In 2021 the Tasmanian Inland Fisheries Service (IFS) received reports of a redfin perch population in a natural lagoon on King Island, Tasmania. Redfin perch (Perca fluviatilis) were introduced from England in 1862 and are regarded as a pest-fish in Tasmania. Although widespread on mainland Tasmania, they were not known to be established on King Island. To determine the overall stock structure and population size of redfin perch in the lagoon, the IFS conducted a survey using box traps, fyke nets, and gill nets in 2022 and caught 214 redfin perch (85 - 405 mm fork length). An additional population of redfin perch was also found on a small golf course dam, resulting in the capture of 14 additional fish, the majority of which ranged in size from 320 to 420mm. The proportion of larger fish (250 - 420 mm) within the King Island redfin perch sampled was much higher than other established populations on mainland Tasmania. The driving factors for this size difference are likely to be due to reduced intraspecific competition as a result of being a newly established population. Cannibalism of smaller size classes was also evident which results in reduced recruitment, and in turn prevents overpopulation and overall stunted growth. Although the two waterbodies on King Island where redfin perch are established are relatively small, the remote nature of the island combined with anecdotal evidence of other established populations means that further assessments are required before an eradication can be considered.