a) Where a fish is returning to spawn for the second time. Gill Maggots, although a freshwater species*, are not killed off when a Kelt returns to the sea and so can survive till it gets back to the river again. In fact, the presence of a significant number of Gill Maggots on a large, fresh, fish is an indicator that it is likely to be a repeat spawner. The photo below shows the Gill Maggots on a returned Spring Salmon of around 13lbs caught on the 26th March 2009; scale reading showed that it had spawned before and was a very fresh fish (it also carried Tide Lice).
b) Where a Spring Salmon has been in the river for a few months, it can then have some Gill Maggots. The photo below shows a dissection of a Spring Salmon, caught on the 28th February 2009, with a half-dozen Gill Maggots. Spring Salmon can enter the river as early as late October.
The Gill Maggots are circled in yellow and the arrow points to the unripe ovaries, the indicator of a Spring Salmon.
* “Gill Maggots”, Salmonicola species: These are, like the Sea and Fish lice, crustaceans, that attach themselves to fish to suck blood and body fluids, but in the gills. The infective larvae are produced by an intermediate stage of the parasite that lives in a freshwater Copepod (small, free-living Crustacean) and these attach themselves to the gills of adult salmon where they begin to grow. Juvenile salmon are too small for them – the larvae cannot get into the gills from the front and the water current passing out from the back is too strong for them to get in from that direction. After five or six months on the gills, they mature and start to reproduce. The “maggots” can survive in salt water, so a salmon that survives spawning, returns to the sea and then returns again to the river for a second time can have Gill Maggots even though fresh run.