The Environment Agency is warning potential poachers that it is being highly vigilant now the salmon and sea trout seasons have ended.
Sea trout season in Cumbria on most rivers ended on 30 September and the last days of salmon fishing were around the end of October.
The agency says the impact of poaching on fish populations is worst at this time of year, when spawning occurs.
It could have a great effect on the number of fish returning next year.
Over the past year, the agency has brought eight prosecutions against poachers in Cumbria, with penalties including fines, probation and curfew orders.
The salmon fishing season on most Cumbrian rivers starts on 1 February and sea trout fishing starts again on 1 April in most areas.