Back in the 1920s and 1930s, the Barents Sea was teeming with cod. That was before its waters substantially cooled off in the decades to follow. Now, with ocean temperatures higher once again, fishermen are seeing more fish.
The entire North Atlantic warmed up during the 1920s and 1930s. More fish appeared not only in the Barents Sea but also off Iceland and Greenland. This warm period reached its peak at the end of the thirties and lasted until roughly 1960, when the waters began turning colder again -- and fisheries resources declined once more.
In recent years, the North Atlantic has shown signs of a new period of warming.
Warm waters then and now
Ken Drinkwater is a senior research scientist at both the Institute of Marine Research and the Bjerknes Centre for Climate Research, located in Bergen. He and his colleagues have been studying the causes of this latest warming trend -- and are finding many answers by poring through the literature describing conditions 80-90 years ago.
Dr Drinkwater rejects the common explanation that the Barents Sea is getting warmer because the atmosphere in the polar regions has warmed.
"This warming is primarily due to currents -- a greater amount of warm Atlantic water is flowing into the North Atlantic and up to the Barents Sea," asserts Dr Drinkwater.
That is what happened in the first half of the 20th century. Although there were large year-to-year temperature fluctuations then, the North Atlantic on the whole remained more temperate than normal until well into the 1960s.
"Many people recorded what they observed taking place in the ocean nearly 90 years ago. If we can determine what occurred during that warming period," believes Dr Drinkwater, "we will better understand what is going on today, plus we'll have more reliable input as to what we can expect in the future."
More and larger fish farther north
The warm period between the world wars led to some major changes in the ecosystem. In the Barents Sea and off Iceland and Greenland, cod catches reached record highs.
"In the 1920s and 1930s, the Arcto-Norwegian cod stock moved north. Fishing of cod was organised around Bjørnøya (Bear Island in the Svalbard archipelago) and more cod were being caught in Russian waters as well. The fishermen were catching cod that were about 50 per cent larger than in the previous decades."
The Arcto-Norwegian cod were not the only fish affected by the milder ocean waters of that period. Capelin and Arctic cod also relocated farther north. Haddock moved towards Novaya Zemlya in nearby Russia. In the 1930s, Norwegian spring-spawning herring had gravitated so far east that a dedicated Soviet fishery emerged off the coast of Murmansk.
Herring was more abundant in the Norwegian Sea than ever before. Off the coast of West Greenland, cod were being hauled in 1,200 kilometres north of their usual range. Icelandic herring flourished, and continued to do so until the late 1960s -- when the herring stock collapsed.
More plankton benefits entire food chain
In explaining how warmer seas could lead to so much more fish in the North Atlantic, all the way to the Arctic, scientists point to the bottom-up effect: Warmer seas result in more phytoplankton, which feeds more zooplankton, providing more nourishment for the herring and capelin that serve as a food supply for cod and other larger fish.
Scientific literature confirms that cod reproduction is typically higher in warm-water years and lower when waters are cold.
Link between ocean temperature and spawning
As part of the recently concluded research project Norwegian Component of the Ecosystem Studies of Sub-Arctic Seas (NESSAS), researchers Svein Sundby and Odd Nakken studied the relationship between ocean temperature and cod spawning
When the two charted spawning along the Norwegian coast in a 1900-1976 timeline, the correlation was unmistakable: In coldwater years, southern coastal areas were of highest importance. When waters warmed up, spawning was most active in the northern areas. Seen over time, their research showed that cod reproductivity was markedly higher when the ocean warmed up, and that spawning moved north.
Spawning grounds off northern Norway
Since 2003, Arcto-Norwegian cod have been observed spawning once again along the coast of Finnmark, Norway's northernmost county. This had not seen since the early 1960s.
Recently, cod catches in the Barents Sea have been on the rise. Within the last few years, catch levels have reached those of the 1920s and 1930s warm period. Off Iceland and Greenland, however, no corresponding increase has been recorded.
Research indicates that knowledge about natural climatic variations in fish stocks deserves a role in the future management of the northern fisheries resources.
US observations corroborate
The correlation between warming ocean waters and more fish has also been investigated in a comparative study carried out by the NESSAS researchers in collaboration with US researchers. They have compared developments in three ocean areas: off the coast of Norway, in the waters of the Gulf of Maine (off the northeast coast of the US), and off Alaska and in the Bering Sea (between Alaska and Russia).
"This comparative study has been interesting," says Dr Drinkwater, "in that the causal factors are completely different for the warming of these three northern seas. Yet in each area, warmer waters have led to longer growing seasons, more plankton and more fish."
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