Does Size Grading Improve Fish Farming? Evidence From a Meta-Analysis

Size grading, sorting a cohort into more homogeneous size classes, is widely used in aquaculture and presumed to enhance production efficiency. However, reported effects on fish growth are inconsistent. To address this uncertainty, we conducted a comprehensive meta-analysis spanning 32 species to systematically evaluate the effect of size grading on fish growth and to identify key moderators of this effect. Overall, size grading reduced the average specific growth rate (SGR) by 4.50%. Species personality type (aggressive vs. docile) was the most important among the moderators tested: grading significantly reduced average SGR in docile species but had no effect in aggressive ones; and in both aggressive and docile species, the effect of grading depended positively on the initial variation coefficient (IVC). We emphasized that aggressive behaviour is closely associated with the effects above and pointed out the role of IVC between frequency of aggressive behaviour (FAB) and daily food intake (DFI) in aggressive species. We also found size grading had opposing effects on changes in size heterogeneity, which is beneficial for aggressive but not docile species. Besides, while size grading significantly reduced cannibalism rate (CR) in both personality types, it was at the cost of overall SGR. Therefore, size grading may not be advisable for docile species due to growth suppression. For aggressive species, size grading generally reduces overall production, but it is recommended from an individual quality standpoint, provided a moderate size heterogeneity is maintained.