Down on the fish farm, workers could not understand why the number of brown trout had suddenly taken a dive. But close observation revealed the reason - an aquatic version of the Great Escape.
The resourceful fish are leaping 3ft out of the water and into an eight-inch pipe which brings fresh water into the farm near Alresford, Hampshire.
Following their instincts, the trout, cousins of the Atlantic salmon, then swim against the flow for 30ft before finding freedom at the other end as they plop into a tributary of the River Itchen.
Simon Johnson, director of the Wild Trout Trust, said: "Brown trout do have migratory tendencies and swim upstream, especially in November and December.
"The water coming down from the pipe is oxygenating the pond and this could be kicking in their natural instincts.
"They might well think it is a waterfall and are trying to head up it to find a place to spawn."