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Bad News For Lake Taupo Fishery

Printed From: The Fishing Website
Category: Freshwater Fishing
Forum Name: Freshwater Fission
Forum Description: The place to discuss all matters related to freshwater fishing!
Printed Date: 02 Dec 2023 at 12:30pm

Topic: Bad News For Lake Taupo Fishery
Posted By: Rainbow
Subject: Bad News For Lake Taupo Fishery
Date Posted: 17 Dec 2022 at 4:58pm

Lake Taupō: Productivity survey spring 2022


Initial analysis of data collected in November suggests Lake conditions are similar to 2019.

Date:  15 December 2022

The condition and number of trout witnessed during the 2022 winter season was exceptional, reflecting the high productivity that existed in the lake throughout 2020 and 2021. However, the productivity this spring, especially in the northern part Lake Taupō, was relatively poor. This marks a return to the levels recorded in 2019, suggesting similar conditions for growing trout.

The illustration below shows the distribution and density of ‘back-scattering data’ which offers a highly visual representation of lake productivity information over time.

Graphs of Lake Taupō's productivity between April 2019 to November 2022. 

Impact on trout

The reduced productivity witnessed this spring could affect trout at four distinct stages of their lifecycle.

For young trout entering the lake this summer, the low productivity may have a strong impact. It may increase natural mortality, so reducing the size of the cohort that will migrate up rivers to spawn in 3 years’ time.

Growth rates may be reduced for 2-year-old fish spending their first full year in the lake, but natural mortality is unlikely to reduce their number.

Fish in their third year, may experience reduced growth rates and low productivity levels could affect the timing of their spawning run next winter. Trout fertility may also be impacted, and we may see a reduction in both the number and size of their eggs. These in turn may affect the recruitment of juveniles in 3-years’ time.

Finally, trout that are recovering from spawning will be negatively impacted. Lower productivity levels will make it harder for these fish to find food, which means it will take longer for them to regain condition. We can expect a higher mortality rate among these fish compared to recent years – next seasons trapping data should confirm.


The good news is this research provides both fishery managers and anglers with a valuable insight into how this wild fishery functions. We know now how quickly productivity in Lake Taupō can change. It is interesting to note, declines in productivity occur much quicker than increases.

A cautious prediction suggests the quality and numbers of trout in 2023 will be similar to 2019. They are unlikely to be as big or numerous as they were last winter.

Data Collection

Collecting lake productivity data involves towing a large specialist transducer behind a boat and recording the stream of information on a computer – a very sensitive, advanced fish-finder! The operation takes three days to complete and covers approximately 110kms. The data is collected along seven paths or transects across the lake, which remain constant from year to year - allowing scientists to make meaningful comparisons over time. A filter is then applied to the raw data which enables the team to differentiate trout, smelt and plankton.

If there is any good news from the above, it is DOC's ability to identify what is happening in the lake and how this might impact the fishery for the next few seasons.     Yes, this is not going to be a one-year downturn, and all will be Ok in 2024.     The impact will be multi-generational with a bad outlook for at least the next 3 or more seasons.      Now that we know what to expect the next question is what can Doc do about mitigating this downturn at least to some extent????

Considering the great spawning run throughout last winter one should expect good recruitment from the numerous spawning streams.     How big depends on what damage the numerous floods had on egg and fry survival.     This can be ascertained with the surveys Dos is carrying out annually.

Lower lake productivity negatively impacts smelt, which is the prime food source for rainbow trout.    If this coincides with the high recruitment of young trout from an exceptional spawning as might have happened last winter, then we are looking at lots of starving trout entering the lake.     One of the lessons from the 2005 smelt collapse was that the starving trout hammered the remaining smelt so hard that this predation greatly prolonged smelt recovery.     

Every farmer knows that if food is scarce his only options are to either buy in more food or destock some of his animals, in other words, adjust down to the right balance.     

Lake Taupo is no different, but adjustments are considerably more difficult.     As little can be done by DOC to increase smelt it follows that its only other option is to reduce trout numbers to achieve some kind of balance with the available food.     

This can be done in several ways.     Encourage more harvest with particular emphasis on removing slabs, perhaps even temporarily scraping the daily catch limit.  Discourage C&R.  Remove a certain number of post-spawners from the spawning streams and reduce the trout population by netting the lake as the last resort.     Brutal as this may sound these are all options to re-establish the all-important balance between trout and smelt.

Prattling on about how anglers have to accept the natural fluctuations in a Wild Fishery is a poor excuse for doing nothing.    Sadly, this is the third option Doc has favoured in the past.



Posted By: Micsam
Date Posted: 17 Dec 2022 at 5:21pm
Interesting reading Rainbow.

My guess is Doc will basically let anglers become accustomed to small skanky starving rainbows again and do absolutely nothing.

They may at best tinker with the catch limit in the hopes anglers want more than 3 skinny grey rainbows!! That though counts most experienced Kiwi anglers out!

Posted By: MB
Date Posted: 17 Dec 2022 at 10:42pm
Deep down, I'm sure DOC want trout gone, but they are a handy revenue stream. Must be confusing for them.

Posted By: Rainbow
Date Posted: 18 Dec 2022 at 5:19pm
Hi All,       It is one thing to monitor productivity changes, but the real deal is to find out what is causing it.      The nutrient inflow from farm runoff. septic tank leaching and enrichment via the Turangi sewage treatment plant is fairly constant and cannot account for the sudden productivity reversal.      In 2005 they said the smelt collapse was the result of the thermocline not vanishing.   Apparently, this stopped the lake water from mixing in winter and with it the benthic nutrients that feed the phytoplankton, which forms the base of the food chain.      There is no indication that this has happened last winter.     So, what else could be the cause for this "sudden" reversal?      Counting the above-mentioned nutrient contributors out one other irregular event is flooding.     Now I would have thought that the massive inflow of organic and mineral debris would boost the lake's productivity, but this may not be the case.      On the face of it there is a tantalizing correlation.       In 2004 there was this huge flood that rearranged the Tongariro and most other feeder streams.      Enormous loads of material would have entered the lake but instead of boosting productivity, the smelt population collapsed in 2005.    It took almost 10years to recover.      Last winter we had a 800+ cm2 flood and productivity survey in November showed a productivity dive with an expected flow-on effect on the fishery.        The smelt are only the canary in the coalmine      I wonder if they actually do regular nutrient sampling to find some correlation with non-regular events.       The tragedy is that after the retirement of the fishery scientist Doc has decided not to replace him as they say they know how the fishery works.      They can show changes in productivity, but I have not seen any information from them of what causes it.   

Bloody hell I was looking forward to another good Tonga year and have just restocked my fly boxes.      Not happy to see the bad news

Cheers and don't let that spoil your Christmas

Posted By: Legacy
Date Posted: 18 Dec 2022 at 6:27pm
Thank you for making us aware of what is going on with the Tonga

Posted By: Reel Deal
Date Posted: 18 Dec 2022 at 9:25pm
This is what can make this site being the info base line NZ needs. I see it from time from time. Basic at the coal front facts ( which is getting rearer these days) and vigorous but often balanced responses ! A big reason I am a long teen member. 

The gods do not subtract from the allotted span of men's lives the hours spent on fishing - Assyrian Proverb

Posted By: Rainbow
Date Posted: 21 Dec 2022 at 12:24pm
Reading the above DOC information, it is very impressive how the data is collected, sorted, and interpreted.     Surprisingly there is not a single word about what might have caused this downturn or what management options exist to respond to it.    Both are vital components of any good management regime.    Imagine a doctor accurately diagnoses some ailment, then telling you at great length what you will be facing but not offering you any treatment.    This is how the DOC information reads.     In other words we told you what you are in for and you better get used to it.    It is precisely because of these fluctuations that sophisticated and innovative management is so important.     This fishery is not just an academic curiosity.     The Taupo fishery sustains a multi-million-dollar downstream industry.    As we have seen for several years following the smelt collapse in 2005 the financial impact was felt well beyond the Taupo catchment.  
It just astounds me how passively anglers and business owners accept what DOC serves up      We have seen this same quiescence when the Waikato Regional Authority creates havoc with their single-minded Tongariro river flood control works.  
 We just roll over.



Posted By: Redfinger
Date Posted: 21 Dec 2022 at 1:29pm
On the ground fishing reports from the Lake right now certainly do not seem toindicate any problem with quality of fish - at the moment. I wonder what lag time is? Also good current reports of smelt numbers too and the high lake level currently is further good news for the smelt eggs hatching etc. I wonder how long it takes for algal blooms (the helpful ones that smelt feed on) to be created - the right mix of sun , water temperature and what else you reckon?   Great article and does appear alarming but maybe affects  on the fish will not happen for a few months.
Imagine a productivity chart comparison over a year or so on Lake Rotoiti would look like?

Posted By: Rainbow
Date Posted: 22 Dec 2022 at 10:41am
 A Rotoiti productivity chart would probably be coloured all red as the lake is a nutrient soup compared with the clear/nutrient-poor Taupo water.   Best of all they never have an over-recruitment problem and can fine-tune this highly productive fishery with targeted hatchery releases.     If they ever find that the nutrients become too low all they need to do is divert more lake Rotorua water through the diversion wall.      I believe this wall is already full of holes.
Piece of cake. really. 



Posted By: Cheeko
Date Posted: 17 Feb 2023 at 11:26am
The fishery certainly isn't producing the quality of fish from last year (that certainly was one of the best in a number of years)!  From my experience this summer there is still some nice fish about but an absolute truck load of slabs has been caught. A lot of them are recovering but those that aren't I have been tapping on the head and either feeding to the koura or putting in the vege garden Smile. There will still be some good fish about for winter but a step down on the past couple of years. In my opinion people could do the fishery a favour and take a few more for the table. Taupo has no shortage of fish for recruitment purposes so it's all about the food supply!! 

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