Crustacean parasite infections caused by the fish louse Argulus are on the increase.
According to a new study by Taylor, Sommerville and Wootten in the Journal of Fish Diseases, the number of infections caused by Argulus in stillwater trout fisheries has been increasing over recent years - and hot sunny weather has been identified as one of the possible causes.
The parasites, which can measure nearly 1cm in length, can smother fish and cause wounds that become infected by other organisms after they break the skin to suck the blood out of the host.
The parasites present a problem for fish farmers and fisheries and the study says that they have led to economic losses through a reduction of anglers, who would prefer to catch clean, healthy looking fish rather than those riddled with ectoparasites.
The infected fish are also harder for anglers to catch, because they can go off their food when heavily infested with parasites.
Of 77 stillwater trout fisheries surveyed in the study, 29% had experienced a problem with Argulus in 2000. In one case Argulus coregoni was responsible, but in all others the fish were infected by the more commonly seen A. foliaceus.
The study provides evidence to suggest that algal blooms, the slow turnover of stock and a drop of up to 1m/39" in the water level are most likely to cause an Argulus problem.
The parasite is also a frequent problem in coldwater fishes from both the UK and abroad. The large parasites cause significant problems for smaller aquarium fishes, such as fancy goldfish and are difficult to treat with conventional aquarium medications.
For more details on the study see the paper: Taylor NG, Sommerville C, Wootten R (2006) - The epidemiology of Argulus spp. (Crustacea: Branchiura) infections in stillwater trout fisheries. J Fish Dis. 2006 Apr;29(4):193-200.