Does the set of the ropes affect mussel spat catches?
Science Centres: Fisheries
Catches of mussel spat are highly variable because of processes such as predation that occur after the spat have settled, but settlement itself is also highly variable.
Frames of spat-catching rope ready to be hung out.
Frames of spat-catching rope hung out.
Settlement marks the end of the swimming larval stage and the start of the sedentary phase of the mussel’s life. We know that current speed affects spat retention, but our data show that it probably also has an influence on the ability of larvae to settle on the spat ropes in the first place.
Suspended spat-catching ropes are likely to alter the flow of water through a farm. To test the effect of the spacing between catching ropes on settlement, groups of ropes were hung at 1 m, 130 mm, and 65 mm spacings on longlines at five sites in the Marlborough Sounds.
Closely spaced ropes form a semi-permeable barrier that partially impedes or slows the water flow, thus allowing the larvae more time to make contact with the settlement surfaces. Therefore, we expected that the more closely the ropes were spaced, the more the water flow would be slowed and the greater the chance of successful settlement. However, at four of the five sites, settlement density was greatest on the medium-spaced (130 mm) ropes, with fewer mussels settled on the closely spaced (65 mm) ropes. This may have been due to the closest rope spacing impeding the water so much that much of it was forced to pass under and around the ropes rather than between them. At all five sites, settlement was significantly less on the ropes that were most widely (1 m) spaced.
Whether the ropes are hung directly from longlines or are wound on to catching frames, it seems that not only do they provide settlement surfaces for the larvae, they can also alter the hydrodynamics to enhance the likelihood of settlement. Further experiments are under way to determine the influence of current flow on settlement and retention, and to determine how the spat themselves influence the water flow around them as they grow. They are part of a series of related studies on the hydrodynamics of mussel farms, spanning spatial scales from large farms to dropper ropes to individual mussels, that are designed to provide information on the best structures for catching spat and growing mussels.