Regional reef fish assemblage maps provide baseline biogeography for tropicalization monitoring.

Climate change, overfishing, and pollution have driven changes in the composition of many extant reef fish assemblages. As global temperatures rise, historically temperate locales are becoming inhabited by vagrant tropical species as their ranges expand poleward in a process known as tropicalization. Severe reef fish biomass declines are predicted by 2100 under high ocean warming scenarios. This redistribution of species has serious consequences for economic development, livelihoods, food security, human health and culture. Determining the tropicalization of Western Atlantic reef fish is challenging because ranges are expansive, their distributions temporally shift with the movement of isotherms, and many dynamic density-dependent factors affect occurrence and density. Since fish assemblages are known to vary with depth, habitat type, topography, and regionality and local benthic habitat map classifications include these descriptors in their scheme, we used benthic habitat maps to spatially represent the zoogeography of regional reef fish assemblages. The outcomes illustrate that the southeast Florida coast is a pivotal location in the transition between tropical and temperate marine fauna for North America driven by geology and Florida Current upwelling dynamics. The southern assemblages had a higher occurrence and density of tropical species, whereas the northern assemblages had a higher occurrence and density of temperate species. Delineating the spatial distribution of reef fish assemblages facilitates monitoring and detection of future poleward tropicalization, as will be evident in temporal changes in percent occurrence, assemblage relative abundance, and the homogenization of assemblage regions. These delineations contextualize restoration expectations and success by providing a spatial reference to focus conservation actions based on expectations from natural environmental variability, especially in transitional areas between ecoregions.