Have you ever taken a piece of fish home from a fishmonger and found a thin black worm in the flesh? If you have it has most certainly put you off that type of fish for life, not unsurprisingly. If you haven’t seen one you may have had a friend that has told you of such an occurrence. This scenario is quite common at this time of year (although there are incidences throughout the year) as fish gorge themselves on food from which the energy they gain is then applied into the production of gametes (eggs and milt). The majority of the bony flat and round fish go through the process of spawning between January and April. So what are these worms and where do they come from? They are parasitic round worms from the species Phocanema and follow the very simple life cycle of most marine parasites. Large marine mammals, generally grey seals, harbour the adult Phocanema who eggs are laid in the stomach of the seal. These eggs are released into the water, hatch, and are then eaten by crustaceans, which are in turn eaten by fish. The worm will then make its way into the flesh of the fish before the fish is eaten by a mammal which completes this very simple cycle. Commonly referred to as Cod worms many species can be affected and some to a huge extent. Cod, Monkfish and John Dory are probably the least affected and easiest to deal with as these three species have nice white flesh making Phocanema easier to spot. Both Hake and Gurnard, more so Tub than Red, can be inundated with Phocanema generally in the stomach and a little in the flesh. Sea Bass can also show signs of Phocanema to some extent at this time of year. I have never seen an incidence in Dover Sole and only once in Lemon sole. Pelagic midwater feeders such as mackerel and herring rarely come into contact with Phocanema.
Most importantly these worms are harmless to humans and die immediately if cooked. Additionally they will die when frozen – probably the reason why worms are never found in many supermarket fish.
So what should you do if you are sold fish with Phocanema in? Firstly, I hope no fishmonger would purposely sell fish inundated with Phocanema, however, if you are buying a whole fish, especially Gurnard, the fishmonger my be unaware of any Phocanema in the flesh. Additionally, if you are buying a very thick piece of cod or Monkfish it could be impossible to see that Phocanema is present. Other than those two scenarios there is no excuse apart from laziness of profiteering to sell a piece of fish with Phocanema in it. If you do see Phocanema in your fish or fillet just put the point of a small knife into the flesh and underneath and lift which will remove the subject. If it is inundated you should consider returning to the fishmonger and asking for a replacement. Be aware the shop is not legally obliged to replace or refund your money, however, I see no reason as why they wouldn’t if they are reputable.