The epidemiology and population ecology of Argulus spp. : Infections in UK stillwater trout fisheries

Abstract

Argulus spp. are branchiuran crustaceans, the majority of which are ectoparasites of fish. In the UK three freshwater species have been recorded: A. foliaceus, A. japonicus and A. coregoni. Argulus spp. have been reported to cause mortality in stillwater trout fisheries, but little data is currently available as to the problems they cause or ways to control them. This study was carried out within the broad aims of the sponsors, which were as follows: a) To review of the current perception of the extent and severity of Argulus spp. infections in UK Stillwater trout fisheries, and identification of the methods currently employed for controlling these infections. b) To review and increase the knowledge of the biology and ecology of Arglilus spp. in relation to these systems. c) To assess the prospects for novel control and management strategies to reduce economic loss. A cross-sectional, questionnaire-based study of 69 randomly selected stillwater trout fisheries showed that Arglilus spp. infections may cause economic loss, through a reduction in the number of anglers attending a fishery. Argulus spp. are perceived to reduce the feeding and therefore catchability of trout. This, in combination with the resulting reduction in condition and aesthetic appeal of fish is believed to reduce angler numbers. In year 2000, 29% of stillwater trout fisheries in the UK suffered a problem infection. Argulus spp. were widely distributed throughout the UK, although problem infections appeared more common in central and southern England and Wales. A. foliaceus was the most common species being found in all but one of the infected study waters. The cross-sectional study identified three factors associated with problem Argulus spp. infections. Presence of an algal bloom / high turbidity, and slow stock turnovers were both associated with an increased risk ofa problem infections, and a drop in water level was associated with a reduced risk of a problem infection. A longitudinal study of the population ecology of A. foliaceus in five trout fisheries of varying management intensity was conducted to identify correlations between risk factors and increase our knowledge of the population dynamics of A. foliaceus. The study also investigated the effects of temperature and identified key points in the life-cycle of Argulus spp. around which interventions could be targeted. Low water clarity and high temperatures were significantly correlated with a high abundance of A. foliaceus. Low water clarity was also associated with reduced stock turnover and would suggest that high numbers of A. foliaceus alone may not affect the catch. The abundance of A. foliacells is greatest towards the end of summer and drops to low levels over winter. The first and second cohorts of the season hatch, in May and June respectively, from eggs that have over-wintered. These cohorts became adult and stal1ed laying eggs in July and August. The over-wintering of hatched stages of A .. J()!iaceus is dependent on a slow winter stock turnover of fish and the presence of reservoir hosts. If these stages over-winter they lay eggs at the end of April that hatch in July. A cohort then hatches every month until September, giving a total of 5 cohorts hatching in a year. The cohorts hatching in July and August lay eggs that over-winter, and the September cohort over-winters as hatched stages. Field studies also elucidated valuable information on the egg laying habits of A. foliacell.\', determining that egg laying occurred between April and September, before stopping over the winter month. Depth of egg laying was found to increase as the season progressed, and was found to occur deeper in clear water than turbid water. Laboratory experiments were used to study the effects of illuminated & darkened conditions, different host species and temperature on the infection success and survival of the parasite. Infection success was high under all conditions. Subsequent survival was similar in all of the experiments, but the likelihood of the parasite reaching adult size was greatest at temperatures of 20°C. Experiments also showed that the metanauplii of A. foliaceus could not survive without a host for more than 3 days. This project has successfully identified the problem posed by Argulus spp. infections to UK stillwater trout fisheries and determined the extent and severity of such problem infections. Several risk factors associated with problem infections were identified and studies were carried out in relation to the parasites population ecology. This work has greatly increased our understanding of the factors controlling A. foliaceus populations, and has led to the development of a series of management recommendations and opened avenues for further research.