• Bimodality;
  • freshwater fish;
  • glaciation;
  • habitat preference;
  • kurtosis;
  • migration;
  • North America;
  • recolonization;
  • skew


Aim  To document continental- and regional-scale variation in the size distributions of freshwater fish and examine some energetic, evolutionary and biogeographic explanations for these patterns.

Location  North America.

Methods  Regional species lists, coupled with habitat and body size information, were used to document the spatial patterns.

Results  At the continental scale, riverine specialist fishes show a unimodal, right-skewed, body size distribution whereas habitat generalist and lacustrine specialist species exhibit bimodal size distributions, with only a slight preponderance of small-mode species. Most large-mode species are migratory. Resident species, unlike migratory ones, show a latitudinal increase in mean size, but the size increase across all species is steeper because the importance of large migratory species increases with latitude. Size distributions change from right- to left-skewed with increasing latitude. Maximum body size does not change with increasing family richness but minimum size declines and skewness increases, consistent with diversification of small species. Skewness does not vary with mean family body size.

Main conclusions  Post-glacial recolonization by large, habitat generalist, migratory species is the main determinant of latitudinal size distribution trends. There is little support for the energetic hypothesis, but the data are consistent with a negative Cope's rule.