Fish investigations in the Barents Sea Winter 2023

Annual catch quotas and other regulations of the Barents Sea fisheries are set through negotiations between Norway and Russia. Assessment of the state of the stocks and quota advice are given by the International Council for the Exploration of the Sea (ICES). Their work is based on survey results and international landings statistics. The results from the demersal fish winter surveys in the Barents Sea are an important source of information for the annual stock assessment.

The development of the survey started in the early 1970s and focused on acoustic measurements of cod and haddock. Since 1981 it has been designed to produce both acoustic and swept area estimates of fish abundance. Some development has taken place since then, both in area coverage and in methodology. The development is described in detail by Jakobsen et al . (1997), Johannesen et al . (2009) and in Appendix 3, and the current survey design and methods for survey index calculation are presented in Appendix 2. The survey manual is available at the internal IMR quality portal here. At present the survey provides the main data input for several ongoing projects at the Institute of Marine Research, Bergen:

• monitoring abundance of the Barents Sea demersal fish stocks

• mapping fish distribution in relation to climate and prey abundance

• monitoring food consumption and growth

• estimating predation mortality caused by cod

This report presents the main results from the surveys in January-March 2023. The surveys were performed with the Norwegian research vessels “Kronprins Haakon” and “Johan Hjort”, and the Russian research vessel “Vilnyus”. Annual survey reports since 1981 are listed in Appendix 5, and names of scientific participants in 2023 are given in Appendix 4.

Table 1.1 presents the vessels participating in the survey in 2023 and IMR trawl station series numbers, and Figure 1.1 shows survey tracks, trawl stations and ice cover.

The coverage of the most northern and most eastern strata differs from year to year. The areas of these strata are therefore calculated according to the coverage each year. Table A1.1 gives the area covered by the survey every year since 1981. In that table “Extrapolated area” reflects the size of areas where some kind of extrapolations/adjustments have been made to take account of incomplete coverage (see also section 3.1). Table 1.3 summarizes the degree of coverage and main reasons for incomplete coverage in the whole period.

Individual lengths are collected from all target species, while otoliths for age determination are taken from cod, haddock, and capelin. For cod and haddock, the otolith readings are key for splitting the survey indices by age.

Table A2.1 gives an account of the sampled length- and age material from bottom hauls and pelagic hauls from 1994 onwards.

Table A2.2. shows the number of age readings per age for cod from 1994 onwards, while table A2.3 shows the same for haddock. The number of age samples for fish age 10+ increased in the second half of the time series, reflecting changing age composition in the stocks.

Details on the calculation of survey indices, including StoX settings for different species are found in Appendix 2.

In 2023, the swept area and acoustic¹ estimation in StoX was based on the following biotic and acoustic snapshot files (versioned trawl and acoustic data):

¹ Acoustic estimation is done for cod and haddock only. The biotic files are used in the acoustic StoX projects to split the acoustic backscatter by age.

3.1 Raising of indices

In 1997, 1998 and 2007, only the Norwegian EEZ (NEZ) and parts of the Svalbard area (S) was covered. The swept-area indices for cod, haddock, and Greenland halibut have therefore been raised to also represent the Russian EEZ (REZ) (Mehl et al . 2016).

In 2006, there was not complete coverage in the southeast due to restrictions. The observations in the partially covered strata 7 were extrapolated to the full strata, and the observations in the partially covered strata 13 were extrapolated to the same area as covered in 2005.

In 2012 the coverage was incomplete in the eastern areas, and the cod and haddock swept area estimates within the covered area were raised by the “index ratio by age” observed for the same area in 2008-2011 (ICES 2012). The scaling factor (“index ratio”) for estimating adjusted total from <Total – area D’> was the average ratio by age for Total/(Total – area D’) in the years 2008-2011 (Aglen et al. 2012).

In 2017, the Norwegian vessel was not allowed to operate south of 70º 10’ N and west of 41º 00 º E, and no Russian vessel participated in the survey. Only a small part of strata 7 was covered, and strata 13, 15, 17 and 20 were not covered. The cod, haddock, and Greenland halibut swept area estimates and cod and haddock acoustic estimates within the covered area were raised following the same procedure as for 2012. The scaling factor for estimating adjusted total from <Total – strata 7 > was the average ratio by age for Total/(Total – (strata 7+13+15+17+20)) swept area indices in the years 2014-2016.

In 2020, coverage was incomplete in strata 17, 19, and 20, and the cod and haddock acoustic and swept area estimates were raised by the “index ratio by age” observed for these strata in 2018-2019. The scaling factor for estimating adjusted total from <Total –strata 17, 19 and 20> was the average ratio by age for Total/(Total – (strata 17+19+20)) in the years 2018-2019.

In 2021, coverage was incomplete in strata 16, 19, and 20. Indices in the partly covered stratum 19 were extrapolated to the entire strata. No trawling was done in stratum 20. As cod and haddock abundances generally are low there, the stratum was partly ice covered and did not have coverage in the last two years, this stratum was excluded from estimation. Only one trawl station was taken in stratum 16. Here the cod and haddock acoustic and swept area estimates were raised by the “index ratio by age” observed for these strata in 2019-2020. The scaling factor for estimating adjusted total from <Total – strata 16> was the average ratio by age for Total/(Total – strata 16) in the years 2019-2020.

The three redfish survey indices were revised in 2022, and no adjustments have been made to the new indices.

In 2023, coverage was incomplete in strata 16, 17, and 20. Coverage was also reduced in strata 9, 13-15, and 24-26, but taken as representative. The main parts of the cod and haddock distributions were, nevertheless, well covered. Given historically low abundances of cod and haddock in stratum 20, this stratum was excluded from the estimation as in previous years. Stratum 16 had only two trawl stations, but given low abundances this year and, historically, they were taken as representative and included in the estimation procedure. Only the southeastern part of stratum 17 was covered. This area has a low abundance of haddock. Therefore, no adjustment was necessary in the haddock indices. For cod, the area of stratum 17 was adjusted to match the 300 m isobath in order to avoid inflating catches in the southwest, which have historically been higher than in the rest of the stratum.

Table 4.1 presents the time series of total echo abundance (mean s_A multiplied by strata area and summed over all strata) of cod and haddock in the investigated areas.

The lowest echo abundances of cod were recorded in the late 1990s, 2004-2007, and in the last few years of the time series (2021-2023), while the highest values were seen in 1994 and 2013-2015. The very low value in 2007 likely reflects the lack of coverage of the Russian zone and is not directly comparable to the others, making 2023 the lowest observed echo abundance in the time series, reflecting the current downwards trend in the stock.

The trend for haddock is similar, but without the dip in 2004-2007 and with peak values five years earlier than cod (2008-2010). The sharp reduction in echo abundance between 2020 and 2021 were seen for both species, but while cod echo abundance dropped from 2022 to 2023, haddock echo abundance remained at similar levels.

¹ not scaled for uncovered areas

For both the bottom trawl and acoustic estimates as well as the diet data, cod with all otolith types (coastal cod included) are included in the calculations.

5.1 Acoustic estimation

Surveys in the Barents Sea at this time of the year mainly cover the immature part of the cod stock. Most of the mature cod (age 7 and older) have started on their spawning migration southwards out of the investigated area and are therefore to a lesser extent covered. There are indications that a higher proportion than normal spawned along Finnmark in some years, e.g., 2004-2006. Thereby, a higher proportion of spawners might have been covered by the survey in those years. Figure 5.1 shows the spatial distribution of acoustic registrations assigned to cod in 2023. The registrations reflect the general distribution of cod in the central and southwestern Barents Sea. The NASC values in 2023 were low, reflecting the overall low echo abundance.

Table A5.1 shows the acoustic indices for each age group by main areas in 2023. 73% of the 1-year olds were found in the extended area (N) in 2023 compared to 26 % in 2022. Age 1 also had the highest percentage in area N of all age groups. The time series of total abundance at age (1994-2023) is presented in Table A5.2 and Figure 5.2.

The acoustic estimates have been variable and increasing in later years, with a peak in biomass in 2013, and this may partly be explained by variable and not complete coverage of the distribution area towards north and east in several years. As cod grow older it gets a more south-westerly distribution during winter, so that it “grows into” the covered area with increasing age. This is especially evident for the strong 2004 and 2005 year classes, which as 6-11-year-olds stand out as the strongest in the time series. The 2019-2020 year classes were among the lowest in the time series at age 1-3 while the 2021 and 2022 year classes were moderate at age 1/2. Table A5.3 shows time series for strata 24-26 (area N) in 2014-2023, which are included in the main time series.

Table A5.4 presents estimated coefficients of variation (CV) for cod age groups 1-14 in 1994-2023. These estimates were obtained by using StoX with a stratified bootstrap routine treating each transect as the primary sampling unit. In addition, a bootstrap routine for all trawl stations by strata was carried out within each run. The estimated CV (Standard Deviation ∙ 100/mean) is estimated from 500 iterations . A CV of 20% or less could be viewed as acceptable in a traditional stock assessment approach if the indices are unbiased (conditional on a catchability model). In 2023 the age groups 2-9 fall into this category. Values above this indicate higher uncertainty of the estimated index, with reduced information regarding year class strength. In all years, CVs for age groups older than 10 years are above what could be considered as acceptable. This is to a large degree related to low catch rates resulting in fewer age samples for these age groups (Table A2.2).

5.2. Swept area estimation

Figures 5.3 - 5.6 show the geographic distribution of bottom trawl catch rates (number of fish per NM² ), for cod size groups < 20 cm, 20-34 cm, 35-49 cm and ≥ 50 cm. Usually, a high proportion of the smallest cod (less than 35 cm) are found in the eastern part of the survey area within the Russian EEZ and in the northern part of the strata system. While this general pattern was still there in 2023, cod abundance in the southeastern Barents Sea was low for all size groups (Fig. 5.3-5.6). The highest catch rates of large cod (≥ 50 cm) were found along the Norwegian coast, around Bear Island and to the west of Svalbard (Fig. 5.6).

Table A5.2 presents abundance indices by main areas and age, and the full time series 1994-2023 is shown in Table A5.5 and Figure 5.7. The bottom trawl indices have fluctuated somewhat for the same reasons as the acoustic indices, and the 2004 and 2005 year-classes stand out as the strongest in the time series. The 2009, 2011 and 2014 year-classes seemed to be strong as 1-year olds, but have later been reduced to average level or below. The year classes 2017 and 2018 also seemed strong at age one, but are more average as 2- and 3-year-olds. The 2019-2020 year classes were among the lowest in the time series both at age 1 and 2 while the 2021 and 2022 year classes were moderate at age 1. 70% of the 1-year olds were found in the extended area (N) in 2023 compared to 24 % in 2022. Age 1 also had the highest percentage in area N of all age groups (Table A5.6). This year, there was hardly any coverage northeast of the extended area, i.e., north of Svalbard outside of the survey stratification, where fair amounts of cod have been observed in previous years.

Table A5.7 presents estimated coefficients of variation (CV) for cod age groups 1-15 in 1994-2023. In 2023, age groups 2-10 have CVs below or equal to 20 %. Values above this indicate higher uncertainty of the estimated index, with reduced information regarding year class strength. In all years, CVs for age groups older than 10 years are above what could be considered as acceptable. This is to a large degree related to low catch rates resulting in fewer age samples for these age groups (Table A2.2).

5.3 Survey mortalities

Table A5.9 and Figure 5.8a-b show the time series of survey-based mortalities (natural log ratios between survey indices of the same year class in two successive years) for the acoustic and swept area indices since 1994. These mortalities are influenced by natural and fishing mortality, age reading errors, and the catchability and availability (coverage) at age for the survey. In the period 1994-1999 there was an increasing trend in the survey mortalities. Most later surveys show lower mortalities, but there are some fluctuations for the same reasons as mentioned for the acoustic and swept area indices. Presumably the mortality of the youngest age groups (ages 1-3) is mainly caused by predation, while for the older age groups the fishery is the main cause. Although the survey mortalities are noisy, the mortalities for age 4 and older correspond well with the strong decrease in fishing mortality around 2007 in the stock assessment. The low survey mortalities in the 2010s, even with “impossible” negative values, could partly be caused by fish gradually “growing into” the covered area at increasing age. 2019-2020 and 2020-2021 estimates suggest higher survey mortalities than in previous years, while mortality decreased for most age groups in 2021-2022 and increased again in 2022-2023.

5.4 Growth and maturity

Tables A5.10-11 and figure 5.9-5.10 present the time series for mean length and mean weight at age. There have previously only been moderate fluctuations, but with a decreasing trend for older fish (8+) in later years. However, in 2020-2021, both length and weight at age was considerably reduced for several age groups, with length at age 4 and 5 and weight at age 4, 5, 6 and 8 in 2021 being the lowest observed in the time series. Growth improved somewhat in 2022 and 2023 (Table A5.12 and Fig 5.11). In 2023, length and weight at age was above the long-term average for ages 1-4, but below for ages 5-8.

The proportion mature at age is presented in Table A5.13 and Fig 5.12. The proportion increased from 2022 to 2023 and was in 2023 above the long-term average for ages 7-9, but below for age 6 and 10. Since 2010, the proportion mature at ages 6-8 has declined, but has in recent years stabilized.

The degree of coverage of the Russian zone (REZ) may also influence the biological parameters, as body size tends to decrease towards the northeast in the survey area. In addition, length, weight and maturity at age of older ages has higher uncertainty due to fewer samples (c.f. table A2.2).

5.5 Stomach sampling

Since 1984, cod stomachs have been sampled regularly during the winter survey. The sampling strategy has generally been the same as that for sampling otoliths. Stomach have been frozen on board and analysed in the laboratory, except for the period 1994-2000, when some of the stomachs were analysed on board and only the main prey categories were identified. For details about the sampling methodology and the Norwegian-Russian cooperation on diet investigations in the Barents Sea, see Mehl and Yaragina (1992) and Dolgov et al . (2007).

The number of stations and stomachs sampled as well as the proportion of empty stomachs and the mean stomach fullness index (SFI, see below) for each of four size groups (≤ 19 cm, 20-34 cm, 35-49 cm, ≥ 50 cm) is given in Table A5.14 and Fig. 5.13. Tables A5.15 - A5.18 and Figs. 5.14-5.17 show the mean stomach content composition by prey species/groups by year for each size group. Note that in the years 1994-2000, blue whiting, long rough dab and Norway pout were included in the category ‘other fish’ when stomachs were analysed on board.

The stomach fullness index is calculated as SFI_i=100*ΣWS_i/W_i, where WS_i is the weight (g) of the stomach of fish i, and W_i is the weight (g) of fish i . For 1987 SFI has not been calculated, because very few fish were weighed that year due to technical problems. The distribution on prey groups has been adjusted by distributing the unidentified component of the diet proportionally among the various components, taking into account the level of identification.

The proportion of empty stomachs is largest for the smallest fish (Table A5.14), a pattern seen for all years. The stomach fullness in 2022 was higher than in 2021 for all length groups except cod ≥ 50 cm. Capelin is the dominating prey for cod ≥ 20 cm, followed by shrimp (Tables A5.16-A5.18), while krill dominates for the smallest cod (Table A5.15). However, in many years capelin is also an important prey for the smallest cod. The proportion of capelin in the diet increased from 2021 to 2022 for all cod size groups.

6.1 Acoustic estimation

The survey covers best the immature part of the haddock stock. At this time of the year an unknown proportion of the mature haddock (age 6 and older) is on its spawning migration south-westwards out of the investigated area. In some earlier years, e.g., 2004 and 2005, concentrations of mature haddock have been observed pelagically rather far above bottom along the shelf edge. The bottom trawl sampling poorly covers these concentrations. There are indications that the distribution of age groups 1 and 2 in some years are concentrated in coastal areas not well covered by the survey. This occurred in the late 1990s and will have strongest effect on estimates of abundance of the poor year-classes. In the later surveys, small haddock have been widely distributed, and the strong year-classes have been found unusually far to the north. Favourably hydrographic conditions and/or density dependent mechanisms might cause this. However, it is difficult to separate the two factors.

Figure 6.1 shows the spatial distribution of acoustic registrations assigned to haddock in 2023. The registrations reflect the general distribution of haddock in the southern and eastern Barents Sea. The overall echo abundance in 2023 was higher than the very low registrations in 2021, but lower than in 2022.

The acoustic abundance indices by age and the main areas in 2023 are presented in Table A 6.1. As in most of the previous years the highest abundance was observed in main area D. The full time series is presented in Table A 6.2 and Figure 6.2. Abundance of age 2 in 2023 increased compared to 2021 and 2022 when it was very low. This is reflected in low abundances of ages 3 and 4 in 2023. Abundance of age 7 (year-class 2016) in 2023 was quite high, while abundance of older fish (age 8+) was low.

The strong 2004-2006 year-classes can be followed through the time series. In later years, the 2009-, 2011-, and 2013-2018-year classes, seem to be fairly strong. In particular, the year classes 2016 and 2017 have high indices at age 1-2. The year class 2019 appears to be much weaker as the abundance of 1-year-olds observed in 2020 is the third lowest in the time series, and the weakest in the time series at ages 2, 3, and 4 year. Abundance of the 2020 year-class, while still low, is still almost 7 times higher than the 2019 year-class as 3 year olds. The 2021 year-class is much stronger and above average in the time series. The 2022 year-class appears a little bit weaker than average.

Table A 6.3 shows indices for strata 24-26 (main area N), which are also included in the full time series (Table A 6.2). The contribution from main area N was rather low in all years, except in 2018 when 29% of age 1 haddock (by number) was found in the extended area, constituting 13% of the biomass. The total abundance in area N in 2023 is 7% by number and 2% by biomass.

Table A 6.4 presents estimated coefficients of variation (CV) for haddock age groups 1-14. In most years, CVs for age groups older than 7 years are above what could be considered as acceptable (approximately 20 %).

6.2. Swept area estimation

Figures 6.3 - 6.6 show the geographic distribution of bottom trawl catch rates (number of fish per NM ² ) for haddock size groups < 20 cm, 20-34 cm, 35-49 cm and ≥ 50 cm. Like in previous years, the distribution extends further to the north and to the east than what was usual in the 1990s.

Table A 6.5 presents the indices for each age group by main areas. Compared to cod a lower proportion of haddock is found in the extended survey area (Table A 6.5). In 2023, the extended area contributed about 6 % of total abundance and about 2 % of total biomass.

The full time series is shown in Table A 6.6 and Figure 6.7. The swept area estimates, too, are highest in the east in area D. The weak 2019 year-class noted for the acoustic index is evident also in the swept area estimates. Overall, this survey tracks both strong and poor year-classes fairly well.

Table A 6.8 presents estimated coefficients of variation (CV) for haddock age groups 1-14. In most years, CVs for age groups older than 7 years are above what could be considered as acceptable (approximately 20 %) .

6.3 Survey mortalities

Survey mortalities based on the acoustic and swept area indices (Table A 6.9, Figure 6.8) have varied between years, and for most age groups there are no obvious trends. However, there are signs of co-variability within years.

6.4 Growth and maturity

Tables A 6.10 and Figure 6.9 present the time series for mean length. Table A 6.11 Figure 6.10 present mean weight at age. Length and weight estimates have been variable with no specific trends in the latest years.

Annual weight increments are shown in Table A 6.12, and Figure 6.11, these are highly variable and show no trends.

The proportion mature at age also shows large variations between years (Table A 6.13, Figure 6.12).

The large variation is one of the reasons that length, weight and maturity at age are modelled from the empirical data in the haddock stock assessment to account for inconsistencies due to high sampling variance and to fill in missing age-year combinations. The assessment input data for these variables may therefore differ from what presented here. The degree of coverage of the Russian Exclusive economic zone (EEZ) may influence the biological parameters, as body size tends to decrease towards the northeast in the survey area. In addition, length, weight and maturity at age of older ages has higher uncertainty due to fewer samples.

Earlier reports from this survey have presented distribution maps and abundance indices based on acoustic observations of redfish. In later years, blue whiting has dominated the acoustic records in some of the main redfish areas. Due to incomplete pelagic trawl sampling the splitting of acoustic records between blue whiting and redfish has been very uncertain. The uncertainty relates mainly to the redfish, since it only makes up a minor proportion of the total value. This has been the case since the 2003 survey, and the acoustic results for redfish are therefore not included in the reports.

7.1 Golden redfish (Sebastes norvegicus)

Figure 7.1 shows the geographical distribution of golden redfish based on the catch rates in bottom trawl. In most years, the distribution is completely covered except towards the northwest. Figure 7.2 and Table A7.1 presents the time series (1994-2023) of swept area indices by 5 cm length groups for the standard area (strata 1-23). The indices were low in many years since 1999 for all length groups. However, in 2016 and 2017 there was an increase in the indices of fish above 25 cm, and in 2018 the total index was at the same level as in 2017, while the total biomass was slightly lower. In 2019 the indices for fish between 35 and 50 cm increased further, and the total abundance and biomass were the highest since 1998. The index for most length groups declined in 2020 and further in 2021 when the abundance of fish < 20 cm was particularly low. However, the 2021 year class appears to have been strong as the number of <10 cm fish was the highest in the series, and the total abundance was the highest since 1998. Table A7.2 present swept area abundance indices by length groups for area N in 2014-2023. Golden redfish was found in this extended survey area in 2014-2023, mainly west of Spitsbergen (strata 24). 17% of the total abundance and 5 % of total biomass was found in the extended area in 2023. Table A7.3 presents estimates of coefficients of variation (%) by length groups. In all years, CVs for most length groups are above what could be considered as acceptable in stock assessment (approximately 20 %).

7.2 Beaked redfish (Sebastes mentella)

Figure 7.3 shows the geographical distribution of beaked redfish based on the catch rates in bottom trawl. Figure 7.4 and Table A7.4 presents the time series (1994-2023) of swept area abundance indices by 5 cm length group for beaked redfish in the standard area (strata 1-23), while Table A7.5 present indices for new strata 24-26 in 2014-2023.

In 2015 and 2016, the estimated indices for 20-39 cm beaked redfish were among the highest in the time series, and in 2017 the indices for 30-39 cm beaked redfish were the highest in the time series, as were the total index and total biomass. The indices for most length groups decreased somewhat from 2017 to 2018 and remained at about the same level in 2019 and 2020 before increasing in 2021. However, the 2020, year class, appears to have been strong as the 2021 estimate of fish < 10 cm and 2022-2023 estimate of 10-15 cm fish were the highest in the time series. The coverage of the beaked redfish distribution was not complete west and north of Spitsbergen (Fig. 7.3). The extended survey area in 2023 contributed about 10% of the total abundance index, compared to around 3 % in 2019 and 2020 and 10 and 9% in 2021 and 2022.

Table A7.6 presents estimates of coefficients of variation (%) by length groups. In most years, CVs for length groups between 10 and 29 cm are at a level that could be considered as acceptable for stock assessment, and in most recent years up to 44 cm.

7.3 Norway redfish (Sebastes viviparus)

Figure 7.5 shows the geographical distribution of Norway redfish. Figure 7.6 and Table A7.7 presents the time series (1994-2023) of swept area indices by 5 cm length groups in the standard area (strata 1-23). Almost all Norway redfish are found in areas ABCD, mainly in main area B, and almost nothing in the extended survey area (Table A7.8). In 2021, the smallest fish (< 10 cm) were found in the extended survey area for the first time and then again in 2022 as the < 15 cm fish. In 2023, fish between 10 and 25 cm were found in the extended survey area.

A few large catches often drive the indices for Norway redfish. There was a large and unexplained increase in the indices of most length groups from 2013 to 2015 to among the highest levels in the time series. Apart from a dip in 2016, the total abundance has remained relatively high since then. The total abundance increased with nearly 50 % in 2021 to the highest observed since 1994, driven by high abundance of 15-30 cm fish. In 2022 and 2023, the abundance of <10 cm increased significantly.

Table A7.9 presents estimates of coefficients of variation (%) by length groups. In most years, CVs for most length groups are far above what could be considered as acceptable for stock assessment.

Figure 8.1 shows the distribution of bottom trawl catch rates of Greenland halibut. The most important distribution areas for the adult fish (depths between 500 and 1000 m along the western slope), are not covered by this survey. The observed distribution pattern in 2023 was similar to those observed in previous years’ surveys. However, areas north of Svalbard was not covered.

The time series (1994-2023) of swept area abundance indices by 5 cm length groups in the standard area is presented in Table A8.1 and Figure 8.2. The abundance indices were low in the 90’s, but increased considerably in 2005 and have remained at a higher level than in the 90’s, with a peak in 2015. After decreasing indices from 2016-2018, there has been an increase in abundance indices since 2019. The abundance in 2023 is at an all-time high, mainly due to an increase in abundance of length groups 30-34 and 35-39 cm.

Swept area abundance indices by length groups for new strata 24-26 in 2014-2023 are found in table A8.2. The abundance index for 2021 and 2022 were higher than in all previous years for individuals smaller than 35 cm, which is related to the coverage of areas close to/inside the ice that has not previously been covered in the survey. The indices for 2023 is among the lowest in the time series.

Table A8.3 presents estimates of coefficients of variation (%) for length groups. In most years, only CVs for length groups between 40 and 59 cm are at a level that could be considered as acceptable for stock assessment.

9.1 Capelin

Although capelin is primarily a pelagic species, small amounts of capelin are normally caught in the bottom trawl throughout most of the investigated area. In Figure 9.1 catch rates of capelin smaller and larger than 14 cm are shown for the winter survey in 2023. Capelin smaller than 14 cm during this period will mainly comprise the immature stock component, while the larger capelin constitutes the pre-spawning capelin stock. Some few trawl hauls show large capelin catches (numbers exceeding 100 000 individuals), and these can probably not be considered representative for the density in the area, because such hauls will either result from hitting a capelin school at the bottom or up in the water column. For this reason, we choose not to present swept area-based indices for capelin in this report.

At this time of the year, mature capelin has started their approach to the spawning areas along the coast of Troms, Finnmark and the Kola peninsula, while immature capelin will normally be found further north and east, in the wintering areas. This is reflected on the maps of capelin distribution, even though some large capelin is always found north of 75°N, and smaller capelin are found sporadically in near-coastal areas. The geographical coverage of the total capelin stock is incomplete, but the maturing component is probably best covered.

It has been noted during several surveys that when sampling capelin from demersal and pelagic trawls, the individuals from demersal trawls are normally larger (and older) than those sampled pelagically. This has led to formation of a hypothesis saying that larger individuals tend to stay deeper than smaller individuals and some even to take up a demersal life. This hypothesis has not been tested, and during the winter surveys there are probably too few pelagic hauls to study the vertical distribution of capelin in a systematic way.

9.2 Polar cod

Polar cod are not well represented in the trawl hauls conducted during the winter surveys (Figure 9.2). This is because this endemic arctic species has a more northern and eastern distribution area in the Barents Sea. During this time of the year, polar cod is known to be spawning under the ice-covered areas of the Pechora Sea and close to Novaya Zemlya. It is not clear whether the concentrations found in open water this time of the year are mature fish either on their way to spawning or from the spawning areas, or if this is immature fish. In 2023, catch rates of polar cod were high in the central Barents Sea close to/inside the ice possibly reflecting the increased abundance of this species in the last years.

9.3 Blue whiting

Since the second part of the 1990s, blue whiting has shown a wider distribution than previously, and echo recordings have indicated higher abundance in the Barents Sea. Figure 9.3.1 shows the geographical distribution of the bottom trawl catch rates of blue whiting in 2023. Since the fish is mainly found pelagically, the bottom trawl does not reflect the real density distribution, but gives some indication of the distribution limits. Acoustic observations would better reflect the relative density distribution. The number of pelagic hauls has, however, been too low to properly separate the pelagic recordings. During the years with high abundance of blue whiting, dense concentrations of blue whiting might have masked recordings of pelagic redfish, haddock and small cod.

Figure 9.3.2 and Table A9.1 shows the bottom trawl swept area estimates by 5 cm length groups for the years 1994-2023. High abundance of fish below 20 cm in several years, e.g., 2001, 2004, 2012, 2015, and 2021 reflects abundant recruiting year-classes (age 1). The distribution of blue whiting in the Barents Sea reflects mostly abundance of younger age groups, i.e., when there are strong year-classes coming into the stock they are seen in the winter survey in the Barents Sea as 1-group the year after. The 2014 year-class is very strong, and this is reflected in the survey in 2015 as fish smaller than 20 cm. 2020 and 2021 year-classes are also regarded as strong.

Relatively high abundance of blue whiting was found in the extended survey area the last years, similar to the situation with abundant recruiting year-classes (Table A9.2). Table A9.3 presents estimates of coefficients of variation (%) by length groups. In most years, CVs for most length groups are above what could be considered as acceptable for stock assessment.

Aglen, A. and Nakken, O. 1997. Improving time series of abundance indices applying new knowledge. Fisheries Research, 30: 17-26.  

Aglen, A., Dingsør, G., Mehl, S., Murashko, P.  and Wenneck, T. de L. 2012. Results from the Joint IMR-PINRO Barents Sea demersal fish survey 21 January – 15 March 2012. WD #3 ICES Arctic Fisheries Working Group, Copenhagen, Denmark 20-26 April 2012.  

Aschan, M. and Sunnanå, K. 1997. Evaluation of the Norwegian shrimp surveys conducted in the Barents Sea and Svalbard area 1980-1997. ICES C M 1997/Y:07. 24pp.  

Bogstad, B., Fotland, Å. and Mehl, S. 1999. A revision of the abundance indices for cod and haddock from the Norwegian winter survey in the Barents Sea, 1983-1999. Working Document, ICES Arctic Fisheries Working Group, 23 August - 1 September 1999.  

Dalen, J. and Nakken, O. 1983. On the application of the echo integration method. ICES CM 1983/B: 19, 30 pp.  

Dalen, J. and Smedstad, O. 1979. Acoustic method for estimating absolute abundance of young cod and haddock in the Barents Sea. ICES CM 1979/G:51, 24pp.  

Dalen, J. and Smedstad, O. 1983. Abundance estimation of demersal fish in the Barents Sea by an extended acoustic method. In Nakken, O. and S.C. Venema (eds.), Symposium on fisheries acoustics. Selected papers of the ICES/FAO Symposium on fisheries acoustics. Bergen, Norway, 21-24 June 1982. FAO Fish Rep., (300): 232-239.  

Dickson, W. 1993a. Estimation of the capture efficiency of trawl gear. I: Development of a theoretical model. Fisheries Research 16: 239-253.  

Dickson, W. 1993b. Estimation of the capture efficiency of trawl gear. II: Testing a theoretical model. Fisheries Research 16: 255-272.   

Dolgov, A. V., Yaragina, N.A., Orlova, E.L., Bogstad, B., Johannesen, E., and Mehl, S. 2007. 20 ^th anniversary of the PINRO-IMR cooperation in the investigations of feeding in the Barents Sea – results and perspectives. Pp. 44-78 in ‘Long-term bilateral Russian-Norwegian scientific cooperation as a basis for sustainable management of living marine resources in the Barents Sea.’ Proceedings of the 12th Norwegian- Russian symposium, Tromsø, 21-22 August 2007. IMR/PINRO report series 5/2007, 212 pp.   

Engås, A. 1995. Trålmanual Campelen 1800. Versjon 1, 17. januar 1995, Havforskningsinstituttet, Bergen. 16 s. (upubl.).  

Engås, A. and Godø, O.R. 1989. Escape of fish under the fishing line of a Norwegian sampling trawl and its influence on survey results. Journal du Conseil International pour l'Exploration de la Mer, 45: 269-276  

Engås, A. and Ona, E. 1993. Experiences using the constraint technique on bottom trawl doors. ICES CM 1993/B:18, 10pp.  

Fall, J. 2020. NEA cod and haddock indices from the Barents Sea winter survey 2020. Working Document # 10 Arctic Fisheries Working Group, ICES HQ (via webex), Copenhagen, Denmark, 16-22 April 2020  

Foote, K.G. 1987. Fish target strengths for use in echo integrator surveys. Journal of the Acoustical Society of America, 82: 981-987.  

Godø, O.R. and Sunnanå, K. 1992. Size selection during trawl sampling of cod and haddock and its effect on abundance indices at age. Fisheries Research, 13: 293-310.  

ICES 2012. ICES. (Aglen, A., Bogstad, B., Dingsør, G.E., Gjøsæter, H., Hallfredsson, E.H., Mehl, S., Planque, B. et al.) 2012. Report of the Arctic Fisheries Working Group, ICES Headquarters, Copenhagen 20-26 April 2012. ICES CM 2012/ACOM: 05. 633 pp.  

ICES. 2020. Benchmark Workshop for Demersal Species (WKDEM). ICES Scientific Reports. 2:31. 136 pp. http://doi.org/10.17895/ices.pub.5548  

ICES. 2021. Benchmark Workshop for Barents Sea and Faroese Stocks (WKBARFAR). ICES Scientific Reports. 3:21. 2015 pp. https://doi.org/10.17895/ices.pub.7920  

Jakobsen, T., Korsbrekke, K., Mehl, S. and Nakken, O. 1997. Norwegian combined acoustic and bottom trawl surveys for demersal fish in the Barents Sea during winter. ICES CM 1997/Y: 17, 26 pp.  

Johannesen, E., Wenneck, T. de L., Høines, Å., Aglen, A., Mehl, S., Mjanger, H., Fotland, Å., Halland, T. I. and Jakobsen, T. 2009. Egner vintertoktet seg til overvåking av endringer i fiskesamfunnet i Barentshavet? En gjennomgang av metodikk og data fra 1981-2007. Fisken og Havet nr. 7/2009. 29s.  

Johnsen, E., Totland, A., Skålevik, Å., Holmin, A. J., Dingsør, G. E., Fuglebakk, E., & Handegard, N. O. (2019). StoX: An open source software for marine survey analyses. Methods in Ecology and Evolution, 10(9), 1523-1528.   

Jolly, G. M., & Hampton, I. (1990). A stratified random transect design for acoustic surveys of fish stocks. Canadian Journal of Fisheries and Aquatic Sciences, 47(7), 1282-1291.  

Knudsen, H.P. 1990. The Bergen Echo Integrator: an introduction. - Journal du Conseil International pour l’Exploration de la Mer, 47: 167-174.  

Korneliussen, R. J., Heggelund, Y., Macaulay, G. J., Patel, D., Johnsen, E., & Eliassen, I. K. (2016). Acoustic identification of marine species using a feature library. Methods in Oceanography, 17, 187-205.  

Korsbrekke, K. 1996. Brukerveiledning for TOKT312 versjon 6.3. Intern program dokumentasjon., Havforskningsinstituttet, september 1996. 20s. (upubl.).  

Korsbrekke, K., Mehl, S., Nakken, O. og Sunnanå, K. 1995. Bunnfiskundersøkelser i Barentshavet vinteren 1995. Fisken og Havet nr. 13 - 1995, Havforskningsinstituttet, 86 s.  

MacLennan, D.N. and Simmonds, E.J. 1991. Fisheries Acoustics. Chapman Hall, London, England. 336pp.  

Mehl, S., Aglen, A. and Johnsen, E. 2016. Re-estimation of swept area indices with CVs for main demersal fish species in the Barents Sea winter survey 1994-2016 applying the Sea2Data StoX software. Fisken og havet 10/2016. Institute of Marine Research, Bergen, Norway. 43 pp.  

Mehl, S., Aglen, A., Johnsen, E. and Skålevik, Å. 2018. Estimation of acoustic indices with CVs for cod and haddock in the Barents Sea winter survey 1994 – 2017 applying the Sea2Data StoX software. Fisken og havet no. 5-2018. ISSN 0071-5638. Institute of Marine Research, Bergen, Norway. 29 pp.  

  Mehl, S. and Yaragina, N.A. 1992. Methods and results in the joint PINRO-IMR stomach sampling program.  Pp. 5-16 in Bogstad, B. and Tjelmeland, S. (eds.): Interrelations between fish populations in the Barents Sea. Proceedings of the fifth PINRO-IMR Symposium, Murmansk, 12-16 August 1991. Institute of Marine Research, Bergen, Norway.  

Mjanger, H., Svendsen, B.V., Fuglebakk, E., Skage, M.L., Diaz, J, Johansen, G.O., Vollen, T, Bruck, S. A., and Gundersen, S. 2021. Handbook for sampling fish, crustaceans and other invertebrates. Version 16.00. October 2021. Ref.id.: FOU.SPD.HB-05, Institute of Marine Research. 146 pp.  

Totland, A. and Godø, OR. 2001. BEAM – an interactive GIS application for acoustic abundance estimation.  

In T. Nishida, P.R. Kailola and C.E. Hollingworth (eds.): Proceedings of the First Symposium on Geographic Information System (GIS) in Fisheries Science. Fishery GIS Research Group. Saitama, Japan.  

¹ Russian EEZ not covered. ² Russian EEZ not completely covered (Strata 7 and 13 in 2006, Area D’ in 2012, strata 7, 13, 15, 7 and 20 in 2017, strata 17, 19, and 20 in 2020, and strata 16, 19, and 20 in 2021). ³ Additional northern areas (N) covered from this year.