Blue mussel spat availability and settlement on longlines in a Faroese Fjord

Eirikur Danielsen and Gunnvør á Norði
Blue mussel farming is currently not an industry in the Faroe Islands. However, they are abundant in the coastal waters around the islands and previous trials have shown a good farming potential. Blue mussels are a low trophic species that show a great potential to increase the food production from the sea in a sustainable manner. Blue mussels are also well tested candidates in Integrated multitrophic aquaculture and various other environmental impact mitigation trials. This study was conducted in collaboration with the fish farming company Hiddenfjord, who wish to test the mitigation potential of blue mussels in relation to fish farming. Since the primary purpose of the blue mussel farms was mitigation, they chose to test low labour intensive self-regulating longlines with natural spat collection. The spat availability, settlement and growth on longlines was investigated from 2018 to 2020. Planktonic Bivalvia larvae were collected with 80 µm and 150 µm mesh plankton nets. In the 80 µm net the majority of the larvae were smaller than reported sizes at settlement. In the 150 µm net the majority of the larvae were around the reported sizes at settling (240-350 µm). The larval abundance was significantly higher in the 80 µm net with peak abundances around 1500 ind m-3 . The abundance of planktonic larvae indicated two peak spawning period, the first in May and June and the second in August and September. However, the interannual variation was high. The mean shell length of larvae captured with the 80 µm net increased steadily until mid-June early-July whereafter it dropped and gradually increased again, indicating that settlement from the first spawning mainly occurred at this time. Three kinds of long lines were investigated Fuzzy rope (FR), Trawl (TR) and a flat rope commonly used at mussel farms in Denmark “Svendsker band” (SB). The settlement on the SB was very poor. The other two showed higher settlement, although the mussel density was quite low after the first settlement compared to similar studies. In 2019 which showed better settlement than 2018, there were 395 ind. m-1 on the FR at 4 m depth and 262 ind. m-1 on the TR. The growth during the first year was similar to previous observations in the Faroe Islands, while it was somewhat smaller during the second and third year. The growth was higher than observed in Nova Scotia, Canada, but significantly lower than observed in the Limfjorden, Denmark where phytoplankton availability is significantly higher than the Faroe Islands. During the second year, new spat settled on the ropes, and the blue mussel size at 4 m depth indicated that the older blue mussels were predated or lost due to mechanical disturbance, while the older blue mussels remained on the rope at deeper depths. After the lines had been deployed for three years, the TR showed highest blue mussel abundances and largest sizes, with 1000 ind. m-1 at 12 m depth and 633 ind. m-1 at 4 m depth, which is within the range of other self-regulating longline trials. The abundance on the FR was 373 and 573 ind. m-1 at 4 and 12 m depth, respectively. Also, the TR showed a cleaner blue mussel culture compared to the FR, where there were quite a variety of other species present on the lines.

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