That depends to a great degree on the level of difficulty of the game. There's a fixed number of fish you can catch before the algae arrives. On Hard, it is a quarter of the number for Easy, and half that for Normal level. I generally play on Easy and usually don't get algae until after the crops have started being harvested. In a couple of games, where I was lucky with events that gave me food, I was able to reach L3 Farming without the algae arriving.