A checklist of parasites of freshwater fish of the British Isles-Kennedy revisited, reviewed and reassessed.

This review addresses the current state of knowledge regarding freshwater fish parasites in the British Isles, highlighting the significant gap since Kennedy's seminal 1974 work (Journal of Fish Biology 6: 613-644). Despite subsequent efforts by others to produce checklists in 1992 and 2016, comprehensive updates to parasite distribution patterns and species records have been limited. The paper emphasises the importance of accurate parasite checklists for understanding the status of and changes to endemic species baselines, climate change impacts, managing fish translocations, and identifying novel infections. Historical records, including paleoparasitological findings from archaeological deposits, provide valuable insights into parasite distribution patterns, though pre-1900s data requires careful interpretation due to taxonomic limitations. This study provides compiled parasite and host lists from published and unpublished records, creating a database that includes host species, parasite nomenclature, organ location, geographical distribution, and reference information. The current compilation details 339 named parasite species in 147 genera from 58 hosts (52 Protista, 3 Microsporidia, 54 Myxozoa, 83 "Monogenea" (79 Monopisthocotyla, 4 Polyopisthocotyla), 58 Digenea and Aspidogastrea, 33 Cestoda, 21 Nematoda, 7 Acanthocephala, 5 Hirudinea, 14 Copepoda, 4 Branchiura, and 5 Mollusca). A further 69 parasites identified to genus or class level have been reported, bringing the total number of parasites reported in the British Isles to 408. Several challenges are identified, including data transposition errors in previous works, sampling method inconsistencies, difficulties in accessing unpublished data, scant repositories to retrospectively evaluate findings, absence of molecular and taxonomic rigour, and the impact of anthropogenic activities on parasite distributions. As a result, any list may represent a significant underestimate of species presence and diversity. The review concludes by recommending future research directions, emphasising the need for combined molecular and morphological approaches, standardised documentation methods, and increased focus on under-represented taxa to enhance our understanding of freshwater fish parasites in the British Isles.