Biogeographic shifts in reef fish size structure along the Southeastern Pacific occur independently of sea surface temperature and chlorophyll-a.

· Body size is a key functional trait linking individual physiology to ecological interactions and community structure.
· Size spectra provide a trait-based framework to quantify how community size structure responds to environmental gradients, yet their large-scale drivers remain poorly understood.
· We tested whether temperature and productivity differentially shape reef fish size structure, examining how sea surface temperature (SST) and chlorophyll-a (Chl-a) influence size spectra and total biomass across a broad latitudinal gradient.
· Reef fish communities were consistently dominated by small-bodied individuals, but cooler, continental systems supported a higher proportion of medium to large fishes.
· Species richness declined with latitude, indicating strong biogeographic filtering across regions.
· Size-spectrum exponents significantly differed among regions, reflecting shifts in the relative contribution of small versus large individuals.
· In contrast, variations in SST and Chl-a had no detectable effect on total biomass.
Synthesis: The broad latitudinal patterns in these fish assemblages are governed primarily by shifts in community composition and size spectra, with SST exerting reduced 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 ecoregional heterogeneity to safeguard size spectra at large geographic scales.