Does Size Matter? Changes in Size Distributions of Six Kelp Forest Associated Fish Species

Size distributions in fishes respond to mortality, usually randomly distributed along a population’s life history. We propose that when a size-selective harvesting effort (non-random mortality targeting larger fish) is applied differentially in a geographic gradient, size structures will present latitudinal patterns. During October-November 2013 we performed underwater ecological surveys at 14 monitoring sites in 291 kms of coast (latitudinally) along the Baja California Peninsula (164  30mX2m transects). Expertly trained divers registered fish richness, abundances and sizes (in 5cm intervals). We compared size structures of 3 exploited species (Paralabrax clathratus, Semicossyphus pulcher and Sebastes atrovirens) and 3 unexploited species (Chromis punctipinnis, Oxyjulis californica and Embiotoca jacksoni) in relation to a high or low fishing effort, and looked for differences in the proportions of potentially mature organisms. We looked for latitudinal patterns in average lengths of each species. For exploited species, size distributions differed between different degrees of exploitation (Kolmogorov-Sminorv, p<0.0001), proportions of potentially mature organisms were always higher for locations with a lower fishing effort, and increments by two-fold in lengths (p<0.01) were observed southward. Unexploited species did not present significant patterns. These results are especially relevant when proposing management strategies, such as minimum catch sizes or the implementation of MPA’s.