Biomass of polar cod in the Barents Sea (Boreogadus saida)
The polar cod is the only strictly pelagic fish species in the Barents Sea. It is adapted to Arctic water masses, with temperatures down to freezing point. The cold water defines its available habitat. The amount of polar cod may give an indication of the conditions for an Arctic population in the Barents Sea.
What is being monitored?
The biomass of polar cod is monitored using acoustics and trawling during the annual Norwegian-Russian Ecosystem Survey in the Barents Sea.
The survey follows transects which are evenly distributed over the Barents Sea, but the entire distribution area of the polar cod stock is not covered, in particular the stock distribution stretches further north.
Standard methodology for biomass estimation with acoustic trawling surveys is used. Acoustic echo sounder recordings along the transects are first discriminated to different target species based on their “acoustic signatures” (i.e. echo strength, response to different frequencies), and the composition of trawl catches. The integrated acoustic values are then converted to biomass. The conversion is based on how much echo an average fish of a certain length gives.
There are several sources of uncertainty with this method, both connected to the actual measurement, detection, interpretation, converting from acoustics to biomass and the degree of coverage. A summary of such uncertainty for equivalent surveying of herring in Norwegian waters is found in Løland et al., 2007.
A fundamental prerequisite for attaining a total estimate for the stock is that the entire distribution area is covered. This is not the case for polar cod in the Barents Sea, and hence the estimate is valid only for the part of the stock present in the Barents Sea during autumn.
Echosounders are calibrated before every cruise, using standard methods described in Foote et al., 1987.
Sampling in laboratory uses methods described in Mjanger et al., 2007.
Data from the joint Russian/Norwegian ecosystem survey in the Barents Sea are available in the Institute of Marine Research’s database. Other metadata are found in the annual reports from the ecosystem survey of the Barents Sea (BESS).
Reference level and action level
There is no reference level or action limit.
Status and trend
The estimated biomass of polar cod in the Barents Sea in 2018 and 2019 has been around the lowest levels since the measurements started in 1987. The survey coverage area in both 2018 and 2019 was reduced which might partly explain the low levels, but the biomass has shown a decreasing trend since 2010, except a sudden increase and subsequent decrease in 2016-2017.
It is not known why the polar cod has shown a decreasing trend in the Barents Sea over the last decade, but an explanation might be that warming of the Barents Sea has reduced available polar cod habitat within the survey coverage area. In addition, predation pressure from cod and marine mammals has likely been high.
The Polar cod stock in the Barents Sea is a potential prey for cod, sea birds and seals. During periods with large stocks of important predators, like the cod, this may be important.
About the monitoring
Polar cod is monitored yearly as a part of the ecosystem cruise in the Barents Sea in the autumn.
Places and areas
The Barents Sea
Relations to other monitoring
- Monitoring programme
- International environmental agreements
- Voluntary international cooperation
- Related monitoring