length of your hand. Some live on a single fi sh, whilst others have complex life cycles with multiple hosts, spanning many years and travelling hundreds of miles before they mature and reproduce. Many parasites lead a benign existence, tolerated by healthy fi sh without causing any obvious distress. However, by their very nature, parasites divert energy from their host for their own survival and reproduction. Consequently, some parasite infections can lead to debilitation of individual fi sh and serious disease problems within populations. Here, Chris Williams and Shaun Leonard give us a brief introduction to some of those parasites and problems. The Fish Louse, Argulus Figure 1: The white, fl uffy fungal The fi sh louse, Argulus, is a resident of rivers infection of Saprolegnia, tends to and lakes and one of the most familiar be a secondary infection on open parasites encountered by anglers. Three abrasions and sores species have been recorded from British freshwater fi sh and all may be found on the skin and fi ns of trout. The largest is Argulus coregoni (Figure 2), a parasite with a preference for running water so most likely to be encountered by the wild trout angler. Home Adults, up to 10mm in size, are light brown and well camoufl aged on the fl anks of trout; the black, beady eyespots can give them away (Figure 3). Suckers allow the parasite to move with surprising agility, yet clamp like a limpet when faced with risk of detachment. Sweet Home Infections of Argulus in the wild are often limited to odd ones and twos, tolerated by A guide to some of the creatures most healthy fi sh. Large aggregations of fi sh, that call a trout’s body home, prolonged low fl ows and warm temperatures can cause parasite numbers to increase. Heavy by Dr. Chris Williams and Shaun Leonard. infections can be extremely damaging, the continual feeding and attachment behaviour arasites are natural ranging environments. Consequently, damaging the skin and draining the fi sh’s components of healthy parasites are everywhere. energy reserves. aquatic ecosystems. They There are reports from the 60s and 70s Over 50 parasite species have been of salmon and sea trout ‘black with lice’ have evolved to infect most recorded from UK trout populations, ranging during extended low fl ows. More recently, Pliving organisms and exist in wide from microscopic protists to tapeworms the dry summers have led to rising abundance of Catch our releases. The magazine for the progressive game angler www.flyfishing-and-flytying.co.uk www.wildtrout.org 45 Salmo Trutta TROUT PARASITES A. coregoni on brown trout in some northern infected with opportunistic organisms like attracted to fi sh odour rather than repelled. rivers, although subsequent rainfall has the fl uffy, white fungus Saprolegnia (Figure Such behavioural changes increase the dispersed these infections. The ubiquitous 1). This adds to the confusion, because chances of the shrimp becoming the next A. foliaceus is the scourge of stillwater trout fi sh without UDN can naturally contract mouthful of a passing fi sh. This is one of fi sheries, threatening the economic viability Saprolegnia, especially spawning male many examples where parasites intricately of some. In such cases, lice may be found in trout and salmon whose thinned skin and re-wire their host to promote their own their thousands on individual fi sh, causing propensity to fi ght each other makes them development and reproduction. severe irritation and rapid condition loss. more vulnerable to infection. Recent research in Poland suggests that UDN-exposed brown Proliferative Kidney Disease The Gill Maggot, trout are less able to maintain the internal Proliferative Kidney Disease (PKD) is a Ergasilus sieboldi chemistry of the body, contributing to the serious problem in rainbow trout culture, Figure 6: the fi sh leech, Piscicola. Note the sucker Figure 7: the freshwater shrimp, Gammarus pulex, This parasite, fi rst recorded in Britain in the fi sh’s death. Some authorities suggest that caused by a tiny myxozoan parasite called at each end of the worm-like body and red infected with the larva of a spiny-headed worm, colouration from blood within. evident as a bright orange spot. 1960s, is most likely to be a problem for trout UDN proliferates where there are large Tetracapsuloides bryosalmonae. Clinical in lakes or gently-fl owing river reaches, since numbers of fi sh congregating in low-fl ow disease signs include swelling of the kidney it is a tiny copepod crustacean (a little over rivers and sunny conditions but again this is that can lead to mortality during the 1mm in length), unable to cope with much not totally consistent. summer months. In Europe, there is some fl ow. Only the females parasitise fi sh, but Large-scale mortalities of salmon and sea evidence to suggest that PKD is increasing quite spectacularly, with modifi ed antennae trout, attributed to UDN, were reported in its distribution and severity. The disease has gripping on whilst mouthparts rasp away at many UK rivers in the 1960s and 70s and recently been linked to the decline of wild the delicate gill tissues (Figure 4). problems reappear periodically. Fish health Figure 2: Argulus coregoni – note the large suckers brown trout in Switzerland, disease of Arctic Heavy infections undoubtedly cause fi sh scientists do not consider UDN to be a for attaching to the fi sh’s skin and numerous char in Iceland and also mortality in salmon mortalities, but thus far, the parasite has only widespread problem in the UK. However, swimming legs. parr in Norway. been a problem in coarse fi sh communities further efforts are needed to monitor this For many years, the life cycle of T. and stocked stillwater trout fi sheries. The condition and new molecular diagnostic bryosalmonae eluded scientists, hindering common name, the gill maggot, derives from techniques could help with fi nding a causative understanding of parasite transmission and Figure 8: a microscopic close-up of the head of Figure 9: cysts of Eustrongylides fi lling the Pomphorhynchus laevis – note the proboscis of abdomen of a brown trout. the appearance of attached females carrying agent in the future. disease dynamics. It was purely by chance the worm lined with attachment hooks and the a pair of white, elongate egg sacs, usually in that a scientist discovered the parasite within bulb, designed to plug the parasite fi rmly within the spring and summer (Figure 5). The Spiny-Headed Worm, free-living freshwater bryozoans – tiny fi lter- the gut of an infected fi sh. Pomphorhynchus feeders that grow freely on tree roots and The Fish Leeches, Piscicola Pomphorhynchus laevis, also known as the rocks in our rivers. The Nematode Worm, As with most parasites, light infections and Hemiclepsis ‘orange peril’, belongs to a group of parasites The parasite responsible for PKD is Eustrongylides are tolerated by trout with few adverse Fish leeches are common, obvious parasites known as the spiny-headed worms or widespread in the UK, but no serious disease Nematodes are common parasites of fi sh, effects. It is only when infections proliferate of trout, especially in stillwaters and slower Acanthocephala. problems have as yet been recorded in our but are often overlooked due to their tiny that problems arise. Recent investigations Figure 3: Argulus coregoni sheltering behind the river reaches, visible as a red-brown, worm- The array of spines that cover the head of pectoral fi n of a brown trout wild trout populations. However, little is size or apparently harmless existence. have revealed unusually heavy infections of like creature up to 30mm in length, attached this parasite make it a master of attachment known about the impact of this parasite in But at over 40mm in length and coloured Eustrongylides in some UK wild brown trout to the fi sh’s skin (Figure 6). (Figure 8). During attachment, the entire the wild and how changing environmental bright red, Eustrongylides is a conspicuous populations. Affected fi sh appear swollen They feed on the fi sh’s blood, biting head of the parasite is forced through the conditions could alter disease emergence in parasite that infects the body cavity of and lethargic. Such infections also cause through the skin, holding position with gut wall into the body cavity. Infl ation of a the future. There is some concern that rising trout (Figure 9). Each parasite lies coiled ovary degeneration, greatly reducing their a sucker at each end of the body. Heavy bulb just beyond the neck plugs the parasite water temperatures and eutrophication could within its own white, fi brous cyst, which reproductive potential. Studies are underway infestations might cause individual fi sh fi rmly in place, the body hanging free within all promote the proliferation of bryozoans provides protection until its host is eaten by the Environment Agency to assess the problems through loss of body fl uids and the fi sh’s gut. Here, the parasite absorbs and the parasites within. Collaborative by a fi sh-eating bird. It is only then that the extent and severity of these cases. leech bite wounds can become infected. Once nutrients from passing food and deposits its studies between the Environment Agency, parasite becomes re-animated, attaching Should you encounter any disease problems the leeches have had their fi ll, they drop off eggs that are fl ushed out with the faeces. Cefas, Aberdeen University and the Natural and maturing within the bird’s gut. Death in trout or require more information on and hide amongst weed and stones, digesting Many trout and grayling anglers will be History Museum are underway to improve of the fi sh can also trigger these nematodes the parasites mentioned, please contact the their meal.