To investigate the population structure of brown trout in Arthurs Lake, we have completed three surveys since December 2022.

There are two reports summarising these surveys which occurred in December 2022, April 2023 and December 2023. These are Arthurs Lake Fishery Performance Assessment April 2023 and Arthurs Lake Electrofishing Survey December 2023. The La Niña years of 2020 to 2022 were ideal conditions for trout to spawn and for fry to drop into the lake. Arthurs Lake has numerous spawning creeks and in wet years there are large numbers of trout recruiting to the fishery.  The survey work has indicated that brown trout have enjoyed three good years of recruitment from the 2020, 2021 and 2022 spawning runs. These wet winters have resulted in a strong population of young fish in the lake.

Young fish now dominate the Arthurs Lake brown trout population. Because of this there has been an increase in competition for food and there is evidence of the older fish losing condition.

In addition, the average size of fish in the spawning run has fallen in the last two years and is likely to fall again this year as fish from 2021 and 2022 mature and spawn.

The chart below shows the average size of spawning trout at Arthurs since 2000. In 2023 the average size was the third smallest recorded since we started monitoring in 1977. The smallest and second smallest average size of fish in the Arthurs Lake spawning runs occurred in 2013 and 2014 respectively, following the wet La Niña years of 2010 to 2012.

Available evidence suggests that the growth potential of fish in Arthurs Lake is being limited by the current population structure. This is likely to continue to drive down the average fish size.

This year we plan to move a limited number of fish from the Arthurs Lake spawning traps over winter and use these small wild fish to stock popular waters. The fish will grow to bigger size in these waters.

The removal of fish from the traps will help those remaining in the lake to grow. There will still be plenty of natural spawning and fish for anglers to catch at Arthurs Lake.