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THIS IS WHY FARMED FISH DIE FROM ALGAE
This is why farmed fish die from algae. While wild fish can swim away from harmful algae, farmed fish are confined in cages, making them more vulnerable during algal blooms.
Author: Beate Hoddevik, Institute of Marine Research
In recent weeks, there have been several blooms in Northern Norway where there has been high mortality in farmed fish .
The blooms have been dominated by two algae that are known to be harmful to fish if the concentration becomes high enough: Chrysochromulina and Phaeocystis .
“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.
Harmful algae causes gill problems
If the density becomes high enough, algae can harm fish in various ways:
- Some can cause mechanical damage to the gills. These algae have ‘hard’ structures on the outside such as spikes or bristles. They can damage the gills and thus cause problems with oxygen uptake.
- Other algae can cause ‘clogging’ of the gills either because they secrete ‘mucus’ or they occur as larger “jelly” colonies.
- The last group is harmful in that they produce a toxin that has hemolytic effects, affecting the gills.
“In general, harmful algae affect the gills of the fish and cause reduced oxygen uptake,” says Naustvoll.
The fish are suffocating
When there are high concentrations of harmful algae, it will quickly affect the gills of the fish so that it is unable to absorb enough oxygen. As a result, the fish suffocate.
“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,” says Lars Helge Stien, who researches animal welfare.
When fish have been affected by potentially harmful algae, they show clear behavioral changes. In the first phase, they avoid the algae itself, and then behavioural changes related to any 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.”
How algae affect fish in Northern Norway
The two algae species that have killed fish in Vesterålen in recent weeks affect the fish in different ways:
- Phaeocystis is a typical spring species that is abundant during the annual spring bloom in Northern Norway. It can occur both as single cells and as colony-forming, the latter of which can lead to gill clogging.
- For Chrysochromulina , the details of its mode of action are somewhat unclear. It affects the gills of fish through biotoxins or other substances. It was this species that dominated the bloom in 2019.
The fish may suffer late effects
In connection with blooms, there is often high mortality when the first front of harmful algae hits a facility.
“In the following days, the number of harmful algae may continue to increase, but without a corresponding increase in mortality,” says Naustvoll.
Even if the number of harmful algae and thus the density decreases, there may be mortality among the 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,” says Naustvoll.
Get effective countermeasures
Once a fish farm is affected by a harmful algal bloom, there are few effective measures.
“The measures are not very full. Most of the measures used in previous blooms are based on reducing contact between harmful algae and fish,” says Naustvoll.
The basis for most measures is monitoring, which provides a warning of the development and what algae are in the area. Based on the species, farmers can choose measures.
Examples of measures include the use of lice skirts, setting up a ‘bubble wall’, using ‘overpressure’ in the system or by pressing down the surface layer. Several people also choose to move fish.
“What all the measures have in common is that they reduce contact between the algae and the fish in the facilities. It is also common to stop feeding so that the fish stay as deep as possible, but that alone is not necessarily enough to prevent mortality.”
Spring blooms, good weather, current conditions and local blooms
Every spring there is a major algae bloom along the Norwegian coast. This is called a spring bloom. It starts first in the south and moves north as the days get longer. Which algae dominate the bloom varies from year to year – and between different areas.
“It is uncertain exactly what causes a particular species to gain the upper hand in individual blooms,” says Naustvoll.
In addition to the annual bloom, a number of local or regional blooms occur throughout the year.
“Many of these blooms consist of species that do not affect fish, such as the calcareous algae Emiliania huxleyi which colours the water green, but some will also have species with a negative effect.
“The ‘extra’ local blooms tend to occur when the weather is nice and sunny from a blue sky. In addition, they need access to nutrients. How large they become in terms of distribution and duration varies greatly.”
During the ongoing situation in Northern Norway, mortality has moved somewhat chaotically between facilities. It is likely that there are many different local outbreaks.
“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,” says Naustvoll.
Notify of mortality
The Institute of Marine Research is closely monitoring the situation, investigating which algae are present and modelling how they might spread. The researchers are asking fish farmers to report any changes in behaviour or increased mortality .
For updated status on the situation:
- Several fish farms are affected by mortality (updated continuously as needed, news story)
- Algae status (here you will find more technical information from the algae analyses and modelling)
Image: The algae Chrysochromulina is present in large quantities in areas where there is now high mortality in farmed fish. Credit: Institute of Marine Research
