Effect of fish shape on gillnet selectivity: a study with Fulton's condition factor

摘要

The Eurasian perch, Perca fluviatilis (L.), was fished with Nordic multimesh gillnets, the mesh size combination of which was based on geometric series. From the test fishing data the fish girth was found to be linearly related to the third root of fish weight and therefore the fish girth could be estimated indirectly with Fulton's condition factor K. Two different response surface models based on B-type gillnet selectivity model were fitted 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% confidence 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 fish length, which violates the classical principle of Baranov's theorem on geometric similarity of fish of different sizes. Therefore the simple model derived from one population is not applicable to another population if there are differences in fish 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.