Hydrodynamic implications of large offshore mussel farms

Around 3000 Ha of New Zealand's coastal waters are presently allocated to mussel farming of the Greenshell mussel Perna canaliculus mainly in small inshore farms of 30 000 to 80 000 m/sup 2/ each. The farms are constructed from blocks of parallel moored long-lines, with loops of spat-impregnated line hanging vertically in the upper 10-15 m of the water column (provided sufficient water depth). When fully laden, the mussel loops can present a rough diameter of 0.20 m or more to the flow. Industry expansion is likely to be in the form of larger offshore farms. Some of the larger proposed farms will have more than 1000 km of mussel line. This presents a potentially significant drag to currents and waves. In this paper, observations are used from one of the first large offshore farms constructed in New Zealand, in conjunction with scaling estimates of energy loss, to investigate the effect of the farm on waves, currents, and stratification. The farm, consisting of 230 long-lines arranged into 20 blocks, measured 650 m /spl times/ 2450 m. The hanging mussel loops at this site reached an average depth of 8 m, and water depth at midtide varied across the site from 10-12 m. Transects were taken through the farm using both conductivity, temperature, depth (CTD), and Acoustic Doppler Profiler (ADP) instruments. A second ADP was moored inside the farm providing velocity time series. The average current was reduced within the farm by 36%-63%. The moored ADP detected an undercurrent beneath the farm with velocities nearly twice that within the farm. Sharp changes in stratification coincide with the start of fully stocked mussel long-lines. Furthermore, other than at the very upstream end of the farm, stratification appears to have prevented significant vertical mixing between the fluid within the farm and that flowing under the farm. Wave energy was low during the 7-d deployment, with significant wave heights peaking at 0.25 m. The attenuation of transitional waves through the farm is examined by comparing simultaneous pressure sensor measurements from offshore and inshore of the farm. The observations show that wave energy attenuation was frequency dependent. Observed wave energy attenuation across the 650 m wide farm was approximately 5%, 10%, and 17% at wave frequencies of 0.1, 0.2, and 0.25 Hz, respectively.