4.6 Snow crab effect on benthos

Benthos habitat. Photo: Haakon Hop, Norwegian Polar Institute

Interactions, drivers and pressures 2018
Typography
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In most of the measured by BESS years, the biomass in the northeast part of the Barents Sea was above the total Barents Sea mean (Figure 4.6.1), but from 2013 and ongoing, the mean biomass was reducing, and was record low (<20 kg/nm) in 2016, and below the total Barents Sea mean. As one of the reasons of this decrease could be assumed develop of snow crab population and it predation on the benthos (including juvenile stages of the megabenthic animals.

In 2017, the biomass extremely increased to 116 kg/nm, the highest value recorded over the entire period of BESS (Figure 4.6.1). But there are some reasons to assume in this year the benthic biomass overestimation due to changes in the trawl tuning. Lack of coverage of the snow crab area let not estimate mean biomass value in the 2018. Thus, the assumption about a decrease in the benthic biomass in the north-eastern part of the Barents Sea by the population of snow crabs, developing here, continues to remain in the hypothetical state.

Figure 4.6.1 Inter-annual fluctuation of the mean megabenthos biomass (excluding Pandalus borealis) for the Barents Sea totally (black line,”Total”) and for it north-eastern section (green line, NE). The years 2012 and 2014 are excluded from “Total” time-series, year 2012 – from NE time-series.Figure 4.6.1 Inter-annual fluctuation of the mean megabenthos biomass (excluding Pandalus borealis) for the Barents Sea totally (black line,”Total”) and for it north-eastern section (green line, NE). The years 2012 and 2014 are excluded from “Total” time-series, year 2012 – from NE time-series.

Current investigations show that the snow crab has very broad diet that includes almost all kinds of benthic invertebrates in the Barents Sea (Manushin et al. 2016). There is a difference in the diet of females and males, juveniles and adults. Juveniles and females prefer shallow areas with communities of bivalve molluscs, males live deeper on slopes and depressions where polychaetes and crustaceans are the most abundant group (Zakharov et al. 2018). Stomach contents are analyzed to determine the species composition and the frequency of occurrence for various benthic taxa. Consumption of food was estimated and compared with data from the Russian seas of the Pacific region (Zakharov et al. 2018).

The total annual grazing of macro-zoobenthos by the snow crab is calculated for its current distribution in the Barents Sea. Snow crab consumes at least 30 thousand tons of benthos annually, which amounts to 0.1-0.2% of the total macrozoobenthos biomass of the investigated area. The snow crab population have the largest impact on the benthic communities in the north-eastern part of the Barents Sea and close to the south side of the Novaya Zemlya Archipelago (fig. 4.6.2). This is an area dominated by the polychaete Spiochaetopterus typicus (deeper areas with adult snow crabs) and the bivalve Macoma calcarea (shallower areas with juvenile snow crabs) (Zakharov et al. 2018).
 

Figure 4.6.2. Total biomass (g/m2) of the benthos consumed/killed by the snow crab population during a nine-year period (2005–2017) (Zakharov et al. 2018).Figure 4.6.2. Total biomass (g/m2) of the benthos consumed/killed by the snow crab population during a nine-year period (2005–2017) (Zakharov et al. 2018).

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