- There are several reasons for the population increase in recent years, scientist Bjarte Bogstad from the Institute of Marine Research explains.
Ups and downs
The Barents Sea cod is also known as Northeast Arctic cod or Arcto-Norwegian cod. The population (fishable stock, i.e. age 3 and older fish) declined from an all-time high of about 4 million tonnes in the mid-1940s to a historical low (around 1 million tonnes) during the 1980s. The population then increased to about 2.4 million tonnes in 1993, before declining again to 1.1 million tonnes in 2000. After that it has increased, and is now estimated to be about 2.5 million tonnes.
As a consequence of this increase in stock size, the quota advice for 2010 (577 500 tonnes) is the highest for more than a decade.
Cod in the Barents Sea is long-lived, average age at first spawning is 6-7 years. Natural variations in recruitment to the stock from year to year are large – a good year-class may be about 10 times as abundant as a poor year-class, although the spawning (parent) stock is the same in both cases. This leads to considerable natural variations in stock size. The stock has been heavily exploited for many years, so fishing also strongly affects the stock abundance.
The stock size is estimated by combining data from annual scientific surveys (trawling and acoustics/echosounder), as well as reports on how much fish is caught annually.
- In such studies it is important to get data not only on the total amount of fish, but also on the size/age composition of fish, says Bogstad.
Quotas and climate
Restrictive quotas are the most important factor behind the recent increase in stock abundance. A management plan was implemented in 2004 by the managing states (Norway and Russia). This plan led to more restrictive quotas than previously set, says Bogstad.
For many years unreported catches were a considerable problem, but such catches have decreased strongly after the introduction of port state control in most of Europe from 2007.
Favorable climatic conditions have also contributed to the growth. Cod has benefitted from higher temperature through increased distribution areas and better food supply, Bogstad points out.
– In the recent warm years the temperature has been about 1°C higher than in previous cold periods. This may not seem much, but the geographical distribution of cod has moved north- and eastwards because of this, as a large part of the Barents Sea now has temperature conditions suitable for cod, says Bogstad.
Stock at high level
Cod generally dislikes water below 0°C, while sea water of normal salinity freezes at -1.9 °C. In previous warm periods also the number of young cod recruited to the stock has been above average. This effect has not been seen in recent years.
The stock increase may not continue.
- In the coming years we expect first a slight further increase, followed by a leveling off and some decrease, but the cod stock will continue to be at a high level, says the cod expert.