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Sub-stock-specific assessment of cod in Öresund - an evaluation of the effects of fishing restrictions introduced in 2009

Meyer, Mathias (2016) BIOM01 20161
Degree Projects in Biology
Abstract
Worldwide, many fisheries are under severe stress and extensive depletion of fish stocks is common. Besides overfishing an important problem preventing depleted stocks from recovery is that fish stocks are commonly managed following administrative rather than biological reasons. This stresses the importance of sub-stock-specific assessments for sustainable fisheries. The present study performed such an assessment for cod in Öresund, where trawling has been banned since 1932. In 2009 more restrictions were introduced in parts of the area and in the adjacent Kattegat to protect cod stocks. The results indicate a decrease in spawning stock biomass of cod in the Sound after 2009, although with an increasing trend in recruitment due to a very... (More)
Worldwide, many fisheries are under severe stress and extensive depletion of fish stocks is common. Besides overfishing an important problem preventing depleted stocks from recovery is that fish stocks are commonly managed following administrative rather than biological reasons. This stresses the importance of sub-stock-specific assessments for sustainable fisheries. The present study performed such an assessment for cod in Öresund, where trawling has been banned since 1932. In 2009 more restrictions were introduced in parts of the area and in the adjacent Kattegat to protect cod stocks. The results indicate a decrease in spawning stock biomass of cod in the Sound after 2009, although with an increasing trend in recruitment due to a very strong year in 2013. In contrast, average recruitment has been lower in the time after 2009 compared with the period before. Average fishing mortality on cod at age 3-5 has been lower after 2009 compared to the time before. However, an increasing trend towards higher fishing pressure has been seen since the implementation of management actions. Overall, the implementation of restriction for fishing has had positive effects on cod in Kattegat, whereas in the Sound no obvious positive effects could be observed. (Less)
Popular Abstract
Cod is a very important species for commercial fisheries. The species is also found in the waters around Sweden. During the 1970ths there has been a lot of cod in the Kattegat, but heavy fishing for decades has led to a very low amount of cod in the region now. In 2009 protected areas were introduced into the Kattegat and a small part of the northern Öresund, where fishing has been restricted to protect the fish. In some of these areas it is now forbidden to fish all year long, whereas in others areas fishing is only allowed during certain times of the year.
There have been evaluations on how cod in the Kattegat has been affected by these management actions, which suggest that the amount of fish has increased during the last seven years.... (More)
Cod is a very important species for commercial fisheries. The species is also found in the waters around Sweden. During the 1970ths there has been a lot of cod in the Kattegat, but heavy fishing for decades has led to a very low amount of cod in the region now. In 2009 protected areas were introduced into the Kattegat and a small part of the northern Öresund, where fishing has been restricted to protect the fish. In some of these areas it is now forbidden to fish all year long, whereas in others areas fishing is only allowed during certain times of the year.
There have been evaluations on how cod in the Kattegat has been affected by these management actions, which suggest that the amount of fish has increased during the last seven years. However, until now, no studies have been performed that investigated the state of the cod population in the Sound. In my master thesis I tried to find out if the protected areas introduced by 2009 in the Kattegat and parts of the Sound have had effects on the cod population in Öresund.
At the moment there is one fishing quota that regulates fishing in a very large area consisting of the Öresund, the Danish straits and the Western Baltic Sea west of Bornholm. In the last years this quota has not been reached (since there is much less fish in the western Baltic Sea than estimated before), which means that fishing in the Sound (where you have a lot of fish) is virtually unmanaged, since one could basically fish without limits.
To see how the fish in the Öresund have been affected by the management actions introduced in 2009 I performed a so called “sub-stock-specific assessment”. This simply means that I used catch statistics from commercial fisheries (how much fish has been caught the last years) to calculate backwards in time how much fish there has been in the sea in the years before- specifically for cod in the Öresund.
What I have seen is that fishing effort was strongly decreased in 2009 compared with 2008. However, fishing has become more intense during the following years and is on average now comparable with the time before 2009. The results also show that a lot of cod were born in 2012. In contrast to this finding, the assessment showed that “spawning stock biomass” (this means just how many tons of mature cod there are) has decreased since 2009. However, one should not conclude that the management actions introduced in 2009 have had negative effects on cod. This negative trend with a decreasing amount of cod could rather be explained by natural ups and downs in a population that could be caused by a lot of factors, e.g. environmental influences.
Compared with other regions, there is a lot of cod in the Öresund. For this reason I think it is important that we keep an eye on the fish there in the future, to keep the stock healthy by avoiding overfishing. To do so, we probably need some changes in the way how fishing is managed in the Baltic Sea. One way could be to set a quota on how much fish that can be caught specific for the Öresund - not as it is done today, for a huge area covering the entire Baltic Sea west of Bornholm up to the coast in northwest Scania. (Less)
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author
Meyer, Mathias
supervisor
organization
course
BIOM01 20161
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
8876310
date added to LUP
2016-06-01 14:03:34
date last changed
2016-06-01 14:03:34
@misc{8876310,
  abstract     = {Worldwide, many fisheries are under severe stress and extensive depletion of fish stocks is common. Besides overfishing an important problem preventing depleted stocks from recovery is that fish stocks are commonly managed following administrative rather than biological reasons. This stresses the importance of sub-stock-specific assessments for sustainable fisheries. The present study performed such an assessment for cod in Öresund, where trawling has been banned since 1932. In 2009 more restrictions were introduced in parts of the area and in the adjacent Kattegat to protect cod stocks. The results indicate a decrease in spawning stock biomass of cod in the Sound after 2009, although with an increasing trend in recruitment due to a very strong year in 2013. In contrast, average recruitment has been lower in the time after 2009 compared with the period before. Average fishing mortality on cod at age 3-5 has been lower after 2009 compared to the time before. However, an increasing trend towards higher fishing pressure has been seen since the implementation of management actions. Overall, the implementation of restriction for fishing has had positive effects on cod in Kattegat, whereas in the Sound no obvious positive effects could be observed.},
  author       = {Meyer, Mathias},
  language     = {eng},
  note         = {Student Paper},
  title        = {Sub-stock-specific assessment of cod in Öresund - an evaluation of the effects of fishing restrictions introduced in 2009},
  year         = {2016},
}