By Vivian Husa and Beate Hoddevik Sunnset
The production of fish for human consumption results in chemical nutrients such as dissolved phosphorous and nitrogen (ammonia) being released. We are particularly interested in the nitrogen compounds, as a shortage of them in summer normally inhibits the growth of phytoplankton. An extra supply of nitrogen at that time of year may result in unusually large phytoplankton blooms. That in return reduces light penetration into the water, and creates oxygen shortages when the plankton is decaying. Large blooms of green macroalgae floating around in mats, covering beaches and bays are other potential symptoms. If this happens, we call it overfertilization or eutrophication. International standards consider bodies of water to be eutrophicated if the biomass of phytoplankton increases by at least fifty percent. It is estimated that Norwegian salmon farms release a total of 8,500 tonnes of dissolved nitrogen into the water each year. Is that enough to result in areas of overfertilization along the Norwegian coast?
New feed types reduce emissions
Most of the salmon production takes place northwards from the county of Rogaland. Along this part of the coast, tidal waves ensures a good water exchange, which means that the chemical nutrients released by fish farms mixes with the coastal waters and are heavily diluted. In the early days of the salmon farming in Norway, the fish farms were often located in bays and sheltered areas, and some of them caused local eutrophication. At date potential locations for all facilities are subject to an environmental assessment, and with few exceptions they are located in places with good water flow. Over the past fifteen years, there has also be a gradual change in the composition of fish feed. Modern feed contains a much higher proportion of plant oils and a lower protein content than in the past. This means that nitrogen and phosphorous emissions per kilo of fish produced have fallen to almost a third of their previous level. In other words, whereas Norwegian salmon production has almost quadrupled since 1994, the increase in the emission of dissolved nitrogen has only been approximately 20 % over the same period.
Hardangerfjorden benefits from good water exchange rate
Hardangerfjorden is one of the areas in Norway with the highest density of farms producing fish for human consumption. The area has large problems with sea lice and escaped farmed salmon, but it does not appear that the nutrients released by the facilities have caused noticeable problems in the region. Two years of monitoring of the fjord has revealed that nitrogen, phosphorous and phytoplankton levels are all within the threshold of values in the coastal water which is not impacted by fish farming activity. The good water exchange in the fjord helps to maintain a good quality of the water, in spite of the release of significant quantities of nitrogen. The results from the monitoring of Hardangerfjorden shows that there is little reason to believe that nutrient emission from the current production level will lead to a general eutrophication of the coastal waters.
However, we cannot exclude the possibility that fish farms will have an local impact on macroalgae communities. Nitrogen, in the form of ammonia are released in pulses when the fish are eating, and can affect benthic algae along the shore in an area approximately five hundred metres downstream from the farm. These pulses of nutrients can stimulate growth of opportunistic filamentous algae, and compete with habitatbuilding species for nutrients and light.