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The mixed wrasse fishery in Irish waters: species composition, population estimates and genetic analysis of the corkwing wrasse
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Date
2025
Authors
Brennan, Jack
Journal Title
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Publisher
University College Cork
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Abstract
The wild wrasse fishing industry in Ireland has developed drastically since it began in the early 1990s with the aim to supply cleaner fish for biological sea lice (Lepeophtheirus salmonis) control to the salmon farming industry. This industry has grown to supply approximately 375,000 wrasse harvested annually from approximately 6-10 bays along the west coast of Ireland. Despite the development of this fishery, no population assessments or genetic studies have been conducted on the Irish mixed wrasse populations until now. This research addresses critical knowledge gaps through comprehensive field surveys combining mark-recapture population estimates, species composition analysis, and genetic assessment using 8 microsatellite markers on corkwing wrasse (Symphodus melops). Species composition was determined from 1,715 wrasse captured across four bays (Mulroy Bay, Donegal Bay, Bertraghboy Bay, and Kenmare Bay) using specific wrasse pots equipped with juvenile escape gaps and size-selective entry points. Corkwing wrasse (Symphodus melops) dominated catches comprising 59.6% of total captures, followed by ballan wrasse (Labrus bergylta) at 22%, goldsinny (Ctenolabrus rupestris) at 12.7%, and rock cook (Centrolabrus exoletus) at 4.7%. Baillon's wrasse (Symphodus bailloni) were captured exclusively in Bertraghboy Bay (16 individuals). Significant size variations were observed between bays, with Kenmare Bay consistently yielding the largest individuals (mean 174.53 mm) while Mulroy Bay had the smallest (mean 126.78 mm). This pattern was consistent across all species and may reflect environmental conditions, as Kenmare Bay represents exposed Atlantic conditions while Mulroy Bay is an enclosed bay system with extremely sheltered conditions. Water temperature showed a strong positive correlation with catch rates (R² = 0.714), with catches varying three-fold between the coldest (Kenmare Bay, 10.9 °C) and warmest (Bertraghboy Bay, 14.4 °C) sites.
Population estimates using Chapman's modification of the Schnabel method revealed substantial wrasse populations across surveyed areas. Conservative density estimates (assuming 1000 m coastline catch assumption) ranged from 5 fish per meter of coastline in Bertraghboy and Kenmare bays to 8 fish per meter in Donegal Bay. Total population estimates were: Kenmare Bay (~1,300,000 wrasse), Mulroy Bay (~784,000), Donegal Bay (~576,000), and Bertraghboy Bay (~490,000). With annual catches averaging 375,000 wrasse across all fishing grounds, current exploitation represents approximately 10 - 12% of surveyed populations, though actual rates are likely lower as wrasse are also harvested from additional bays not included in population surveys.
Genetic analysis of 171 corkwing wrasse from six Irish bays plus comparative samples from Scotland and Norway revealed no significant population structure within Ireland. Pairwise FST values among Irish populations ranged from -0.005 to 0.014, indicating high gene flow and connectivity. This genetic homogeneity extended to Scottish populations, effectively forming a single genetic cluster across the Irish Sea. All populations demonstrated high heterozygosity (0.77-0.83) and low inbreeding coefficients (FIS = -0.04 to 0.04), suggesting healthy, randomly mating populations. However, Norwegian samples formed a distinct genetic cluster (FST = 0.055-0.073 compared to the Irish and Scottish populations), confirming the North Sea acts as a biogeographic barrier to gene flow.
These findings provide essential baseline data for evidence-based management of Ireland's wrasse fishery. The absence of genetic structure supports current practices of sourcing wrasse within broad regional areas, while the significant size differences between bays and presence of unique populations like Baillon's wrasse in Bertraghboy Bay highlight the need for continued monitoring and potentially bay-specific management considerations. Further research and long-term monitoring are necessary to ensure the sustainability and health status of this valuable fishery supporting Ireland's organic salmon farming sector.
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Keywords
Corkwing wrasse , Symphodus melops , Cleaner fish , Wrasse fisheries , Population assessment , Mark–recapture , Fisheries management , Population genetics , Genetic connectivity , Microsatellites , Salmon aquaculture , Sea lice control , Organic salmon farming , Irish coastal waters , Atlantic Northeast
Citation
Brennan, J. 2025. The mixed wrasse fishery in Irish waters: species composition, population estimates and genetic analysis of the corkwing wrasse. MRes Thesis, University College Cork.