Shellfish environmental and biological monitoring programme: Feasibility Study

The blue mussel, Mytilus edulis (L., 1758), is common throughout the British Isles and is an important industry in Shetland valued at over £2 million annually which contributes to 54% of the annual rope grown mussels in Scotland. The aim of the study was to address the feasibility of monitoring biological and environmental conditions in order to assist the industry in optimising their production in future years. In order to carry this out, four sites, Booth, Riskaness, Sellivoe, and Siggi Bight, on the west of Shetland were sampled on a weekly basis from May through to September. The study was broken down into four main areas:
1) Concentration of D-larvae in the water column at each sampling site
2) Variation in D-larvae concentrations between two sampling techniques
3) Influence of tidal cycle on D-larvae concentration
4) Estimating mussel recruitment using rope droppers, on a weekly basis, from each site

Main Findings
• D-larvae concentration varied between sites with Booth having the highest concentrations and Riskaness the lowest. D-larvae were recorded throughout the study at Booth with a reduced concentration lasting up to mid August for Riskaness and early September for Sellivoe and Siggi Bight.
• Peaks in D-larvae concentration were recorded at Booth on the 5th September (100 000 m-3), Riskaness on the 13th August (8 000 m-3), Sellivoe on the 11th August (56 000 m-3), and Siggi Bight on the 18th July (80 000 m-3).
• A higher D-larvae concentration was recorded using the sampling hose (44 000 m-3) compared with a phytoplankton net (5 540 m-3).
• Higher D-larvae concentrations were recorded during the ebb tide which also had a greater variation in concentration compared to the flood tide.
• Recruitment was found to be greatest at Booth on the 4th June with 5 800 mussels/10 cm of dropper, lowest at Riskaness (138 mussels/10 cm) on the 16th July, with 804 mussels/10 cm recorded at Sellivoe on the 9th July.

Conclusions
The data clearly shows a distinct biological difference between the site at Booth and the remaining sampling sites. Similar patterns in recruitment were recorded at both Riskaness and Sellivoe which are geographically close to one another.

Sampling equipment and time of sampling seem to be highly influential when estimating D-larvae concentration. Although the sampling hose recorded a higher concentration of D-larvae, the present consensus is that phytoplankton nets are a more accurate means of estimating larval concentration as they sample a larger quantity of water. Care should be taken to ensure that samples are taken at the same point in the tidal cycle each week.

It was clear from this study that further, long term sampling would benefit the industry including starting sampling earlier in the year (e.g. late April/early May), using phytoplankton nets, and incorporate in environmental data such as temperature and salinity using CTD recorders.