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.

Summary

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.

Cheers

Rainbow