Fennell, fisheries officer of the board in those balmy days when Cliff Richard was still topping the pop charts with the Shadows, innocently put 97 finger-length zander into the Great Ouse Relief Channel in Cambridgeshire. He said at the time: 'The Relief Channel is a still water from which the fish cannot escape.' They are now in all fenland drains and rivers, as well as in waters of neighbouring counties, plus Kent, Worcestershire, Oxfordshire, Surrey, Staffordshire and Yorkshire.
Warned by fishermen, who were catching 4lb zander just two years later, Fennell dubbed anglers 'biologically naive' and said it was fantasy to suggest that the zander were spreading throughout the Great Ouse system. It might not have been a problem if zander had co-existed with the resident fish. But like a busload of drunken heavy-metal fans at a WI jam-making seminar, zander rampaged through the roach, bream, bleak and dace. However, even Iron Maiden enthusiasts don't eat their hosts. A couple of years later, all the small fish had gone. One conservative estimate is that the zander were eating more than two million fish a year.
Introducing non-native species can be biologically disastrous. You would have thought the Australians, after their experiences with rabbits, cats and foxes, would know better. (Even the dingo is not a native Australian species but was introduced by the Aborigines and has probably wiped out a few home-grown animals.)
But Queensland waterways are now teeming with an African fish, the Mozambique mouth-brooder. Also known as tilapia, the fish is believed to have been smuggled into Australia to supply the aquarium market. It is remarkably hardy and can survive in polluted water, storm drains and even brackish water. Tilapia are not naturally carnivorous like the zander, but will eat anything, particularly the eggs of other fish. Many Queensland waters now hold tilapia and nothing else.
This spread is attributable to their brilliance as parents. The female protects the eggs and more than 1,000 newly-hatched fry in her mouth, while both parents protect the young until they are old enough to look after themselves. The fish becomes sexually mature in the year of its birth, and can have several broods in a year.
Australian scientists have been trying to find a way to curb the African invaders, and researchers at the James Cook University of North Queensland have just isolated a virus that is said to work exclusively on tilapia and not affect other fish. Ironically, the same virus is being used to curb the giant cane toad, another daft Australian import.
Keith Ransom, from the Institute of Aquaculture at Stirling University, is concerned that the virus, which attacks the fish's kidneys and red blood cells, should not be introduced without extensive trials. 'I think it would be terribly irresponsible to try this virus without several years' research work,' he said.
It appears that the Australians have not thought of farming the fish. Tilapia, a popular table fish throughout Africa, is delicious and would be a blessing to a country where fresh fish is even more expensive than in Britain. Strangely, zander - which have now been caught weighing more than 20lb in Britain - are very good to eat as well. It's a shame Fennell did not use them as fish stock instead of stock fish.Reuse content