Benthos is one of the main components of the marine ecosystems. It can be stable in time, characterizing the local situation, and is able to show the ecosystem dynamics in retrospective. 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. 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 join Russian-Norwegian Ecosystem survey. This includes mainly large bodied animals with long lifespans. This investigation started in 2005, but compared with the timelines of plankton and fish investigations, this is a short timeline. This means that interpretation of long-term trends in the megabenthic data must be considered with respect to this limitation.
Benthos. catch with the standard demersal trawl at the Ecosystem Surveys, have been registered annually by benthic taxonomic specialists since 2005 on board Russian vessels, while annually 2007–2013 and in 2015–2016 on board Norwegian vessels. The identification has been to lowest possible taxonomic level, but in case there were no specialist available on board (2007–2008 in northern Barents Sea in Norwegian sector), the benthos bycatch has been identified to main benthos groups. There is ongoing work between IMR and PINRO to harmonize and improve species identification and benthos catchability in the trawl among the specialists and vessels. Several publications have been made based on the fine taxonomic resolution data (Anisimova et al., 2011; Jørgensen et al., 2015a; Jørgensen et al., 2015b).
The distribution of the large benthos groups shows that Porifera (mainly the Geodia group) dominates in biomass in the west, while Echinodermata (mainly brittlestars) dominates in the east. In the NE, Cnidaria (soft corals, such as the sea pen Umbellula encrinus, and sea anemones) dominates together with Echinodermata, while Crustacea dominates together with the Echinodermata in the SE (Figure 3.4.1).
Figure 3.4.1. The main benthos group distribution (in biomass). The data are the integrated mean for the period 2009–2014.
Statistical analyses of data from the monitoring show that there are four distinct zones of benthos in the Barents Sea (Jørgensen et al., 2015a, Figure 3.4.2). These four zones are characterized with temperate species in the southwestern part, cold-water species in the eastern part, arctic species in the northern and northeastern part, 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 3.4.2. 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.
The status of the megabenthos in 2016 and possible trends.
In the short “long-term” monitoring of the distribution of benthos (g/nm trawling) shows relative stable large-scale patterns with high biomass particularly in the SW, but also stable, though changing biomass in the north east. The central part of the Sea is fluctuating in biomass and shows that the biomass distribution has a high level of spatial and temporal variety (Figure 3.4.3) which are difficult to explain due to the short period of monitoring.
Figure 3.4.3. The annual Interpolated biomass (log) distribution of the Barents Sea from 2005 to 2016. In 2012, the biomass of the Russian zone was overestimated and shall be considered as an error. In 2005 and in 2014 only parts of the Norwegian zone was covered.
In order to estimate the long-term dynamic of the benthos state, the interannual changes of the mean biomas total for the sea and separetely in four sectors (NE, NW, SW and SE) was analysed.
The total Barents sea
The 2005–2016 fluctuation in total mean biomass of the Barents Sea (excluding large,
i.e. >1 T, sponge catch in the SW) shows a decreasing trend from 2007 to 2010 (Figure 3.4.4). The biomass fluctuation from 2010 and onwards is not possible to identify due to missing data in 2012 and 2014 (i.e. technical problems and missing coverage of the benthos). The biomass of 2016 seem to be at the mean value and in the range of interannual variations (Figure 3.4.5), but due to missing data, a longer time serie is neccessary.
Figure 3.4.4. The annual total mean biomass (kg/n.ml) of the Barents Sea from 2005–2016. Broken line mean missing coverage. Total Barents Sea = South of 80°N and 15°E but excluding all stations W and N of Svalbard to 62°E and all sponge catches >1 T excluded.
Northwest (NW) and southwest (SW)
Although sponge catches >1 T was excluded the biomass in the SW was higher than in the NW during the period of long-term monitoring. The mean biomass of the NW, SW and the total Barents Sea was at the lowest in 2010 (Figure 3.4.5). Since then all three values was increasing until 2013. The value for 2014 is unknown due to missing coverage, but the mean biomass was observed from 2015 and 2016 to the highest measured (54 and 39 kg/nml respectively) for the western Barents Sea since 2005. The long-term variation of the mean biomass of the NW, SW and the total Barents Sea show strong correlations. This might indicate that the western Barents Sea are driven by a factor common for the total Barents Sea.
Figure 3.4.5. The interannual mean biomass fluctuation of the SW (red, all <1 T catches of sponges excluded) and NW (green) from 2005–2016. The dotted line is the Barents Sea mean biomass (see also Figure y3). NW = 74–80°N and 15–40°E but excluding all stations W and N of Svalbard, SW = 65– 74°N and 10–40°E. All stations west and north of Svalbard and all sponge catch >1 T excluded.
Southeast (SE) and northeast (NE)
In the SE area (Figure 3.4.6) benthos had a record high maximum in 2007 (60 kg/nml), due to one extra-large catch of king crab, but has stayed low (<25 kg/nml) all years after this, and below the total Barents Sea mean. The SE are experiencing high level of commercial trawling which might be the cause of the low benthos-biomass. This become more evident when the SE is compared to the high biomass of benthos in the NE which have no trawling activity (Ljubin et al., 2011).
The highest biomass (sponge catch >1 T excluded) in the Barents Sea was recorded in the NE (>60 kg/n.ml). In most of the measured years, the biomass in the NE was above the total Barents Sea mean (Figure y5). But from 2013 and ongoing, the mean biomass (see also black arrows in Figure 3.4.6) was reducing, and was record low (<20 kg/nml) in 2016, and below the total Barents Sea mean.
Figure 3.4.6. The interannual mean biomass fluctuation of the SE (red line with yellow circles) and NE (blue). The dotted line is the Barents Sea mean biomass (see also Figure y3). NE=74-80°N and 40-62°E, SE=65-74°N and 40-62°E.
The area of reduced benthos biomass after 2013 (black arrows in Figure 3.4.3. for 2013-2016) are overlapping with the maximum distribution of the snow crab (see below) and with increasing bottom temperatures (chapter 3.1). We therefore suggest the strong decrease in benthos biomass to be an effect of multiple impact factors.
State of selected benthic species
The snow crab is a new species in the Barents Sea. There are several theories of the appearance of crab including invasion via ballast water or natural spreading from the west or east. The first record of the crab was done in 1996 (Figure 3.4.7) and regular annual investigation of the snow crab population started on the Ecosystem Survey in 2004.
Figure 3.4.7. The invasion period, the first record, and the dynamics (data from Ecosystem Survey 2004–2016) of the snow crab population in total number of individuals (grey) and mean number of individuals (black) per trawl (Strelkova, 2016).
Since these early observation, the crab has mainly been recorded west of Novaya Zemlya (Figure 3.4.8) and studies on size structure of the population show two strong generations of snow crab juveniles in 2009 and in 2012 (Figure 3.4.9). The 2009 generation resulted in a subsequent increase in abundance in 2011–2014 while the 2012 generation will have a peak in nearest future.
Figure 3.4.8. The dynamics of the snow crab population in number of individuals of the Barents Sea from 2005–2016 (according to Strelkova, 2016 with additional data of 2016).
Figure 3.4.9. The sex and size structure of the snow crab population from 2006–2015 (Bakanev and Pavlov, 2016)
In 2012 the abundance of snow crabs in the Barents Sea reached a maximum, and started to decrease from 2013 to 2016 (Figure 3.4.9). This resulted in a reduction of the snow crab population to half of the size compared to the peak years. But as this species lives seasonally patchy, and often in dense pods, stock assessment is difficult, with autumn samplings giving the most reliable assessments (Mullowney et al., 2014)
The calculation of the population abundance index (Bakanev et al., 2016) show that the snow crab population reached 4346 millions individuals in 2012 that was spread across an area of 569 000 km2 (Table 3.4.1).
Table 3.4.1. The spreading area, mean abundance and total stock indices of the snow crab in the Barents Sea 2004–2014 (Bakanev et al., 2016).
Analysis of the snow crab distribution show a temperature preference from -1.9 to 9.3°C, with the densest aggregations from -1.5 to 3.0°C (temperature optimum) (Bakanev et al., 2016). This means that the temperature is the strongest factor limiting the spread of the snow crab to the south and western part of the Barents Sea. This could also explain that the Ecosystem Survey did not record a significant spread of crab toward the west during 2004–2016. Probabilistic estimation of the snow crab distribution in the Barents Sea (Bakanev, 2016) shows that if the bottom temperature will decrease with 1°C, the snow crab is expected to spread westward and reach the Hopen Deep and the areas round Svalbard (Figure 3.4.10B), but if the temperature increase by 1°C, the population is not expected to reach the Hopen Deep or the shallower parts of the sea around Svalbard.
Figure 3.4.10. The probability of occurrence (%) of snow crab in the Barents Sea in 2010–2014 (A) and a forecast of the distribution if the water temperature stay at the long-term mean (В) in the case of temperature decrease of 1°C (C) and an increase of 1°C (Bakanev, 2016).
The temperature is not limiting the eastward spread of the snow crab. This was shown by the PINRO trawl investigation in 2007 and in 2013 in the Kara Sea and St. Anna trough, where the number of snow crabs increased from zero to several individuals per station (Figure 3.4.11).
Figure 3.4.11 Station coverage of PINRO trawl surveys and ecosystem surveys into the Kara Sea and St. Anna trough in 2007 and 2013 without (black dots) or with (red dots) snow crab records (according to Strelkova, 2016)
Northern shrimp (Pandalus borealis) is widely distributed in the Barents Sea (Figure 3.4.12). The highest densities are recorded on silty grounds on the slopes of banks, troughs and the northern continental slope facing the Arctic basin, as well as in the western fjords and northern sound of the Svalbard. Usually are the highest densities found in the frontal zones between the Arctic and boreal waters, but has in the recent years shifted eastwards (NIPAG 2015). The optimum bottom temperatures for the densest concentrations of the northern shrimp are between 0 and 2°C. (Berenboim, 1992).
Figure 3.4.12. The mean density of Pandalus borealis in the Barents Sea from 2004 and 2015 with highest values in red.
After a minimum shrimp stock size in the last half of the 1980s, the size of stock has increased but is fluctuating. The results from the ecosystem survey in 2015 and 2016 (Figure 3.4.13) suggest a slight increase in the stock compared to 2014 and an estimate slightly above the average for the ecosystem survey period (2004-2016).
Figure 3.4.13. The temporal development in the shrimp stock 1970–2016 (relative abundance estimates from assessment model, provided by C. Hvingel), in ICES Subareas 1 and 2.