Biomass of polar cod in the Barents Sea
Last updated 5 September 2022
The polar cod (Boreogadus saida) is the only strictly arctic pelagic fish species in the Barents Sea. It is adapted to Arctic water masses, with temperatures down to the 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?
Biomass of polar cod
The figure shows the development of polar cod biomass in the Barents Sea, measured with acoustics during autumn from 1986 to 2021. There are large fluctuations in the biomass throughout the monitoring period. The most obvious explanation for the fluctuations is a combination of change in population size and change in distribution. The polar cod population has a distribution reaching further north and east than the area covered by the annual monitoring survey in the Barents Sea.
(Cite these data: Institute of Marine Research (2022). Biomass of polar cod in the Barents Sea. Environmental monitoring of Svalbard and Jan Mayen (MOSJ). URL: https://mosj.no/en/fauna/marine/polar-cod.html)
Details on these data
| Last updated | 5 September 2022 |
| Update interval | Yearly |
| Next update | May 2024 |
| Commissioning organization | Ministry of Trade, Industry and Fishery |
| Executive organization | Institute of Marine Research |
| Contact persons | Georg Skaret |
Method
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 than the survey area.
Standard methodology for biomass estimation with acoustic trawling surveys is used. Acoustic echo sounder recordings along the transects are first allocated 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.
Quality
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.
Other metadata
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 increased markedly in 2020. While the estimates in 2018 and 2019 showed the lowest levels since the measurements started in 1987, the 2020 estimate was among the highest in the time series. The estimate was completely dominated by 1-year-old fish. The amount of polar cod in the Barents Sea has shown a decreasing trend since 2010, but also in 2016 a sudden increase in biomass was observed due to large amounts of 1-year-olds. This increase quickly passed, and the survey in 2021 will show whether the big increase in 2021 is also a transient one.
Causal factors
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 (Eriksen et al. 2015; Huserbråten et al. 2019). In addition, predation pressure from cod and marine mammals has likely been high.
Consequences
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
- None
International environmental agreements
- None
Voluntary international cooperation
- None
Related monitoring
- None
Further reading
Links
Publications
- Eriksen E, Ingvaldsen RB, Nedreaas K, Prozorkevich D (2015) The effect of recent warming on polar cod and beaked redfish juveniles in the Barents Sea. Regional Studies in Marine Science 2:105-112
- Foote, K. G., Vestnes, H. P., MacLennan, D. N., og Simmonds, E. J. 1987. Calibration of acoustic instruments for fish density estimation: a practical guide. ICES Cooperative Research Report, 144: 69 s.
- Huserbråten MBO, Eriksen E, Gjøsæter H, Vikebø F (2019) Polar cod in jeopardy under the retreating Arctic sea ice. Communications Biology 2:407
- Løland, A., Aldrin, M., Ona, E., Hjellvik, V., og Holst, J. C. 2007. Estimating and decomposing total uncertainty for survey-based abundance estimates of Norwegian spring-spawning herring. ICES Journal of Marine Science, 64: 1302-1312.
- Mjanger, H., Hestenes, K., Olsen, E. M., Svendsen, B. V., Wenneck, T. d., og Aanes, S. 2007. Håndbok for prøvetaking av fisk og krepsdyr. Bergen Institute of Marine Research, 3.
- Simmonds, E. J., og MacLennan, D. N. 2005. Fisheries acoustics theory and practice, Blackwell publishing, Oxford. 437 s.
