And, as sewin and salmon feed on elvers, the food chain is interrupted even before it begins as the bigger "fish" - those at the top of the aquatic food chain such as seals - feed on salmon and sewin.
Unfortunately, there has also been a decline in the number of sewin running Welsh rivers.
Those of us with long memories recall the days when most rivers in Wales supported a good head of sewin.
It is sad to learn that this deadly chemical is still being used by some authorities - and as a consequence will enter the river system killing the creepy-crawlies which are the food of young sewin and trout.
Many bailiffs who patrolled our rivers two decades ago reckon that the rivers do not hold half the sewin stocks they once did.
There is a belief among some anglers that the sewin no longer return in great numbers to our rivers because of their slaughter at sea, when huge nets scoop up everything including sewin which are then thrown back dead when the nets are hauled into the boat.
There are lean years ahead for the sewin - which will lead to a decline in the number of anglers on our river banks.