Targeting aggregations of invasive goldfish ( Carassius auratus ) for active removal using acoustic telemetry

Management options for established populations of invasive fishes are limited, yet critical to mitigate the negative impacts on aquatic habitat and native fishes. Goldfish ( Carassius auratus) is considered a high-risk invader, increasingly found in high abundance across freshwater ecosystems, including the Laurentian Great Lakes. Here we tagged and tracked goldfish across Hamilton Harbour, Lake Ontario and applied a telemetry-informed traitor fish approach. Acoustic telemetry data revealed targetable aggregations occurred in April and May, from dusk into the night, within a small sheltered area where a major tributary drains into the harbour. Four targeted sampling events occurred within April and a total of 642 adult goldfish were removed from the system. Aggregations were predominantly located in the northern half of the sampled area in warmer water, typically close to shore, at shallow depths. Results presented here may be used to further refine goldfish removal within Hamilton Harbour and inform the development of targeted removal efforts throughout their invaded range. Such efforts are critical to protecting aquatic resources and ultimately supporting the recovery of native fish populations.