The reason New Zealand is so good at exporting farmed mussels is that the endemic green-lipped mussel is not found anywhere else in the world. It is bigger and can live at greater depths than the blue mussel.
These mussels once formed great carpets on the sea floor down to 50M underwater. Mussels are filter feeders that remove particles from the water, early sailors remarked on the incredible clarity of NZ waters. Unfortunately by the late 1960s nearly all of the mussels had been lost to dredging and the water quality has declined. All exported mussels are farmed.