Just go to Boom City, find some large firecrackers with waterproof fuses and add your own weight (small rocks) with biodegradable adhesive tape and voila'... Seal Bomb!

To successfully deploy, run your boat directly on top of the fish thief until your line angle is nearly vertical, light the fuse and drop the bomb! Make sure the weighted 'cracker is at least a foot away from your line & boat hull.

Repeat as necessary and don't rinse your fish off in the water right away...
Mr. Pinniped will be lurking around your boat for a while...