Taupo trout

General Facts

Percentage of rainbow and brown trout: approximately 90% of Taupo trout are rainbows and 10% are browns.

Age: Majority of rainbow trout live to be 3 to 4 years old, whereas brown trout have a longer life span of 4 or more years.

Size: Over recent years, our fish trap programme has produced some very big brown trout from Lake Taupo measuring over 70cm in length and weighing almost 6kg (~13lbs). However, brown trout generally average 60cm & 2.8kg. Rainbow trout tend to be smaller than browns averaging 54cm & 1.8kg but the odd 4.5kg+ (10lb+) rainbow does exist in the lake.

The legal size requirement for keeping a Taupo trout is 45 cm (from tip of nose to the mid-V point in it's tail).

Life Cycle: Rainbow trout spawn all year round in Taupo rivers but the main spawning event occurs during September and October. Brown trout are more concise and spawn during the first half of the year between January and July.

• Stage 1: Adult females (hens) lay several thousand eggs in the clean river bed gravels. Adult males (jacks) discharge sperm, also called milt, which fertilizes the eggs.
• Stage 2: After 1-3 months the eggs hatch into alevins which remain in the gravel living off their yolk sack.
• Stage 3: Several weeks later they emerge as 25mm long fry.
• Stage 4: By late summer the young trout have grown into parr (50-70mm long).
• Stage 5: 12 to 18 months after hatching the young trout enter the lake as fingerlings.
• Stage 6: Once mature trout swim back up the rivers from which they originally hatched in order to spawn.

Movement: Spawning rainbow trout travel at an average speed of 320m per day while swimming up the Tongariro River to spawn although this can be influenced by rainfall and flooding.

Rainbow trout mainly move upstream during the day between midday and midnight with the majority of upstream movement occurring between 4pm & 8pm. Rainbows don't travel during the darkest hours of the night (midnight to 4am) with a small burst of movement occurring at first light.

Rainbow trout also adjust their upstream movement in relation to changing day length - i.e. they move later in the morning as the day length shortens. This further emphasises how day light may play an important role in navigation.

Food source:

A rainbow trout is known as a pelagic feeder or in other words it will search for its food in the open water. The brown trout is a littoral feeder meaning that it lives and feeds in shallow, weedy waters. Therefore, rainbow trout spend a large amount of energy feeding off smelt that swim in the open water column of Lake Taupo, and the brown trout will eat anything that it can find along the edges of the lake. Their diet consists of mainly bullies, catfish, frogs, rats and mice. While residing in the river environment, the two types of trout have diets that consist of macro-invertebrates like mayflies, stoneflies and caddisflies. In both environments brown trout have a much greater tendency than rainbows to eat other fish.

Caddisworm

Mayfly

Smelt by Stephen Moore

Fishery Management:

Fishery Management Goal = Self-Sustaining Trout Population

The management of a fishery can be illustrated by thinking about a farmer managing his own farmland and stock. A fishery manager is similar to that of a farmer as they both want to be able to count stock numbers, look after the stock's health, and also determine how much of the stock can be culled for maximum profit but also ensuring that there is enough stock to sustain the population in the following year. However, some would say the fishery manager's task has an added challenge due to the trout population being largely invisible beneath the water's surface.

Fish traps help to monitor the adult trout population

Data loggers around the lake collect information from tagged trout about their specific habitat requirements

The electric fishing method is used to monitor the juvenile trout population

There are many things that a Taupo fishery manager does in order to estimate the total trout population. Scientific research is one tool that the manager uses. The more the manager knows about trout ecology and their habitat the better decisions he/she can make in regard to protecting the trout. The most recent scientific study that the Taupo Fishery scientist, Dr. Michel Dedual, has completed identified trout movement and behaviour in Lake Taupo over a one year period. This study was completed by using data from acoustic transmitters put in 36 fish and recorded by an automatic logging system. A large set of data was collected about the tagged fish's vertical and horizontal movements in the lake as well as the temperature of the fish. From this data Michel was able to make some fascinating conclusions about the distances and speeds at which the fish travel, the locations they tend to visit and the reasons behind these movements.

Fishery managers also do a lot of monitoring in order to identify the trends of the trout population and the health of these fish. Fish traps help managers monitor the adult population of trout whereas an electric fishing technique helps them monitor the juvenile population. The population of trout is not the only population monitored. Both the catfish and smelt populations and are also kept track of as a significant change in their numbers would most likely effect the well-being of the trout.