Penstock Lagoon a stocking success

Inland Fisheries Service
Long touted as the jewel in the crown of Highland fisheries, Penstock Lagoon has been a consistent high quality fishery since 2000. Although only fly fishing is permitted, it has been ranked in the top 10 lake fisheries on an annual basis for the State since the 1999/2000 season, in terms of angler visitation. Given that it is a small water body and that only those fly fishing can fish there it is an extremely popular fishery. During the 2008/2009 season Penstock sustained a very high angler effort which was in part due to low lake levels and less than ideal conditions at a number of other popular fisheries such as Arthurs Lake. Despite this, the fishery has consistently produced good size and conditioned fish at reasonable catch rates.

Penstock Lagoon was formed in the 1930s to pool water for the Waddamana power station. During its operation there were good inflows to sustain natural recruitment of the rainbow and brown trout that the fishery was famous for. Once the Waddamana power station was decommissioned in 1994 the supply of water during the crucial times for trout spawning and fry development was cut. Trout stocks at the lagoon are now maintained solely through stocking by the IFS.

During the late 1990s Penstock Lagoon suffered from poor water quality, both in terms of clarity and nutrient loading resulting in algal blooms. The reason for the decline in water quality was the change in status of the lagoon from a flow through system to a static water body, due to the water no longer passing through the lagoon on its way to the power station. The IFS (or IFC as it was in those days) and Hydro Tasmania were committed to re-establishing the lagoon as a premium fishery. By monitoring water quality and experimenting with various water levels an optimal water management regime was established that maintained the water quality and ensured improved conditions for fishing. For the fishing to improve it was vital that fish stocks were maintained at healthy levels as well. In the absence of the flow through regime that Penstock once had it was necessary to maintain fish populations through stocking.

At the end of 1998 the IFS commenced the restocking of Penstock Lagoon, firstly with rainbow trout fry that were hatched from eggs collected from lagoon itself. During 1999, to fill a hole in the population and to provide instant angling the IFS stocked 250 adults from the brown trout spawning run at Liawenee. The stocking of adults continued with a peak of 750 being transferred annually in 2003 and 2004, this was reduced gradually till 2009 when only 100 were transferred. Since 1999s there has been an annual stocking of rainbow and brown trout fry and fingerlings since 1999 there has been less need to rely on Great Lake transfers. By 2009 it has become evident that the juvenile stockings are now maintaining the fishery and the need for adult transfers has diminished to negligible.

The IFS undertook a tagging study in 2002 to assess the status of the trout populations at Penstock Lagoon. The 500 adult brown trout transferred from the Liawenee Canal spawning run were all tagged and released into the lagoon prior to the commencement of the 2002/2003 season. An electrofishing survey was conducted at the lagoon just prior to the season opening showed a high recapture of tagged fish relative to the untagged adult population of brown trout. Of the 500 released 167 were recaptured by anglers after the season opened. This represents a high recapture rate and was indicative that the lagoon at this point in time was fairly reliant on the stocking of adults to sustain the fishery. This electrofishing survey did show promise that the stocking program was effective as large numbers of juvenile trout of both species were captured and or sighted. This was evidence that the stocking program was paying dividends as a number of age classes of young fish were encountered and whilst adult stockings may be required in the short term the IFS was confident that stocking of juveniles would eventually sustain the fishery. In recent seasons this has definitely been the case.

Penstock Lagoons recovery from the mid 1990s doldrums was to a large extent due to the recovering water quality and establishment of a regime that maintained conditions favourable to fly fishing. Water clarity is important to fly fishers and its improvement can result in better catch rates. This improvement is important to insect life and the steady appearance of duns during the 1999/2000 season would have made the trout more visible and catchable targets for the fly fisher. Table. 1 shows the number of anglers estimated to have fished the lagoon for each season. These estimates are based on responses from the IFS Angler Postal Survey (APS) which has been running continuously since 1985. Following a recovery from poor water quality during the season 1999/2000, significantly more anglers visited the lagoon than for the any season during the 15 years prior. Despite a slight drop in effort during the 2000/2001 season angler effort has steadily increased overall for all but the 2005/2006 season.

With fisheries such as Penstock Lagoon reliant on stocking it is important to get the balance right when angler effort is increasing. Catch rates are important for monitoring the success or otherwise of stocking programs in these types of fisheries. The catch rates for Penstock Lagoon can be seen in the Table 2. Whilst catch rate has been satisfactory at over 1 fish per day for both species combined on average, it is the maintenance of catch rate in the face of an increasing angler pressure that has been the key to the success of this water as a popular and fruitful fishery.

The harvest at the lagoon for rainbow trout has remained fairly consistent while for brown trout it has risen substantially. The trend shown from APS data indicates in spite of increasing angler effort that catch rate has not suffered and this indicates that trout stocks are not a limiting factor. This would not happen if the fish stocks were not at a sufficient level.

Stocking is only part of the management of the fishery, the implementation of fishing regulations are another important factor. Penstock Lagoon is managed with a low bag limit at three fish per day. The low bag limit has contributed to the maintenance of catch rate at this fishery as has the minimum size limit of 420 mm. By setting the minimum size limit at 420 mm young fish have time to grow to a size where they are meet angler expectation of the quality of fish expected from a premium fishery.

Penstock Lagoon continues to impress as a stocked fishery and reports from this current season (2009/2010) indicate that the fish are in good condition and at a size that would please all anglers that fish there. With the good spring rains lifting the lake levels at a number of other fisheries the pressure on Penstock should dissipate this season but in any event it will continue to impress despite the high angler pressure.

Inland Fisheries Service
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