Building better offshore mussel farms

Building better offshore mussel farms

Measuring mussel float motion by radar. The radar is equivalent to 30 police radars monitoring different points on the water surface.

Mussel farms near the shore face increasing space constraints, but building further offshore is not a matter of simply ‘beefing up’ an inshore mussel farm.

Existing large farms in areas like Golden Bay and the Firth of Thames are relatively sheltered. It is much more difficult to build farms on less sheltered coastlines.

We are working with the University of Canterbury, OCEL Consultants, and the University of New Hampshire, USA, to improve mussel farm design to cope with the rigours of waves and currents. We have adapted a computer model to show how aspects of the farm design, such as the stretchiness of the rope, or length of the longlines, reduce or enhance the effect of waves. We’re also conducting laboratory experiments and taking field measurements of such factors as water turbulence and farm structure motion.

The main findings of the research so far are these.

  • Designs can’t simply be scaled up. If a farm is hit by waves twice as big, it’ll have to withstand more than twice the amount of drag, especially if there are reasonable currents. Predicting the likely drag and its effects will help not only in farm design, but also in site selection, consideration of mussel food availability, assessment of environmental effects, and preparation of permit applications.
  • Surface long-lines can be hit directly by waves, but even submerged backbones experience significant loading in ocean swell.
  • Large farms do affect the waves, currents, and mixing of water around them. Drag from mussel farms has a strong effect on water stratification. This discovery will help us to develop more reliable estimates of phytoplankton removal and productivity, and better understand how brackish plumes from rivers spread through a farm.

The research was initiated by the Royal Society ISAT Linkages Fund, and is now an integral part of the new Sustainable Aquaculture project funded by the Foundation for Research, Science & Technology.

Research subject: Oceans