Effect of ®sh shape on gillnet selectivity: a study with Fulton's condition factor M. Kurkilahti a,* , M. Appelberg b , T. Hesthagen c , M. Rask d a Finnish Game and Fisheries Research Institute, Ryma Èttyla È Fisheries Research Station, FIN-21150 Ro Èo Èla È, Finland b Institute of Freshwater Research, Fish Monitoring Group, S-178 93 Drottningholm, Sweden c Norwegian Institute for Nature Research, Tungasletta 2, N-7485 Trondheim, Norway d Finnish Game and Fisheries Research Institute, Evo Fisheries Research Station, FIN-16970 Evo, Finland Received 29 February 2000; received in revised form 29 September 2000; accepted 21 November 2000 Abstract The Eurasian perch, Perca ¯uviatilis L.), was ®shed with Nordic multimesh gillnets, the mesh size combination of which was based on geometric series. From the test ®shing data the ®sh girth was found to be linearly related to the third root of ®sh weight and therefore the ®sh girth could be estimated indirectly with Fulton's condition factor K. Two different response surface models based on B-type gillnet selectivity model were ®tted to the empirical data: one model with Fulton's K and one without simple model). The Fulton's K-model had narrower Monte Carlo simulated 95% con®dence interval than the simple model. Simulation studies showed that the traditional simple model without Fulton's K does not have a linear relationship between mesh size and ®sh length, which violates the classical principle of Baranov's theorem on geometric similarity of ®sh of different sizes. Therefore the simple model derived from one population is not applicable to another population if there are differences in ®sh condition. However, if as is the case for Nordic multimesh gillnets with a mesh size combination based on geometric series like Nordic multimesh gillnets, this error is minimal because adjacent mesh sizes cover each other and correct this error. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Condition factor; Multimesh gillnet; Geometric series; Net selectivity curves; Mathematical model 1. Introduction Thegirthisthemainsizefactordeterminingthesize of ®sh caught by different mesh sizes. Nevertheless, most gillnet selectivity models have been derived for ®sh length, not for the ®sh girth as they strictly should be. The main reason for this is that measuring girth is dif®cult, time-consuming, and therefore expensive if compared to length and weight measurements of ®sh. Another reason is that we are usually inter- ested in ®sh size, which is routinely measured as length,andthereforethe®shgirthbasedmodelwould be adjusted to ®sh length anyway. Nevertheless, the length based gillnet selectivity models are justi®ed if the ®sh girth and ®sh length are linearly related to each other. This relationship is also important because the ®sh girth is the main factor in determin- ing the optimum mesh size that will catch a ®sh of certain size and the mesh size/®sh length relationship isusuallyassumedtobelinearseeHamley,1975,for example). Fisheries Research 54 2002) 153±170 * Corresponding author. Tel.: 358-20-57-51-731; fax: 358-20-57-51-731. E-mail address: mika.kurkilahti@rktl.fi M. Kurkilahti). 0165-7836/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0165-783600)00301-5