A general methodology for the estimation of gillnet size-selectivity and population length frequencies

Abstract The size selectivity of gillnets can be estimated using the catch data from experimental fishing of gangs of net panels having different mesh sizes. Gillnet selectivity curves can take a wide variety of shapes, and currently their estimation requires consideration of several different parametric forms, with right-skewed or bimodal curves typically being preferred. Here it is shown that the generalized additive model (GAM) framework provides a convenient and more flexible alternative. The GAM approach also generalizes the scope of analysis by permitting the population length frequencies of encounter to be jointly estimated as a smooth function of length. Moreover, the GAM framework allows for the inclusion of covariates such as sex or a condition index, and accommodates hierarchical sampling designs and spatial or temporal effects. Relative fishing power of the different sized meshes can also be included, notwithstanding that care with non-identifiability is required. The ease of use of the GAM approach is demonstrated on previously published lake trout data.