Rising sea temperatures are facilitating range shifts of tropical biota towards higher latitudes. The rabbitfish Siganus fuscescens, a tropical herbivore, has become a regular sighting along rocky reefs and in seagrass meadows of temperate southwest Western Australia and southern Victoria. Posidonia-dominated habitat was trawled in two nearshore locations in Perth, WA, to determine whether the diets of S. fuscescens were similar to a range of temperate seagrass-associated omnivorous fish species through gut content analysis, and whether it is likely to enhance seagrass grazing rates in temperate meadows. While seagrass was consumed by S. fuscescens in large quantities at one site, corticated terete rhodophytes (Hypnea & Chondria) and leathery heterokontophytes (Ecklonia & Sargassum) were consumed in greater quantities at the other site. In comparison, the temperate omnivores Haletta semifasciata, Heteroscarus acroptilus and Scobinichthys granulatus consumed seagrass in high quantities, but showed distinct interspecies differences in their diets.

While there is some dietary overlap between all native species, niche partitioning is likely to reduce competition between the tropical and temperate species. Regardless, the migration of the tropical S. fuscescens into temperate seagrass meadows is likely to increase grazing on seagrass as it establishes its population in temperate seagrass meadows.