Normally, the lake is a bit sparse from what I read. suckerfish and arctic char mostly. Maybe some other introduced salmonids, eels. One article suggests sturgeon.

However, the migratory salmon and salmonids probably are there once or twice a year in good numbers.

A giant creature migrating to and from the ocean, one round trip per year would probably be noticed cruising through downtown Inverness. Or more likely climbing over the Dochfour Weir. There is simply no getting past the weir without being exposed.