Aquaculture didn’t cause the algal bloom
Published: 28.05.2019 Updated: 29.05.2019
There are fish farms in both Ofotfjorden and Astafjorden, but their emissions are too low to provide sufficient nutrients for an algal bloom of the size we are experiencing.
“The aquaculture in Ofotfjorden is less intensive than in many other Norwegian fjords. Over the past two years, there has been an average of 16 tonnes of fish/km2. There is only a risk of eutrophication when you reach 200 tonnes fish/km2”, says Vivian Husa, who does research on emissions from aquaculture.
Calculations show not many inorganic nutrients added
To calculate how much emissions from fish farms can increase the level of inorganic nutrients in a given volume of water, researchers must use a theoretical method that assumes stationary water and no inorganic nutrients being wasted. In the natural environment, on the other hand, currents mean that nutrients spread out and are significantly diluted, so the actual increase in plankton production will be much less than implied by the results of the theoretical method.
“Calculations show that the theoretical increase in phytoplankton production is 7.3% in Ofotfjorden, which is relatively low. Taking into account the effect of currents and water circulation in the fjord, we know that the real increase in phytoplankton production is far lower”, says Husa.
In Astafjorden the aquaculture intensity is somewhat higher than in Ofotfjorden, with an average of 51.5 tonnes fish/km2 in 2017-2018.
“Here too the levels were well within the range that we consider to present a low risk of eutrophication. Here the theoretical potential increase in phytoplankton production was 22.5%, but once again in practice the increase will be much lower than this”, says Husa.
Monitoring of the environmental conditions of coastal waters in areas with aquaculture of a similar intensity to Ofotfjorden and Astafjorden has found that there is no measurable increase in inorganic nutrient concentrations or phytoplankton quantities.
The international definition of eutrophication is a 50-100% increase in phytoplankton production.
May prolong, but not cause, algal blooms
“Along the Norwegian coast, there are many fjords where the amount of fish produced per unit area is much higher than in Ofotfjorden and Astafjorden, frequently in the range 50-200 tonnes fish/km2”, says Husa.
In a small number of areas, production exceeds 200 tonnes fish/km2, and in an even smaller number it approaches 600 tonnes fish/km2.
“No major harmful algal blooms have been recorded in any of these fjords with high production levels over the past 20 years”, says Husa.
If emissions of inorganic nutrients from aquaculture had been the cause of harmful algal blooms, you would expect regular blooms in fjords with highly intensive aquaculture.
“However, we cannot rule out the possibility that inorganic nutrients from fish farms help to prolong blooms once they start. Even then, we also know that fish farmers stop feeding their fish during blooms, to encourage the fish to swim down and away from the algae. This also cuts off the supply of inorganic nutrients”, she says.
What is the cause?
“Inorganic nutrients are important to phytoplankton production, but there are also other factors that play a vital role in algal blooms”, explains Lars-Johan Naustvoll, who does research on algae at the Institute of Marine Research.
The presence and quantities of harmful algae, the rate of water circulation, vertical stability, weather conditions, grazing marine animals and competition with other species are all factors that affect the ability of a harmful alga to bloom at a given point in time. It is also important to understand that different harmful algae respond differently to an increase in inorganic nutrients.
“The dominant alga in a bloom will vary. Generally it is completely harmless, like Emiliana, while other times the environmental conditions are more favourable to one of the toxic algae, resulting in a toxic algal bloom like the one we are currently experiencing”, says Naustvoll.