Natural England commissioned this study to assess the distribution, extent and range of subtidal maerl communities within the Falmouth to St Austell Bay Special Protection Area (SPA). ENVISION analysed sidescan acoustic and underwater imagery data collected by the Cornwall Inshore Fisheries and Conservation Authority (CIFCA) from summer 2023 to May 2024. Imagery analysis included quantification of live and dead maerl, identification of habitats and biotopes to inform interpretation of geophysical data for habitat mapping, and classification of data using a maerl categorisation system (Axelsson, 2023).
The predominant habitats predicted throughout St Austell Bay consisted of coarse sediment with varying amounts of maerl (live/dead), often observed in mobile waves of dead maerl and maerl sediment with a low percentage cover of live maerl at the peripheries. Smaller areas in the centre of the bay were classified as maerl bed biotopes (≥5% live maerl).
Comparisons were made with previous extents of maerl from data collected by CIFCA during a previous survey in 2016 but were limited due to methodological differences. The current mapping process is likely to have been biased as ground truth samples were targeted towards maerl habitats, potentially overestimating their distribution. Challenges in applying the maerl categorisation system (Axelsson, 2023) were summarised.
Recommendations were made for review and clarification of maerl bed habitat definitions, refining the maerl categorisation system and adjusting survey sample design of future surveys.
This study provides insights into the nature and distribution of maerl habitats in St Austell Bay and can be used to inform management and conservation efforts for the SPA and other areas of maerl habitat.