Reef-fish size spectra along the Southeastern Pacific are influenced by Sea Surface Temperature and not by Chlorophyll-a

● Body size has been described as the single most important biological trait for determining both an organism’s vital rates and ecological interactions.● Size-based indicators, such as size spectra, are useful for measuring changes in individual body size and community structure, from anthropogenic and environment-driven effects. However, how size spectra vary along broad-scale environmental gradients is still unclear.● In this study, we investigated how size-spectra and total biomass of reef fish communities were influenced by oceanographic variables, sea surface temperature (SST) and chlorophyll a concentration (Chl-a) at four ecoregions, Rapa Nui and Salas y Gómez, North Chile, Central Chile, and South Chile.● Between 2012 and 2021, fish species and lengths were recorded by scuba divers using Underwater Visual Census methods and the Reef Life Survey methodology.● A total of 82 fish species were surveyed, ranging in size from 2.5 to 75 cm, and encompassing 37,796 individuals.● Composition of fish communities at all ecoregions was predominantly smaller fish species (&lt; 20 cm), whereas the ecoregions of continental Chile had a greater proportion of medium to larger sized fish (20 - 60 cm).● Species richness was highest at Rapa Nui and Salas y Gómez, and lowest at the ecoregion of South Chile.● When we compared fish size spectra between ecoregions, the mean exponents (lambda) significantly differed between Rapa Nui and Salas y Gomez and North Chile.● Lambda decreased nonlinearly as SST increased, indicating the contribution of large-sized fish increased toward colder waters, from Rapa Nui and Salas y Gómez to South Chile.● Despite variation of Chl-a between ecoregions, there was a non-significant effect on total biomass of fish communities.Synthesis: The broad latitudinal patterns in these fish assemblages are governed primarily by shifts in community composition and size spectra, with sea surface temperature exerting region-specific effects, whereas total biomass shows limited variation across environmental gradients. This stresses the importance of conserving size structure—especially larger individuals—alongside biomass, and supports management approaches that incorporate spatial heterogeneity in thermal conditions to safeguard size spectra across ecoregions.<br>