FISHING LINES: Tiddlers nestle in family tree

My Buddhist readers who haven't been pulling as hard as they should on the donkey-cart of life should beware of being reincarnated as a small fish next time round. It is as bad as pulling "dung beetle" out of the big hat. When you are a tiddler, everything wants you for lunch, and at this time of year, there is not much for you to eat either. But the biggest winter enemy of baby fish, especially those that live in rivers, is a flood.

When you are only an inch long, a trip to the seaside means certain death. Each year, millions of fish die because they have no way of resisting strong currents. In East Anglia, this is a perennial problem because waterways are often flood relief channels: featureless aquatic motorways with no bends in which to hide from the torrent.

This problem is compounded by the National Rivers Authority, which seems to think the perfect river is a 50-mile water-trough with an even depth and no nasty weeds or trees to stop that headlong seaward charge. On the Hampshire Avon, for example, heavy weed-cutting takes place every summer to protect agricultural land from flooding. But this strips away essential cover for fish and other aquatic life.

A more recent worry is a root disease that has already affected an estimated 20,000 alders, probably our most common riverside tree. The Forestry Commission reckons it has the same destructive potential as Dutch Elm disease. Phytophthora is mainly affecting trees on river banks, which seems to show it is water-borne. If it continues to spread, the result will be increased erosion and even fewer places for small fish to hide.

This problem of fish and floods is not exclusively British. Some countries have been dumping bundles of tyres to provide shelter and get rid of all that unwanted rubber. A few years ago, there was even talk of using decommissioned oil rigs to achieve thesame thing. These are made of high-grade steel, so they would last as long as 100 years, unlike old cars.

But in the US, scientists have come up with an even more original idea to protect fish stocks, and it is a scheme that is particularly appropriate to this time of year. Don't know what to do with that used Christmas tree, now looking forlorn in the back garden and shedding its clothes faster than a stripper on piece-work? The dustmen refuse to cart it away, conveniently forgetting the £10 Christmas box you gave them only two weeks ago. It is useless for firewood, even if the children would allow such a horrible fate, because you've got gas central heating. There's no point in planting it, because it probably has no roots.

If it has, you will be forced to dig a hole in rock-hard ground and plant the thing, where it will die slowly rather than quickly. If it survives, you are in for an even bigger problem. A fir tree will dominate the garden and kill your flowers by depriving them of light and sunshine.

I am not sure whether the solution discovered by Ohio State University was a carefully controlled experiment, the result of several years' work, or simply a lucky guess by someone desperately trying to get rid of a Christmas tree. But the result is a practical use for millions of redundant fir trees.

Attached to a concrete base, the trees were put in rivers. Regular checks showed that areas which had previously been devoid of fish, or where there had been only a few, suddenly enjoyed a population explosion. Put simply, the fir trees provided shelter from strong currents for all the fish incapable of resisting flood conditions. And environmentally, it has proved a winner because the trees became more than a 12-day wonder.

The scientists extended the experiment to lakes and reservoirs, where floods are less of a danger than marauding pike, lake trout and other predators. But as well as giving the small fish shelter, ensuring higher survival rates and improved stocks, therewere beneficial side-effects. Tiny invertebrates moved in, giving the immature fish winter rations, while at the other end of the food chain, the lunkers soon learnt that hanging around the trees like a child on Christmas morning could often result in aregular fish supper. It didn't stop there. Anglers cottoned on that a Christmas tree mysteriously growing out of a lake probably meant good fishing, because big fish would be somewhere about.

So it's a Christmas story with a happy ending for everyone and everything: trees, small and big fish, scientists and anglers. If only I could work a couple of shepherds into it . . .