Reduced catchment inflows from anthropogenic modification of natural systems can drastically alter estuary mouth state. Prolonged closure of estuary mouths has consequences for fishes that may utilise freshwater, estuarine and marine environments during different life stages. From 2017–2023, we investigated the effect of an ongoing, seven-year mouth closure on a large-bodied fish assemblage in Lake George, South Australia, with focus on the abundance and demographics of a marine species, Aldrichetta forsteri. Since estuary mouth closure in 2017, recruitment of A. forsteri in Lake George was undetected through fine-mesh seine and multi-panel gill net sampling, and there was a decline in gill net catch-per-unit-effort from 2017 (32.9 ± 10.1 fish/net) to 2023 (20.3 ± 7.8). In 2023, the sampled population (322–454 mm total length) consisted of 6- (11%) and 7-year-olds (89%), corresponding to spawning during 2017 and 2016. The evidence indicates there has been negligible recruitment of A. forsteri within Lake George since mouth closure and suggests the fishery depends on immigration from the sea. Without mouth reconnection or other intervention, persistent recreational fishing mortality and natural mortality will deplete the A. forsteri fishery in Lake George.