Selectivity of gill-nets used in curvine (Cynoscion othonopterus) fishery in the upper Gulf of California, Mexico

Abstract

Results are presented for the evaluation of gillnet selectivity of gulf curvine (Cynoscion othonopterus) in the Upper Gulf of California (UGC). In April 2009, 187 commercial fishing hauls were evaluated using three different mesh sizes, registering a total of 15.970 fish sizes, based on a multistage probability sampling design. A first exploratory data analysis revealed a spatial pattern of fishing effort, related to astronomical tidal movements and schools of Gulf curvine moving toward the Colorado River Delta. Given this information, we performed a stratification of data according to the day of the tide and fishing area, to evaluate the selectivity of different gill nets used by the fishing fleet. This was accomplished by means of a frequency analysis of the sizes of fish caught in each mesh, using generalized linear models. The analysis assumes that the distribution of the number of fish of a given size is asymptotic to a Poisson probability distribution. The different size distributions, according to the day of the tide and fishing area, were fitted to three different exponential probability functions including normal function, log-normal and Gamma in order to obtain the best model in each case, using maximum likelihood as fit criterion. Results show that only the use of gill nets with 14.6 cm mesh size maintains by catch below 35% for individuals smaller than 65 cm, as required by regulation.