Abstract
Cockle health in the UK: a case study for the implementation of holistic investigative approaches to explain complex mortalities and long-term population declines
Introduction:
Cerastoderma edule (C.edule ) populations of the Wash estuary embayment in eastern-England, UK have suffered long-term declines linked to recurrent annual mortalities. Essential to local fisheries, and wildlife populations, Wash cockle stocks were largely wiped out due to a mass mortality event in 2008, occurring annually ever since. Whilst disease investigations have provided significant insights into key pathologies impacting cockle health, the various mechanisms contributing to cockle mortality, including environmental, ecological and anthropogenic variables, have yet to be fully determined. Contributory factors may be sub-lethal individually, but act in a synergistic manner to drive mortalities. Hence, a more holistic approach is required to understand the complexed nature of cockle population decline.
Methodology:
Wash cockle populations have been sampled over multiple years. In a traditional disease investigation, samples were taken for molecular screens and histopathology for identification of pathogens and pathologies. Additional tissue samples were collected for chemical analysis including metals and pharmaceuticals, as well as environmental samples, including bulk water, incubation water, bulk sediment and sediment cores, taken for eDNA analyses and chemical analyses. A systems mapping exercise was conducted to identify key variables influencing adult cockle population size. The suitability of existing data was assessed, with analysis of trends in data and cockle population dynamics.
Results:
Three key pathologies were identified from C.edule individuals: Marteilia cocosarum (a paramyxid parasite previously linked to similar cockle mortalities in the Burry Inlet), a novel iridovirus Bivalve Iridovirus 1 (BilV1), and disseminated neoplasia (DN) types A and B. However, no one pathology alone or in combination with others can fully explain the observed mortalities. A subset of cockles underwent tissue chemical analyses to explore differences between individuals of varying health status, alongside environmental chemical exposure revealed from sediment samples. Age-depth relationships were obtained from sediment cores and linked to chemical and contaminant levels to identify historical disturbance. A systems map output provided expert consensus on key factors affecting cockles in the Wash, determining potential synergistic drivers controlling adult population size.
Conclusion:
Findings have identified three likely deleterious pathologies previously linked to population declines. On-going analysis will provide an overview of cockle health in relation to the broader ecosystem. Development of stakeholder and cross-agency collaboration will strengthen our understanding of the Wash ecological system and advance current research campaigns to improve existing data. Additional metagenomic and transcriptomic work undertaken on 2024 cockle samples will provide further insight into metabolic and host responses to stressors.