Title Offshore finfish mariculture in the Strait of Juan de Fuca
Authers J.E. Jack Rensel, Dale A. Kiefer, John R.M. Forster, Dana L. Woodruff and Nathan R. Evans
Citation Bull. Fish. Res. Agen. supplement No. 19,113-129, 2007
Finfish mariculture has existed in the U.S. Pacific Northwest for over thirty years, but for the past 15 years most effort has focused on culture of Atlantic salmon in protected, inshore cage sites. The Strait of Juan de Fuca (the “Strait") is a large area with relatively sparse shoreline development and several apparent advantages for mariculture using offshore technology.
This study provides an overview of pertinent hydrographic conditions and possible water quality effects of marine or salmonid finfish culture in the Strait for commercial harvest or stock rehabilitation. Circulation studies, current and wave meter deployments, acoustic Doppler current profiles and phytoplankton assessments were conducted in three different regions distributed throughout the Strait near the southern, U.S. shore. Results were compared to existing inshore fish farms and analyzed with a simulation model (AquaModel) that accounts for growth and metabolic oxygen demands of caged fish and the response of phytoplankton to nutrients and grazing. An available benthic submodel was not used as current velocities throughout the water column and near the sea bottom far exceed known threshold rates for salmon farm waste resuspension. Such strong currents allow for dispersal of the organic wastes and their aerobic assimilation into the food web.
The field study results and modeling indicate no probable adverse effect of large scale fish mariculture in the Strait with regard to sedimentation or water column effects. Phytoplankton growth stimulation as a result of fish culture will not occur because nutrients do not limit microalgal growth. The area is naturally replete with dissolved inorganic nitrogen and sunlight is the primary factor limiting phytoplankton growth. Similarly, background nitrogen levels exceed half-saturation rates of seaweeds and farm plume dispersal is mostly parallel to shore in deep water so no effect on seaweeds is anticipated. Fish-killing harmful algae were rarely observed and then only in sparse numbers, although harmful Heterosigma akashiwo are known to occur throughout the waters of the Strait, Puget Sound and adjacent waters of the Pacific Ocean. Growing season phytoplankton abundance is much lower in the Strait than in nearby bays or Puget Sound.
Previously undetected and persistently lower sea surface temperatures were observed in satellite imagery for the central Strait region, especially during the summer and early fall. Surface-layer water temperature was positively correlated with dissolved oxygen concentration during the same seasons. Accordingly, there could be significantly reduced dissolved oxygen content of surface waters of the central Strait during this period, but this finding requires field verification. Eastern and western areas of the Strait may be marginally better for fish culture on this account, depending on fish species cultured.
We conclude that effects of marine fish mariculture on water quality or benthic conditions would be insignificant and that fish culture is technically feasible in the Strait. However, the high energy environment and challenging conditions will necessitate revised and novel management techniques to insure successful operations.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull19/13.pdf