The current status of serpulid reefs, horse mussel beds and flame shell beds in Loch Creran SAC and MPA

The report presents the results of site condition monitoring of the Loch Creran SAC biogenic sub-features ‘serpulid reefs’ and horse mussel beds’, and the Loch Creran MPA protected feature ‘flame shell beds’.

A scientific dive team led by Heriot-Watt University with members of SNH made detailed assessments of the habitats under investigation for this SCM survey. Included in the assessment is all information available from previous research carried out in Loch Creran by Heriot-Watt University.

Widespread fragmentation and marked reduction in tube occupancy of the serpulid reefs of the loch are reported to have taken place between 2005 and 2014-19, with loss of habitat estimated at around 20%.  It is proposed that a phase of reef degradation without compensatory replenishment forms part of a long-term, natural cycle.

Horse mussel beds covering a total area of 30 ha have been mapped, representing the biotopes SS.SBR.SMus.ModHAs and SS.SBR.SMus.ModT. Localised reductions in horse mussel density were recorded in three of the beds between 2005 and 2018, with evidence of long-term, poor recruitment in the most extensive, upper basin bed. Only slight changes in species composition and diversity of the community associated with horse mussels were recorded over the years 2005 - 2017. Temporal changes in the horse mussel beds were consistent with natural temporal variation.

In view of the decline in the condition of the sub-feature serpulid reefs, the condition of the reefs feature has been referred to the category Unfavourable Declining.

Three flame shell beds covering a total area of 35 ha have been mapped.  An increase in the recorded extent of the habitat between 2012 and 2017-18 was due to the increase in spatial spread of survey sites. Mean byssal coverage of the sea bed and diversity values of the associated biota were similar in 2012 and 2017-18 and the feature was considered to be in Favourable Condition.