Rainbow trout

Image: DOC

Introduction

Taupō fishery scientists have summarised 2018 trout data for anglers.

Number of trout

Trapping and escapement counts indicate that the overall population of fish in Lake Taupō is slightly lower than last year. Yet fish counts from October in both the Waimarino River and Tauranga-Taupō River are among the highest recorded in 10 years.

Previous radio tracking studies reveal that early in the season trout rely strongly on ‘freshes’ to move up the rivers. This year July was unusually wet, so river levels were elevated much of the time – creating a continuous fresh.

As a result, trout did not accumulate at river mouths in large numbers, waiting before travelling upstream in a group. Instead they formed a small constant trickle of fish that moved passed anglers. They were difficult to target and fishing was consistent but slow.

Near the end of the spawning period there were longer spells of dry weather. This allowed fish to build up close to river mouths as they awaited another fresh. The arrival of wet weather encouraged trout to move upstream in a larger group – providing improved fishing opportunities for anglers.

Condition of trout

‘Patchy’ might be the simplest way to summarise fish condition in 2018. The graph below (figure 1) indicates that the overall condition of the fish was not as good as in 2017.

Looking at the width of the boxes (represents 75 % of the fish) and the lines (represents 95% of the fish), we can see that this year the extremes are wider than last year.

This means the poor conditioned fish from 2018 are worse than those from the previous year. It also indicates the good fish 2018 are better than the good fish last year.

Figure 1: Graph - condition of brown and rainbow trout at Waipa.
Figure 1: Condition of brown and rainbow trout at Waipa
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Trout to food ratio

So why such a dichotomy? The condition of the fish is dictated by the ratio of fish abundance to the amount of food. With fish abundance this year comparable to last year, the wider differences in condition must be related to the food quantity or availability.

The abundance of food in the lake can be indexed by what is left in the lake if we filter adult trout out. This is precisely what we do during our bi-annual echo-sounding survey.

The figure below (figure 2) shows that this year the overall food abundance in April 2017 was comparable to April 2018.

A closer look indicates that the difference between high and low areas was again greater this year. In other words, it was not as uniform as in 2017. We remember that in 2017 trout caught in the lake looked all like siblings. This year it’s different with the good ones having done very well but the poor ones not so well.

Figure 2: Lake productivity acoustic monitoring 2017 - 2018
Figure 2: Lake productivity – acoustic monitoring
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Some fish have a good food supply and others struggle

We can conclude that some fish have found a good food supply while others for some unknown reason have struggled.

Previous studies have shown us how extensively trout move around the lake but we don’t yet know how smelt and zooplankton move.

Another possibility is that fish spawning in a particular stream move in a specific way on their return to the lake. If so, then the distribution of food in the lake could have a substantial bearing on the potential condition these fish can achieve during growth and post spawning recovery.

To find out more about this we plan to do another tracking experiment where we will follow fish in the lake. The key is we will have identified which river these fish come from before tracking.

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