In Virtual Villagers 2: The Lost Children the seventh puzzle is Sustainable Fishing and the objective is to solve the problem of of the ocean to the west which has developed algae due to being over-fished. Check out our How to get rid of Algae guide to find the answer to this problem.
The ocean to the west will eventually become contaminated with algae and your fisherman will refuse to fish and will say they are disgusted by the algae. To solve this problem you will will need to have Level 3 Farming and have Master Scientist and a Master Farmer. Once you have these requirements take both the Master Farmer and Master Scientist to the pond and drop them in.
You will then be told that that they have identified algae-eating fish. This species of fish looks like the Canary-Fire-Arrow from Fish Tycoon. If you now let the villager transport this fish from the pond to the ocean about 10 times you will get rid of the algea and restore fishing.
Check out our Virtual Villagers Origins 2 walkthrough and guide to find out how to solve the different puzzles in the game.