OK lets try this then.
Putin is known to favour clandestine means of eliminating his enemies.
Not only his warships but his fishing boats (ever heard the term 'Russian Trawlers') often operate close to our shores.
Huge numbers of lobsters and crabs in specific waters off the north east coast of England are dying due to some heretofore 'unknown' type of virus killing the crustacea; the government don't appear to know what this virus is or how it got there to cause such devastation.
Putin is fully capable of getting his minions to develop such a virus; he also has the means (via the aforementioned trawlers) to 'seed' the local waters off Whitby and elsewhere with it.
He knows he's on a hiding to nothing with his present campaign against Ukraine so he turns to other means to try to destroy our fishing industry.
Fantasy? You tell me; I think it's perfectly feasible (and no I don't like lobsters or crabs; can't stand the taste)