Evaluation of different combined collectors used in longlines for blue mussel farming

Longlines are used for blue mussel farming in Norway. Rigs using combined spat collectors and growth bands are preferred, because they make the production more cost effective. An experiment with four different types of combined collectors (Ålesund band I and II, Fiskevegn stocking, Swedish band) was conducted on two production sites to investigate, which factors affected the attachment of spats. Ålesund band I and II are 5 cm wide flat bands made of a combination of polypropylene (PP) and polyester (PES) fibres. In Ålesund band I the PP threads are grouped in seven vertical lines, whereas in Ålesund band II the PP threads are spread uniformly over the width of the band. The Fiskevegn stocking is a loose crochet rope made of a combination of PP and PE (polyethylene) fibres. The Swedish band is a 5 cm wide white PP band. The results showed that the amount of mussels that can be harvested was highly dependent on the type of collector. On both sites the amount of harvested mussels was largest for the Swedish band, which averaged between 25 and 30 kg mussels per collector of length 6.1 m. The average amount of harvested mussels on the three other collectors was between 14 and 18 kg. Both shape and surface structure of the collector seems to be important. A dense structure, such as in the Swedish band, provided several attachment points for the byssal threads. The attachment between the mussels and the collector surface was also stronger. In the experiment, the mussels showed no preferences for collectors with thready and filamentous surfaces (Fiskevegn stocking). A collector band must be stiff enough to prevent folding of the band as the weight of the mussels increases. Folding occurred on both Ålesund band I and II, but not on the Swedish band. Suboptimal structure and surface quality of the collectors may result in large initial attachment of spats followed by a large spatfall later in the growth period. In the experiments, the number of spats was over 4500/m after the first growing season. At harvesting, the mussel densities had been reduced to 600-800 mussels per m collector.