The gill maggot (Salmonicola salmonea) is a parasite which can be found attached to the gill filaments of adult salmon and is frequently of interest to anglers who normally associate it with kelts.
It is in fact not a maggot at all but is a copepod crustacean which lacks a shell. Larval gill maggots attach themselves to the gills of adult salmon during the period when adults are in freshwater and then grow by sucking blood. The gill maggot then remains on the adult fish and requires the fish to return to the sea and successfully return to freshwater in order to complete it's life cycle.
Thus, gill maggots are usually seen on salmon which have been in freshwater for a considerable time, like kelts, although they can sometimes be found on fish which have been in freshwater for only a few months. The larger adult gill maggots are found on salmon which are returning to spawn a second time. Gill maggots on a fresh run fish are therefore a good indicator that the fish might be a previous spawner.
Since, at the present time, very few of the salmon returning to freshwater are previous spawners (as little as 0.5%), the lot of a gill maggot does not seem to be a particularly productive one!
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Gill maggots (Salmonicola salmonea) are usually found on kelts or previously spawned adult salmon.
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