Processes controlling spatial and temporal variations in cockle Cerastoderma edule (L.) abundance and distribution

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The common cockle Cerastoderma edule (L.) is an ecologically and commercially important species in Wales, and other North West European tidal flats and estuaries. Cockle populations often exhibit high inter-annual variability in recruitment strength but also spatially within population distributions. The post-larval stage is a critical period for determining recruitment strength in cockle populations, and likely to be a key period for creating spatial structure. This thesis aimed to record and quantify the spatio-temporal changes in post-larval cockle (newly settled 0-group individuals) density over large and small spatial scales, and investigate what processes may create or maintain these patterns. I found that over large scales (100s of metres) the initial settlement can be restricted to certain areas of the shore with high densities, but as time progresses, post-settlement redistributions move post-larvae across the shore. These redistributions recorded over two years extended post-larvae from initial low shore settlements into the mid and high shore, and resulted in a distribution similar to that of the adult population. Over small spatial scales (10s of metres) highly patchy and intense initial settlement of C. edule was also observed. Over time the spatial distribution of post-larvae became more dispersed resulting in significant positive correlation with the adult population post-settlement. To understand what is driving changes in density, experiments were conducted in the field and laboratory. Lugworms and adult cockles can be very abundant and disturb surface sediment during feeding and movements, which may kill or negatively affect C. edule post-larvae. Lugworms and adult cockles were excluded from areas of sediment in the field and the effect on post-larval densities over time investigated. Removing lugworms increased the densities of post-larvae but removing adult cockles decreased the post-larval density. Laboratory experiments looking at the possible role of lugworms and adults cockles causing direct mortality or increasing benthic and pelagic migrations showed no evidence that these were producing the results seen from the field experiment. I also tested if patchiness of post-larval cockles, compared to a uniform distribution, was beneficial for survival under predation from juvenile shore crabs Carcinus maenas, which experience a negative effect of interference competition on their foraging efficiency. Due to the behaviour of ‘take away foraging’ in the juvenile shore crabs, C. edule post-larvae still had higher mortality when aggregated. It is concluded in this thesis that spatial distribution and structure of post-larval C. edule change significantly during the post-settlement period, over large and small scales, and that macrofaunal interactions and predation may be a significant factor in producing these observations.


Original languageEnglish
    Award dateJan 2013