ESTIMATING THE SIZE-SELECTIVITY OF FISHING GEAR BY CONDITIONING ON THE TOTAL CATCH

A conditional maximum likelihood model is used to estimate the size-selectivity of trawls, gillnets, and books when the data are obtained by simultaneous fishing with meshes or hooks of different size and/or shape. Size-selectivity is here by the selection curve, r(l), the probability that a fish of length l, if contacting the gear, will be retained (caught). In many selectivity studies r(l) is fitted either by eye, by heuristic means, or by improper application of generalized linear models. Then it is not possible to make legitimate statistical inference about r(l), or about assessments of the state of the fishery if those assessments use r(l). It is shown here that by conditioning on the total catch, selectivity data can be modeled as binary data, or polytomous data on interval scales. Application of the model to trawl and hook data demonstrates that selection curves can be fitted using generalized linear models, which may require nonstandard link functions or link functions with parameters.