While it is true that the vast bulk of a trout’s diet in streams and rivers is nymphs, this is not always true in all pools and lakes.
In some lakes, Lake Taupo is a good example, the primary feed source is baitfish. In Taupo’s case it is [smelt]. In fact the [Taupo fishery] only exists because smelt was introduced as a food source, sometime after trout were introduced. Where baitfish exist, up to 75% of trout’s food intake can be made up from this source.
But any fish smaller than any trout becomes food - cock-a-bullies, smelt, whitebait, smaller trout, tadpoles - the list goes on.
An important food source for trout is the winged form of many insects and bugs.
Most aquatic insects need to reach the water surface to transform into the winged form. Trout try to intercept them on the journey, or at the surface as they transform.
Many insects, once they have changed into their winged form, mate, females lay their eggs, and die. This can take just a few hours, to a few days.
The dead and dying insects fall down onto the water where trout feed on them. Other insects mate in the air, then the females return to the water surface to lay their eggs. A dangerous practice if trout are lurking below.
Insects that live in vegetation close to water also change from their grub or caterpillar form into winged insects, again to breed. Breeding over, these weak and dying terrestrial insects can be blown from grasses, bushes, and trees onto the water. Trout feed voraciously on this bounty. Sometimes this can be quite spectacular. Whole ‘mats’ or 'rafts' of dead insects drifting in the current, with trout gulping from this mat as it drifts above them.
It is important to remember that winged insects form only a small part of a trout’s overall diet.
But while a hatch is on, or insects are being blown into the water in large numbers, trout will often feed on this food source to the exclusion of all else. This can be for a period of a few minutes, hours or sometimes days.
Other creatures that can form a significant part of a trout’s diet are largely dependent on the water where you are fishing. Many waters have local food sources that have significance in that water, but may not be available in others. These food sources cover a wide range of creatures such as snails, freshwater crayfish, damsel flies, dragonflies, moths, grasshoppers, cicadas, frogs, whitebait, smelt, mice - there are many more.
When fishing new water it is important that fishermen gain as much information as possible on local food sources.
In fact many fishermen carry a small net – the type used to catch goldfish in a bowl is ideal. This net can be used to make a small ‘trawl’ along the bottom of the river to catch the nymphs and bugs that lurk there. Selecting a fly that most closely imitates or represents the predominant nymph can help the angler ‘match the hatch’ more closely.
A good deal of what a trout eats is governed by the seasons. For instance cicada are an important food source in some areas, but will only be available from mid to late Summer.
Grip and Kill
The way a trout is held when taking a photo, (aka 'Grip and grin'), can easily turn into 'grip and kill' if the fish is not handled carefully and correctly.
The area above the pectoral fins, (the fins just behind and below the gills) contains the fish's heart and other organs; too much pressure applied to this area can lead to the fish's death.
For the full story on releasing fish with best chance of survival: