A temporal investigation of Europe-wide drivers of cockle Cerastoderma edule biology, health and population dynamics

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Mahony, Kate Ellen
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University College Cork
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The common cockle Cerastoderma edule is an economically, ecologically and culturally important bivalve species and provides a range of ecosystem services. However, increasing reports of cockle mass mortalities, coinciding with reduced harvests, are a concern for various stakeholders. The factors responsible for these mass mortalities (e.g. parasites, weather extremes) are likely to be exacerbated by climate change, making it necessary to examine past and present trends in cockle biology, in order to make plans for the future protection of this species. To examine the key drivers of cockle populations, historic analysis and current field surveys were conducted. Historic data were collated to understand past trends of abundance, spawning and harvesting, at both a local and global scale. The current impact of biotic (parasites) and abiotic (latitude, environment and fishing) factors on cockle populations were investigated in a 19-month survey. This field study was conducted across latitudinally varying European sites (Ireland to Portugal) and combined histological techniques and morphometric analysis to examine gametogenesis, health and growth, and the key biotic and abiotic modulators of these characteristics. The evident trends were then considered in the context of future climate change. The variability in cockle populations was affirmed, particularly at a local scale. At a global scale, climate was found to historically influence cockle populations, with warmer waters resulting in reduced abundance. However, historic data collection was lacking in coordination across borders and between stakeholders. The current study, which was more coordinated, also demonstrated the potential influence of climate. In terms of reproduction, cockles at warmer southern sites exhibited prolonged spawning. Additionally, growth was found to be reduced at warmer temperatures, likely due to a diversion of energy to gametogenesis. This highlights a potential impact on the growth and spawning of northern cockles as a result of climate change. Furthermore, trematodes (both metacercariae and sporocysts) were found to negatively impact gametogenesis and growth. Previous studies highlighted the influence of climate change on increased trematode transmission, and this was supported in this thesis, with increased sporocyst prevalence linked with warmer water. Furthermore, additional information was discovered relating to the potential influence of climate change on a range of mortality inducing parasites (e.g. bacteria, Trichodina ciliates, sporocysts, metacercariae), which are likely to increase as a result of the previously predicted warming seas and increased precipitation. The results presented in this thesis have a number of implications for the protection of cockles and ensuring future sustainability of European populations. In particular, regular monitoring at a local scale is necessary, due to the evident micro-variability of cockle population characteristics. This regular monitoring, in particular of reproduction and growth rates, was shown to be necessary in creating management strategies, in particular for setting minimum harvest size. A lack of data mobility was noted between scientists, managers, conservationists and the public, highlighting the necessity of data sharing efforts not just for cockles, but for all commercially exploited species. Finally, climate change is evidently a major threat to cockles, however this study and the future resulting research, which has been widely disseminated to a variety of stakeholders, will assist the protection of the common cockle.
Cockles , Gametogenesis , Growth , Parasites , Trematodes
Mahony, K. E. 2020. A temporal investigation of Europe-wide drivers of cockle Cerastoderma edule biology, health and population dynamics. PhD Thesis, University College Cork.
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