Lake Fishery Maintenance.

Lake fisheries are largely dependent on permanent inflows for annual recruitment of trout and in some regions lake limit populations of salmon. When tributary flows fail then that fishery may become  “put and take”, where release of fish species from other waters is necessary. Providing gravel deposits are clean and plentiful the fish are successful reproducers however some high country tarns do not have such a resource and must be stocked to ensure angler satisfaction.

Lake Alexandra, often regarded  as the most productive and popular of South Canterbury’s  lakes is reliant on a small and rather insignificant inflow named Scotts Creek. Without this stream spawning gravels for trout would be almost nonexistent excepting for the outlet stream into Lake Macgregor.

In a previous Reel Life the regravelling of Scotts Creek was described, the former success of this spawning enhancement technique prompting another makeover which , judging by past experience should be just as successful.

Mark Webb makes the following  comments on Lake Alexandrina and the all important Scotts Creek. 

“We fully expected this enhancement to work as there was no obvious reason why it should not if we followed the same process as we did in 1986 when it was last done. Sadly this does not appear to be the case.

While we followed the same strategy in terms of area regravelled, size of gravel and depth of gravel the results do not look as good as 1986. We believe a contributory factor has been reduction in the flow of Scotts Creek over the last 20 years and while this may not be apparent to the casual observer our flow gaugings show an average flow of less than 100 l/s in the creek now compared to 125 l/s in 1986. For further evidence of declining surface water resources in the Tekapo area you need look no further than Lake Murray which dried out for the first time ever in 2007. The loss of 25% of the flow in Scotts Ck and the continuing decline may not seem much but it is significant in terms of the water depth and velocity requirements for trout spawning. When we regravelled we did not take this in to account and the result is likely to have been that spawning conditions are not ideal for trout through being too shallow i.e. the gravel depth or width is too great. We hope we have addressed this with mechanical excavation in early April this year.

The regravelling in 2005 only covered half of the stream length as the stream bed in the top half of the creek was less effected by accumulated silt. Trout should still have ideal spawning conditions here.

The continuing decline in stream flow is a recently realized problem. Previously we thought siltation of the Scotts Ck bed was the biggest problem threatening the sustainability of the Alexandrina fishery. We are considering options for augmenting Scotts Creek flow – diversion of Muddy Ck, realigning Scotts Ck to produce a steeper gradient bed, supplementing with bore water.

The best action out of Alexandrina for many years has been formation of the Alexandrina Conservation Trust. It is a very positive group who let their actions speak for them. If they have a complaint they seek to fix it and they have very good relationships with F&G, DoC and local landowners. I would recommend any hut holder or dedicated angler to join the trust. I believe their project to enhance spawning in the Outlet Creek is an excellent one and supported by F&G”.

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