This is the type of dangerous question that leads to interminable lists of laws. If there is any problem and we don't know of another solution that works as well, let's go with the one we do know. It also happens to be the easiest and we know it will work. One wonders why the same solution doesn't apply to paper that is also a serious littering problem? And it's always this same type of appeal to emotion. If you cared about the fish so much, why not just outlaw all plastics? Answer: because people only care about things as much as it doesn't affect them to solve. That's how regressive laws happen.

I could put forth a guess or two on attempts at action, but I am not familiar enough with the science to argue them wholeheartedly. But I can say the political decisions are rarely science based and more often based on perceived righteousness (which is why it's a tax and not illegal).