Despite what the description says, this achievement is actually tied to your players total number of Perch caught across all lakes. Once you have caught 50 Perch total you just need to catch 1 more whilst fishing on Bergsee to pop the achievement.
The total number of Perch caught can be checked in the menus in the players stats.
The same applies to the 2 other similar achievements on Hunters lake and Lilies:
As for actually catching Perch, live baits such as Maggots, Bloodworm, Dendrobaena Worm or Casters usually work. Fishing with smaller hooks (10 or 12) close to the shore on Bergsee seemed to be where I had most success although due to the luck based nature of the game, results will vary.