Like almost all species, plant or animal, salmon have at times to bear a large parasite burden. Intestinal worms and flukes, skin and fin parasites, lice and leeches; salmonids can get them all under the right conditions. The best known salmon parasites are undoubtedly sea lice. Sea lice are entirely marine parasites which die and drop of the fish within a few days of entry back into freshwater. On return to freshwater the salmon can become infected with Argulus, a freshwater louse; in Ayrshire I once saw a salmon that had high numbers of both sea and freshwater lice at the same time, an unusual ocurrance. Thankfully Argulus don’t seem to be present in the Spey.
One parasite that that is fully adapted to the salmons migratory lifecycle is the gill maggot, Salmincola salmonea. The gill maggot is not actually a maggot at all rather it is a crustacean although it is easy to see why it is commonly called a maggot. Gill maggots live exclusively on the gills of salmon, although adults only, they are not found on parr or smolts. Nor are they found on brown or sea trout; this parasite is specific to the atlantic salmon.
Anglers are normaly delighted when they catch a salmon with “long-tailed” sea lice, a sign of a really fresh fish, although they are not actually tails. What looks like tails are the twin egg sacs that are borne by mature females. Gill maggots also have similar twin egg sacs which can contain up to 900 eggs. But given the contrasting lifecycles of these parasites anglers are not normally so elated to catch a fish bearing “long-tailed maggots”!
Once shed from the egg sac the eggs of gill maggots drop to the riverbed where they hatch. Once hatched they have a free swimming stage, which lasts a few days, during which time they have to find and attach to a host, in this case an adult salmon. It is assumed that the gill maggot larvae are sucked in through the mouth of the fish and passed out via the gills as it respires. If it successfully manages to get a grip on the gills it initially attaches via a filament. Once attached the larval maggot moults and grows considerably. Adult maggots are always found close to the tips of the gills and usually on the outside edge. The female adults establish a permanant attachment to the gills from which they hang like a gymnast on parallel bars. Thier head and mouth are then in the correct position to feed on the gills of the fish, which are of course well supplied with blood.
Once attached to the gills it takes several months before the female is mature and capable of producing eggs. Every angler knows that kelts, which have of course generally been in the river for many months at least, carry gill maggots, but so do previously spawned fish which may be returning for the second or extremely rarely, more times to spawn. These fish are relatively scarce in Scotland and only a few percent of all the fish running the river in any one year will be previous spawners. Whilst at sea the gill maggots continue to grow but don’t produce eggs. If the fish survives to return to the river they start to produce eggs on reaching the inner estuary or freshwater.
Any parasite has of course to evolve a strategy to perpetuate itself. In the case of the atlantic salmon returning previously spawned fish can infect maiden fish entering the river for the first time but kelts, which are present in most rivers until late spring also carry mature gill maggots, and the larvae they produce can infect spring fish. In rivers where there are few spring fish the previously spawned fish are important carriers of mature maggots to ensure that the survival of the species in that particular river. In the Spey at this time of year there will be a few previously spawned fish returning but there is a big population of kelts which ensures that practically every spring fish will be infected with maggots which will mature over the course of the year.
The average number of gill maggots on previously spawned fish is about 20 and at this low level they are not thought to cause much harm to the fish, although is the damage to kelts gills a contributary factor in the low return rates of previously spawned salmon compared to for example sea trout, which often spawn many times? The picture below shows the gills of a previously spawned male fish from May 2012. A few gill maggots are present but more extensive damage to the gill structure can be seen with loss of the gill tips and scar tissue.
Salmincola salmonea seems to be a highly successful parasite as they are present in almost every kelt in the river by midwinter. They can be a useful identification factor for kelts and whilst most previously spawned fish carry gill maggots not all do. Classically previously spawned fish are supposed to be identifiable by the heavier spotting on the gills and flanks but the few I have had the opportunity to inspect have not been that obvious and I suspect there is considerable under reporting. Our scale reading usually reveals a few each year but rarely are there any notes on the scale packet to indicate the fact.
Most of the information for this blog came from the classic paper on gill maggots written by G.F. Friend, a lecturer at University of Edinburgh in the 1930s. He conducted most of his research at the salmon netting stations on the Tay where he was able to examine many thousands of fish. Mr Friend, I thank you for your still hugely valuable contribution to our understanding of this little “maggot”.