Well let's be fair. The power to impound has existed in EU regs for breaches of quota for a long time. The real problem isn't in the channel, it's in the North sea, part of which is technically international waters despite arbitrary lines etc and to the south is continental shelf disputed by us France and to some extent Spain. The real question is if there's enough boats in our fleet to actually make that much difference. The environmental damage for the EU CFP has already been done according to some conservationists, amongst which is a family friend who worked for CFAS in Scotland.Don't need to catch them all, just at a steady enough rate to be a real deterrent. And preferably deep in what will be UK waters, so they can't plead minor navigational error.
After a number of trawlers get impounded, the number trying it on will drop off.
It would also be prudent for UK trawlers to stay well away from EU waters - Marcon in particular will be itching to impound them in retaliation.