Today I talked to Urs over the fish. As he has reminded of a lake in Flumserberg, where he once fishermen to see how they have taken on a running pike with leeches ...
Could it be the Seebensee ? I think so. Here a picture . There
All about to maps etc. it as a PDF here . And here is the link to Hotel, which is accessible only by foot ... Day ticket is CHF 35 - and only available at the hotel. Guests staying longer than 3 nights are also picked up ....
fishing season from mid June to late September. Fishing boats available, cost unknown.
PS: mountain stream fishing should also be really great ;-)