Fishing lines: The fish are biting - count your fingers

A St Helens canal was renowned for the many tropical species you could catch there

Readers of Carl Hiaasen's books on the abuse of Florida's environment will be unsurprised to hear there are now more than 50 non-native species swimming in its lakes, ponds and canals.

We're talking seriously exotic here: mollies, cichlids, guppies, gouramis, siamese fighting fish, angelfish and climbing perch, plus oddball catfish such as the ripsaw, granulated and bristlecheek. You can buy fresh tilapia, once only found in Africa, in any Florida fish market.

Good news or bad for anglers? Well, the chance to catch arawana or peacock bass seems, sadly, to have overriden their environmental concerns. Many guides now run trips to catch peacock bass. Dangle a line in certain Florida waterways and you may even catch a piranha. To my knowledge, three species have been recorded there.

So what relevance does this have for British anglers? More than you think. The discovery of a dead red-bellied piranha on a Thames Water barge at Dagenham last week may be just the tip of a foreign invasion.

The theory is that a pet owner dumped it in the Thames but the fish turned belly-up, whereupon a seagull spotted it. A Thames Water spokesman said reassuringly: "Piranha could not survive below 15C for more than a few days; the temperature of the Thames is currently closer to 10C."

Oh yeah? Years ago, I kept a couple of piranha. Nasty little buggers they were, too. Unbeknown to me, their water heater wasn't working properly. The result was that the temperature dropped. And the fish were fine.

Now I don't want to scare you, but with global warming and an increasing number of idiots dropping unwanted pets in local rivers or ponds, we're going to go the way of Florida. A stretch of canal in St Helens was once famed for the number of tropical species you could catch there (they flourished because of a warm-water outlet). I've heard of cichlids, walking catfish and blue carp being caught here on rod and line. Are piranha that ridiculous?

So did a seagull really drop old red-belly, proudly trumpeted as the 119th species to turn up in the Thames? Well, the theory is fine, except that gulls are not known for being picky about their food. I reckon if one had found the piranha, it would now be just another splat of gull poo on your car roof.

The only thing I would add is that piranha are horrible to eat. Some years ago, we caught them in the Ecuadorian jungle and, as revenge for them trying to bite our fingers off, cooked them. Yuk! They tasted like toilet paper impregnated with tiny needles. We fed them to the ants. Even they weren't that keen.

Perhaps that gull took a closer look at what he'd grabbed, and realised it was a piranha. My theory is that he thought: "Oh no, I'm not eating another one of those."

So maybe there's a piranha or two in a pond near you. If you've cut your finger, I'd advise you not to wash it in the local lake. Just in case.