This is done carefully since the byssal attachment may be weak enough that the mussels may drop from the rope. Harvesting from rafts can also be done by winch and hoist. However, larger companies such as Taylor United are using sink floats that are maneuvered under the rafts. The culture ropes are cut and the mussels fall onto the sink float and can be transported to the processing point. Typically growers will be responsible for washing and declumping their stock prior to delivery.
Some markets also prefer the mussels to be de-byssed which adds cost to processing.
There are machines available for these processes. A Handbook of Northern Mussel Culture . Skidmore, D. and Chew, K. 1985. Mussel Aquaculture in Puget Sound . Quayle, D.B. and Newkirk, G.F. 1989.