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

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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] 1 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.
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