New Zealand Self Drive Adventures

February 05, 2020

Why are New Zealand Trout so Big

The better question is why do some rivers and streams have such large trout? It really comes down to the habitat as opposed to anything genetic. Brown trout from Britain and Rainbow trout from western North America were introduced into New Zealand, there were no native trout in NZ.

The size a trout will grow to is a basic equation:

Amount of available food

Amount of oxygen available to convert the food into energy

Suitable temperature to allow maximum rate of digestion and conservation of energy.

Amount of food available is more about the foliage on the banks of the waterway and the stability of the bed of the waterway. In other words, the conditions in which insects and their larva thrive.In many of the rivers and streams in NZ the beds of the waterways are stable rather than having shifting gravel (scouring). Insect larva (nymphs) are large in numbers and constantly flowing from under the rocks. Thus, nymphing is the most practiced and successful method of fishing in NZ.

There are insects on top of the water as well and Dry fly fishing during the summer can be fantastic but this is seasonal and not the reason trout are so large. This food source is inconsistent and requires a “hatch” that is dependent on multiple factors that are less reliable than those responsible for the high level of nymphs.

The temperature and oxygen content of many NZ rivers is in the range that allows trout to conserve energy, digest food easily and gain in size. Where this is combined with an abundant food source and minimal angling pressure the result is large trout.

Angling pressure in some of the more famous rivers is having an effect but overall there are so many rivers and so few people fishing that big trout are still relatively common. Flooding during unusually wet winters appears to have more of a negative effect than does angling pressure. Farming and logging also negatively impact the quality of the water and therefore oxygen levels, food sources etc.

30 years ago, there were definitely more and bigger trout in New Zealand. Today it is still one of the premier fly fishing destinations in the world and catching a large trout in a small stream is very achievable if you have the right information on how and where to do it. If you are reasonably good fisherman then a 5lb trout is a very realistic expectation. In the better locations a 7 or 8lb trout is likely if you have the right conditions on the day. A trophy (+10lb) trout is possible but is less likely without a guide, advanced fishing skills and several days to devote to the task.

Aside from trout caught in lakes, the avid New Zealand fly-fisherman will have at best, caught a handful of trophy trout in a lifetime of trying. That same angler may well have caught a hundred 5lb plus trout.

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Author: David Turner