Subtidal seed mussels are the sole resource of juveniles in the bottom-grown mussel industry in the Republic of Ireland. Understanding recruitment patterns allows for better management of the seasonal fishery and provides the industry with some production sustainability. From their planktonic larval phase to the fast-developing young seed or spat, mussels are subjected to various external factors such as shelf sea currents, wind-driven processes, sea temperature, food availability and substrate type, resulting in spatio-temporal variability in recruitment success through the years. This thesis proposes a methodology to assess the biomass of subtidal seed mussel beds and examines the effect of sea temperature on see mussel sizes at two locations in the Irish Sea.
Seed mussel surveys have been taking place on the coast of Ireland since 1970, providing settlement locations and indicative biomass figures to the bottom mussel farming industry and the regulatory bodies. Unlike intertidal settlements, subtidal seed mussel beds are permanently submerged, making their assessment more challenging. During 2020, using side scan sonar acoustic imagery and ArcGIS, it was possible to visualize and establish the extent of two subtidal seed mussel beds located along the southeast coast of Co. Wexford, one near Rosslare and a second in the Rusk Channel. Random sample locations for both sites were generated on mapping software, providing extensive ground-truthing, which was carried out using grab samples. Using the weight of seed mussel in each grab and the Inverted Distance Weight (IDW) interpolation method on ArcGIS, seed mussel biomass was estimated for each of the settlements. Post-fishery surveys were carried out using the same method, and the remaining biomass was estimated, indicating that less than 10% of the original estimated biomass remained after the fishing effort. The original biomass estimations were then compared with the combined post-fishery biomass and the reported fishing figures from the industry. The variation between both figures indicated that the combined original biomass for both beds had been underestimated by less than 3% (or 182 metric tonnes), thus validating this estimation methodology.
Historically, the recruitment of subtidal seed mussel beds on the southeast coast of Ireland has been variable from year to year. According to the literature, the sea temperature is one of the main factors influencing the mussel life cycle. The seed mussel length records for the two main recruiting areas on the southeast coast of Ireland, namely Wexford-Rosslare and Cahore-Rusk Channel, collected from 2009 to 2022, give valuable insight into the seed length variation. Using a linear mixed model including the modelled sea temperatures for the expected larval phase until the first settlement (April, May, and June) and the recorded seed mussel length for each year, it was possible to assess the effect of sea temperature variations on the recruitment size. The model results for both areas indicated that smaller seed mussel size was related to lower sea temperature from April to June, in particular in 2013, when the lowest spring sea temperatures were observed. However, uncertainties generated by the prediction models indicate that low temperature may not be the only factor affecting the size of the seed mussel. Further scenarios, including other variables such as food availability, sediment mobility, and larval dispersion, should be considered.