You need level 3 (Level 2)of the faction you chose in order to get the next herb. Then you can make the potion to get rid of the sharks.

In the meantime, the absolute best thing to do, in my humble opinion, is to not play your game for 4 or 5 days; maybe even a week. Pause your game and turn it on once a day in order to have your chief make food. I know it's hard not to play them, but it's the best way to save them. It's possible to do it by changing the clock on your computer (while paused) but it seriously messes with events and collectibles and has the potential to really mess up other things as well. ...

You need to have at least 200 food in order for them to work and not be worried about food. However, with so many villagers that amount will not last long. I recommend at least a week of your game paused except just long enough for the chief to do his/her thing.