Researchers from DTU Aqua have decoded the behaviour of Norway lobsters and cod and used the results to develop a selective trawl. This so-called SELTRA-trawl ensures that fewer cod end up as by-catch in the Norway lobster fishery in the Kattegat.
Despite the fact that the cod fishery in the Kattegat is subject to strict fishing quotas, a substantial amount of cod have ended up as by-catch in the Norway lobster fisheries. But after July 15 2011, more cod have escaped the lobster trawl. From this date, the Danish Ministry of Food, Agriculture and Fisheries has decided, that all Norway lobster fishing in the Kattegat is to be conducted using a selective trawl, called the SELTRA-trawl.
”The Norway lobster population in the Kattegat is doing well, and the Norway lobster fishery is the most economically important fishery in the Kattegat. In 2010 alone, 1700 tonnes of Norway lobsters were caught here. The cod population, on the other hand, has declined severely in the last 20-30 years. If it had not been possible to reduce the by-catch of cod by implementing the SELTRA-trawl, the Norway lobster fishery would have to be reduced significantly in order to protect the cod”, says senior research scientist Niels Madsen DTU Aqua who has been in charge of developing and testing the SELTRA-trawl during a project funded by the Ministry of Food, Agriculture and Fisheries and the EU.
The cod escapes
A trawl is a funnel-shaped net, which distends when it is pulled after a vessel. When it is pulled along the bottom of the sea, it catches Norway lobsters and bottom-dwelling fish on its way. The catch then falls back towards the rearmost end of the trawl and ends up in the so-called codend.
The challenge for the researchers at DTU Aqua has been to design a trawl that selectively catches Norway lobsters while letting cod and other unwanted by-catch escape through the meshes. Norway lobsters are relatively small, and a small mesh size is thereby required to retain them in the codend. These small meshes also retain fish the size of or larger than the Norway lobsters which is the reason that previously there has been a great deal of by-catch when fishing Norway lobsters.
The researchers came up with the idea of replacing the traditional round codend with a codend shaped like a square box. This square-shaped box proved to be more stable in the water enabling the researchers to take advantage of the cods’ and Norway lobsters’ behaviour.
When using this codend, the researchers discovered that the Norway lobsters were passive and preferred the bottom part of the codend, while the cod were more active and had a preference for the upper part of the codend and tried to swim against the current to escape.
Based on the knowledge of the differences in behaviour, the researchers at DTU Aqua created the so-called sorting box that has a larger mesh size and is placed in the front end of the SELTRA-trawl allowing the cod to escape. Thereby, they had come up with the basic idea for the SELTRA-trawl.
The seven meter long SELTRA-trawl has been designed to replace the rearmost end of the fishermen’s own trawl. The square codend is 50 cm high and 50 cm wide. The complete trawl is around 30 meter long. The mesh size in the square codend is only 90 mm, thereby retaining both Norway lobsters and larger fish. In the sorting box, the mesh size is 180 mm and thereby large enough to allow the cod to escape. The research group at DTU Aqua is now developing a computer programme that can simulate the catch process. This will enable further enhancement of the mesh size and shape and thereby optimise the selectivity of the SELTRA-trawl. Illustration: Niels Madsen, DTU Aqua.
To be placed on the fishermen’s own trawl
In order to keep the costs of the SELTRA-trawl relatively low, the SELTRA-trawl was developed to be added to the fishermen’s own trawl:
”The fishermen fishing for Norway lobsters has their own trawl already, and all they need to do is to place the seven meter long SELTRA-trawl with the sorting box and the square codend instead of the rearmost part of their own trawl. In this way, the fishermen do not have to buy a complete new trawl,” explains Niels Madsen.
Testing the trawl
Project SELTRA was initiated in 2005 and completed in the end of 2008. Since then, the SELTRA-trawl has been tested in the Norwegian company SINTEF’s flume tank at the North Sea Science Park in Hirtshals.
”Through the co-operation with the Danish Fishermen’s Association, fishermen and net makers we got ideas on how to design the SELTRA-trawl, so that it is convenient and useful for the fishermen and easy to construct for the net makers,” says Niels Madsen.
The SELTRA-trawl has been used on commercial fishing vessels in the so-called closed areas in the Kattegat. The closed areas are areas, in which the Danish Ministry of Food, Agriculture and Fisheries has prohibited cod fishing in order to protect spawning cod.
”In some places in the closed areas, the fishermen have been required to use the SELTRA-trawl when fishing for Norway lobsters. The fishermen, who have now used the SELTRA-trawl for a couple of years, say that they have not experienced significant reductions in the amount of Norway lobsters that they catch,” says Niels Madsen and continues:
”Furthermore, the SELTRA-trawl has proved to allow the main part of the cod to escape. During the development work and the following tests we have seen up to 90 % of the cod escape from the SELTRA-trawl”.
By Kristine Bohmann