It's amazing the lengths they'll go to to have a pop though. Sending out fishing trawlers to deliberately seek out our submarines and them try to catch them in fishing nets isn't exactly clever though. Maybe they need to consider using a bigger net next time.
With the European dominance of the Spanish trawler fleet in the field of fishing, France could be looking to redeploy its redundant trawlers as ASW vessels thereby killing two birds with one stone, i.e. cheap ASW and employment for redundant fishermen.
Simply deploy a line of trawlers across your submarine threat axis and wait - when one or more of the trawlers disappears from view, you've found your submarine - now you've just got to vector an appropriate ASW weapon into the vicinity of the gap!