This mussel seed collection operation in the Oosterschelde estuary, southwestern Netherlands, deploys approximately 2,000 coir rope collectors annually to capture wild blue mussel (Mytilus edulis) spat for transfer to bottom culture plots. For three consecutive years (2017-2019), the operation experienced declining spat yields, with barnacle fouling (principally Balanus crenatus and Austrominius modestus) on collector ropes increasingly outcompeting mussel larvae for settlement space and reducing usable spat harvest by up to 35 percent in the worst-affected years.
Working with researchers from Wageningen Marine Research, the operation conducted a deployment-timing trial over the 2020 and 2021 seasons. Collectors were deployed at two-week intervals from early March through late May, with spat yields and barnacle coverage assessed at harvest in October. The results showed a strong timing effect: collectors deployed in late April consistently captured 40 percent more mussel spat than those deployed in early March, primarily because the early-deployed ropes accumulated heavy barnacle fouling during the peak barnacle settlement window (March-April) before mussel larvae became available in sufficient numbers (May-June). By delaying deployment to late April, the barnacle settlement peak was largely avoided while still capturing the full mussel spatfall. The operation has since adopted a staggered deployment starting in mid-April, recovering most of the lost yield at no additional cost.
Regional deployment timing for mussel seed collection can be optimised using the fouling season calendar. The barnacle species involved are profiled in the organisms database, this site appears on the aquaculture farm map, and the cost calculator can quantify the economic value of improved spat yields.
Outcomes
Barnacle fouling on seed collectors reduced mussel spat yield by 35%. Timed deployment to avoid peak barnacle larval settlement improved yields. Collectors deployed in late April captured 40% more mussel spat than those deployed in early March.