Discussion

Comparison of our data with those of Ivanov (1998) show that the boreal clubhook squid is less frequently caught near the bottom than within the upper (0-50 m) pelagic layer (<1% vs. 17.1% respectively). On the one hand, these data are in

w 10

n=41

Fig. 19.5 Weight frequencies of the boreal clubhook squid in the Pacific waters off the northern Kurile Islands and southeastern Kamchatka, 1993-2002 (n = number of squids weighed).

<2.5 2.6-5.0 5.1-7.5 7.6- 10.1- 12.6- 15.1- 17.6- 20.0- 22.6- 25.1- 27.6- 30.1- 32.6- >35 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0

Body mass, kg

Fig. 19.5 Weight frequencies of the boreal clubhook squid in the Pacific waters off the northern Kurile Islands and southeastern Kamchatka, 1993-2002 (n = number of squids weighed).

80 100 Mantle length, cm

Fig. 19.6 Length-weight relationship in the boreal clubhook squid in the Pacific waters off the northern Kurile Islands and southeastern Kamchatka, 1993- 2002 (n = number of squids weighed and measured).

80 100 Mantle length, cm

Fig. 19.6 Length-weight relationship in the boreal clubhook squid in the Pacific waters off the northern Kurile Islands and southeastern Kamchatka, 1993- 2002 (n = number of squids weighed and measured).

contradiction with the suggestion (Hochberg and Fields, 1980; Nesis, 1982; Bizikov, 1996) that this squid primarily inhabits near-bottom layers of the outer shelf and bathyal zone. On the other hand, this may not reflect real species' abundance or distribution patterns near the bottom and in upper ocean layers but is the result of different recovery capability of bottom trawls (vertical opening 5-7 m) and large midwater trawls (vertical and horizontal openings 50-80 m).

The data available on the species' frequency of occurrence make it possible to assume that this species is much less abundant in the eastern part of the North Pacific. One proof of this is that, as some data indicate, only six individuals of the boreal clubhook squid were caught during the last 29 years in the waters off the coast of Washington, which yield over 100,000 metric tons of fish annually (Miller, 2003). According to other sources (Anderson, 2002), somewhat over 50 squid of this species were found in the Puget Sound area (Washington) between 1949 and 2002. In our case, over 60 individuals of this species were recorded only in bottom trawl catches during the decade of research.

The data available on the depths of the species' habitat are scarce, and they are somewhat contradictory. There is a view that this species is common at great depths (Meachum, 2002). Nesis (1982) pointed out that this squid inhabits the near-bottom layers of the lower sublittoral and upper bathyal zones. The only paper suggesting a concrete vertical range for the species of 100-600 m is Hochberg and Fields (1980). A lot of papers indicated frequent and even abundant captures of this squid in the epipelagic layer (Pearcy, 1991; Radchenko, 1992; Shuntov et al., 1993; Radchenko et al., 1997; Nagasawa et al., 1998; Ivanov, 1998; Sinclair et al., 1999; Ishida et al., 1999; Ivanov and Sukhanov, 2002). There have been records of capturing the boreal clubhook squid in the Bering Sea from depths of 300-450 m (Bizikov and Arkhipkin, 1997; Miller, 2003) whereas it was caught at 700-910 m off the US West Coast (Anon., 2002; Hines, 2002). The pattern of its vertical distribution in the area studied is similar to that of the abundant northwest Pacific near-bottom Commander squid Berryteuthis magister (both species have similar ranges and inhabit biotopes of the same sort); according to various sources, the latter squid's concentration density is greatest in the range of 150-600 m (Railko, 1983; Nesis, 1989; Fedorets et al., 2000).

The temperatures in the species' habitats are virtually unknown. Anderson (2002) pointed out that the water temperature around Puget Sound fluctuates from 6°C to 13°C, while it is about 7°C at the depth of 250 m off the coast of Washington. In temperate waters, boreal clubhook squid occurs in the ocean at surface temperatures ranging from 6°C to 25°C with maximum frequency from 6°C to 9°C (Pearcy, 1991). In terms of thermal conditions, the species' habitat is close to that of the Commander squid whose major concentrations tend to be found at bottom temperatures exceeding 3.65°C (Verkhunov, 1996).

The boreal clubhook squid ranks second by size among the squids occurring in the North Pacific, the first being Architeuthis japonicus whose mantle can be up to 6 m of the total length of over 20 m (Roper et al., 1984). The maximum length data on the clubhook squid are quite contradictory. Akimushkin (1963) suggests that the maximum size is 6.8 m (total length with tentacles) and 2.47 m (mantle length). The maximum length data provided by Miller (2003) are close to that (6.1 m). Other authors put forward the values of 2 m (Roper et al., 1984) or 2.5 m (Nesis, 1982) as maximum lengths of the mantle, or over 4 m as an overall length (Anderson, 1996). Several papers (Armstrong, 2001; Anon., 2002; Hines, 2002) refer to information that the boreal clubhook squid can reach the length of 914 cm (30 ft.) which appears unlikely since there have been no documented reports of finding squids of this size. Our data agree with those of Hochberg and Fields (1980) that the squids of this species with a mantle of about 1 m were most frequently caught. Akimushkin (1963) reported that 1.5-2 m squids are not rare. Of the species found in bottom trawl catches in the western Bering Sea, 11 individuals had 97.5-141.5 cm long mantles (Bizikov and Arkhipkin, 1997). Some dead squids, 152-335 cm long have sporadically been found on the US West Coast (Armstrong, 2001; Meachum, 2002; Miller, 2003), though it is unclear which length of the dead specimens was measured: total length or that of the mantle. Consequently, the shortage of data prevents a current judgment regarding similarity or disparity in the size composition of the boreal clubhook squid in various parts of its range. In the western Bering Sea, squids captured were approximately of the same size (97.5-141.5 cm long mantles), age, sex, and stage of maturity (immature females) (Bizikov and Arkhipkin, 1997). This fact, according to the opinion of those authors demonstrates that the western Bering Sea serves as a feeding ground for immature females of the boreal clubhook squid. Unfortunately, we have no data on sex composition of our squid catches. The longevity of this species is unknown though it is suggested that its life span exceeds one year (Bizikov and Arkhipkin, 1997). The size composition (very small number of squids with 75-80 cm and 100-105 cm DML) of our catches may reflect the complex age structure of the population with the existence of two or three age groups. Juvenile boreal clubhook squids are known mostly from waters off Japan (Tsuchiya and Okutani, 1991). Specimens with mantle less than 95 cm do not occur in the western Bering Sea (Bizikov and Arkhipkin, 1997) but are very abundant (52.4%) in the Pacific off the northern Kurile Islands and southeastern Kamchatka (our data). Therefore, it is possible to suggest that the tropical and subtropical waters from Taiwan and Japan to California are the spawning grounds of the boreal clubhook squid. The temperate and boreal waters of the North Pacific (including Bering Sea) serve as feeding grounds for this species. It is likely that squids perform feeding migrations gradually with growth and return to spawning grounds while mature; the life span may take about 2-3 years.

The body mass data of the species in question are contradictory as well. Roper et al. (1984) suggested 50 kg as the maximum weight. According to other sources (Miller, 2003), the weight of the boreal clubhook squid may reach 181-272 kg, though findings of squids having such a body mass have not been documented. The recorded weight of the individuals near the US West Coast was 36.3-45.4 kg (Armstrong, 2001; Meachum, 2002; Miller, 2003), which agrees with the previously published data (Roper et al., 1984).

The mantle length-body mass relationship in the species under review has not been previously investigated. The exponential coefficient in the equation obtained is close to that of the Commander squid (2.5312-2.8028 depending on the sex and state of maturity) of the western Bering Sea (Bizikov and Golub, 1996), which may be evidence of the similarity in the mechanisms of growth in both species.

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