Effects of temperature on global patterns of tuna and billfish richness

Abstract

Although tunas and billfishes are of substantial economic importance and conservation concern, global patterns of diversity and distribution remain poorly understood. Many species are highly migratory and able to tolerate a wide thermal range. In the present study, ambient water temperature data for 18 species of tuna and billfish from 190 literature sources were combined according to geographical location. An empirical modelling approach was used to relate temperature tolerances of tunas and billfishes to their global diversity patterns. Mean preferred and tolerated temperature ranges were calculated for each species in the adult and juvenile life stages. Mean tolerance data were then overlaid in order to fit models relating the species richness of tunas and billfishes to ambient water temperature. The best-fit model was used in conjunction with gridded water temperature data to predict global species richness patterns. Cumulative species richness pre- dictions from water temperature data were positively correlated with observed longline-derived rich- ness data (r = 0.577, p < 0.0001). Diversity consistently peaked at intermediate latitudes (10 to 35° N and S) in a manner similar to other pelagic taxa. This analysis provides evidence that the ambient water temperature tolerances of tunas and billfishes can be used to predict broad species richness patterns on a global scale.