Sea temperature and habitat effects on juvenile reef fishes along a tropicalizing coastline

Abstract

Aim: Temperate marine systems globally are warming at accelerating rates, facilitating the poleward movement of warm-water species, which are tropicalizing higher-latitude reefs. While temperature plays a key role in structuring species distributions, less is known about how species’ early life stages are responding to warming-induced changes in preferred nursery habitat availability. We aim to identify key ecological and environmental drivers of juvenile reef fishes’ distributions in the context of ocean warming. Location: South-eastern Australian coastline from 30 to 37°S. Methods: We used a decade of underwater visual census data to uncover latitudinal distribution patterns of juvenile reef fishes and habitats across 1000 km of coastline, from subtropical to temperate latitudes. We modelled how benthic habitat cover, depth, wave exposure and sea surface temperature influence distributions of warm-water and cool-water juvenile reef fishes on temperate rocky reefs. Results: We found sea surface temperature was typically the most important factor influencing densities of juvenile fishes, regardless of species’ thermal affinity or latitudinal range extent. Juveniles of tropical and subtropical range-expanding fishes responded more strongly to warmer temperatures and lower wave exposure, while juveniles of temperate species responded more strongly to benthic habitats. Species’ responses to greater availability of temperate reef habitat-formers such as kelp and other macroalgae contrasted, being positive for temperate and negative for tropical and subtropical juvenile fishes. Main conclusions: The availability of both suitable habitat and sea temperatures for species’ early life stages is important considerations when predicting changes in reef fishes’ distributions in the context of ocean warming. Warming-induced isotherm shifts and feedback loops constraining the persistence of key temperate reef habitat-formers will favour range-expanding tropical reef fishes colonizing higher-latitude reefs, while disadvantaging some macroalgal-associated resident temperate species. Such varying responses to warming-induced environmental changes may strongly influence the structure of emerging tropicalized reef assemblages.