Estimating annual CPUE indices for perch (Perca fluviatilis) from monthly logbook data of a gill-net fishery in the Bothnian Bay, Baltic Sea

Catch per unit effort (CPUE) indices, derived from daily logbooks of open sea fisheries or from standard gill-net surveys have traditionally been used as a source of information on trends in fish stock biomass or abundance. Nowadays, the systematic collection of catch and effort data from coastal small-scale fisheries has gradually been launched as well, e.g., in the Baltic Sea. The data from coastal fisheries is typically collected in monthly logbooks. We demonstrate that raw CPUEs, CPUEs based on subsetted data and CPUE indices based on linear mixed models (LMMs) from coastal gill-net fishery data covering a 20-year period yield different results on trends in abundance. The use of advanced counting methods is recommended to reduce the amount of the annual variation not attributable to changes in abundance. Thereafter, CPUE indices based on the monthly logbook data can produce valuable and cost-effective information on fish abundance.