During the last week of September the Inland Fisheries Service undertook a survey of the brown trout population at Arthurs Lake.  The survey was planned as a pilot study in preparation for a larger tagging survey being considered for 2017.  Despite being on a smaller scale, this survey produced some interesting information about the Arthurs Lake trout fishery.

Over four days a total of 240 box traps were set, capturing 277 brown trout of all sizes.  Box traps are a way of capturing fish so they are able to swim away unharmed after being weighed and measured.  Most fish were captured on the eastern side of the lake between the Morass and Tumbledown Bay, along the back of the islands in depths of 1.5 to 4.5 metres.  The Jonah Bay and Cowpaddock arm produce very few fish, while catches at Hydro and Phantom bays were reasonable.  All up however, the catch rate across the lake was at the lower range of expectation.  The good news from survey was that despite the lower than expected numbers, there were great signs of consistent recruitment of fish over the last five years (see figure 1 below).

Normally young fish (under two years old) are difficult to sample with the methods used, but the fact they were present in detectable number is a good sign.  Three and four year old fish are the prime spawners and indications are they are present in significant numbers.  The presence of large numbers of five year old fish indicates good survival of fish from a large spawning event in 2011.  Fish from this year’s (2016) spawning event are yet to drop into the lake from the spawning streams, so they were absent from the survey.

Figure 1: Length frequency plot showing the percentage of brown trout in each length (age) class.

Further good news was the condition of the fish captured was impressive, with many typical fat plump Arthurs Lake fish present.  While the average size in not large (see table 1 below) the condition of fish is good!

Table 1: Summary data for brown trout Arthurs Lake.

Over the four days, just three tagged fish (from 1,349 tagged and released earlier during July 2016) were recaptured.  This indicates that if a reasonable estimate of the population is to be attempted during 2017, a much higher number of fish will need to be tagged, and a larger recapture survey will be required.

While anglers are experiencing lower than usual catch rates, the number of fish within the lake at present is not a major concern.  There are very positive signs with a good base number of fish from all size classes, with consistent recruitment a feature over the past five years and good survival of larger fish.

The fishery is well and truly on the improve and anglers should adjust their angling methods to suit the current conditions, with fish generally holding in the deeper sections of the lake over weed beds in the 1.5- 4.5 metres depth within the eastern basin.