocks the fish belonged to.

"This is knowledge that simply wasn’t available at that time, because they didn’t have the same methods as we do today."
Professor Einar Eg Nielsen, DTU Aqua

“If you don’t examine which stocks are caught, you cannot explain why the fishery collapsed. So this is knowledge that simply wasn’t available at that time, because they didn’t have the same methods as we do today," explains the fish geneticist.

But what should they have done back then to prevent the collapse, had they known what we know now?

“They should probably have concentrated the fishery in the southern part of West Greenland which had the largest population of Icelandic cod, or generally have caught far less cod,” says Professor Einar Eg Nielsen.

Improves management today

Today, the new genetic knowledge is used directly in the fisheries management of cod in Greenland. Here, thanks to the new understanding, it has been possible to open up for cod fishing from fjords (where the cod thrives), because we now know that it does not have an impact on the stock from the banks in the open sea on the west coast, which has still not recovered:

“This is the way to harvest cod in the future, and which is made possible when using the genetic knowledge and including it in the management of fish stocks: to fish in areas having many fish from stocks that are doing well, but not in areas where those doing less well are living.”

Einar Eg Nielsen and his colleagues from DTU Aqua are currently looking into cod fisheries in the North Sea, the Kattegat, and the Baltic Sea, which also involve mixed stocks. As fish management is traditionally based on quotas for species in specific geographical areas without taking into account which genetic stock they originate from, the new stock-based management approach requires a number of new monitoring tools for providing advice on fishing. This includes new models for stock estimation that can incorporate the information about the proportion of the different stocks in the catches.

The studies of the causes of the West Greenlandic cod fishery collapse was carried out by DTU Aqua in cooperation with Aarhus University and the Greenland Institute of Natural Resources and was funded by the Greenland Climate Research Centre and the Nordic Centre for Research on Marine Ecosystems and Resources, NorMER.

The scientific article:

Bonanomi, S. et al. Archived DNA reveals fisheries and climate induced collapse of a major fishery. Sci. Rep. 5, 15395; doi: 10.1038/srep15395 (2015)