What a wonderful trout fishing season this has been! Every day anglers have expressed their amazement at the improvement to trout condition in the Tongariro River and ask why?
TRM try to remind them that these are not river fish but lake fish on their annual spawning runs. So it all goes back to what happened in the lake. This has been discussed previously on June 25 – with inmates opinions – and July 3 with timing of spawning runs. But a more scientific explanation is needed…
Previous TRM reports have already commented on the most prevalent rumour that the higher lake levels resulted in massive explosion in smelt population as it provided better conditions for spawning along the edges. Smelt are the most important food source and account for more than 80% of their total diet. So the following is our interpetation of ta possible reason for the situation – we emphasise as usual we are not qualified to comment but SWMBO is an expert at everything.
Lake Taupo – a large deep lake covering around 616 square kilometres – is a ‘low-nutrient’ lake – scientific description is “oligotrophic”. Every year about now – late August – the whole lake is essentially one temperature, approximately 10C. When Spring arrives at end of September the surface water absorb the heat and the water temperatures rise. By mid-summer the water temperature is near 20C. with the warming extending down about 20-25 metres. It is at this point the lake waters become separated or stratified into three horizontal layers, each with a different temperature.
The warmer upper layer (epilimnion) lies on top of the middle layer or thermocline. This is where there is a rapid change in temperature over a small change in depth, typically from 15C to 10C in only a few metres. By February the surface water temperature can reach 22C. and by April the warmer top layer can penetrate more than 50 metres. Then as winter approaches the top layer begins to lose heat as it becomes less dense again.Wind and currents also encourage the mixing so by the end of July the entire lake is back to one temperature again.
At this point the lake has “turned over”. This is important as it affects the food chain for trout. In Lake Taupo zooplankton, smelt and trout all display similar temperature sensitive behaviour. Although they are able to tolerate higher temperatures for short periods, most of the food chain congregate predictably around the thermocline. There is very little mixing between the layers and this has a crucial effect on the lake ecology and the distribution of trout – which, we acknowledge, is all you really want to know about. But the trout distribution link to water temperature and the food chain is what drives seasonal migrations, so you will need to understand this for a basic understanding of the ecology. OK?
The Food Chain
At the start of the food chain in Lake Taupo is phytoplankton – an algae. Above we started to mention the low nutrients in Lake Taupo due to its depth. The major source is rainfall which is filtered through pumice soils around the lake catchment. A feature of this water source is that the nutrient make-up is low in nitrogen but high in phosphorus, not conducive to plant growth and free of aquatic algae and weeds. This gives Lake Taupo its famous clear blue colour.
These are not good conditions for other aquatic weeds and phytoplankton to thrive. This in turn affects the next next link in the food chain, zooplankton, which are mainly tiny crustaceans. The next – third – step in the food chain are the introduced smelt. Originally these were native koaro which were wiped out after trout were introduced. Their place in the food chain was replaced with smelt originally from Rotorua lakes. In Lake Taupo the trout follow the migration of the food chain.
That is enough for your biology lesson today. You (and I) really should be out fishing…
If you wish to learn more about all this lake ecology then refer to: “Volcanic Trout” (arguably the best book for Taupo anglers) by Brendon Mathews published 2003 (from where I pinched most of the above info. so as compensation have added his image of introducing his daughter to the art of fly fishing on right – he was supposed to be baby sitting!)
“Lake Taupo” 1983 publication by DSIR by D. J. Forsyth & C. Howard-Williams.
We are saved!
Since starting this post, a local fishing guide – Rob Henderson – has taken on the task of investigating the issues in a more scientific manner to check on the lake levels and temperatures over previous years. Thank you Rob – we all look forward to your research results.