The Cornelis Gert Jan is a British trawler that was impounded by the French authorities in the UK-France fishing dispute, part of the residual outcome of issues in the Brexit trade deal with the EU which were "kicked down the road" for future resolution in order to get the deal done in time for 31 December 2020.
But the most important part of this dispute is not Brexit. The ship is a scallop dredger, an irresponsible and destructive type of "fishing" which scoops up layers of the sea bed, destroying habitat and ruining fishing for years to come. You can see the heavy metal [no! not Black Sabbath, my musical friends] scoops used here:
openseas.org.uk tells us "Scallop dredging is known to have the most severe ecological effect of all UK marine fisheries because of the damage and mortality it causes to the seabed habitat and species that live there." These effects are known to governments and they typically designate a limited number of areas where bottom dredging is allowed. A 2014 report for the University of York states "The king scallop fishery is the fastest growing fishery in the UK and currently the second most valuable. The UK is also home to the largest queen scallop fishery out of all of Europe ... the majority of scallop landings (95%) are made by vessels towing scallop dredges."
I tried reasonably hard to find out in what proportion of the coastal waters of the UK dredging is allowed but, unable to raise enough enough enthusiasm to wade through academic reports, I suggest we all get someone to submit a Freedom of Information request to answer the question.