In January last year, the remnants of a degraded fish carcass found on the banks of the popular private fishery Hearne lagoon was reported to the IFS. This fish was confirmed to be an invasive tench (Tinca tinca) and was of concern as they are not known to be established in this area. Hearne lagoon is part of the upper Shannon catchment, where until now, tench have only been found in the lower reaches of this catchment.

Extensive surveys were undertaken in the Shannon River, Ripple Creek, and Hearne Lagoon to determine the distribution, population structure, and source population. A mixture of backpack electrofishing, box traps, and fyke nets were used. Small numbers of tench were found in Ripple Creek, upstream and downstream of Hearne Lagoon. The source population was eventually detected in April 2024, in a farm dam on Ripple Creek.  During the initial survey work, 71 tench ranging in size from 27 to 231 mm were captured in this dam using box traps and fyke nets.     

On the 28 and 29 May 2024 the source population dam was drained down and treated with a selective fish poison called rotenone. In total, 359 tench were kept for further processing, with the fish ranging in size from 27 to 345 mm. Many more tench were collected post-treatment, but were not retained. Initial analysis of the data suggests that there were likely to be at least five to six year classes of fish.

Over two days in January 2025, IFS staff resurveyed the source population dam using box traps, fyke nets, and electrofishing. No tench were caught which suggests the eradication efforts in the dam was successful. Complete eradication of tench from the upper Shannon catchment is unlikely due to the establishment of fish in numerous areas of Ripple Creek, therefore the aim will be to manage the tench population by suppressing it to low numbers.