Statistical power in coastal fish monitoring with passive gears - effects ofsampling strategy

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Development of the coastal fish fauna has been monitored for up to three decades in the Baltic Sea, Kattegat and Skagerrak. Although aims of the monitoring programmes vary, all address changes in fish community and population characteristics over time. Programmes cover monitoring of fish resources, recipient studies and long-term studies of natural variation in undisturbed reference areas. Monitoring data provide the basis for developing indicators for assessing changes in coastal fish communities and populations, and is partly used within HELCOM for following up Ecological Quality Objectives. Sampling is performed using passive gears; small fyke nets and gillnet series are used along the Kattegat and Skagerrak coast, whereas multi mesh gillnets and gillnet series are used in the Baltic Sea. In the present paper we assess the statistical power of coastal fish sampling using catch per unit effort (CPUE) as test variable. The results show that statistical power (1-β) of the measured variables in time series monitoring based on gillnet and/or fyke net sampling may provide sufficiently high statistical power, at least when evaluating changes in CPUE for total fish community and dominating species. In Before After Control Impact (BACI) studies using passive gears for sampling, a randomised sampling design with sufficiently high number of samples might provide adequate results also in variable environments.