Punny!

A Marine Biologist discovered that he could prolong the life of dolphins almost indefinitely if they were fed on certain rare species of sea-gull. 

Naturally he decided to feed his own dolphins on this diet. Unfortunately, he began to run out of supplies from the coast and had to start obtaining his seagulls from an inland lake. 

This necessitated crossing a national park filled with sleeping lions. He carefully moved around the lions on tip-toe so as not to wake them ... 

However a park ranger came along and arrested him for transporting gulls across sedate lions for immortal porpoises.