Standardisation of commercial CPUE A. Salthaug * , O.R. Godù Institute of Marine Research, PO Box 1870, Nordnes, N-5817 Bergen, Norway Received 2 November 1999; received in revised form 20 March 2000; accepted 8 April 2000 Abstract A model for standardisation of ®shing power for individual vessels in a commercial ®shing ¯eet is developed. Catch rates of vessels are compared when they are ®shing close together in time and space, and their ®shing power relative to a standard vessel is calculated. The model is applied to a logbook data base from the Norwegian bottom trawler ¯eet, and effects of varying model parameters are explored. Relative ®shing power from the model are correlated with the vessel's length and engine power. The model seems to be robust when varying catch composition criteria, minimum number of comparisons required and standard vessel. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fishing power; Effort standardisation; Commercial data; CPUE 1. Introduction When using catch per unit effort (CPUE) from a commercial ®shing ¯eet as an index of ®sh stock abundance or density, the measure of one unit effort has to be equal for different vessels (Gulland, 1983). Individual ®shing vessels often tend to show large differences in ®shing power, i.e. their CPUE for the de®ned effort unit will differ when ®shing on the same density of ®sh at the same time and place (Beverton and Holt, 1957). The effort in CPUE observations from different vessels should, therefore, be standar- dised or adjusted to the same level. Factors most likely to cause differences in ®shing power between vessels are size, engine power, skipper, age of vessel and differences in ®shing technology (Gulland, 1983). Differences in vessel-generated noise may also be important (Enga Ês et al., 1991). A vessel's ®shing power also differs for different species and for sub-groups within the same species, and the ®shing power may vary with season. This is so because different groups of marine organisms have different spatial distributions and totally different behaviour in relation to the ®shing gear, which also may vary seasonally (Ferno È and Olsen, 1994). Over time, the ®shing power of vessels is expected to change due to technological improvements (Gulland, 1983). Indivi- dual vessels are also expected to have different plans for making these improvements, thereby having dif- ferent pace in enhancing ®shing power. A realistic measure of ®shing power for commercial ®shing vessels should be obtained when the vessels target the species of interest. In today's multi-species ®sheries, the skipper's target species often change rapidly because of prices, quotas and catch regula- tions. Different vessels may also target different spe- cies within the same time period. The catch composition may, however, be used as an indicator of the vessel's actual target species (see, e.g., Ketchen, Fisheries Research 49 (2001) 271±281 * Corresponding author. Tel.: 47-55-23-86-73; fax: 47-55-23-86-87. E-mail address: ares@imr.no (A. Salthaug). 0165-7836/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0165-7836(00)00204-6