Zero-group fish are important consumers of plankton and are prey for other predators, and, therefore, are important for transfer of energy between trophic levels in the ecosystem. Estimated total biomass of 0-group fish species (cod, haddock, herring, capelin, polar cod, and redfish) varied from a low of 165 thousand tonnes in 2001 to a peak of 3.4 million tonnes in 2004 with a long-term average of 1.7 million tonnes (1993-2017) (Figure 3.5.1). Biomass was dominated by cod and haddock, and mostly distributed in central and northern-central parts of the Barents Sea.
In 2018, the biomass of 0-group fish was not estimated due to lack of spatial coverage.
Figure 3.5.1. Biomass of 0-group fish species in the Barents Sea, August–October 1993–2017.
Capelin, young herring, and polar cod constitute the bulk of pelagic fish biomass in the Barents Sea. During some years (e.g., 2004–2007 and 2015–2016), blue whiting (Micromesistius poutassou) also had relatively high biomass in the western Barents Sea (east of the continental slope). Total biomass of the main pelagic species during 1986–2017 fluctuated between 0.5 and 9 million tonnes; largely driven by fluctuations in the capelin stock. During 2017-2018, the cumulative biomass of capelin, herring, polar cod, and blue whiting was close to the long-term average (Figure 3.5.2). Data on blue whiting for 2018 are still being processed.
Figure 3.5.2 Biomass of main pelagic fish species (excluding 0-group fish) in the Barents Sea, August-October 1986–2017.
Estimated abundance of 0-group capelin varied from 952 million individuals in 1993 to 995,101 million individuals in 2008 with an average of 314,184 million individuals during the 1980-2017 period (Figure 3.5.3). In 2018, the total abundance index for 0-group capelin was not estimated due to lack of coverage.
Figure 3.5.3. 0-group capelin abundance (corrected for trawl efficiency) in the Barents Sea. Red line shows long-term mean for the 1980–2017 period; blue line indicates 0-group abundance fluctuation.
In 2018, the western Barents Sea (west of the Norwegian-Russian border) was sampled and spatial indices were estimated for eight regions (South West, Bear Island Trench, Thor Iversen Bank, Hopen Deep, Svalbard South, Svalbard North, Central Bank, and Great Bank (ICES WGIBAR 2018 Annex 4). 0-group capelin were distributed mainly in central and northcentral regions (Thor Iversen Bank, Hopen Deep, and Central Bank) (Figure 3.5.4). In these eight regions, highest abundance was observed when strong year classes occurred in 1989, 2008, 2012 – 2013, and 2016. Intermediate abundance of 0-group capelin was observed in western and central regions indicating an average year class in 2018. However, capelin usually occur in southern and eastern areas also; therefore, 0-group abundance was likely underestimated in these regions.
Figure 3.5.4. Percentage of 0-group capelin abundance in western, central, and northern regions of the Barents Sea (1980–2018). Red line shows total abundance for these eight regions.
Distribution of 0-group capelin has varied during the last four decades. The total area of distribution was smallest during the 1990s, has been largest during the current decade, and associated with the occurrence or non-occurrence of strong year classes (Figure 3.5.5) Capelin have expanded distribution in the southeastern and northeastern direction (Eriksen et al. 2017).
Figure 3.5.5. Distribution of 0-group capelin abundance in the Barents Sea during the 1980s, 1990s, 2000s, and 2010s. Abundance was log-transformed (natural logarithms) before mapping. Fish density varied from low (blue) to high (yellow). Red dots indicated sampling locations. The map is from Eriksen et al. (Progress in Oceanography, under revision).
Sampling the main area of capelin distribution during 2018 was timely and well covered; but some areas where young capelin might occur were not surveyed. This increases the uncertainty in estimating recruitment. It is believed, however, that the stock assessment was sound, and results are comparable to last year.
The geographic distribution of capelin density is shown in Figure 3.5.6. Capelin distribution in 2018 was spatially comparable to 2017, but abundance decreased in northern and northeastern regions of the Barents Sea. The main capelin schools were observed along the western edges of the Great Bank, while fewer were observed in eastern and northern regions.
Average length of capelin in 2018 was 12.7 cm; average weight was 11.4 g, approximately the same as in 2017. For age 1 capelin, increasing trends in length and weight were observed (Figure 3.5.7). For age 2 capelin, average fish weight and length were both almost the same as in 2017. For age 3 capelin, both average length and weight decreased slightly but remained above the longterm average. In general, all biological characteristics of capelin were at the average long-term level. This is most clearly observed in age 2 fish (Figure 3.5.7). Usually this age group forms a large component of total stock and reflects general trends in condition of the capelin stock.
Dynamics of changing average weight-at-age reflect capelin feeding conditions during the summer-autumn period. These conditions are determined not only by the stock size, but also by the state of the plankton community in the Barents Sea. It is evident that in 2018 the capelin food base (zooplankton abundance and species composition) was favourable.
Figure 3.5.6 Geographic distribution of capelin in 2017 (left) and 2018 (right). Circle size corresponds to SA (area back-scattering coefficient) values per nautical mile.
Figure 3.5.7 Biological characteristics of capelin during August-September (1973-2018).
The total adult capelin stock was estimated to be approximately 1.6 million tonnes in 2018, which is below the long-term average (2.9 million tonnes) and represents a 36% decrease from 2017. About 66% (1.06 million tonnes) of the 2018 stock was above 14 cm in length and considered to be maturing (Figure 3.5.8).
Age 2 capelin (2016 year class) dominated the stock composition (43%); the 2015 year class (age 3) made up 15.3% of the stock. The recruiting age 1 (2017) year class) was estimated at 58.6 billion individuals, which is below the long-term average value. However, as noted above, there is some uncertainty in the estimation of age 1+ individuals. It is likely that the actual number of age-group 1+ individuals is larger. The estimated number of older (age 4+) individuals (0.32 billion) was relatively low (Figure 3.5.9).
Figure 3.5.8. Capelin biomass based on 1972–2018 acoustic survey data: maturing stock biomass (MSB) and total stock biomass (TSB).
Figure 3.5.9. Capelin stock age composition (age 1-4) during 1972–2018. (Note: age 5 and older was removed due to negligible numbers in the total stock).
Due to significant spawning mortality, the natural mortality of capelin can be estimated indirectly only. Figure 3.5.10 shown total mortality (Z) calculated as the decrease from age 1 to age 2 in the autumn survey. Negative mortality values are most likely the consequence of underestimation of age 1 fish in the survey. Сapelin natural mortality varies significantly between years.
Figure 3.5.10. Capelin natural mortality from age 1 to age 2, estimates based on acoustic survey data. X axis shows cohorts.
Spatial distribution of capelin in the Barents Sea depends on environmental and stock conditions, primarily: position of the ice edge; distribution of zooplankton; and capelin stock size and structure. In years with a large stock, capelin is distributed widely. Juvenile capelin are distributed further south than adults. During the 1972-1979 period, the capelin stock was large and widely distributed. During 1980-1989, the stock decreased and distribution was more southward. Since the 2000s, capelin began movement north- and eastwards. During 2010-2017, the stock was in good condition and moved significantly northward into ice-free waters (Figure 3.5.11). This represented a shift northward an average of 60-80 nautical miles further than observed in the 1970s. During more recent years, capelin stock size has decreased; the area of distribution has decreased as well. In general, during periods of warming in the Barents Sea, capelin move further north and north-eastward to find feeding grounds with high plankton biomass. However, at low stock levels, capelin have adequate food availability, and temperature does not appear to be a key factor driving northward expansion.
Figure 3.5.11a. Estimated capelin biomass during August-September by decade (1970s, 1980s, 1990s, 2000s, and 2010s). Biomasses presented for World Meteorological Organization (WMO) squares system of geocodes which divide areas into latitude-longitude grids (1° latitude by 2° longitude). One dot is equal to 500 tonnes.
Figure 3.5.11b. Estimated capelin biomass during August-September for recent periods of record high temperature condition and increased cod stock size. Time periods are further broken down into sub-periods (2004-2009, 2010-2014 and 2015-2018). Biomass is presented for WMO squares. One dot is equal to 500 tonnes.
Estimated abundance of 0group herring varied from 37 million individuals in 1981 to 773,891 million individuals in 2004 with a long-term average of 163,247 million individuals for the 1980-2017 period (Figure 3.5.12). In 2018, the total abundance index for 0-group herring was not estimated due to lack of coverage.
Figure 3.5.12. 0-group herring abundance in the Barents Sea for the 1980–2017 period, corrected for trawl efficiency. Orange line shows the long-term average, while the blue line shows abundance fluctuation.
In 2018, the western Barents Sea (west of the Norwegian-Russian border was surveyed and spatial indices were estimated for eight regions (South West, Bear island Trench, Thor Iversen Bank, Hopen Deep, Svalbard South, Svalbard North, Central Bank, and Great Bank) (ICES 2018 Annex 4). 0-group herring were distributed in southwestern and central regions (Figure 3.5.13.). Abundance in these eight regions was highest when strong year classes occurred during 1996-1998 and 2004. Very low numbers of 0-group herring were observed in these areas likely indicating that a strong year class did not occur in 2018. However, herring usually occur in southern areas also, likely indicating that 0-group abundance was underestimated.
Figure 3.5.13. Percentage of 0-group herring abundance distributed in the Barents Sea (1980–2018) in the South West, Bear Island Trench, Thor Iversen Bank, Hopen Deep, Svalbard South, and Svalbard North. More detail about these spatial indices can be found in ICES WGIBAR 2018 Annex 4.
Spatial distribution of 0-group herring varied over the last four decades, was most limited during the 1980s and has increased since that time. Extent of the area occupied was assosiated with the occurance or lack of occurance of strong year classes (Figure 3.5.14.). Hhigher densities of herring have been observed in the northwestern areas during the last decade than during the previous three decades.
Figure 3.5.14. Distribution of 0-group herring abundance in the Barents Sea during 1980s, 1990s, 2000s, and 2010s. Abundance estimates were log-transformed (natural logarithms) before mapping. Fish density varied from low (blue) to high (yellow). Red dots indicate sampling locations. The map is taken from Eriksen et al. (Progress in Oceanography, under revision).
Herring age 1-2
Figure 3.5.15 shows biomass of age 1 and 2 herring in the Barents Sea, calculations are based on the last ICES assessment for age 2+ herring, assuming M=0.9 for age 1. During 2013–2017, abundance of young herring sampled during the ecosystem survey has been relatively stable, while it increased from 2017 to 2018 due to the strong 2016-year class. Biomass of young herring in 2018 was the highest since 2005, and well above the long-term average. Figure 3.5.16 shows herring distribution in 2018 with highest amounts in the southern Barents Sea, but sampling coverage was incomplete.
Figure 3.5.15. Age 1 and 2 Norwegian Spring Spawning herring biomass in the Barents Sea – based on Working Group on Widely Distributed Stocks (WGWIDE) VPA estimates (ICES 2018b).
Figure 3.5.16. Estimated distribution of herring, August-October 2018. Circle sizes corresponding to SA (area back-scattering coefficient) values per nautical mile.
Polar cod is a true Arctic species with a circumpolar distribution. Historically, the world’s largest population of this species has been observed in the Barents Sea.
Young of the year
Estimated abundance of 0-group polar cod varied from 201 million in 1995 to 2 billion individuals in 1994 with a long-term average of 317,204 million individuals for the 1980-2017 period (Figure 3.5.17). In 2018, the total abundance index for 0-group polar cod was not estimated due to lack of coverage in the main 0-group area (the southeastern Barents Sea).
Figure 3.5.17. 0-group polar cod abundance (corrected for trawl efficiency) in the Barents Sea during the 1980–2017 period. Red line shows long-term average, while the blue line indicates 0-group abundance fluctuation.
In 2018, the western Barents Sea (west of the Norwegian-Russian border) was covered and the spatial indices were estimated for eight regions (South West, Bear Island Trench, Thor Iversen Bank, Hopen Deep, Svalbard South, Svalbard North, Central Bank, and Great Bank) (ICES 2018 Annex 4). Within these eight regions, highest abundance was observed during 1990-1991 and 2000-2002. 0-group polar cod were distributed mainly in the Svalbard South and Svalbard North regions (Figure 3.5.18.). Abundance of polar cod in these two regions is not associated with occurrence of strong year classes as the most important 0-group area is situated in the southeastern Barents Sea (Eriksen et al. 2017). In 2018, abundance of 0-group polar cod was low, as observed in previous years.
Figure 3.5.18. Percentage of 0-group polar cod abundance in South West, Bear Island Trench, Thor Iversen Bank, Hopen Deep, Svalbard South, and Svalbard North regions of the Barents Sea during the 1980–2018 period. More detail about these spatial indices can be found in ICES 2018 Annex 4.
The distribution of 0-group polar cod varied over the last four decades, and was largest during the 1990s and 2000s. Size of area occupied was associated with the occurrence or non-occurrence of strong year classes from the Pechora Sea, southeast of the Barents Sea (Figure 3.5.19.).
Figure 3.5.19. Distribution of 0-group polar cod abundance in the Barents Sea during the 1980s, 1990s, 2000s, and 2010s. Abundance estimates were log-transformed (natural logarithms) before mapping. Fish density varied from low (blue) to high (yellow). Red dots indicated sampling locations. The map is from Eriksen et al (Progress in Oceanography, under revision).
Adult polar cod
In 2018, the area of polar cod distribution was not well covered. The total stock was estimated to be 46 thousand tonnes, which likely does not reflect the actual stock size. Thus, there is no new information about polar cod stock in 2018. Polar cod density in 2018 in the same area was lower than in 2017. Thus, it can be assumed that the polar cod stock in the Barents Sea remains at a low level. In 2017, estimated total abundance and biomass of polar cod in the Barents Sea decreased significantly. Estimated total-stock biomass (TSB) was only 357 thousand tonnes; approximately 38% of the 2016 estimate. Total stock number (TSN) was only about 23% of the 2016 estimate. The 2015-year class decreased from an estimated 95 billion in 2016 to 8.27 billion individuals in 2017 (Figure 3.5.20). Such a decrease in the polar cod stock may be the result of increased natural mortality due to consumption by cod and other predators; it may also be due to a significant portion of the polar cod stock having been distributed outside the survey area (Figure 3.5.21).
Figure 3.5.20. Total abundance in billions (coloured bars / left axis) and biomass in millions of tonnes (green line / right axis) of polar cod in the Barents Sea (acoustic survey and BESS data) collected August-September during the 1986–2017 period. (2003 values based on VPA due to poor survey coverage. A reliable estimate is not available for 2018).
Figure 3.5.21. Estimated distribution of polar cod during August–October 2018. Circle size corresponds to SA (area back-scattering coefficient) values per nautical mile.
Acoustic estimates for the Barents Sea blue whiting stock have been made since 2004. In 2017, the BESS data time-series was recalculated using a newer target strength equation (Pedersen et al., 2011), and a standardized area; this resulted in an overall reduction in estimated biomass to about one third of previous estimates. During 2004–2007, estimated biomass of blue whiting in the Barents Sea was >200 000 tonnes (Figure 3.5.14) but decreased abruptly in 2008 and remained low until 2012; after which time it has been variable. In 2017, blue whiting biomass was estimated at about 115 000 tonnes; a decrease from 2016 (Figure 3.5.22). Blue whiting migrate from the Norwegian Sea into deeper parts of the Barents Sea (Figure 3.5.23) when the stock is large and sea temperatures are high. Survey coverage for blue whiting during BESS 2018 was complete, but final estimates are not available yet.
Figure 3.5.22. Total abundance in billions (coloured bars / left axis) and biomass in millions of tonnes (green line / right axis) of blue whiting in the Barents Sea (BESS data revised in 2017) collected August–September during the 2004–2017 period. (The 2018 estimate is being updated).
Figure 3.5.23. Estimated distribution of blue whiting during August-October 2018. Circle size corresponds to SA (area back-scattering coefficient) values per nautical mile.