Benthos is an essential component of the marine ecosystems. It can be stable in time, characterizing the local situation, and is useful to explain ecosystem dynamics in retrospect. It is also dynamic and shows pulses of new species distribution, such as the snow crab and the king crab, and changes in migrating benthic species (predatory and scavenger species such as sea stars, amphipods and snails with or without sea anemones). The changes in community structure and composition reflect natural and anthropogenic factors.
Benthos
L.L. Jorgensen (IMR), N.A. Strelkova (PINRO)
There are more than 3000 species of benthic invertebrates registered in the Barents Sea (Sirenko, 2001), but here we only present the megafaunal component of the benthos collected by trawl and registered (species, abundance and biomass) during the Barents Sea Ecosystem Survey (BESS). This includes mainly large bodied animals with long lifespans. This investigation was initiated in 2005 only – a short timeline relative to investigations related to plankton and fish. Accordingly, interpretation of long-term trends for megabenthic data must be pursued with caution.
Benthos collection. Benthos, collected with the standard demersal trawl gear during the BESS, have been registered annually by benthic taxonomic specialists since 2005 onboard Russian vessels; annual surveys have been conducted during 2007–2013 and 2015–2016 onboard Norwegian vessels. Species identification has been to the lowest possible taxonomic level. In cases where there no specialist was available onboard (2007–2008 in northern Barents Sea in the Norwegian sector), the benthos has only been identified to major benthic group. Work is ongoing between IMR and PINRO specialists to standardize and improve species identification, as well as the catchability of benthos between different trawls and vessels. Several articles have been published based on the resulting high-resolution taxonomic data (Anisimova et al., 2011; Ljubin et al., 2011, Jørgensen et al., 2015a; Jørgensen et al., 2015b, Jørgensen et al., 2019).
Megafauna description. The distribution of large benthos groups shows that Porifera (mainly the Geodia group) dominate biomass in the west, while Echinodermata (mainly brittle stars) dominate in the east. In the Northeast, Cnidaria (soft corals, such as the sea pen Umbellula encrinus, and sea anemones) dominates along with Echinodermata, while Crustacea dominates along with the Echinodermata in the Southeast (Figure 3.4.1.1).
Figure 3.4.1.1 The main benthos group distribution (in biomass). The data are the integrated mean for the period 2012-2017.Statistical analyses of monitoring data show that there are four distinct zones of benthos in the Barents Sea (Jørgensen et al., 2015a, fig. 3.4.1.2). These four zones are characterized with temperate species in the southwestern zone, cold-water species in the eastern zone, arctic species in the northern and north-eastern zone, and an area in the eastern Barents Sea where the snow crab, a new non-indigenous large benthic species, are expanding. The period with warmer water entering the Barents Sea has led to migration eastwards and northwards of temperate species and groups (Jørgensen et al., 2015a). The retreating ice front opens for new areas for human impact as well as imposing changes in the planktonic production and annual cycles, with possible impact on the benthic zones.
Figure 3.4.1.2 The baseline map of the Barents Sea mega‐benthic zones in 2011, based on fauna similarity (see Jørgensen et al., 2015a for methodology, results and discussion) with the northern (green and blue) and southern (yellow and red) region where the black full line is illustrating the “benthic polar front” in 2011. The grey full line is the approximately oceanographic Polar Front. Dotted line: Is partly illustrating a west‐east division. Red: South West subregion (SW) Yellow: Southeast, banks and Svalbard coast (SEW). Green: North West and Svalbard fjords (NW). Blue: North East (NE). Source: IMR.Jørgensen et al. (2019) show a recent increase in community mean temperature ranks (P=0.0011), indicating an increased importance of species with affinity for warmer waters and a reduced importance of cold water species. Commercial fish stocks and snow crab are expanding into the western part of the Barents Sea, thereby simultaneously increasing the exposure of both large immobile species to trawling and of small prey species to crab predation. Overall, we have found a high-level of vulnerability to the three investigated exposures in the northwestern Barents Sea, which may lead to alterations in community structure and diversity.
The status of megabenthos in 2018
The Barents Sea was only partly covered in 2018, and ten Russian and Norwegian experts were involved in the megabenthos by-catch processing across the four BESS vessels. The main results from this work are given in table 3.4.1.1 while the megabenthos distribution are given in figure 3.4.1.3
Table 3.4.1.1 The main characteristics of the megabenthic by-catches during BESS 2018.
Figure 3.4.1.3 The A) number of taxa, B) number of individuals and, C) biomass per nautical mile (nml) according to BESS 2018.The results showed a high number of taxa (fig. 3.4.1.3 A) in the northwest and low number in the east, and the southern areas, except for some few places with elevated taxa-richness. The abundance (fig. 3.4.1.3 B) showed high numbers of individuals in the north east and east of Svalbard, including the Spitsbergen Bank. Individuals were few in the south and along the slope in the west. The biomass (fig. 3.4.1.3 C) were high in the northeast, in the southwest, on the Spitsbergen Bank and locations west of Svalbard.
Long-term trends in distribution of the megabenthic biomass.
Spatial distribution. The relatively short monitoring time-series for distribution of megabenthos biomass (g/nml trawling) shows relative stable large-scale patterns, with high biomass particularly in the southwest; biomass is also stable in the Northeast, but more variable. In central Barents Sea, biomass has a high level of spatial and temporal variability which is difficult to characterize due to the relatively short data time-series (fig. 3.4.1.4).
Figure 3.4.1.4 Distribution of the megabenthos biomass (excluding Pandalus borealis) in the Barents Sea from 2005 to 2018.Figure 3.4.1.4. show that in the years 2005, 2014 and 2018 there were a lack of station coverage in the Norwegian or Russian areas. In 2014, 2015, and 2016 the Loop Hole area was not sampled because a commercial snow crab fishery. In 2012 and probably 2017 the biomass was overestimated in the Russian zone due to technical issues with the trawl tuning. Such lack of coverage and non-standardised processing makes it difficult to calculate benthic long-term monitoring.
Inter-annual fluctuation of the mean megabenthos biomass. To estimate long-term dynamics of the megabenthos, inter-annual changes of the mean biomass were calculated for the total Barents Sea and separated for the four sectors – northeast, northwest, southeast and southwest part of the sea (fig. 3.4.1.5 A, B) (ICES WGIBAR Report 2017, 2018). Despite fluctuations, the minimum mean benthos biomass was recorded in 2010 both for the Barents Sea totally and for it separates sections. This allows us to assume that this could be a “negative reaction” of the megabenthos to the cold year of 2003 (fig. 3.4.1.5 C) approximately seven years earlier. This hypothesis is supported by previous investigations documenting response of the
Barents Sea macrobenthos to changes of environmental conditions with a delay in approximately four years (Lubina et al., 2011, 2016; Denisenko, 2013). Difference in duration of the delay may be caused by different mean size and longevity of the lifespan of macro and mega benthic organisms.
Figure 3.4.1.5 The inter-annual fluctuation of the mean megabenthos biomass in total Barents Sea and in it eastern (A) and western (B) sections and variations of the mean temperature in the water layer 0-200 m in the 3-7 stations of the Kola Section (http://www.pinro.ru) (C). “Total” – Barents Sea within 68-80° N, 15-62° E, “NE” – north-eastern sector (74-80° N, 40-62° E), “NW” – north-western sector (74-80° N, 15-40° E), “SE” – south-eastern sector (68-74° N, 40-62° E), “SW” – south-western sector (68-74° N, 15-40° E). Biomass of Pandalus borealis and all catches more than 1t are excluded. Red circles show cold years which could cause decrease of the biomass in 2010 and probably in 2018.Due to lack of station coverage in 2018, only the south-western and the north-western sectors of the Barents Sea could get an added mean megabenthic biomass to the time series. In both these sectors a decrease in the mean biomass value was recorded (fig. 3.4.1.5 B.).
This drop in the mean biomass is 7-year after the cold year in 2011. This agrees with the hypothesis above, but due to the short time-series and high uncertainty of megabenthos quantification, such an assumption can only be taken as a working hypothesis.
TIBIA-polygon mapping and monitoring of the main benthos parameter. Due to high variation of the inter-annual data and an annual lack of coverage, we averaged three-year periods 2004-2009, 2010-2013, and 2014-2017 of station data to each of the TIBIA polygons. The results of this averaging are presented in the figure 3.4.1.6.
Figure 3.4.1.6 Mean megabenthic A) biomass (g/nm), B) abundance (number of individuals per nm), C) body weight (g) per TIBIA polygons for periods 2005-2009, 2010-2013 and 2014-2017.The mean biomass increased in the central area, the NE area and around Svalbard from 2005 to 2017 (fig. 3.4.1.6 A). A reduction of mean biomass was observed in the Bear Island trench, the Hopen Deep and partly in the SE.
The mean abundance values demonstrated the same tendency, but not so strongly expressed, as the biomass. High numbers of individuals are always recorded in north (east) while always low west and north of Svalbard and partly (NB-due to large Geodia catches) in the south (fig. 3.4.1.6 B).
The mean body weight of megabenthic organisms are always large in south and in north (east), while always low in central/west Barents Sea (fig. 3.4.1.6 C). The body weight has increased west, north and east of Svalbard from 2005 to 2017.
State of selected benthic species
D.V. Zakharov (PINRO)
Snow crab
The snow crab (Chionoecetes opilio) is a non-indigenous species in the Barents Sea and was first recorded in 1996 on the Goose Bank area (Strelkova, 2016). Several theories about the cause of the arrival exist, and the introduction via ballast water is one of them. The introduction of the snow crab to the Barents Sea is believed to have occurred during 1996–1993.
Regular annual monitoring of the snow crab population began with BESS in 2004. This survey is, currently, the most important source of information on snow crab population status.
Assessments of snow crab dynamics based on BESS data (Table 3.4.2.1 and figures 3.4.2.1 and 3.4.2.2) indicate that in the Barents Sea the snow crab population is still developing (spreading, population increase).
Table 3.4.2.1 Characteristics of the snow crab catches during BESS 2005-2017.
Figure 3.4.2.1. The temporal distribution of the snow crab population in the Barents Sea (number of individuals/nm) according to BESS 2005-2016.
Figure 3.4.2.2. The dynamic of the snow crab population in the Barents Sea given as the total number of crabs (blue bars) and the number of trawl hauls with crabs (red line) during the BESS 2004–2017.In 2017, as in previous years, the densest aggregations of snow crabs (more than 1000 ind/nml) were concentrated in the central part of the Barents Sea in the Loop Hole area and near Novaya Zemlya archipelago within the Russian Economic Zone. Since 2017, the snow crab was for the first time recorded at Svalbard. One record was made in Stor-fjord at 162 m depth (two immature males with 47 mm and 48 mm carapace widths); the other was northwest of Svalbard archipelago at 506 m depth (one juvenile male with a 14 mm carapace width) (fig. 3.4.2.3).
Figure 3.4.2.3 Distribution of the snow crab (Chionoecetes opilio) in the Barents Sea in August-October 2017 (according to BESS data).Studies of snow crab population size structure indicate that abundant generations appear periodically, and that this affects the overall population dynamics in the Barents Sea. During the ecosystem survey period, abundant generations were recorded with 3 years’ interval - in 2009, 2012, and 2015–2016 (fig. 3.4.2.4) (Bakanev & Pavlov 2016).
Figure 3.4.2.4 The sex and size structure of the snow crab population from 2006–2017 (Bakanev & Pavlov 2016, with editions). On vertical axes: 2006–2015 – number of individuals; 2016–2017 – abundance, %.Since 2003, snow crabs in the eastern part of the Barents Sea, have been recorded in stomachs of bottom fish species (cod, haddock, catfish, American dub, and starry ray).
In recent years, snow crabs have become one of the most important prey species for cod. In 2011–2012 it made up about 2% of the cod stomachs examined, in 2013–2014 it made up 4–7%, and in 2015–2016 it made up 5–6%. All size categories of snow crab (up to 120 mm carapace width) are eaten by cod. Cod feeding on snow crabs was most intensive (up to a quarter of total stomach content) during autumn at Novaya Zemlya, Great Bank, Central Banks.
Northern shrimp
Northern shrimp (Pandalus borealis) is common and widely distributed in the Barents Sea above the deep (250-350 m) muddy flats of the Barents Sea and in temperatures between 2 - 3°C. During the 2017 BESS survey, it was recorded at 281 of the 376 trawl stations with a biomass that varied from a few grams to 439.8 kg per nautical mile, with an average catch of 13.8±1.7 kg/nml across 281 station. The densest concentrations of shrimp were registered in central Barents Sea, around Spitsbergen, and in Franz Victoria Trough (fig. 3.4.2.5). In 2017, the northern shrimp biomass index (method of squares) was 314.2 thousand tons; 1.5% higher than in 2016, and 8% lower than the average index value.
Figure 3.4.2.5 Distribution of the Northern shrimp (Pandalus borealis) in the Barents Sea, August–October 2004–2017.During the BESS 2006-2017 average catches of the shrimp varied from 4 to 11 kg (Figure 3.4.2.6), all stayed stable around the average level. The increase of biomass in 2017 may be connected with the investigations in Franz Victoria Trough were large biomasses of shrimp were recorded.
Figure 3.4.2.6. Average catches of the Northern shrimp (Pandalus borealis) in the Barents Sea during the BESS 2006-2017. The red line shows mean value over the all years.Biological analyses of the northern shrimp population in the eastern part of the BESS were conducted in 2017 by Russian scientists. Similar to 2016, the bulk of the population consisted of younger individuals: males of 12–27 mm carapace length; and females of 17–30 mm carapace length (fig. 3.4.2.7).
Figure 3.4.2.7 Size and sex structure of catches of the Northern shrimp (Pandalus borealis) in the eastern Barents Sea, August–October 2006–2017.In the western survey area, as in the eastern part of the Barents Sea, smaller shrimp (males 11–23 mm carapace length, and females 18-28 mm carapace length) were most abundant; comprising up 64% of catches (fig. 3.4.2.8).
Figure 3.4.2.8. Size and sex structure of catches of the Northern shrimp (Pandalus borealis) in the western Barents Sea, August–October 2017.