Typical well conditioned six pound Tongariro River Rainbow landed by Grant Alley on 28 July 2017.

All Taupo anglers have been amazed at the sudden improvement in the trout condition this season.  Anglers cannot recall returning so many four pound plus Rainbow trout in anticipation of a five pounder being caught.  So many have lost big fish estimated at well over five pound.  What a season!  It has been the best in our experience over the last twelve years with the best still to come… 

So what has happened to explain the improvement in the trout so much compared to the last 12 years of very ordinary fishing. DOC are the Taupo Fishery managers.  Based on previous seasons results, for the new 2017-18 season DOC reduced the minimum size from 40 cm to 35 cm and increased the daily limit bag from three to six.  This attempt to find a better balance between trout food and trout condition appears to have been unnecessary.  It was originally recommended back in 2013 when the trout condition was suffering, but took four years to happen.  So why the change?

It all goes back to Lake Taupo where the trout spend about three years maturing as adults before they return to the rivers to spawn.  In the lake their main diet is simply smelt.  So why has the smelt size and population suddenly increased? 

We put that question to Tim at Greig Sports who has watched the increased size of smelt on his fish finder over the last year.  He puts it down to the high lake levels maintained over the last two years without the fluctuations in levels experienced in the previous decade. 

(TRM have to bite their tongue on this as previous reports have been critical of the power companies maintaining continuing high lake levels as they increase the danger of flooding from the Tongariro River.  During wet conditions when the river floods it has nowhere to go as it hits the hydro dam called Lake Taupo and quickly backs up.  This is a major threat to the stability of river banks and to Turangi township.)

For the last year there have been many letters to the editor complaining about the constantly high lake levels causing severe erosion around the foreshore of Lake Taupo.

Other TRM angles are convinced the high levels are only part of the story.  They suggest that when the lake is higher the smelt can then spawn in the foliage – weeds and grass – instead of on pumice beaches.  Then the threat of predation by catfish sucking up their eggs is greatly reduced. 

Or is it more to do with a better balance in the lower order of the food chain – phytoplankton or zooplankton which feeds the smelt? 

Firstly, what are smelt?  TRM pinched the following description from Tony Bishop’s BISH & FISH website.

Smelt

Retropina retropinna (New Zealand)

Small silvery fish that occurs off beaches, in estuaries, lowland rivers and some lakes. Known only in New Zealand and Australia. Average size 6-10 cm.

This fish is the key to the Taupo (and other central North Island of New Zealand) trout fisheries.

Smelt were first introduced into Lake Taupo in 1934 to provide an alternative food source for the ailing trout population. Gone were the halcyon days of huge trout and twice in the preceding years the condition and size of the trout had crashed as the population of native fish, and the koura (freshwater crayfish), collapsed under the predatory pressure. On the first occasion, authorities allowed controlled netting in an attempt to reduce the size of the trout population and between 1913 and 1920, 103,000 trout were sold at markets in Wellington and Auckland to defray expenses.

The trout population made a remarkable recovery and the mid 1920s probably reflect the best years of the Taupo fishery, as epitomized in Zane Grey’s writings. However, the recovery was short-lived and by the 1930s it was again apparent that the trout population had outgrown its food supply. Coincidentally, a similar situation was also occurring in Lakes Tarawera and Okataina and smelt which had been successfully released into Lake Rotorua in 1907 were in turn released into these lakes.

Within two years a huge turnaround occurred in the size and condition of the trout and so, not surprisingly the release of smelt from Lake Tarawera into Lake Taupo was trialled. Releases of smelt continued at intervals through to October 1939 and by 1940 smelt were clearly established in the lake. By 1942 smelt were a large part of the diet of Taupo trout which has remained unchanged today. In turn, trout size stabilized and their condition returned as the population finally achieved some sort of balance.

Lake Taupo high levels:

A news report last November 2016 reported:

With heavy rain battering the Taupo region recently, Lake Taupo is nearing 100 percent capacity.

With more wet weather forecast, preventing the lake from filling up even more may be difficult.

Measuring levels show the lake has less than 25 centimetres left before it reaches capacity.

If it continues at its current rate, service holes, stormwater drains and sewerage pipes could all take a hammering.

Waikato Regional Council’s Adam Munro says the situation is worrying.

“We’ve had a lot of rain in that catchment over the last three or four months. We’ve got 30 tributaries that flow into Lake Taupo and there’s only one outlet.”

But he says they are working to find a solution.

“We were with Mercury [Energy] on agreed procedures around how we can either change the gate control at Lake Taupo or we can communicate that out to the public to make sure they aren’t aggravating the situation for example by not trampling on sensitive dunes and vegetation.”

One of the biggest impacts of Lake Taupo filling up so quickly is the erosion happening along the shoreline.

 Physics of Lake Taupo – the mixing cycle

Following description has been extracted from 1983 DSIR Division of Marine and Freshwater Science publication “Lake Taupo” co-ordinated by D. J. Forsyth and C. Howard-Williams. (P 56)

“The complete mixing in Lake Taupo occurs in winter. Temperatures are then the same at all depths and circulation extends to the bottom of the lake, mixing highly oxygenated water from the surface and redistributing the plant nutrients released from the lake sediments during the period of stratification (non-mixing).  The surface waters of the lake begin to warm early in September-October but it is not until December that the distinct layers typical of most lakes in summer begin to form.  From December onwards a warm layer of mixing water (the ‘epilimnion’ extends to 25 m or deeperand floats on the colder, denser water beneath(the ‘hypolimnion’).  Separating the two layers is a region the ‘thermocline’ where temperatures change rapidly with depth. etc.” 

Now you know…(?)  DOC referred to this stratification problem about ten years ago as the main reason for the lack of smelt.  That mixing of the nutrients provides food for zooplankton and smelt.  A University based study on the thermocline behaviour has been under way for several years and we are still awaiting their review.