Some fishermen, indicated by the numbers in the puzzle, each caught a fish. The numbers show how many squares are occupied by their lines (including the fish, excluding the fisherman). Lines never cross and every square, except there where plants grow, is used. We will solve a small puzzle as an example.
Puzzles in this genre
These three fishermen all have just one square next to them, so the first parts of their lines can already be drawn. The squares in the upper left, in the upper right and in the lower right all have only two possible entrances/exits. Because all squares have to be occupied we can draw lines there as well.
The line of fisherman 4 now passes through three squares. It means he must have caught the fish located towards the upper right. The square to the right of the bottommost fish only has two possible entrances/exits, so these can be drawn as well. The line of fisherman 5 is complete.
Fisherman 7 will have to make use of all seven remaining squares, and obviously there is just one way to do that.