317Agnalt, A.-L. and K.E. Jørstad. 2010. Recent Trends in Distribution and Abundance of the Snow Crab (Chionoecetes opilio) Population in the Barents Sea. In: G.H. Kruse, G.L. Eckert, R.J. Foy, R.N. Lipcius, B. Sainte-Marie, D.L. Stram, and D. Woodby (eds.), Biology and Management of Exploited Crab Populations under Climate Change. Alaska Sea Grant, University of Alaska Fairbanks. doi:10.4027/bmecpcc.2010.23

© Alaska Sea Grant, University of Alaska Fairbanks

Recent Trends in Distribution and Abundance of the Snow Crab (Chionoecetes opilio) Population in the Barents SeaAnn-Lisbeth Agnalt and Knut E. JørstadInstitute of Marine Research, Nordnes, Norway

Valery A. PavlovPolar Research Institute of Marine Fisheries and Oceanography (PINRO), Murmansk, Russia

Erik OlsenInstitute of Marine Research, Nordnes, Norway

AbstractThe snow crab (Chionoecetes opilio) is distributed in the Bering Sea (North Pacific) and the northwestern Atlantic, including Canada and West Greenland. A few specimens were first recorded in the northeast Atlantic in 1996. Since 2004 snow crabs have been recorded routinely during annual bottom-trawl surveys (winter and autumn) in the Barents Sea. These investigations confirmed previous fishery observations of snow crabs in the northern region of Goose Bank, but also detected snow crabs over a much broader geographic area. The crabs were found at depths of 40-380 m, with a majority between 100 and 300 m. In 2008, a total of 664 snow crabs were captured during the autumn survey, sug-gesting that the abundance is increasing. In total, about 40% of the snow crabs consisted of juveniles under 50 mm carapace width (CW), provid-ing evidence for successful recruitment. These small-sized crabs were exclusively found in eastern Barents Sea, indicating this to be the main nursery area. Egg-bearing females have been found since 2004. The smallest female with extruded eggs measured 65 mm CW; all females >80 mm CW were ovigerous. Collectively, these data demonstrate the successful establishment of a snow crab population in the Barents Sea.

318 Agnalt et al.—Snow Crab Trends in the Barents Sea

Comparative genetic studies are needed to identify the source popula-tion of this species introduction, and extensive sampling of size, sex, maturity status, distribution, and abundance are necessary to monitor the potential growth of this new snow crab population.

IntroductionThe first five specimens (one female, four males) of snow crab (Chionoecetes opilio) in the Barents Sea were captured by Russian fisher-men in the eastern Barents Sea (Goose Bank) in 1996. The sizes ranged from 41 mm carapace width (CW) to 123 mm CW; crabs were captured at depths from 100 to 280 m (Kuzmin et al. 1999). It is believed that the snow crab invaded the Barents Sea via ballast water from unknown areas where the species is commercially abundant (Kuzmin et al. 1999). Since 1996, additional crabs were reported, mainly as bycatch in bottom trawl fishery in the Barents Sea. Species identifications and samples have been collected during the annual bottom trawl surveys by the Institute of Marine Research (Norway) and the Polar Research Institute of Marine Fisheries and Oceanography (Russia) since 2004.

In the Barents Sea, another large-bodied crab species, the red king crab (Paralithodes camtschaticus) was successfully introduced from waters off the Kamtschatka Peninsula for purposes of establishing a new commercial fishery (Orlov and Ivanov 1978, Sundet and Hjelset 2002). Possible negative ecological consequences of the red king crab introduction are, however, poorly understood. In contrast, negative impacts of other introduced crab species, such as European green crab (Carcinus maenas), have been documented in other regions of the world (e.g., Grosholz and Ruiz 1995, Griffen and Byers 2009).

As a precursor to studies of potential impacts of the non-native snow crab in the Barents Sea, the purpose of this paper is to summa-rize the distribution and abundance of snow crab from trawl surveys conducted during 2004-2008.

Materials and methodsNorway and Russia have established a long-term joint research and monitoring program of fishery resources in the Barents Sea, includ-ing annual hydroacoustic and bottom trawl surveys for demersal fish, mainly targeting Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). The combined surveys are conducted in autumn (August-September), with two or three large research vessels participating each year (G.O. Sars, J. Hjort, F. Nansen, or Smolensk) to cover this large geographic area. All surveys are conducted with a Campelen 1800 shrimp trawl with 80 mm (stretched) mesh size in the front, and a 20 mm codend with a 116 mm cover net (Alvsvåg et

319Biology and Management of Crabs under Climate Change

al. 2009). Norway also conducts a survey in winter (February-March). Annual survey effort has typically been 10-14 vessel-weeks and about 450 bottom-trawl hauls. Depth was recorded at all trawling stations. In some years (e.g., 2007) the survey area during winter was reduced mainly due to ice cover or administrative obstacles to gaining access to Russian waters. The autumn survey, on the other hand, covers almost the entire area of the Barents Sea (Stiansen and Filin 2008).

Crab abundance was estimated as number of crabs per nautical mile of trawl tow distance for each winter and autumn survey over 2004-2008. The observations obtained during the winter surveys over 2004-2006 were reported by Alvsvåg et al. (2009) and are mainly used here as reference for evaluating recent changes in the crab population. Snow crabs were identified based on morphological characteristics described by Jadamec et al. (1999). Since 2004, a large fraction of the crabs was frozen onboard and brought back to the laboratory in Bergen for fur-ther analysis. In the laboratory, CW and body weight were recorded to the nearest mm and gm, respectively. Sex was determined according to the shape of the abdomen (Jadamec et al. 1999). Brood development of egg-bearing female snow crabs was estimated using the criteria of Sainte-Marie (1993).

ResultsThe total annual number of captured snow crab in the Barents Sea stead-ily increased from 39 in 2004 to 720 in 2008, except in 2007 when only 66 crabs were captured (Table 1). In 2008, 93 of the trawl hauls captured snow crabs, and as many as 79 crabs were found in one trawl haul (Russian vessel) at 111 m depth, while 28 crabs (all >50 mm CW) were captured in a single trawl haul at the Central Bank (Norwegian vessel). Trends over time were consistent for both autumn and winter surveys, although fewer crabs were captured during winter surveys compared with autumn surveys. In general, one to three crabs were captured per nautical mile; however, in autumn 2008 catches averaged >11 crabs per nautical mile (Table 1). Snow crabs were captured at depths of 40-380 m, with the majority between 80 and 300 m depth (Fig. 1). During autumn 2008 crabs were found at bottom temperatures of 0.7-4.1 (Fig. 2). Although most snow crabs were captured in the eastern Barents Sea, a large number of crabs also were captured in the central Barents Sea. In 2007 most crabs were distributed in northeastern regions, whereas crabs were found over much larger areas in subsequent years, including south of Novaya Zemlya.

During 2004-2008 biological information was collected from 239 crabs (161 males and 78 females) by Norwegian surveys. In general, males were larger than females (Fig. 3). The largest female measured 93 mm CW, and the largest male was 136 mm CW. About 40% of the males were larger

320 Agnalt et al.—Snow Crab Trends in the Barents Sea

than 90 mm CW, but only two females were found at these sizes. A signifi-cant fraction of crabs of both sexes was small, between 10 to 30 mm CW. These small crabs were found during both February/March and August/September surveys in 2006 and 2008, respectively. Peaks in the overall size distribution occurred at 22 mm CW and 30 to 32 mm CW for both sexes.

Ovigerous females (N = 23) were found throughout the entire inves-tigative period. Females with recently extruded eggs (bright orange in color) were captured during winter and autumn surveys, but females with eggs close to hatching (dark brown coloration) were found in February only. The smallest and largest female with extruded eggs mea-sured 65 mm and 93 mm CW (Fig. 4), respectively. No females <55 mm CW were ovigerous, but all females >80 mm CW were ovigerous. Size at which 50% of females were ovigerous was 66 mm CW.

DiscussionThe presence of snow crab was discovered in the Barents Sea in 1996 when five specimens were captured in a Russian trawler (Kuzmin et al

Year, monthNumber of

stations

Number of stations with

crabs

Number of crabs per nmi trawl (CPUE)

Number of crabs

2004 Feb-March 517 13 1.66 37

2004 Aug-Sep 825 1 2.00 2

2005 Feb-March 493 19 2.66 85

2005 Aug-Sep 646 3 3.06 5

2006 Feb-March 298 17 3.43 95

2006 Aug-Sep 663 21 2.24 31

2007 Feb-March 339 6 1.04 11

2007 Aug-Sep 581 23 2.83 55

2008 Feb-March 481 19 1.78 56

2008 Aug-Sep 412 74 11.32 664

2009 Feb-March 407 13 3.0 30

Table 1. Total number of bottom-trawl stations at each survey, number of stations with captured snow crabs (Chionoecetes opilio), estimated number of crabs per nautical mile trawling (CPUE), and total number of crabs captured. Note that snow crabs were captured as bycatch in the surveys targeting cod and haddock.

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1999). In the 1990s, occasional individuals started being captured in bycatch of the commercial cod fishery in Norway and Russia (Alvsvåg et al. 2009). Since 2004, snow crabs have been recorded as part of the bycatch in annual bottom trawl surveys run by the Institute of Marine Research in Norway and the Polar Research Institute of Marine Fisheries and Oceanography in Russia. Catch observations during 2004-2006 sug-gested an increase in abundance and size of the area occupied by snow crabs (Alvsvåg et al. 2009). Findings during the survey run in autumn 2008 suggested a further increase in number of crabs in the Barents Sea.

Figure 2. Bottom temperature at stations where snow crabs were captured during the 2008 autumn survey.

0

20

40

60

80

100

0 100 200 300 400

Num

ber o

f cra

bs c

aptu

red

Depth (m)

Figure 1. Depths at which snow crabs (Chionoecetes opilio) were captured in the Barents Sea during surveys from 2004 to 2008.

322 Agnalt et al.—Snow Crab Trends in the Barents Sea

The preponderance of small crabs (10 to 40 mm CW) of both sexes indicates recruitment. The peak in size distribution in the Barents Sea population at 22 mm CW corresponds to instar VI, and the peak at 30 to 32 mm CW to instar VII (Alunno-Bruscia and Sainte-Marie 1998), with corresponding ages of 28 months and 3.3 years, respectively. However, research trawls may select for specific size groups. Small crabs bury in muddy sediments and as crabs become larger they become more susceptible to capture by trawls.

The presence of a significant number of ovigerous females and small juveniles was interpreted as evidence for a reproductively viable popu-lation of this non-native crab species in the Barents Sea (Alvsvåg et al.

Figure 3. Size frequency distributions of (a) female and (b) male snow crabs (Chionoecetes opilio) captured in the Barents Sea from 2004 to 2008.

323Biology and Management of Crabs under Climate Change

2009). To date, the reproduction area seems to be established only in the eastern part of the Barents Sea, a conclusion also drawn by Alvsvåg et al. (2009). The size at which 50% of the females are ovigerous (~66 mm CW) and size (80 mm CW) above which all females are ovigerous is much larger than those (50 and 60 mm CW, respectively) in the Gulf of St. Lawrence (Watson 1970, Jewett 1981, Elner and Beninger 1995). Minimum size at maturity in male snow crab in the Barents Sea was esti-mated by to be 45 mm CW, and all males above 55 mm CW were mature (E.A. Filina and V.A. Pavlov, Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, pers. comm., 2009).

Although snow crab appear to have been unintentionally intro-duced to the Barents Sea, possibly through ballast water (Kuzmin et al. 1999), there are substantial distances to important fishing areas, such as the northern Bering Sea and northwest Atlantic (Canada and West-Greenland). Little information is available about ship traffic in the most eastern and remote part of the Barents Sea, where the first known crabs were reported. An alternative possibility is that the appearance of snow crab in the Barents Sea is from a trans-arctic interchange. About 3.5 million years ago the Bering Strait, between Alaska and the Russian Far East, was open during a warm period, and species could move between the Atlantic and Pacific oceans (e.g., Reid et al. 2007, Vermeij and Roopnarine 2008). Examples of arctic invasive species are Pacific herring (Jørstad 2004), the gadoid fish Theragra finmarchica (Christiansen et al. 2005), the Pacific copepod Calanus marshallae (Sundt

Figure 4. Percentage of ovigereous female snow crabs (Chionoecetes opilio) with size. Observations come from all surveys were combined. Sample size is given above each triangle.

324 Agnalt et al.—Snow Crab Trends in the Barents Sea

and Melle 1998), and bivalve mollusks (Väinölä 2003, Nikula et al. 2007). The Bering Strait also has been ice free in this century (Gerland et al. 2008, Reid et al. 2010), and another species interchange could possibly have taken place.

In other areas snow crab distribution seems to be controlled by bottom substrate and temperature (Coulombe et al. 1985, Robichaud et al. 1991, Dawe and Colbourne 2002). Hence, further expansion of this new snow crab population in the Barents Sea will possibly be closely linked to climatic conditions as this species prefers temperatures below 5ºC. Extensive monitoring programs in Norwegian and Russian areas should be established. Since 2007, Russia has conducted annual assess-ment surveys during August to September (Pavlov 2008). Based on these assessments, Pavlov (2009) estimated that the population consisted of 7.7 million individuals. This estimate appears high and needs to be reevaluated by more comprehensive surveys. Comparative genetic studies are needed to identify the source of this species introduction, and extensive sampling of size, sex, maturity status, distribution, and abundance are necessary to monitor the potential growth of this new snow crab population.

AcknowledgmentsWe are indebted to Institute of Marine Research staff on the research surveys for sample collection, and especially Eva Farestveit for technical assistance. We would also like to thank Elin Hjelset for her contribution on the environmental data. Part of the work was supported by grants from the Norwegian Ministry of Foreign Affairs.

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