Suppose that many people who earn a living catching fish decide they can make more money selling t-shirts and switch occupations. how will this affect the number of fish supplied by producers?

This will reduce the quantity of fish supplied in the market.

Explanation:  

When many people who earn a living catching fish decide they can make more money selling t-shirts and switch occupations. Once these people switch occupation, there will be less catcher of fish and as a result the supply of fish in the market will fall. This will shift the supply curve for fish to the left driving up the price of fish. The quantity of fish supplied by producers will decrease.