TR 2008/37

J. W. Oldman (NIWA)

Abstract

Environment Waikato is currently scoping a plan change to allow for the diversification of aquaculture within existing aquaculture management areas in the Region. This plan change may allow for the cultivation of species other than mussels, including finfish. The biggest aquaculture management area in the Region is the Wilson Bay Marine Farming Zone located in the Firth of Thames. Currently, Area A of the Wilson Bay Zone is consented for 470 ha of mussel longlines, and Area B of the Zone, once developed, will comprise an additional 520 ha. In addition to this, 220 ha of older smaller sized farms exist within Wilson Bay. 

To provide some background to assist in scoping an aquaculture diversification plan change, this report presents estimates of the scale of expected benthic effects that maybe associated with an individual cage within a fish farm in the Wilson Bay Marine Farming Zone. These estimates are based on the dispersal of waste material (i.e., feed pellets or fish faeces) due to measured currents, bathymetry, a range of realistic fall velocities, best estimates of cage dimensions (cage area and depth) and a range of depths below the cage that could be achieved within Area A.

Data presented in this report show that less than 11% of the waste material from an individual cage may end up being deposited directly beneath a cage. As cages are placed in deeper water, less farm waste will be deposited directly beneath the cage. If cages are placed in an area of higher flows, less waste material will be deposited directly beneath the cage, and if farm waste material has low fall velocity, less waste material will be deposited directly beneath the cage.

Results presented in this report give the likely level of deposition (g/m2/day) for a hypothetical waste load of 1000 g/day from an individual cage. Data from the modelling exercise has shown that in the lateral direction (i.e., in the cross shore direction) there is unlikely to be any cumulative effects between individual cages if cage are placed at least 100 m apart. The modelling has shown that, in the long shore direction, waste material from a cage can be dispersed up to 700 m away from the cage. However, based on a threshold of measurable effects, the longitudinal footprint of an individual cage within a farm will be much smaller than this and may range between 100 and 200 m. Once the actual cage waste loading (based on cage dimensions, stocking rate and feed conversion ratios) is known, it will be possible to determine the actual extent of the cage footprint using the results presented in this report. Once data on the baseline sediment conditions are known the extent of the measurable effect or benthic footprint can be determined.

Seafloor Footprint Estimates of Waste Deposition from Potential Finfish Farms in the Wilson Bay area of the Firth of Thames [PDF, 423 KB]