A FISH farmer from Hampshire who uses computerised heating to produce giant trout claims he will produce a 70-pounder, the weight of a large dog, in next year's crop.

Nigel Jackson, from Dever Springs day-ticket fly fishery at Barton Stacey near Andover, this year broke all three records for cultivated trout. He produced his record-breaking rainbow, brown and brook trout after just five weeks' exposure to his fish husbandry techniques which he believes are unique.

He does away with seasonal variations in temperature by giving the fish a 12-month 'summer' throughout the second year of their lives. In their first year Mr Jackson's trout grow normally, to about 3lb. During the second year they would ordinarily grow to about 9lb, but can reach far greater weights if kept warm.

Each fish has an inch-long 'hatpin' inserted between its cerebral lobes, using ultrasound to make sure it is positioned correctly. This implant carries a tiny transmitter linking the fish to a central computer so its temperature can be monitored. The fish is identified via a bar code stuck on its side which carries coded information on its genetic lineage.

The fish are kept in a carbon- lined concrete pond bathed in

infra-red light. The water temperature is kept at about 12C and the fish themselves at about 20C, slightly above the temperature they would reach during an ordinary summer, but the optimum for food conversion.

Dever Springs uses 'triploid' fish, with three sets of chromosomes rather than the normal two. These genetically-altered fish are sterile, so food that would normally be used to produce ovaries and eggs is converted efficiently into bulk weight.

The largest trout Mr Jackson has produced so far weighed 40lb, but died before it could be put into Mr Jackson's ponds. 'I hadn't quite perfected the technique then . . . I think I cooked him.' He is confident of producing a 70lb fish by next September - three times the normal weight - as it will be the first to receive a full year's treatment. The fish should reach about 4ft 6in in length with a 60-inch girth.

Mr Jackson said his giant fish have more silver colouring than normal, but could not say whether they taste any different as he does not like trout - and most people who catch the large fish are more interested in displaying them than eating them.

(Photograph omitted)