why is it legal or allowed to catch tuna but not sharks and dolphins?

First of all dolphin are mamals and protected under the same laws that protect whales .

 Shark fishing laws vary geographicaly bassed on population and overall species status , ie in New England Spinny Dogfish can be kept where Smooth Dogfish must be released. Travel 1000 miles south and the situation is reversed . Others such as the whale shark are globally protected, while the great white sees global attention as a spots fish and tends to be the subject of persecution .

I live in Maine and we can catch all the sharks we want. Dolphin no.

Because you stupid man.

This is too asinine to answer.

Dolphins are mammals and highly intelligent.

There are way more tuna in the wild than sharks or dolphins. Also, killing animals lower on the food chain does less damage to the ecosystem.