Distribution, Abundance, and Biology of Polar Cod, Boreogadus Saida (Lepechin 1773), in Icelandic Waters

ICES CM 2012/M:04

Distribution, abundance, and biology of polar cod, Boreogadus saida (Lepechin 1773), in Icelandic waters

Olafur S Astthorsson

Distribution, abundance, and biology of polar cod, Boreogadus saida, was studied in the waters around Iceland based on material sampled during demersal fish surveys in March 1985-2011 and in pelagic 0-group surveys in August-September 1974-2003. Demersal polar cod were most often caught on the outer the shelf to the northwest of Iceland but during the years of highest abundance and widest distribution it was also caught on the north and northeastern shelves. Pelagic 0-group polar cod was only caught sporadically and confined to the waters over outer part of the northwestern shelf. Both distribution and abundance showed variations related to bottom temperature. Demersal polar cod was most widely distributed and peaks in abundance highest in the cold years of 1989-1999, 1994-1995 and 2002. A fourth peak in both abundance and distribution was observed during the somewhat warmer period of 2007-2009. On average highest numbers of polar cod per haul were caught at temperatures of 1.4° C and 0.14° C, and at depth ranges 55-100 m and 300- 400 m, respectively. The length of demersal polar cod ranged from 5-32 cm while the fish caught in the pelagic trawl ranged from 2.2-19 cm. The polar cod in north Icelandic waters most likely originates from east Greenland or even possibly Svalbard waters.

Key words: Polar cod, Icelandic waters, distribution, abundance, biology.

Contact author: Olafur S Astthorsson, Marine Research Institute, P. O. Box 1390, Skulagata 4, Reykjavik, Iceland. [email protected]

Introduction Polar cod is an arctic species circumpolar in distribution (Christansen and Fevolden, 2000; Sunnanå and Christiansen, 1997) and it is also the species of fish which has been found farthest to the north in the Arctic Ocean (Leim and Scott, 1966). In the Arctic Ocean and adjacent seas it is important in transferring energy from zooplankton to several fish species, seabirds and marine mammals (e.g. Bradstreet et al ., 1986; Ajiad and Gjøsæter, 1990; Lønne and Gulliksen, 1989a; Welch et al ., 1992, 1993; Lilly et al ., 1994; Sakshaug and and Kovacks, 2010). On the Atlantic side of the Arctic polar cod occurs as far south as northern Norway, in the White and Barents Seas, off Spitsbergen, northern Iceland and around southern Greenland (Christiansen and Fevolden, 2000; Sunnanå and Christiansen, 1997; Svetovidov, 1986). Limited published information is available on polar cod in Icelandic waters and most of it is confined to isolated records of occurrence. Around Iceland demersal polar cod has been found from Bjargtangar, off the northwest coast, clockwise to Ingolfshofdi, off the southeast coast (Saemundsson, 1926, 1949; Jonsson, 1992; Jonsson and Palsson, 2005). Since 1985 the Marine Research Institute in Reykjavík has undertaken an annual survey of the abundance and distribution of demersal fish stocks in Icelandic waters (Palsson et al . 1989, 1997). With few exceptions (e.g. Stefansdottir et al ., 2009), the analysis of these data has mainly concentrated on the commercially exploited species while valuable information on less important and/or rarer species has hitherto received a limited attention. Polar cod has been caught almost every year in these surveys (Bjornsson et al ., 2007) but no detailed analysis of the material has been undertaken. Schmidt (1909) reported 0-group (pelagic young) polar cod from one station in Hunafloi off the north coast of Iceland. More recent surveys of 0-group fish (see Astthorsson et al ., 1994) in Iceland and east Greenland waters, undertaken during 1974-2003, have occasionally recorded pelagic polar cod (e.g. Vilhjalmsson and Magnusson, 1980; Magnusson and Sveinbjörnsson, 1991; Sveinbjörnsson and Hjörleifsson, 2002) but no biological information on the specimen from these surveys nor discussion of the findings has hitherto been presented. In order to obtain more information on polar cod in Icelandic waters the information from the groundfish surveys is here analysed to study the abundance, distribution and biology of demersal polar cod. Further, information from 0-group surveys in Icelandic waters has been analysed for information on the abundance, distribution and biology of pelagic polar cod. Polar cod is one of relatively few fish species of arctic or polar origin in Icelandic waters and in the area it is also near its south-eastern distribution range. In light of the effect of recent warming on many fish stocks in Icelandic waters (e.g. Astthorsson et al ., 2007, 2012; Valdimarsson et al ., 2012) it is of particular interests to also consider polar cod in this context.

Material and methods Data on the distribution abundance and length of demersal polar cod were obtained form the annual Icelandic groundfish surveys conducted at standard stations in late March during 1985-2011. The survey of each year has been conducted with similarly equipped commercial trawlers and research vessels at standard stations on the shelf area all around Iceland, from shallow water (ca 50 m) down to about 500 m depth. The number of stations has changed slightly from one year to another during the investigation period, ranging from 509 in 1998 to 600 in 1995 (average 560 stations, Figure 1A). The demersal trawl deployed had 17 m between the wing ends, a 135 mm mesh in the front part of the net, 80 mm in the belly and the extension piece and the

2 cod-end was covered with 40 mm netting. The standard tow length was 4 nautical miles and basic data obtained were number of polar cod per station or haul. Initially the polar cod were only counted in the samples while from 1996 they have also been measured for length. For further information on the design and sampling methodology of the Icelandic ground fish survey see Palsson et al . (1989, 1997). Available data from 0-group surveys conducted in Iceland and east Greenland waters in August-September 1974-2003 were analysed for information on distribution, abundance and length of pelagic polar cod. The sampling was conducted using a Harstad pelagic trawl with a 18 x 18 m opening and a 0.5 x 0.5 cm mesh at the cod-end. For most of the investigation period a more or less fixed survey route has been operated (Figure 1B) covering the whole of Icelandic waters and ice free part of the east Greenland shelf between Iceland and Greenland at 65-68º N. The exact number and positions of trawling stations have not been fixed but in the distribution area of polar cod (northwest, north and northeast of Iceland) dealt with here they have ranged from 76 in 1974 to 207 in 1997 (Table 1). Throughout the survey period standardised survey methods have been used with trawling usually made at depths of 20-50 m. On deck all polar cod was counted and their length measured. For more details on the pelagic sampling see Astthorsson et al . (1994). The seawater temperature was registered continuously during the demersal trawling with a “Scanmar" temperature sensor fastened to the headline of the trawl. For the present study the registration recorded at the onset of trawling is used as a measure of environmental temperature at each station. An average for all stations in the main distribution area of polar cod (cloackwise from the Westfjord Peninsula at ca. 66ºN to the middle of the east coast at ca. 65ºN) was then calculated to get an measure of temperature conditions for a given year (Figure 3). The temperature data have also been used to calculate mean temperature for different depth ranges at the stations which demersal polar has been caught (Figure 4).

Results Demersal trawl surveys Distribution and abundance A total of 15147 demersal trawl hauls have been taken during the 26 years of investigation. Polar cod has been found in 786 of them (5.2 %) and total of 2293 polar cod were caught. The largest haul contained 83 individuals and it was taken at a depth of 342 m on the north-eastern shelf in 1995. Most of the hauls with polar cod present (71.9 %) contained, however, only 1-2 individuals. Most of the hauls (55.6%) were taken between 201-300 m depth and most individuals (1137 or 49,6 % of total caught) came from this depth range. As pointed out below, densities (number of individuals per haul were, however, higher at other depth ranges). Polar cod was usually caught in the demersal trawl on the outer shelf to the northwest and northern of Iceland at ca. 66º 30' - 67° 30' N and between ca. 18-24° W (e.g. Figure 2, year 1989, 2003). Off the northwest peninsula polar cod was very seldom caught west of ca. 24° W. During years of greatest distribution (and abundance, e.g. 1990, 1994, 1995, 2009 in Figure 2) the distribution extended farther to the east and closer to the north and north-eastern shores and as far south along the east coast as ca. 65° N (1994, 1995). Long term variations in number of stations with polar cod and the mean number of polar cod caught per station are show in Figure 3. The similarity between the two lines is quite striking. This may reflect that the same factor, such as e.g. a given temperature regime or currents from the north, are at the same time bringing in

3 more polar cod and also expanding their distribution. More limited distribution and low abundance were observed during the first years of the investigation (1985-1988). A peak was then observed in 1989-1990 followed by a low during 1991-1993. A second somewhat larger peak was observed in 1994-1995 followed by a low in 1996- 2000. A third peak was observed in 2001-2003 while a low occurred in 2004-2006. The forth peak was observed in 2007-2009 while during 2010-2011 a low again was observed. The bottom temperature in the distribution area of polar cod (the waters northwest, north and northeast of Iceland) is shown in Figure 4. The figure shows lows in temperature in 1989-1991, 1995, 2002, and to a lesser extent in 2008 or approximately during the same years as a peak was observed in the distribution and abundance of polar cod (Figure 3). This indicates that as the temperatures are low the distribution of polar cod expands and the abundance increases. The negative relation between temperature and number of stations with polar cod is further demonstrated by the significant scatter plot in Figure 5 (r 2 = 0.3112, P< 0.05).

Depth distribution in relation to temperature In the waters around Iceland the overall depth distribution of polar cod ranged from 55-479 m and at bottom temperatures of -1.6-5.2 C° (Figure 6). The average catch was highest at depths of 55-100 m, i.e. relatively close to the shore where the temperature at the stations was on average 1.4 C° and then at 301-400 m where the temperature was 0.14 C°. Catch was also relatively high in the 401-500 m depth interval where temperature was 0.07 C°.

Length distribution The length of polar cod caught in the demersal hauls in 1996-2011 is shown in Figure 7A. The length of polar cod ranged from 5-32 cm. The combined material shows only one length mode or cohort with a mean at 15.6 cm. Length distributions from single years (e.g. 2003 which is the single year from which measurements are most numerous) indicated that at least in some years there may be two length modes in the population, one at ca. 14-15 cm and another one at ca. 19-20 cm (Figure 7B).

Pelagic 0 gr. surveys Distribution and abundance Polar cod were only been caught sporadically in the pelagic 0-group surveys during the 30 years of investigation between 1974-2003 (Table 1). Most of the individuals of which the length was measured were <70 mm, i.e. mainly of 0-group (c.f. Falk- Petersen et al ., 1986). The limited data does not allow analysis of long term changes in abundance while some information on the distribution of can be obtained. Pelagic 0-group polar cod was caught mainly in water of >500 m depth in the Denmark Strait, i.e. the area between the northwest Icelandic shelf and the east Greenland shelf (Figure 8). The highest number of hauls with 0-group polar cod was observed in 1991 and 1997 (in 12 and 10, respectively, Table 1). The highest number of 0-group polar cod caught in a given year was caught in 1980 and 1991, respectively, 60 (in 4 hauls) and 72 (in 12 hauls) individuals. The highest number of 0-group caught in a single haul (55 individuals, ranging from 2.4-3.7 mm in length) was taken in 1980 on the east Greenland shelf at 66º 38' N, 32 º 33' W. The limited distribution and the low number of 0-group polar cod probably reflects that they are brought into Icelandic waters through physical events at the boundary between the cold East Greenland Current and warm North Icelandic Irminger Current. Their main habitat is possibly in

4 the areas closer to the ice but this is, however, very difficult to sample effectively with large pelagic trawls.

Length distribution Polar cod caught in the pelagic trawl ranged from in length from 2.2 -19 cm (Figure 9). The length distribution showed a peak at 3.0-3.9 cm while fish ca <7 mm are considered to belong mostly to the 0-group (c.f. Falk-Petersen et al ., 1986). Older length/age classes cannot be visualized from the length frequency distribution. Polar cod >7 cm (i.e. fish of age 1 year and older) occurred most often in the pelagic trawl only as single individuals (Table 1).

Discussion In the Icelandic waters demersal polar cod was most commonly found on the outerpart of the northwestern and middle northern shelves. Similarly pelagic 0-group polar cod was most commonly observed deep off the north western shelf (Figures 1, 8). This distribution pattern seems to reflect the hydrographic conditions in the area and the northern orgin of polar cod. Deep off the northwest coast of Iceland the relatively warm northward flowing North Icelandic Irminger Current forms a front with the cold southward flowing East Greenland Current. Further, sea ice which is encountered almost annually off the northwest coast of Iceland is only to a very limited extent formed locally. Most of this ice originates in the Arctic Ocean and is then brought southwards along the east coast of Greenland with the East Greenland Current. In the Denmark Strait the distance from the Greenland shelf and to the Iceland shelf is only ~60 nautical miles and towards there the sea ice easily brought by currents or wind. Polar cod spawns in the vicinity and under the sea ice close to the surface during winter (from end of December to end of March, with peak usually in January to February) (Ponomarenko, 2000; Graham and Hop, 1995; Bradstreet, 1986; Sameoto, 1984; Craig et al ., 1982; Rass, 1968). Just as transporting sea ice the currents may transport eggs, larvae and juveniles of polar cod from the Greenland shelf and towards the shelf waters to the northwest and north of Iceland. The limited information on polar cod from east Greenland are mainly early records occurrence and at present there appears to be no information on polar cod spawning grounds in East Greenland waters but a few larvae (9.0, 13.5 and 16.5 mm) and a fish in the size range of 0-group (44-74 mm) have been reported form e.g. Danmarks Havn (ca. 77°N) and at the south coast of Jameson Land (ca. 71° N) (Jensen, 1909; Johansen, 1912). Possibly there are unknown spawning areas of polar cod along the east Greenland coast but to my knowledge the confirmed spawning area of polar cod nearest Iceland is to the east and north east of Svalbard at ca. 77-81°N and 25-30° E (Gjøsæter, 2009, 1973; Gjøsæter and Anthonypillai, 1995). The surface currents between Svalbard, east Greenland and northwest Iceland (Hansen and Østerhus, 2000) suggest that there may be a connection between the polar cod in these three areas. Based on information on spawing time of polar cod, the time it takes for polar cod eggs to hatch in the cold water (Ponomarenko, 2000; Sameoto, 1984; Rass, 1964), the current speed of the East Greenland Current near the surface (ca. 10-15/cm sec, Steingrimur Jonsson pers. com., based on unpublished measurements from Greenland Sea Project 1988-1991) and the distance of ca. 1300 km between spawning grounds at Spitsbergen (at ca. 80º N) and the waters between Iceland and Greenland (at ca. 68 ºN) it seems possible that early juveniles in the size range of 2.0-4.0 cm (c.f. Figure 9) may be transported from the spawning grounds at Spitsbergen to the Greenland- Iceland area in 3-5 months if they are not confined to areas where flow is much

5 reduced. Further studies in this respect are, however, needed before a firm conclusion can be made. In this context it is of interest to point out that genetic studies using Allozyme and scnDNA on polar cod from the Pechora Sea, off Svalbard and in the Denmark Strait (Fevolden and Christiansen, 1997) showed negligible polymorphism. This contrasts findings from RAPD-analysis of the same material along with additional samples from Disco bay (western Greenland) and Posangerfjord (northern Norway) (Fevolden et al ., 1999) where many of the RAPD primers revealed multiple polymorphism. However, both studies were consistent in revealing the absence of heterogeneity among samples. Also, in the latter study the relative low numbers of individuals analysed for some locations, impairs conclusions that can be drawn about stock diversification. In addition, based on mitochondrial DNA (mtDNA) Palsson et al . (2009) reported little genetic differentiation amongst polar cod sampled from Greenland, Spitsbergen and Iceland. Additional biological and genetic studies are of interest in order to reveal the relationship between the stocks of polar cod along its distribution range. Both the distribution and abundance of polar cod in the demersal trawling showed a decreasing trend with bottom temperature in the study area to the north of Iceland (Figures 3, 4, 5). Previous studies both from the Barents Sea (Monstad and Gjøsæter, 1987) and from Labrador and Newfoundland waters (Lilly et al . 1994) have similarly demonstrated extended distribution of polar cod when temperatures got lower. Further, Falk-Petersen et al . (1986) reported from Spitsbergen coastal waters that a larger portion of hauls taken in temperatures below 0° C caught more fish than those taken above 0° C which was considered as indicating the preference of polar cod for low temperatures. It should, however, be pointed out that the temperature tolerance of polar cod seems to be quite wide and in the waters to the north of Iceland it was caught at temperatures ranging from -1.6-5.2° C. In Spitsbergen water polar cod was caught at temperatures ranging from -1.6-3.2° (Falk-Petersen et al ., 1986). This may again explain why in fairly warm years like 1994 and 2008-2009 the abundance of polar cod was quite high and distribution wide. In the waters north of Iceland polar cod was found to be most common in the 0-100m and 300-400 m depth ranges were temperature at the stations was respectively 1.37 and 0.14 ºC on average (Figure 5). The high numbers observed at depths <100 m relate to “large” hauls at one station in the years 1993, 1994, and 1995 (30, 38 and 16 fish respectively). As stations with polar cod in this depth range are few (only 26 stations) compared to other depth intervals the calculated mean has to be taken with some caution. However, in other areas polar cod have also been reported to form large patches in relatively shallow water. Thus in Spitsbergen coastal waters, Falk-Petersen et al . (1986) observed some of the highest abundances of polar cod (>1000 per trawl hour) at a depths only slightly greater than 100 m and at similar temperatures as the Icelandic shallow water records. Welch et al ., (1993) also reported high densities in large schools at only a few meters depth in the Barrrow Strait region of the eastern Canadian arctic. Possible the observations of the high abundance of polar cod at some stations in the shallow water north of Iceland may therefore be reflecting actual higher concentrations of fish in the coastal regions. The size of polar cod caught in the demersal hauls in Icelandic waters ranged between 5-32 cm while the majority of the fish ranged from 12-16 cm. This is a size range similar to that reported by Falk-Petersen et al. (1986) in demersal trawling during July-August in Spitzbergen coastal waters (5-30 cm). Falk-Petersen et al .

(1986) also reported an indication of a size/depth segregation in distribution with smaller fish usually occurring in shallower areas and the larger ones at greater depths. According to Figure 9 in Falk-Petersen et al . (1986) the different age groups of polar cod in Spitsbergen waters have approximately to following mean lengths: age-1 = 10 cm, age-2 = 13 cm, age-3 = 15 cm, age-4 = 16.2 cm, age-5 = 17.5 cm, and age 6 = 20 cm. Gjøsæter and Ajiad (1994) also demonstrated significant difference between years in the growth of polar cod from the Barents Sea. During the three years they studied (1987-1989) age-1 fish ranged from 10.9-12.5 cm and age-2 fish form 13.9-15.3. Based on the above length-at-age information the majority of polar cod caught in the demersal hauls in Icelandic waters was probably mostly 2-4 years old. Marked changes have been observed in the distribution of many fish species in Icelandic waters during a warming in recent years (Astthorsson et al ., 2007, 2012, Valdimarsson et al ., 2012). Many southern species, both commercially exploited (e.g. haddock, monkfish, mackerel), rare and vagrant (e.g. greater fork beard, blue antimora, snake pipefish, sea lamprey, Ray’s bream), have extended farther north. An arctic exploited species (capelin) has also retreated into colder water. The present paper has further demonstrated changes in the distribution and abundance of polar cod (Figures 2, 3, 4, 5) that appear to be related to temperature variations. The distribution of polar cod along the east coast of Greenland (and also into north Icelandic water) seems to follow closely the east Greenland ice edge (Sunnanå and Christiansen, 1997; Ponomarenko, 1968). The persistent decline of sea ice in the Greenland Sea (Comiso, 2010) during recent warming is therefore along with the local warming likely to affect distribution of polar cod along the east coast of Greenland and also limit its extrusions into Iceland waters. Further warming may eventually lead to polar cod disappearing in Icelandic waters.

Acknowledgements The present analysis would not have been possible without the hard work and often during difficult sea conditions of the many staff members of MRI who through the years have participated in the 0-group and groundfish surveys. All these people I wish to thank. I also thank Christophe Pampoulie for commenting on the genetic related discussion.

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Table 1. Summary of polar cod caught during pelagic 0-group surveys in Icelandic and east Greenland waters during 1974-2003. During the years not listed in the table (1976, 1977, 1981, 1983, 1985, 1987, 1988, 1989, and 1990) no polar cod was caught. Year No. No. Total no. Total no. Total no. Total no Total no. trawling stations polar cod polar cod polar cod polar cod of polar stations w. polar caught counted measured <7 mm cod >7 mm cod 1974 76 1 1 1 1975 83 2 2 1 1 0 1 1978 119 1 1 0 1 0 1 1979 113 6 27 8 19 2 17 1980 130 4 60 0 60 58 2 1982 134 1 1 0 1 0 1 1984 149 1 1 0 1 0 1 1986 185 1 1 0 1 1 0 1991 190 12 72 11 61 61 0 1992 191 2 8 0 8 8 0 1993 184 3 5 0 5 1 4 1994 197 2 6 0 6 2 4 1995 145 1 1 1 1996 155 4 9 0 9 8 1 1997 207 10 42 26 14 13 1 1998 196 1 17 0 17 17 0 1999 178 6 6 0 6 6 0 2000 204 2 14 0 14 0 14 2001 183 4 19 0 19 19 0 2002 108 7 57 20 37 37 0 2003 122 2 15 0 15 15 0

Figure 1A: Standard stations in the Icelandic groundfish survey in March. Stations have been more or less fixed for the whole investigation period (1985-2012). Green and blue are additions in recent years (for details see Material and methods). From www.hafro.is

Figure 1B. Typical survey grid in the Icelandic 0-group surveys during 1975-2003 (during 1998-2003 Greenland waters were not covered). From Astthorsson et al ., 1994.

Figure 2 . Catches of polar cod (number/standard haul) in the Icelandic groundfish survey in March in four selected years (1985, 1990, 1995, 2000) during the period of investigation (1985-2000). The small grey dots show trawling stations while blue circles size reflect number of polar cod caught. Largest circle indicates highest catch recorded, i.e. 84 individuals at a station in 1995.

80 0,50 No. stn w. polar cod 70 0,45 No. polar cod/no. stn 0,40 60 0,35 50 0,30 40 0,25 30 0,20 0,15 No. stn w. polar cod polar w. stn No.

20 stn polarcod/no. No. 0,10 10 0,05 0 0,00

Figure 3 . Variations in the number of stations with polar cod and in number of polar cod per station (based on all stations taken) in the Icelandic groundfish survey in March 1985-2011.

3,5 3 2,5 2 1,5 1 Temperature C C ̊ Temperature 0,5 0

Year

Figure 4 . Average bottom temperature at groundfish survey trawling stations northwest, north and northeast of Iceland in March 1985-2012.

80 70 60 y = -12,461x + 52,923 50 R² = 0,3112 40 30

No. stn polar cod polar stn No. 20 10 0 0 0,5 1 1,5 2 2,5 3 3,5 Temperature

Figure 5 . Relationship between temperature and the total number of polar cod caught in different years in the Icelandic groundfish survey in March 1985-2011.

6 2 Avg. no. polar cod stn 1,8 5 Avg. bottom temp. 1,6 1,4 4 1,2 3 1 0,8 2 0,6 Avg. bottom temp. bottom Avg. Avg. no. polar cod stn polarno.cod Avg. 0,4 1 0,2 0 0 55 -100 101 -200 201 -300 301 -400 401 -500 Depth interval Figure 6 . Average catch of polar cod (no/station at stations where polar cod was caught) at different depth intervals and the average temperature at the stations in the same depth intervals in the Icelandic groundfish survey in March 1985-2011.

180 160 1996 -2003 140 2003 120 100 80 Number 60 40 20 0 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 Length cm

Figure 7 . Length frequency distribution of polar cod in the Icelandic groundfish survey in March 1985-2011.

Figure 8 . Records of polar cod (number/station, coloured dots) in the Icelandic 0- group survey in August. Black dots are trawling stations were polar cod was not caught. Three selected years (1979, 1991, 1999) from the investigation the period of investigation (1975-2003) are shown.

150 125 100 75

Number 50 25 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Length, cm

Figure 9. Length frequency distribution of polar cod in the Icelandic 0- survey in August 1974-2003.