Global variation in marine fish body size and its role in biodiversity–ecosystem functioning

Abstract

To date, investigations of the positive re- lationships between biodiversity and ecosystem func- tioning among global marine ecosystems, and the management initiatives derived from these studies, have focused exclusively on species richness. We hypothesize that body size variations that occur among size-structured aquatic ecosystems subjected to ex- ploitation may also contribute to these patterns. Our analyses of maximum lengths of 12 151 fish species from 56 exploited large marine ecosystems (LMEs) revealed a 2-fold variation in geometric mean lengths, that were strongly and negatively correlated with spe- cies richness. LMEs characterized by larger mean sizes were also skewed towards dominance by large spe- cies. The expected positive and significant correlation between mean fish length and latitude (characterized as Bergmann's Rule) was observed among North Atlantic, South Pacific, and Indian Ocean (southern lat- itudes) LMEs, but not in 3 other regions, including the North Pacific. In the North Atlantic, anomalous long- term declines in top predator catch rates (one measure of ecosystem functioning) occurred, and within tem- perate North Atlantic LMEs, species size distributions were more skewed towards large species relative to North Pacific LMEs. Our global analyses indicate that fish body size may act as a factor of considerable importance in mediating the relationship between global marine fish species richness and ecosystem functioning, and that management initiatives to ensure ecosystem stability in the face of exploitation should include the restoration and maintenance of this impor- tant functional trait.