Impact assessment of non-native parasites in freshwater fisheries in England and Wales

Abstract

Non-native parasites pose a significant threat to aquatic bio-diversity and fishery development. Many factors have facilitated the introduction of non-native parasites into England and Wales. Identifying the effects of these parasites and the importance of any changes to wild fish populations represents a considerable challenge. However, in order for the Environment Agency to identify future disease threats, effectively manage resources and implement practical and sustainable control measures, the risks posed by these parasites to fisheries must be better understood. A structured, risk-assessment process for non-native freshwater parasites is proposed. This approach includes procedures for hazard identification, impact assessment, risk management and communication. A quantitative approach to hazard identification provides an initial prediction of impact at the time of introduction to inform decisionmaking and guide preliminary control measures. This is followed by a more comprehensive framework for impact assessment that promotes understanding of changes at host, population and fishery levels. These are placed into context with the economic and ecological value of native resources. An independent 'non-native parasite review group' has been convened to ensure consistency of policy decisions and clear communication of disease risks to interested bodies. It is hoped that this wil assist the management of future invasions and provide a scientifically robust foundation on which to base proportionate control measures. The Category 2 parasites are defined as "species having a significant disease potential when introduced into waters where they do not already exist, or are non-indigenous species with unknown pathogenicity and distribution". Understanding of the dangers posed by these parasites varies considerably. Ergasilus sieboldi, Anguilicola crassus, Bothriocephalus acheilognathi and Lernaea cyprinacea, are well recognised non-native fish pathogens that pose a considerable threat to fisheries. Conversely, the effects of other introduced parasites remain poorly understood. In many cases, a paucity of published literature at the time of introduction has limited a reliable assessment of impact. Four non-native parasites were considered a priority for further study. These were Paraergasilus longidigitus (Copepoda: Poecilostomatoida), Ergasilus briani (Copepoda: Caryophylldae) Poecilostomatoida ), Atractolytocestus huronensis (Cestoda: and Philometroides sanguinea (Nematoda: Philometridae). Pathological, epidemiological and experimental investigations were undertaken to assess the effects of these parasites at host and population levels. P. longidigitus can cause pronounced pathological changes to the olfactory epithelium of infected fish. Although this damage provides the potential for disruption to spawning through reduced sensitivity to reproductive chemical cues, experimental observations combined with seasonality studies suggest that the parasite is unlikely to disrupt reproduction within infected cyprinid fisheries. Similarly, studies conducted on the caryophylidean cestode A. huronensis provide little evidence to suggest that the parasites poses a threat to car fisheries. Damage within the intesinal tract was characterised by relatively mild mechanical and inflammatory changes. Infections of up 213 parasites had no adverse affect upon the conditon of common carp. Based on these findings and the application of the aformentioned risk model, P. longidigitus and A. huronensis are considered to be of low disease risk to fisheries. The pathology of E. briani and P. sanguinea are described for the first time. Literature suggests that both parasites are pathogenic to juvenile fIsh. E. briani causes a number of pathological changes within the gils of juvenile cyprinids, although these remain localised due to the very strict site specificity of the parasite. Migrations of the nematode P. sanguinea causes damage to the fins and caudal musculature of juvenile crucian car. For both parasites, the extent and severity of pathological changes are inversely proprtional to host size. Further studies at the population level are needed before an assessment of impact to fisheries can be made. The difficulties associated with studying the effects of introduced parasites to juvenile fish populations in the wild are recognised. Recommendations for further study are given.