Development and intercalibration of methods in Nordic freshwater fish monitoring

The awareness of the effects of transboundary pollution has increased the necessity to use comparable methods and to initiate joint studies between countries in environmental monitoring. In freshwater fish monitoring a number of different methods have been used, strongly reducing the possibilities to comparative assessments between countries. In 1990, a workshop on freshwater fish sampling was initiated in order to develop and intercalibrate methods used in freshwater fish studies in the Nordic countries. During a three year period, a new type of multi-mesh gill net to be used for fish monitoring in Norway, Finland and Sweden have been developed. Comparative studies and gillnet-selectivity assessments show that these new multi-mesh gillnets better describe the actual population structure of European perch (Perca fluviatilis), roach (Rutilus rutilus) and Arctic char (Salvelinus alpinus) than do the traditional giunet series used in the Nordic countries. Ageing of fish is central in most environmental studies, however, the comparability of analyses performed at different laboratories may be low. Comparative age analyses between the three countries have been performed for a number of fish species. The results of ageing E. perch, whitefish (Coregonus sp.) and roach indicate that differences between laboratories can be reduced by intercalibration. In the future, the workgroup will be focused on a further development of joint methods within studies of freshwater fish and on joint internordic assessments on species distribution, abundance and life history characteristics in relation to airborne pollutants and liming.