Impact of Harmful Algal Blooms on Farmed Fish in Vesterålen

Recent observations along the Vesterålen coast have revealed concerning trends with harmful algal species dominating local waters. “Both species are naturally present along our coast, but in some areas in Vesterålen they have dominated in recent weeks. This has resulted in a negative/dangerous effect for farmed fish,” says researcher and algae expert Lars-Johan Naustvoll.

The Effects of Harmful Algae

As Naustvoll explains, “In general, harmful algae affect the gills of the fish and cause reduced oxygen uptake.” When these algae thrive in high concentrations, they rapidly compromise the fish’s gills, leading to suffocation due to insufficient oxygen absorption.

Salmon exhibit behavioral responses to the presence of harmful algae. “The salmon gets a clear signal that large amounts of algae are something it needs to escape from. Whether it is in the form of pain, irritation or severe discomfort is difficult to say,” adds Lars Helge Stien, a researcher focused on animal welfare.

Behavioral Changes in Affected Fish

Fish impacted by algal blooms display clear changes in behavior. Initially, they avoid the harmful algae; however, as the situation escalates, they exhibit further reactions relating to any potential damage.

“Fish that are affected by algae often have bleeding in their gills and stand apathetic at the surface. In the end, they suffocate. Fish that are less affected stand deep in the cage and are stressed. In some cases of irritation in the gills, the fish will gasp for air at the surface,” explains Naustvoll.

Species of Concern

• Phaeocystis: This species typically blooms in spring along the Northern Norway coast, appearing as both single cells and colony-forming groups, which can clog fish gills.
• Chrysochromulina: The exact mechanism by which this species affects fish gills is somewhat unclear; it may do so through biotoxins or other substances. Notably, this was the dominant species during the blooms of 2019.

Long-Term Effects on Fish

The Risk of Mortality

Algal blooms often lead to high mortality rates when the initial waves of harmful algae strike a fish farm. “In the following days, the number of harmful algae may continue to increase, but without a corresponding increase in mortality,” Naustvoll warns.

Even as the concentration of harmful algae begins to decrease, there may still be losses among farmed fish. “Fish that have been exposed to an algal bloom are somewhat weakened, and mortality often occurs in fish following the bloom due to late damage and if the fish are exposed to stress,” he adds.

Mitigation Strategies

Once a fish farm is impacted by harmful algal blooms, effective countermeasures are limited. “The measures are not very full. Most of the measures used in previous blooms are based on reducing contact between harmful algae and fish,” states Naustvoll.

The basis for these measures lies in rigorous monitoring, providing alerts about the development of algal blooms and their respective species, allowing farmers to implement appropriate responses.

Examples of mitigation strategies include:

• Using lice skirts to create a barrier.
• Setting up a ‘bubble wall’ to discourage algae contact.
• Implementing ‘overpressure’ systems to protect fish.
• Diving deeper into the water column by stopping feeding to encourage fish to stay submerged; however, this is not always effective in preventing mortality.

The Annual Cycle of Algal Blooms

Each spring, a significant algal bloom occurs along the Norwegian coast, starting in the south and moving northward as daylight increases. The dominant algal species can vary each year and by region.

“It is uncertain exactly what causes a particular species to gain the upper hand in individual blooms,” Naustvoll notes, emphasizing the complexity of the phenomenon.

In addition to the seasonal blooms, local or regional blooms can occur throughout the year. Many of these blooms consist of harmless species, such as the green water-forming calcareous algae Emiliania huxleyi, though some can still be detrimental to fish health.

“The ‘extra’ local blooms tend to arise during periods of clear, sunny weather coupled with availability of nutrients, and their size and duration can vary greatly,” he adds.

Currently, mortality rates among fish farms in Northern Norway are fluctuating chaotically, likely due to a combination of the spring bloom, the presence of harmful Chrysochromulina, and specific water conditions. “The reason for the somewhat chaotic picture is probably a combination of spring bloom, the presence of Chrysochromulina and how the water flow is right now,” Naustvoll concludes.

Ongoing Research and Monitoring

The Institute of Marine Research is actively monitoring the situation, analyzing which algal species are present and modeling potential spread patterns. Researchers urge fish farmers to report any changes in fish behavior or increased mortality rates.

For up-to-date information regarding this ongoing situation, please refer to the Institute of Marine Research.

Image: Algal blooms of Chrysochromulina are currently prevalent in areas experiencing high mortality rates among farmed fish. Credit: Institute of Marine Research