There is evidence that organisms have become smaller during the past periods of global warming. Global change has substantial effects on biodiversity, with body size reduction being the third most common response to global warming. Body size allometry in ectotherms needs to be explored further; the objectives of this study were to better understand the mechanisms regulating body size in fish by testing: i) Bergmann’s rule with temperature and elevation, ii) additional environmental drivers, iii) the role of isolation, iv) ecoevolutionary hypotheses comparing native and exotic species, and v) the role of migration propensity in comparing migratory and resident species. We analyzed an extensive dataset of Chilean fish composed of 75,198 records which included 25 species from 12 different families between latitudes -28.80 to -51.42 using linear mixed models to discern the best environmental variables contributing to body size changes, as well as incorporating factors related to dispersal capabilities, biogeographic isolation, and levels of exotic/native interactions. Bergmann’s rule is supported by changes in elevation, and our study shows that freshwater fish body size also increases with increasing environmental heterogeneity and productivity. In general, inland native fish tend to be smaller than coastal ones, supporting the island rule with evidence of gigantism or dwarfism in selected species. Ecological variables affecting fish body size do not differ between native and exotic fish unless other factors are considered, such as dispersal capacity (migrating vs. resident fish) or mechanisms related to their isolation. Although temperature is not a direct driver of body size in Chilean fish, heterogeneity, productivity, geography, migratory ability, and species origin may affect body size. A better understanding of the mechanisms driving body size in ectotherms will aid in determining management priorities in the face of global climate disruption.

This dataset, compiled using several databases of existing collections of past projects (EULA Centre at Universidad de Concepcion), represents the raw data. Not all body size measurements were available from the fish collected, therefore, some measurements were calculated based on other measurements. For example, total length was not recorded for all the fish, therefore it was calculated based on the values of standard length. The criteria used to predict body size are provided in the methods section of the paper, as well as the methodology used to filter the data according to the analyses used. This dataset includes body size (standard length and total length) as well as the latitude and longitude of exotic and native fishes collected throughout southern Chile. Each row represents a different individual fish.