Pelagic fish

Photo: Bjørn Frantzen, Norwegian Polar Institute.

Pelagic fish 2019
Typography
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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 3.5.1.1).

Biomass was dominated by cod and haddock, and mostly distributed in central and northern-central parts of the Barents Sea. In 2018 and 2019, the biomass of 0-group fish was not estimated.

Total biomass

Figure 3.5.1.1. Biomass of 0-group fish species in the Barents Sea, August–October 1993–2017. Figure 3.5.1.1. Biomass of 0-group fish species in the Barents Sea, August–October 1993–2017.

Abundance estimates and mean length were calculated for the new 15 subareas (Fig. 3.5.1.2) by MatLab software for period 1980-2018 and that summarized for the entire Barents Sea. This was done due to the use of new software and new strata system (ICES 2019-WGIBAR). The «new» 0-group indices are very close to those calculated before by other software (SAS, MS Access and etc.). Abundance estimates and fish length, presented in the report, takes into account capture efficiency of the trawl (Dingsør 2005, Eriksen et al. 2009). For calculation indices in 2019 using StoX software (Johnsen et al. 2019). These indices were not checked for comparability with previous calculations. Thus, the 2019 indexes are preliminary and will be verified later.

Figure 3.5.1.2. Map showing subdivision of the Barents Sea into 15 subareas (regions) used to calculate estimates of 0-group abundance and fish length based on the BESS (more description in ICES 2018). Figure 3.5.1.2. Map showing subdivision of the Barents Sea into 15 subareas (regions) used to calculate estimates of 0-group abundance and fish length based on the BESS (more description in ICES 2018).

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 1986–2019 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 3.5.1.3). In 2019, the total biomass of pelagic fish in the Barents Sea is below 2 million tons, this is less than the long-term mean and at its lowest level over the past 23 years.

Figure 3.5.1.3. Total biomass of pelagic fish component (excluding 0-group) in the Barents Sea in 1986-2019. Figure 3.5.1.3. Total biomass of pelagic fish component (excluding 0-group) in the Barents Sea in 1986-2019.

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Capelin

Young-of-the-year

Estimated abundance of 0-group capelin varied from 2082 million individuals in 1993 to 1 251 469 million individuals in 2008 with an average of 397 222 million individuals for the 1980-2019 period (Figure 3.5.2.1). In 2019, the total abundance index for 0-group capelin was estimated in 564,080 million individuals which is above the long-term average.

Figure 3.5.2.1. 0-group capelin abundance estimates and fluctuation 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johnsen et al. 2019).    Figure 3.5.2.1. 0-group capelin abundance estimates and fluctuation 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johnsen et al. 2019).

In 2019, the spatial indices were estimated for the all main regions. The highest numbers of capelin were found in the Hopen Deep and Thor Iversen Bank area (west-central) and South East area (south-central Barents Sea). High local densities of 0-group capelin were also found to west and north of Svalbard (Spitsbergen) and near to Norwegian coast, however with relative low average numbers for these regions (figure 3.5.2.2).

Figure 3.5.2.2. Percentage of 0-group capelin abundance in the 15 regions of the Barents Sea 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018 WGIBAR), while for 2019 using StoX (Johansen et al. 2019). Figure 3.5.2.2. Percentage of 0-group capelin abundance in the 15 regions of the Barents Sea 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018 WGIBAR), while for 2019 using StoX (Johansen et al. 2019).

2019-year class dominated by fish of 4.5 - 5.5 cm length. The largest capelin (with an average close to 6 cm) were observed to the north-eastern boarder of their distribution in the Franz-Viktoria Land and North East areas, while smallest capelin (with an average length of 5.1 cm) were found close to the coast in the South West and South East areas.

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 3.5.2.3) Capelin have expanded distribution in the southeastern and northeastern direction (Eriksen et al. 2017).

Figure 3.5.2.3. 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. Figure 3.5.2.3. 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.

Adult capelin

Sampling the main area of capelin distribution during 2019 was timely and well covered. To north from Spitsbergen (81°N) capelin was distributed outside survey area, but probably insignificant amounts of fish were outside the survey area. Thus, capelin stock assessment in 2019 was well done and comparable to last year. The geographic distribution of capelin density is shown in Figure 3.5.2.4. Capelin distribution area in 2019 was significantly smaller compared to 2018, the main concentrations were found close to the coast east of Edge Island between 77° and 78°N, and on the eastern part of Great Bank at similar latitude. Very little capelin was found in the east and to the north. In all regions, capelin concentrations were significantly lower than in 2018.

Figure 3.5.2.4. Geographic distribution of capelin in 2018 (left) and 2019 (right). Circle size corresponds to SA (area back-scattering coefficient) values per nautical mile. Figure 3.5.2.4. Geographic distribution of capelin in 2018 (left) and 2019 (right). Circle size corresponds to SA (area back-scattering coefficient) values per nautical mile.

Average length of capelin in 2019 was 12.63 cm; average weight was 11.8 g, it is very close to values 2018. For age 1 capelin, no evident trends in length and weight were observed (Figure 3.5.2.5). For age 2 and 3 capelin, average fish weight and length were higher compared to 2017. In general, all biological characteristics of capelin were at the average long-term level. This is most clearly observed in age 2 fish (Figure 3.5.2.5). 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 2019 the capelin food base (zooplankton abundance and species composition) was stable.

Figure 3.5.2.5. Biological characteristics of capelin during August-September (1973-2019). Figure 3.5.2.5. Biological characteristics of capelin during August-September (1973-2019).

The total capelin stock was estimated to be approximately 0.41 million tonnes in 2019, which is significant below the long-term average (2.95 million tonnes) and represents a 75% decrease from 2018. About 73% (0.3 million tonnes) of the 2019 stock was above 14 cm in length and considered to be maturing (Figure 3.5.2.5).

Age 3 capelin (2016-year class) amount only 3.3% of total stock by number; the 2017-year class (age 2) made up 26.5% of the stock by number. The recruiting age 1 (2018 year class) was estimated at 17.46 billion individuals, this age group is dominated the stock composition by number (50%), but this is far below the long-term average value, lowest since 1995 and close to the minimum for the historical observation period. Thus, in 2019, the stock of Barents Sea capelin is at its lowest level since the period of collapse 2003-2005 (Figures 3.5.2.6, 3.5.2.7).

Figure 3.5.2.6. Capelin biomass based on 1972–2019 acoustic survey data: maturing stock biomass (MSB) and total stock biomass (TSB). Figure 3.5.2.6. Capelin biomass based on 1972–2019 acoustic survey data: maturing stock biomass (MSB) and total stock biomass (TSB).

Figure 3.5.2.7. Capelin stock age composition (age 1-4) during 1972–2019. (Note: age 5 and older was removed due to negligible numbers in the total stock). Figure 3.5.2.7. Capelin stock age composition (age 1-4) during 1972–2019. (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. Since fishing mortality for ages 1 and 2 is absent or very small, it can be assumed that total mortality for age groups is natural. Figure 3.5.2.8 shown natural mortality (M) calculated as the decrease from age 1 to age 2 in the autumn survey. In some years were a negative mortality values obtained. It is most likely the consequence of underestimation of age 1 fish in the survey.

Figure 3.5.2.8. Capelin natural mortality from age 1 to age 2, estimates based on acoustic survey data. (Negative mortality has been removed). Figure 3.5.2.8. Capelin natural mortality from age 1 to age 2, estimates based on acoustic survey data. (Negative mortality has been removed).

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 3.5.2.8). This represented a shift northward an average of 60-80 nautical miles further than observed in the 1970s. During 2018-2019, capelin stock size has decreased; the area of distribution has decreased as well (Figure 3.5.2.9). 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 3.5.2.9a. 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 3.5.2.9a. 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 3.5.2.12b. Estimated capelin biomass during August-September for recent periods of high temperature condition and cod stock size. Note that cod abundance peaked in 2013 and that temperature decreased in 2018-2019. Time periods are further broken down into sub-periods (2004-2009, 2010-2014 and 2015-2017 and 2018-2019). Biomass is presented for WMO squares. One dot is equal to 500 tonnes. Figure 3.5.2.12b. Estimated capelin biomass during August-September for recent periods of high temperature condition and cod stock size. Note that cod abundance peaked in 2013 and that temperature decreased in 2018-2019. Time periods are further broken down into sub-periods (2004-2009, 2010-2014 and 2015-2017 and 2018-2019). Biomass is presented for WMO squares. One dot is equal to 500 tonnes.

Herring

Young-of-the-year

Estimated abundance of 0-group herring varied from 93 million individuals in 1986 to 940,773 million individuals in 2004 with a long-term average of 188,586 million individuals for the 1980-2019 period (Figure 3.5.3.1). Low total abundance of 0-group herring (17,245 million individuals) was estimated in 2019, which was significantly lower than the long-term level and the lowest in last 18 years.

Figure 3.5.3.1. 0-group herring abundance estimates abundance estimates and fluctuation1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018 WGIBAR), while for 2019 using StoX (Johnsen et al. 2019).   Figure 3.5.3.1. 0-group herring abundance estimates abundance estimates and fluctuation1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018 WGIBAR), while for 2019 using StoX (Johnsen et al. 2019).

In 2019, distribution of 0-group was covered well. The spatial indices were estimated for the all regions. Herring were distributed in the central, north western and south-central Barents Sea. Very few herring were found in the southeastern part of the Barents Sea. Most of 0-group herring were found in the west-central areas: Bear island Trench and Thor Iversen Bank (Figure 3.5.3.2).

Figure 3.5.3.2. Percentage of 0-group herring abundance in the 15 regions of the Barents Sea 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johansen et al. 2019).   Figure 3.5.3.2. Percentage of 0-group herring abundance in the 15 regions of the Barents Sea 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johansen et al. 2019).

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 3.5.3.3). Hhigher densities of herring have been observed in the northwestern areas during the last decade than during the previous three decades.

Figure 3.5.3.3. 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. Figure 3.5.3.3. 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.

Herring age 1-2

During 2013–2017, abundance of young herring in the Barents Sea was relatively stable. It increased from 2017 to 2018 mainly due to contribution of the strong 2016-year class, and then decreased again in 2019. Figure 3.5.3.4 shows herring distribution in 2019 with highest amounts in the south-west and south-eastern parts of the Barents Sea. Figure 3.5.3.4 shows biomass estimates of age 1 and 2 herring combined in the Barents Sea based on the last ICES assessment for age 2+ herring, assuming M=0.9 for age 1.

Figure 3.5.3.4. Estimated distribution of herring, August-October 2019. Circle sizes correspond to SA (area back-scattering coefficient) averaged over 1 nautical mile. Figure 3.5.3.4. Estimated distribution of herring, August-October 2019. Circle sizes correspond to SA (area back-scattering coefficient) averaged over 1 nautical mile.

It should be noted that in the herring surveys in the Barents Sea in 2019 age 3 herring dominated. Neither the abundance of age 1-2 as shown in Figure 3.5.3.5 nor the abundance estimates from the surveys (June survey and BESS) carried out on young herring give a coherent picture. The estimates from the assessment depend heavily on the assumption of M and also it varies between years whether or not also age 3 herring is present in the Barents Sea. On the other hand, there are not survey data for all years and they are often inconsistent between years (e.g. the abundance of the 2016-year class was approximately the same both at age 1, 2 and 3). Thus, an analysis to determine a time series for young herring abundance in the Barents Sea, taking all data sources into account, should be given high priority.

Figure 3.5.3.5. Estimated biomass of Norwegian Spring Spawning herring Age 1 and 2 in the Barents Sea – based on Working Group on Widely Distributed Stocks (WGWIDE) VPA estimates (ICES 2019b).Figure 3.5.3.5. Estimated biomass of Norwegian Spring Spawning herring Age 1 and 2 in the Barents Sea – based on Working Group on Widely Distributed Stocks (WGWIDE) VPA estimates (ICES 2019b).

Polar cod

Polar cod is an Arctic species with a circumpolar distribution. Historically, the world’s largest population of this species has been observed in the Barents Sea. In recent years, there have been significant changes in the stock size and distribution of polar cod.

Young of the year

Estimated abundance of 0-group polar cod varied from 519 million in 1995 to 2488460 million individuals in 1994 with a long-term average of 438,152 million individuals for the 1980-2019 period (Figure 3.5.4.1). In 2019, a low abundance for 0-group polar cod was observed within the standard survey area.

Figure 3.5.4.1. 0-group polar cod abundance estimates and fluctuation1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johansen et al. 2019).   Figure 3.5.4.1. 0-group polar cod abundance estimates and fluctuation1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johansen et al. 2019).

In 2019, the distribution area of 0-group polar cod increased significantly compared to previous years. Polar cod were widely distributed with denser concentration south of the Svalbard (Spitsbergen) and south of Novaya Zemlya. In 2019, the spatial indices were estimated for the all regions. The highest abundance was observed in the Svalbard South and Svalbard North regions (Figure 3.5.4.2).

Figure 3.5.4.2. Percentage of 0-group polar cod abundance in the 15 regions of the Barents Sea 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johnsen et al. 2019).   Figure 3.5.4.2. Percentage of 0-group polar cod abundance in the 15 regions of the Barents Sea 1980-2019. Note that estimates were calculated for the new 15 subareas in the Barents Sea for the period of 1980-2018 in MatLab (ICES 2018), while for 2019 using StoX (Johnsen et al. 2019).

Since 2015, the proportion of 0-group polar cod in the southeast of the Barents Sea (Pechora region) has been decreasing (Figure 3.5.4.2.). Low abundance of 0-group cod in the traditional core area, the Pechora Sea, most likely due to redistribution of spawning sites out of the Barents Sea and into the western part of Kara Sea. This is indirectly confirmed by 2019 studies in the Kara Sea, where a significant amount of the 0-group polar cod were found.

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, in the southeastern corner of the Barents Sea (Figure 3.5.4.3.).

Figure 3.5.4.3. 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. Figure 3.5.4.3. 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.

Adult polar cod

In 2019, the area of polar cod distribution was covered partly. The BESS survey area has been regularly decreasing in recent years, especially in the north-eastern part. Probably, polar cod distributed north-east Barents Sea, outside the surveyed area, as significant concentrations were found on the border of the survey area (Figure 3.5.4.4). Main concentration of polar cod were found in the same places as previous years, around Franz Josef bank. Small, isolated concentrations were also recorded to south and to north of Svalbard (Spitsbergen). In all areas, the density of polar cod was low.

Figure 3.5.4.4. Estimated distribution of polar cod during August–October 2019. Circle size corresponds to SA (area back-scattering coefficient) values per nautical mile.  Figure 3.5.4.4. Estimated distribution of polar cod during August–October 2019. Circle size corresponds to SA (area back-scattering coefficient) values per nautical mile.

The total stock was estimated to be 87.2 thousand tonnes, which likely does not reflect the actual stock size. Thus, there is no reliable new information about polar cod stock abundance in 2019. Polar cod density in 2019 in the distribution area was slightly higher than in 2018. Anyway, the polar cod stock inside the Barents Sea border remains at a low level. From 2012 total abundance and biomass of polar cod in the Barents Sea has decreased significantly. Only one year-class of 2015 gave a short-term increase the polar cod stock in 2016 (Figure 3.5.4.5), then the stock quickly decreased again. The issue of the situation with stocks of polar cod in the Barents Sea and adjacent areas remains open. Such a decrease in the polar cod stock may be the result of increased natural mortality due to increased consumption by cod. Assuming that a significant part of the polar cod stock migrated outside the Barents Sea, why did some of the polar cod remain in the Barents Sea? It is obviously that polar cod populations of the Barents and Kara Seas are related. However, how mutual populations interchange going on is not yet clear. To solve this problem, it is necessary to carry out a complete study of the Barents Sea, especially the northeastern regions which are not covered annually.

Figure 3.5.4.5. 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 1986–2019 period. (2003 values based on VPA due to poor survey coverage. A reliable estimate is not available for 2018). Figure 3.5.4.5. 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 1986–2019 period. (2003 values based on VPA due to poor survey coverage. A reliable estimate is not available for 2018).

Blue whiting

Acoustic estimates for the proportion of the blue whiting stock present in the Barents Sea have been made since 2004. In 2017, the BESS data time-series were recalculated using a newer target strength equation (Pedersen et al., 2011), and a standardized area. The revised estimates were on average about one third of previous estimates. During 2004–2007, estimated biomass of blue whiting in the Barents Sea was >200 000 tonnes (Figure 3.5.5.1) but decreased abruptly in 2008 and remained low until 2012. In 2012 and 2013 the strong 2011-year class contributed to an observed increased abundance of blue whiting and in 2015 and 2016 the even stronger 2014-year class contributed largely to the total estimated biomasses >150 000 tons in 2015 and 2016 (Figure 3.5.5.1). With strong year classes the young blue whiting are abundant along the shelf break to the Norwegian Sea and partly distribute into the Barents Sea (Figure 3.5.5.2). In 2018 and 2019 the blue whiting abundance in the Barents Sea was very low, and dominated by older fish.

Figure 3.5.5.1. Total abundance in billions (coloured bars / left axis) and biomass in millions of tonnes (gray line / right axis) of blue whiting in the Barents Sea (BESS data revised in 2017) collected August–September during the 2004–2019 period. Figure 3.5.5.1. Total abundance in billions (coloured bars / left axis) and biomass in millions of tonnes (gray line / right axis) of blue whiting in the Barents Sea (BESS data revised in 2017) collected August–September during the 2004–2019 period.

Figure 3.5.5.2. Estimated distribution of blue whiting during August-October 2019. Circle size corresponds to SA (area back-scattering coefficient) averaged over 1 nautical mile. Figure 3.5.5.2. Estimated distribution of blue whiting during August-October 2019. Circle size corresponds to SA (area back-scattering coefficient) averaged over 1 nautical mile.

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