The Net Effect/The Only Catch
The essential premise is represented by the earth as a fish-bowl; somewhere we cannot escape and whose welfare is intrinsic to our existence. The graphic is provocative but the copy offers hope.

Solution

A print media idea where I wish to convey that the issue of over-fishing is a global one.
As per the brief, didacticism has been minimized while  constructive action is inspired by the polite suggestion to 'Encourage' sustainable fishing.

Few people like to be (however symbolically) shouted at and told what to do.
The slogan "The only catch is moderation" is a pun which hopefully makes people smile while suggesting how people can be part of the solution.