Hawaii Site Evaluation
Simulations were run near the NW coast of the big island of Hawaii where we forcasted water column and benthic effects.
Studies were run of both a single modestly-sized fish farm site and, for six slightly larger farms in the same region.
We studied potential interactions and effects on nutrients, phytoplankton and nearshore areas using the far field modeling system of AquaModel. The hypothetical farms were located relatively nearshore (1.5 to 2 km) but in deep water (~ 100m) and in areas of strong currents. The direction of water flow at each of the far field sites varied slightly, but was dominantly to the northeast and therefore onto the shore a few km away.
We found that the strong currents and modest production schedule resulted in essentially no measurable adverse effect upon the seabottom anywhere in the modeling domain. This was expected and the small amount of total organic carbon containing waste fish feces or waste feed that reached the bottom would be resuspended, moved, aerated and eventually assimilated by bottom dwelling organisms from bacteria to invertebrates and demersal fish.
The water column effects of the six farms were found mostly to be independent of each other, with the exception of two or three sites in the main bight of the Kohala coast.
Waste nitrogen excreted by the fish is rapidly moved in several directions, but we were particularly interested in transport that would impinge upon the nearby shoreline areas that are replete with coral reefs. Nutrients in the water column around coral reefs can be detrimental to reef survival by enhancement of epiphytic growth of attached benthic algae, although the subject is controversial.
Measurable amounts of nitrogen were seen to occur at the farm sites, but data “capture cells” located about one-half the distance to shore from the net pens show an order of magnitude decline of dissolved nitrogen concentration due in part to dilution and uptake of nitrogen by phytoplankton.
Nearest shore, the concentration of net pen origin nitrogen would be near zero except for brief periods, particularly when the fish biomass was approaching maximum carrying capacity. At such times the concentrations nearshore would increase from the naturally low level of about 0.1 μM to 0.2 μM but would average much less.
Larger or more numerous fish farm sites could increase the concentrations near shore and possibly initiate a problem for the coral reefs.
We used a conservative estimate of dissolved nitrogen production that included both urea and ammonia produced by the fish, as well as other, usually neglected components through our own studies of the physiology of the targeted culture fish, moi.
Study of the distribution and amount of phytoplankton resulting from the single and six fish farm operations indicated that there were more optimal and less optimal sites in terms of phytoplankton accumulation and density. Although most phytoplankton are considered beneficial and part of the base of the food web of the sea, it would be prudent to minimize nearshore production of phytoplankton in the Hawaian Islands as there are already numerous anthropogenic and natural sources of nutrient flowing into the sensitive near shore areas.
We found that effects nearshore were most pronounced from pens located inside the Kohala Coast bight, a sort of open bay that tended to reduce water flow. Effects were at times noted around the fish farm locations, but due to the lag period of nutrient production to uptake and cell division, the location of increased production due to the six fish farm operation could be some distance from the fish farm sites.
Minimal nearfield effects were recorded for Site 5 near a major headland known as Keahole Point, due to the northeasterly currents that had no immediate nearby shore in the down current direction. The simulation shows evidence that dissolved nitrogen is taken up by photosnthetic phytoplankton, as downstream there are minor plumes of phytoplankton but the concentration of nitrogen in these plumes is similar to ambient conditions.
In all cases, the production of phytoplankton at all locations was minimal, only a few tenths of a part per billion in units of chlorophyll a (a surrogate density measure of phytoplankton abundance).
This analysis indicates that some level of net pen based fish farming would be sustainable along the subject coastal area, but that there are limits or a carrying capacity where the effects would shift from non measurable or insignificant to measurable and potentially adverse. Presently, AquaModel is the only computer software in the world that is specifically designed for forecasting these effects.