Abstract
The first confirmed observations of wild spawning behaviour in the tropical Gulf Damselfish Pristotis obtusirostris were recorded within the temperate Port Stephens estuary, south‐eastern Australia. In 2025, large aggregations occurred at two sites, with males guarding adhesive eggs within defined nesting areas containing densities of up to 1.1 nests per m 2 and dozens of nests per site. During nesting, males displayed reproductive behaviours including colour darkening, nest tending and egg aeration. These findings demonstrate successful reproduction well beyond the species' typical range and highlight accelerating tropicalisation within temperate reef fish communities.