Noodzaak van schadebeperkende maatregelen voor vissen bij schonen en baggeren

The Dutch District Water Boards have adapted a code of conduct to prevent damage to protected species during maintenance work like removing vegetation or dredging waterways. One of these measures is an inspection walk on the banks in order to put fish back into the water. To assess the need for this, research was done within sections of 250 meters in 18 different waterways where maintenance was carried out in a normal manner. We counted the number of fish on the banks and the number that remained in the water (based on electro fishing) for 1) 100% dredging, 2) removing 75% of the vegetation and 3) removing 100% of the vegetation. In many of these sections the proportion of fish removed from the water during the maintenance work was over 25% and sometimes even well over 50%. The highest percentages were seen in sections that had been dredged or where 100% of the vegetation had been removed. We also found that the way the maintenance was carried out had much effect. The opportunities for fish to escape are better when the vegetation is removed on the opposite shore, when the machine goes slowly, and when only plants and no mud is removed. Leaving as much vegetation as possible and working in a manner with opportunities for fish to escape will minimize the number of fish taken from the water. For the sake of protecting fish we recommend this more ecological way of maintenance rather than walking the banks and putting fish back into the water. it is also important to spread the dredged mud in thin layers so that any fish taken up can be found. Jaarlijks worden door de waterschappen duizenden kilometers sloten geschoond en gebaggerd om de watertransport- functie te waarborgen. Hierbij bestaat het risico dat beschermde planten of dieren worden verstoord, verwond en/of gedood. Om dit te voorkomen is er in 2006 door de Unie van Waterschappen een gedragscode opgesteld, die is goedgekeurd door het Ministerie van Economische Zaken. In de gedragscode zijn onder andere schadebeperkende maatregelen als het sparen van minimaal 25% van de vegetatie en het terugplaatsen van vissen opgenomen. Om de effectiviteit van deze maatregelen te bepalen is in 2015 een onderzoek uitgevoerd naar het aandeel van de vissen dat met baggeren en schonen op de kant terecht komt.