The big concern around fishing gear is that it is specifically designed to catch marine life, so when it is lost or deliberately discarded in the ocean, it continues to kill for years for no reason at all.

The movie Seaspiracy makes this point in a very graphic way: https://www.youtube.com/watch?v=1Q5CXN7soQg

This fact-check runs through all the points of the video in 15 minutes: https://www.youtube.com/watch?v=s7QRLZp63Eo