Bridging the gap between science and industry

: Biology, ecology and management of the European eel (Anguilla anguilla L) in Lough Neagh

Abstract

Fish are among the world’s most important natural resources, providing humans with numerous ecosystem goods and services, including an annual harvest of over 100 million tonnes of wild biomass worldwide. Current evidence suggests that many fish species and stocks are under pressure. The Food and Agriculture Organization of the United Nations estimate that approximately 47% of fish stocks are exploited to their maximum sustainable threshold. A further 18% are estimated to be over-exploited, and 10% are depleted. A number of causal agents have been identified in the decline of fish stocks globally. Of the many threats, invasive species, pollution, overexploitation, climate change, habitat loss and degradation are of major concern. In light of these concerns, a paramount challenge of conservation biology is to identify and manage those species facing the greatest risk. The European eel, Anguilla anguilla is a valuable biological resource targeted by fishers throughout its distributional range. The A. anguilla population have undergone a dramatic decline, and the species is now classified as critically endangered. Lough Neagh supports the largest wild eel fishery in Europe, producing approximately 14% of the entire EU wild eel catch annually. However, the eel stock of Lough Neagh is no exception, with natural recruitment much reduced and fishery yields have declined considerably over the years. Notwithstanding these declines, commercial fishing of yellow and silver phase eel on Lough Neagh remains intensive. To compensate for reduced levels of recruitment, the Lough Neagh Fishermen’s Co-Operative Society (LNFCS), the fishery owner is now heavily reliant on the purchase of juvenile eels from outside the catchment to maintain yields and to also ensure sufficient escapement of silver eel to meet EU obligations. Increased concerns over decreasing catches by the LNFCS, paralleled with a continued decrease in recruitment of glass eels has highlighted the need for greater understanding of many aspects of the species biology and ecology. Consequently, there is a critical need for empirical data that address concerns around the sustainable management of the species. To address these knowledge gaps, five novel studies were used to investigate different aspects of A. anguilla biology, ecology and stock assessment. Fishermen’s ecological knowledge has the potential to inform scientists and policy makers on individual species ecology and improve fisheries management. Due to their cryptic nature little is known about the spatial distribution and habitat preferences of juvenile yellow eel in Lough Neagh. Chapter 2 of this thesis aimed to verify the value of local ecological knowledge in comparison to that derived from scientific evidence and demonstrated that these different source of knowledge could be effectively combined to provide a better “synergistic” approach for the management and conservation of Lough Neagh eels. The International Council for Exploration of the Seas (ICES) has identified a major methodological gap in quantifying the stock of yellow eel in large waterbodies such as freshwater lakes and transitional waters. Chapter 3 of this thesis tested a new survey method “Enclosure net” developed in the Baltic. Quantitative densities of yellow eel were achieved and results indicate its suitability for use in large eel producing waterbodies in the United Kingdom and Republic of Ireland. Surprisingly given the economic importance of A. anguilla, many aspects of its biology remain understudied. In particular, very limited scientific literature is currently available on the general biology and ecology of male yellow eel. Large, commercial size female eels form the main proportion of landings in most yellow eel and silver eel fisheries. As a direct consequence, females have been the focus of intensive study. Similar data on male eel, however, was clearly needed. Some of these knowledge gap are addressed in Chapter 5 and 6 of this thesis. Chapter 5 investigates sex specific differences in the biology and ecology of yellow eel from Lough Neagh. Sexual dimorphism was clearly evident in a number of biological variables. The diet and metazoan parasite assemblages of the different sexes are compared and discussed. Life history traits such as sexual dimorphism, longevity, delayed sexual maturity and semelparity make A. anguilla vulnerable to overexploitation. Finally, chapter 6 examined the health status and key biological characteristics of migrating male silver eel from three important eel producing catchments. Results suggest that fitness loss due to the presence of the introduced parasites A. crassus, coupled with low fat content could impair the migration and spawning success of migrant silver eels. The results from this thesis can ultimately be used to aid the management and conservation of the species. The presumed panmictic nature of the European stock requires that management and conservation strategies be addressed at a local, national and international level.

Thesis embargoed until 31 July 2024.

Date of Award	Jul 2022
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Keith Farnsworth (Supervisor) & Paulo Prodohl (Supervisor)