Harvey Clark: On the nose ... and out of luck

If troutfishers think of themselves as nice clean blokes, they should think again. They stink. They send a pong through the current that makes trout race for cover.

It is widely known that trout can easily see and hear the angler, especially if he or she is careless.

But their keen sense of smell is lesser known, and some interesting observations have been coming to light.

For example, this advice from South Island guide Peter Langlands is well worth taking: "Avoid walking downstream at the start of a day's angling. Brown trout have a remarkable sense of smell. They will spook when they smell you as you cross the river. The fish can then become very hard to catch, so make sure you walk upstream and spot fish as you go."

Such advice applies mainly to the middle and especially the upper reaches of rivers where the water is clean and the prey can easily detect foreign smells such as human odour.

In a wilderness river, too much wading can ruin the day's fishing for yourself and others.

South Island angler-scientists John Hayes and Les Hill give excellent examples of human odour spooking trout in their book, The Artful Science of Troutfishing, published last year.

They were preparing their gear to fish the tail of a clean backcountry pool where they had spotted two fine browns feeding.

Two hundred metres upstream, a hunter emerged from the bush and crossed the river. Two minutes later, when his smell reached the pool, the feeding trout took off and were not seen again.

Hunters don't go into the bush to put on lipstick and look pretty. They go to hunt, and they don't bother washing for a few days - so it is not hard to imagine trout gagging as they get a whiff of backblocks body odour.

Hayes and Hill give other examples of seeing large feeding trout racing unexpectedly for cover for no apparent reason until, minutes later, someone heading downstream comes into view.

Research shows that lowland rivers are affected by chemicals, farm runoff, cattle and other contaminants, so the smell of an angler here would be insignificant.

But "in clear backcountry rivers human odour can be a strong enough foreign stimulus to scare trout", say Hayes and Hill.

The fish can hear people on the bank having a conversation, especially if they are speaking in loud voices.

I saw a graphic example of this when freediving near the Whareroa boatramp on Lake Taupo, watching several large trout feeding on cockabullies, which was an education in itself. They fed voraciously as I lay quietly 2m away. Above, a boy ran on to the ramp and shouted to his friend. All the trout took off.

At night, especially, voices will carry and scare trout nearby.

This is especially true on the popular shallow stream-mouths such as Waiteti and Ngongotaha on Rotorua, or Waipehi, Omori or Whareroa at Taupo.

Voices will bounce off the bottom, vibrate along the trout's lateral lines and send it scurrying away.

How often have you heard someone shouting, ranting or talking loudly to his mates at night? How many fish do they catch?

Best to stay well away from people who can't shut up, and fish quietly on your own. Remember one of angling's golden rules - tread softly.