Fishing lines: Never carry a torch for the Brownies

Taking a gap year has become standard practice for school-leavers. But there's also a fish that cuts loose and sets off to see the world. The funny thing is, nobody knows why sea trout leave family and friends to holiday at the seaside.

Taking a gap year has become standard practice for school-leavers. But there's also a fish that cuts loose and sets off to see the world. The funny thing is, nobody knows why sea trout leave family and friends to holiday at the seaside.

And here's an even odder thing: if the salmon-like fish stayed in its home river or lake, it would be just another brown trout. But that travel bug turns it into a totally new fish.

You can imagine those family rows: tears, raised voices, accusations of "nobody has left this family and made anything of themselves downriver". But when the prodigal comes back - always assuming it hasn't been eaten by seals, pike, ospreys, goosanders or cormorants - it's bigger, healthier and sporting a shiny new coat (Salmo Trutta and the the Amazing Silver Dreamcoat).

Even my old school chum Dr David Solomon, who knows more about sea trout than anybody, can only hazard informed guesses as to why thousands of brown trout get the migratory urge, while others in the same river never leave the family ranch. He told a symposium last week that nine factors seemed to be important. These include poor growth in the home river; high juvenile populations; competition from other species; lots of predators; a safe or easy route to and from the sea; and good feeding conditions when they taste the salty air. Decent travel brochures may also be a factor here, but he seems to have missed that one. This might all seem of no relevance to anglers - except that sea trout have achieved raised status in the past couple of years. This is largely because Irish drift-netters have nobbled all the salmon, leaving sea trout the only migratory fish left.

Sea trout don't grow as big as salmon: just over 40lb, compared with the largest salmon of 79lb 2oz. But they typically grow four or five times larger than their stay-at-home brothers and sisters.

Unfortunately, that sea journey also seems to have imparted wisdom. If you want to catch sea trout, you probably have to fish at night. There are certain conventions, most of which are probably part of the mystical status that anglers give to sea trout. But you have to wade along a river as if you're trying to sneak past a sleeping alligator. That's not easy when you can't see where you're going.

Using any sort of light is frowned upon. It will scare sea trout for miles, say the books. The tiniest glimmer to tie on a hook is just about acceptable, but one of those 10m-candlepower torches will get you drummed out of the Brownies.

England has hardly any sea-trout rivers; Wales is well blessed. Scottish waters, once great, are a shadow of their former selves, and fish farms have screwed up most of the Irish stocks. So the Welsh are more likely to set you up for a date with their lovely daughter than tell you their hot sea-trout spots. Tony Davies is an exception. I'll tell you about him next week.