Institutes Look Into Sea Bed Damage By Trawls
by Quentin Bates
(from Fishing News International)

In addition to its normal gear research activities, the Belgian Sea Fisheries department has been involved in a programme co-ordinated by RIVO and NIOZ in the Netherlands to study the impact of fishing gear on the sea bed. 

"There were 13 institutes involved, but most of them were studying the biological aspects, while we concentrated on the physical aspects and the changes made to bottom topography and sediment features," Ronald Fonteyne tells FNI "Using 4 and 10 metre beam trawls, sometimes on chartered vessels, the Department studied vulnerable areas in inshore waters along the Belgian and Dutch coasts as far north as Scheveningen. The ground was scanned using side scan sonar and Roxann in later phases, before and after trawling, and all the changes were monitored." 

"This is an area with quite a lot of tide, and we found that the changes in bottom topography lasted for a maximum of 54 hours. After that we couldn't see any more marks. The ground recovers very fast with the action of tide and storms. Once we had to go back to harbour because of a storm, and when we went back out 20 hours later, there were no marks at all to be seen." 

"The biological aspects are something else, and there is always damage to some extent. But beam trawling makes food available for scavengers, and fish feed on what has been disturbed. We are working on devices that will allow the benthic species, whelks, crabs and the like, to escape, by using large mesh in the lower panels to provide dropout zones." 

As well as looking into the changes on the grounds, the Department has also measured bottom penetration, showing that these beam trawls penetrate between one and five centimetres into the seabed, depending on the seabed sediment and the rigging of the trawl. 

By using load cells on the trawl warps, and two pressure sensors fitted to each trawl head, the Department measured the pressure on the seabed from the sole plates of the beam trawls. 

"The pressure is between 170 and 200 grammes per square centimetre, which is about the same as an 80 kilo man walking across the beach." The trials also showed that trawling with the tide at six knots showed normal ground contact, but when the Department's chartered Eurocutter turned to tow against the current with a relative increase in speed, the trawls showed almost no contact at all and were practically off the bottom. 

"We are looking at the selectivity of shrimp beam trawls, and ways of reducing the amounts of juvenile flatfish that are caught, and we are looking at the economic consequences if these fish are saved. Fishermen will also have to use sorting grids or sieve nets under new legislation, 
and we are looking into that." 

Sea trials carried out by the Department have shown that grids can be quite efficient in sorting out the unwanted by-catch. However, there were too many hauls where the grid was clogged with starfish and debris resulting in significant shrimp losses. As it stands now, grids are not yet acceptable to the commercial fishery but the research is being continued.

Link to gear effects directory