Fish Sci (2014) 80:117–126 DOI 10.1007/s12562-014-0716-z
SPECIAL FEATURE: REVIEW ARTICLE
Social-ecological systems on walleye pollock under changing environment: Inter-disciplinary approach
Comparison of factors affecting recruitment variability of walleye pollock Theragra chalcogramma in the Pacific Ocean and the Sea of Japan off northern Japan Tetsuichiro Funamoto • Orio Yamamura • Osamu Shida • Kazuhiko Itaya • Ken Mori Yoshiaki Hiyama • Yasunori Sakurai
•
Received: 1 July 2013 / Accepted: 11 December 2013 / Published online: 15 March 2014 Ó The Japanese Society of Fisheries Science 2014
Abstract The Japanese Pacific stock (JPS) and the northern Japan Sea stock (JSS) of walleye pollock Theragra chalcogramma are mainly distributed in the Pacific Ocean and the Sea of Japan off northern Japan, respectively. This paper summarizes and compares the factors affecting the recruitment variability of these two stocks. Spawning season is from December to March for both stocks. JPS recruitment has a positive relationship with the water temperature in January and February, whereas that of JSS has a negative relationship with the water temperature in January, February, and April. One possible reason for
This article is sponsored by the Fisheries Research Agency, Yokohama, Japan. T. Funamoto (&) O. Yamamura K. Mori Hokkaido National Fisheries Research Institute, Fisheries Research Agency, 116 Katsurakoi, Kushiro, Hokkaido 085-0802, Japan e-mail:
[email protected] O. Shida Central Fisheries Research Institute, Hokkaido Research Organization, 238 Hamanaka, Yoichi, Hokkaido 046-8555, Japan K. Itaya Wakkanai Fisheries Research Institute, Hokkaido Research Organization, 4-5-15 Suehiro, Wakkanai, Hokkaido 097-0001, Japan Y. Hiyama Japan Sea National Fisheries Research Institute, Fisheries Research Agency, 1-5939-22 Suido-cho, Chuou-ku, Niigata, Niigata 951-8121, Japan Y. Sakurai Graduate School of Fisheries Science, Hokkaido University, 3-1-1 Minato, Hakodate, Hokkaido 041-8611, Japan
this is that pollock larvae have an optimum growth temperature of approximately 5 °C in the field. Drift of early life stages also appears to be an important influence on the recruitment of both stocks. Because the current generated by the northwest wind carries eggs of JPS into the main larval nursery ground, JPS recruitment is enhanced in years when the northwest wind is predominant in February. On the other hand, early life stages of JSS are transported into the nursery ground by the Tsushima Warm Current. However, this current also carries early life stages into the Sea of Okhotsk and offshore, resulting in poor JSS recruitment in years when this current is strong in March. In contrast to JPS, the recruitment of which is significantly impacted by cannibalism, young pollock have not been found in the stomachs of adult JSS. Warm temperatures in the Sea of Japan seem to induce the separation of young and adult pollock, and the shape of the stock–recruitment relationship also suggests that cannibalism is not important for JSS. Based on this knowledge, and on the hatch date distributions of larvae and juveniles, we propose mechanisms that can explain the recruitment fluctuations for JPS and JSS pollock. Keywords Japanese Pacific stock Northern Japan Sea stock Recruitment fluctuation mechanism Regional comparison Stock characteristics Walleye pollock
Introduction Recruitment projection is a herculean task but an important one for stock assessment and the management of fishery resources. Precise recruitment projections make it feasible to simulate future biomass fluctuations, resulting in more reliable reference points such as the allowable biological
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Fig. 1 Biomass, spawning stock biomass (SSB), recruitment, and recruitment per spawning (RPS) for Japanese Pacific stock (JPS, left panels), and northern Japan Sea stock (JSS, right panels) of walleye pollock
catch (ABC). Recruitment projections use results from prerecruit surveys and model predictions. When a model prediction is adopted, it is mandatory to demonstrate the mechanism by which each covariate modulates recruitment. Hence, understanding the mechanisms that drive recruitment fluctuations is not only of biological interest; it is also important for stock assessment and management. Walleye pollock Theragra chalcogramma (hereafter simply ‘‘pollock’’) is widely distributed in the North Pacific Ocean from off central California to off northern Japan [1]. The pollock catch was more than 3 million tons in 2010, which was the world’s second largest single species catch after that of Peruvian anchovy Engraulis ringens [2], indicating the commercial importance of pollock. Around northern Japan, there are four pollock stocks: the Japanese Pacific stock (JPS), northern Japan Sea stock (JSS), southern Okhotsk Sea stock, and Nemuro Strait stock [3]. These stocks are mainly or temporarily distributed around Hokkaido Island (hereafter simply ‘‘Hokkaido’’). Although total allowable catches (TAC) are proposed annually for all four stocks, biomass has only been estimated for JPS and JSS using virtual population analysis. In contrast to JPS, which has exhibited a stable biomass since the 1980s, the stock of JSS has approached critical levels in recent years (Fig. 1) [4, 5]. From 2006 to 2010, abiotic and biotic factors affecting the recruitment variability of JPS and JSS were investigated under the ‘‘Dynamics of Commercial Fish Stocks’’ program, which identified several important drivers of recruitment for both stocks. The results from a
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Fig. 2 Early life history and related ocean currents of walleye c pollock. a Japanese Pacific stock, b Coastal Oyashio, Oyashio, Tsushima Warm Current, and Tsugaru Warm Current, c Northern Japan Sea stock
comparison of these drivers are expected to improve our understanding of the mechanisms controlling recruitment fluctuations of pollock. In this paper, we first summarize the stock characteristics of JPS and JSS from the viewpoints of life history and its environment. Second, we introduce the four main factors that affect JPS and JSS recruitment: water temperature, transport, predation, and spawning stock biomass (SSB), and investigate how each factor controls recruitment. Finally, based on this knowledge and hatch date distributions, we propose comprehensive recruitment fluctuation mechanisms for JPS and JSS.
Stock characteristics Life history JPS is distributed along the Pacific coast of northern Japan from off Joban to around Etorofu Island (Fig. 2a) [5, 6]. Although local spawning of JPS is observed around Kinkazan, the southeastern coast of Hokkaido (Doto area), and Etorofu Island, spawning mainly occurs just outside Funka Bay from December to March [5, 7–10]. Eggs
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spawned around Funka Bay are transported into the bay by the Coastal Oyashio (Fig. 2b) and the northwest wind, and larvae and juveniles remain inside the bay until approximately June [9, 11–13]. Afterward, juveniles migrate from Funka Bay, and most of them reach the Doto area after August [14, 15]. Juveniles settle in the Doto area and reside in this area or further east until they reach first maturity at 3 or 4 years of age [5, 16]. JPS begins maturity at age 3, and about 80 % of JPS mature at age 4 [5]. After reaching first maturity, adult JPS migrate seasonally between spawning (around Funka Bay) and feeding (around Doto) areas. JSS is mainly distributed in the Sea of Japan off Hokkaido (Fig. 2c) [4, 6]. In the 1970s and earlier, JSS spawned around Rebun Island, Rishiri Island, Musashi Bank, off Ofuyu, Ishikari Bay, Iwanai Bay, and off Otobe (Hiyama area) [17–19]. However, in recent years, few eggs at early developmental stages have been found in Ishikari Bay and further north, indicating that the main JSS spawning grounds are currently restricted to Iwanai Bay and the Hiyama area [20]. The spawning season for JSS is the same as that for JPS, ranging from December to March [4, 19]. Eggs and larvae are transported northward to the coast of northwestern Hokkaido (nursery ground) by the Tsushima Warm Current [20, 21]. Juveniles settle and reside around this nursery ground until their first maturity [4]. JSS starts maturity at age 3, and more than 80 % of JSS walleye pollock have matured by age 5. After reaching first maturity, the JSS migrates to Iwanai Bay or the Hiyama area for spawning; however, its post-spawning migration remains unclear. It is thought that JSS returns to the area off northwestern Hokkaido or migrates westward after spawning [17, 22]. Thereafter, adult JSS repeat spawning and feeding migrations. The recruitments of JPS and JSS to the fishery occur at the age of 2 or older [4, 5]. Hence, recruitment of both stocks is defined as the number of age-2 fish in this paper. Habitat environments The distribution of JPS is influenced by four ocean currents: Coastal Oyashio, Oyashio, Tsugaru Warm Current, and Kuroshio (Fig. 2b) [23]. Of these, the Coastal Oyashio and the Oyashio are cold currents that govern the thermal conditions along the Pacific coast of Hokkaido, which includes the main spawning and nursery grounds of JPS. On the other hand, the Tsushima Warm Current dominates the oceanographic conditions of the habitat of JSS. Therefore, although both JPS and JSS are located near the southern end of the range of this species, ambient temperatures are lower for JPS than for JSS. For example, the sea surface temperature (SST) around the spawning ground in February (one month of the spawning season) ranges from approximately 1 to 6 °C for JPS and from 4 to 9 °C for JSS [24].
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Factors controlling recruitment Water temperature JPS recruitment is positively related to January and February SSTs around the spawning ground [24, 25]. In contrast, a negative relationship is observed between JSS recruitment and winter (January–March) SST off western Hokkaido [24, 26]. However, the relationship between March SST and JSS recruitment is weak (Funamoto, unpublished data). Instead, a strong negative relationship with JSS recruitment is observed for April SST in addition to the January and February SSTs. Although the underlying mechanisms linking water temperature and pollock recruitment are still unclear, there are at least three plausible explanations. The first possibility is that water temperature directly affects the larval growth of pollock. Hamai et al. [27] examined the growth of pollock larvae at 2, 6, and 10 °C in the laboratory, and found that growth was strongest at the higher temperature. However, the difference in growth between 6 and 10 °C was quite small. In addition, larval food availability in the field is generally lower than that in a rearing environment, although the patchy distribution and/or higher energies of prey organisms in the wild can cause favorable food conditions. Under limited food conditions in the field, it is possible that the optimum growth temperature for larvae is lower than that under unlimited food conditions in the laboratory (e.g., [28]). The ambient temperature of pollock larvae is around 2–6 °C for JPS and 4–8 °C for JSS (Funamoto, unpublished data). Hence, if pollock larvae have an optimum growth temperature of around 5 °C in the field, warmer temperatures would result in increased larval growth for JPS and decreased growth for JSS, consistent with a positive relationship between water temperature and recruitment for JPS and a negative relationship for JSS. The second possibility is an indirect effect of water temperature on the survival of pollock larvae through food availability. The most important prey of pollock larvae at the first feeding stage is copepod nauplii [29, 30], and Funamoto [24] found a positive correlation between February SST and the density of copepod nauplii in Funka Bay. This implies higher prey abundance for larval JPS at higher water temperatures, likely resulting in higher JPS recruitment under warmer conditions. Unfortunately, the density of copepod nauplii has not been examined in relation to the distribution of JSS. It is necessary to investigate the larval food availability of JSS. The third possibility is an influence of water temperature on spawning locations. Based on acoustic observations, Shida et al. [31] suggested that JPS tends to spawn mainly off Iburi in years with high water temperatures, but off
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Fig. 3 Schematic of the recruitment fluctuation mechanism for Japanese Pacific stock of walleye pollock
Oshima in years with low temperatures (Fig. 2a). Eggs spawned off Iburi are transported toward the mouth of Funka Bay by the Coastal Oyashio. This can increase the number of eggs which are transported into Funka Bay, because the northwest wind can carry eggs only around the bay mouth into the bay (see ‘‘Transport’’). In contrast, eggs spawned off Oshima are carried further away from the mouth of Funka Bay by the Coastal Oyashio, probably decreasing the number of eggs which are carried into the bay. Eggs of JPS at early developmental stages are primarily found outside Funka Bay [32]. On the other hand, larvae of JPS are abundant in the inner area of the bay. These observations suggest that successful transport of eggs into Funka Bay is important to JPS recruitment [33]. Therefore, JPS recruitment is expected to be enhanced in years with high temperatures due to an increased drift of eggs into Funka Bay. The Coastal Oyashio affects the thermal conditions around Funka Bay, and temperatures around the bay are generally high in years when the influence of the Coastal Oyashio is weak. In such years, the survival of JPS can be enhanced by successful egg transport (due to spawning off Iburi) and fast larval growth (due to the direct effect of temperature and high larval food availability) (Fig. 3). For JSS, the interaction of water temperature and the bottom topography of the Sea of Japan significantly affects their spawning location [34]. Spawning of JSS was observed even around Rebun Island, Rishiri Island, Musashi Bank, off Ofuyu, and Ishikari Bay in the 1970s and earlier, when the water temperature was low (Fig. 2c) [17– 19]. However, these areas are relatively shallow, with a bottom depth of about 200 m, indicating that water temperature can increase throughout the water column from the surface to the bottom in years with high temperatures.
These high temperatures prevent adult pollock from accessing these areas for spawning. In contrast, the bottom depth around Iwanai Bay and the Hiyama area, the main spawning grounds of JSS in recent warm years, is more than 400 m. Therefore, water temperatures around these spawning grounds are low in the deep layer, even in years with high surface layer temperatures, enabling adult pollock to spawn. However, these spawning grounds are far from nursery areas off the northwestern coast of Hokkaido, so it is likely that the proportion of the early life stages that are transported offshore or encounter unfavorable environments on the way to the nursery ground increases in years with high temperatures [20]. This could cause the negative relationship between water temperature and JSS recruitment. Transport The successful transport of eggs into Funka Bay is important for JPS recruitment, and this egg transport is affected by the Coastal Oyashio and northwest wind [11– 13, 33]. Previous studies have indicated that the Coastal Oyashio transports eggs originating off Iburi to the entrance of Funka Bay, and that the currents generated by northwest winds carry the eggs around the entrance into the bay. For the latter transport, the combination of the northwest wind and the parabolic bottom topography of Funka Bay induces a current that flows northwesterly at the bay mouth, in the opposite direction to the winds from the northwest, and this current transports the eggs near the bay mouth inside the bay. The Far East Zonal Index (FEZI) reflects the shift in the axis of mid-latitude westerlies, and the northwest wind is predominant around Japan when the FEZI is negative [35]. Funamoto et al. [25] analyzed the
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relationship between FEZI and JPS recruitment and observed high recruitment under a negative February FEZI, providing empirical support for enhanced JPS recruitment in years when the northwest wind is predominant in February. Some fraction of the pollock eggs that cannot enter or remain in Funka Bay are transported to the Pacific coast of the Tohoku region (Tohoku area) [36, 37]. Transport to the Tohoku area is influenced by the interaction of the Coastal Oyashio, the Oyashio, and the Tsugaru Warm Current [23]. The Tohoku area was an important nursery ground for larvae and juveniles of JPS in the 1980s because the abundance of age-0 pollock in this area was high during this period [38]. However, there is currently no information on the contribution of these age-0 pollock in the Tohoku area to the year-class strength of JPS [23]. On the other hand, in the 1990s and since then, the abundance of 0-age pollock in the Tohoku area has been low [5, 38]. Thus, it is likely that, at least since the 1990s, successful egg transport into Funka Bay has been more important than that into the Tohoku area. This is supported by the observation that the relationship between February FEZI and JPS recruitment becomes stronger when only data since the 1990s are included (Funamoto, unpublished data). Because few JPS juveniles have been found in the offshore area, most of the JPS carried offshore (i.e., not into Funka Bay or the Tohoku area) during their early life stages are thought to die. Although the eggs and larvae of JSS are transported towards the northwestern coast of Hokkaido (nursery ground) by the Tsushima Warm Current, this current carries early life stages even further into the Sea of Okhotsk [39]. Between 1986 and 1989, when the distribution of pollock larvae and juveniles was examined using trawl nets, recruitment per spawning (RPS) was found to be low in 1989 (Fig. 1), when the proportion of larvae and juveniles transported into the Sea of Okhotsk was high. Therefore, it is believed that the larval and juvenile transport to the Sea of Okhotsk is one-sided, and that the extent of this loss is one of the key factors driving the recruitment variability of JSS. In addition, transport models indicate that some early life stages of JSS drift offshore (Okuno, personal communication). Similar to juveniles of JPS, few juveniles of JSS have been collected in the offshore area, suggesting the death of most juveniles that are flushed offshore. Funamoto [26] statistically investigated the relationship between the volume transport of the Tsushima Warm Current in March and JSS recruitment, and found a reduction in recruitment in years with a strong Tsushima Warm Current. When this current is strong, enhanced drift of early life stages offshore and/or into the Sea of Okhotsk seems to result in a weak year-class of JSS. The Tsushima Warm Current affects the thermal conditions off northwestern Japan, and temperatures off
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Fig. 4 Schematic of the recruitment fluctuation mechanism for the northern Japan Sea stock of walleye pollock. SSB spawning stock biomass
western Hokkaido are generally low in years with a weak Tsushima Warm Current. In such years, successful transport of early life stages to the nursery ground (due to weak currents and northward expansion of spawning grounds) and fast larval growth can lead to high survival of JSS (Fig. 4). Predation Juveniles of JPS are found in the stomach of some groundfishes in the Doto area, where most JPS settle [40, 41]. These predators include large-sized (about [300 mm body length) Pacific cod Gadus macrocephalus, Kamchatka flounder Atheresthes evermanni, and pollock. Catch per unit effort (CPUE) of the bottom trawl fishery can be used as an index for the abundance of these predators in the Doto area because these predators are the main targets of this fishery. Funamoto et al. [25] investigated the relationships between these CPUEs and JPS recruitment and found significant negative relationships between them, suggesting the importance of predation by these groundfishes on JPS recruitment. However, it should be noted that the relationships between these CPUEs and JPS recruitment have been weak since the 2000s (Funamoto, unpublished data). Unfortunately, information on the stomach contents of groundfishes in the Sea of Japan off Hokkaido is scarce. Based on limited data, no pollock juvenile has been found in the stomach of an adult pollock in the nursery ground of JSS [42, 43]; hence, adult pollock of JSS do not appear to be cannibalistic. This can probably be attributed to thermal conditions in the Sea of Japan, which has the warmest water temperatures among the major stocks of walleye pollock in the Pacific Ocean. Juvenile pollock (potential
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prey) are found in waters up to 16 °C, but adult pollock (potential predator) are generally distributed in waters \6 °C [15, 16, 44]. Therefore, warm temperatures in the nursery ground of JSS can lead to a separation between juvenile and adult habitats due to the vertically and horizontally limited distribution of adult pollock. However, further research on predation on JSS pollock is needed. SSB Temperature-dependent stock–recruitment models were adopted for JPS and JSS, and a constant stock–recruitment model in which SSB is not incorporated as an explanatory variable was selected as the best model for JPS [24]. Constant recruitment with increasing SSB implies that JPS recruitment is density dependent, but that there is no overcompensation, as in the Ricker model [45]. However, as mentioned in the section ‘‘Predation’’, a strong negative relationship is found between the CPUE of large pollock in the nursery ground (i.e., the abundance of cannibalistic adults) and JPS recruitment [25]. It appears that SSB is not a good index of cannibalism potential for JPS because the SSB does not take into account when and where cannibalism occurs. On the other hand, a density-independent stock–recruitment model in which SSB and recruitment are linearly related was chosen as the best model for JSS [24]. Although a relationship between abundance of cannibalistic adults and recruitment has never been examined for JSS, this density-independent stock–recruitment model suggests that cannibalism is not important for JSS, because the most probable density dependent mechanism for pollock is cannibalism [24, 46, 47]. In addition, a positive linear relationship between SSB and recruitment indicates that a higher SSB directly results in higher recruitment for JSS. Recruitment data at low SSB are scarce for both stocks, especially for JPS (Fig. 1). Hilborn and Walters [48] indicated that in order to understand how recruitment responds to SSB, the stock needs to be observed over a broad range of SSB values. It is desirable to reanalyze the stock–recruitment relationships with a sufficient range of SSB levels for both stocks because a stock–recruitment relationship is fundamental for determining appropriate ABC levels.
Recruitment fluctuation mechanism JPS Nishimura et al. [49] estimated the hatch dates of juvenile JPS by otolith microstructure analysis, and found that strong year-classes were characterized by early hatching.
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Namely, newly settled juveniles of the strong 2000 yearclass consisted mainly of individuals that hatched in February, whereas those of the weak 2001 and 2002 yearclasses hatched mostly after March. In the 1990s, newly settled juveniles of the strong 1995 year-class were dominated by individuals that hatched in January and February, whereas those of the weak 1996 and 1998 year-classes showed the peak hatching period in March [50, 51]. Thus, high survivorship of the individuals that hatch in January and February (hereafter ‘‘early-hatching individuals’’) up to settlement appears to be a necessity for good recruitment of JPS. The survival of early-hatching individuals around Funka Bay can be regulated by water temperature in January and February, as it has a positive relationship with JPS recruitment. For example, under warm conditions due to a weak Coastal Oyashio, it is possible that spawning occurs primarily off Iburi, which is favorable for the successful transport of eggs into Funka Bay, and that larval growth is improved by direct and indirect (through food availability) effects of temperature. Hence, the survival of early-hatching individuals is expected to be enhanced in years with high temperatures in January and February. In addition, a predominance of the northwest wind in February can also elevate the survival of early-hatching individuals through increased transport of eggs into Funka Bay. However, high survivorship of early-hatching individuals around Funka Bay does not result in a strong year-class of JPS when the abundance of predators in the Doto area is high. For example, recruitment was low for the 1997 year-class, which experienced high abundances of predators in the Doto area, although the peak hatching of newly settled juveniles occurred in February for this year-class [51]. High survivorship of early-hatching individuals before settlement seems to be a necessary but not sufficient condition for good recruitment. In other word, high survivorship of early-hatching individuals up to recruitment is likely to be a necessary and sufficient condition for achieving a strong year-class for JPS. Interestingly, the relationships between predator abundances in the Doto area and JPS recruitment have been weak since the 2000s. The reason for this is still unclear, but it is possible that the importance of the Doto area as a nursery ground has faded in recent years because the number of age-1 pollock in this area was rather low in some years, such as 2007 and 2009 [5]. The coastal region to the north of the Doto area may also be the main nursery ground of age-1 pollock in recent years. The reason for the high survivorship of earl- hatching individuals is thought to be their high abundance, because the number of newly hatched larvae caught in January and February is much higher than that in March [52]. That is, the number of age-2 fish can reach a level equivalent to good recruitment only when the survival of early-hatching
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individuals (the majority group) is high up to recruitment. Conversely, even if the survival of individuals hatching in March (the minority group) is high up to recruitment, the number of age-2 fish is too low to become a strong yearclass without high survivorship of early-hatching individuals. Therefore, high survivorship of early-hatching individuals (the majority group with high abundance) due to warm temperatures, a northwest wind, and low predator abundance seems to be important for good JPS recruitment (Fig. 3). JSS For JSS, otolith analysis indicated that juveniles of the 2006 year-class, which showed high RPS (Fig. 1), were mainly composed of individuals hatching in March, and that the hatching peak for juveniles of the 2008 and 2010 year-classes, whose RPSs were expected to be low, were late January or early February (Chimura, unpublished data). In the 1980s, larvae and juveniles of the 1986–1988 year-classes with high RPSs consisted mainly of individuals hatching in late February and March [39]. These suggest that high survivorship of individuals hatching in late February and March (hereafter ‘‘late-hatching individuals’’) is necessary for a high RPS of JSS. January, February, and April water temperatures (under the influence of the Tsushima Warm Current), which show strongly negative relationships with JSS recruitment, are thought to have an impact on the survival of late-hatching individuals. For example, it is possible that January and February temperatures regulate the spawning location, and that spawning occurs in Iwanai Bay and around the Hiyama area in years with high temperatures in January and February, probably causing low survival of late-hatching individuals due to a long distance between spawning and nursery grounds. On the other hand, in April, water temperature can control larval growth, and warm April temperatures appear to lead to low survival of late-hatching individuals due to slow larval growth. In March, the Tsushima Warm Current can also influence the survival of latehatching individuals through transport, and more early life stages seemed to be carried into the Sea of Okhotsk or offshore under a strong Tsushima Warm Current, resulting in low survival of late-hatching individuals. In contrast to the importance of the water temperature and transport during the pelagic phase, cannibalism after settlement is likely to have a negligible effect on JSS recruitment, because no pollock juvenile has been found in the stomach of an adult pollock in the nursery ground. The high correlation between juvenile abundance in May and JSS recruitment also supports the idea that year-class strength is largely determined before settlement (Chimura, unpublished data).
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The significance of the high survivorship of late-hatching individuals is unclear, but, similar to JPS, one possibility is a high abundance of late-hatching individuals. Although there is little information on the temporal abundance of eggs and newly hatched larvae for JSS, it was found that the peak spawning period of the 2006 year-class (a strong year-class), as estimated from egg data, is consistent with that estimated from hatch data of juveniles (late-hatching individuals) (Chimura, unpublished data), suggesting that late-hatching individuals originated from the true peak spawning period (i.e., majority group with high abundance) for this year-class. According to the stock–recruitment relationship, a negative density dependence is not apparent for JSS, indicating that increases in SSB result directly in higher recruitment [24, 26]. High SSB along with a high survivorship of late-hatching individuals (majority group) due to cold temperatures and a weak Tsushima Warm Current appear to be important for good JSS recruitment (Fig. 4).
Future issues The processes determining the recruitment of JPS are believed to be very complicated. For example, although the early life stages of JSS are only subject to the Tsushima Warm Current, those of JPS are under the influence of the Coastal Oyashio, Oyashio, Tsugaru Warm Current, and Kuroshio [23]. Furthermore, juveniles of JPS change their nursery ground from Funka Bay to the Doto area by moving against ocean currents [15]. Shida et al. [38] depicted these complicated processes by invoking eleven switches representing the factors that are probably related to the survival of JPS. These switches include egg quality and pre-settlement predation, which remain poorly understood. In addition, the life history of JPS spawned around Etorofu Island and their contribution to the year-class are still unclear. To deepen our understanding of the processes controlling the recruitment of JPS, it is necessary to investigate these unexplained factors. Stock recovery is urgently required for JSS because the biomass of JSS has been declining since the 1990s (Fig. 1). The positive stock–recruitment relationship of JSS, with no negative density dependence, indicates the need for SSB recovery to produce high recruitment. For JSS, a TAC system has been employed as a management measure, but the SSB has been low in recent years. More severe measures, such as drastic cuts in the TAC and extensive marine protected areas around the spawning ground should be adopted for JSS. Both JPS and JSS recruitment are related to water temperature, and recruitment projections based on temperature forecasts were performed for pollock in the eastern Bering Sea [53]. Therefore, recruitment projections
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may also become possible for JPS and JSS, although forecasts of factors other than temperature are probably difficult. These recruitment projections are expected to result in the proposal for a more reliable ABC and consequently TAC for both stocks. In addition, for JPS, the ABC has been recommended based on the stock status of JPS alone, i.e., single-species management has been conducted [5]. However, JPS recruitment is related to the abundance indices of Pacific cod and Kamchatka flounder, suggesting that it is important to shift to ecosystem-based management, considering the interactions with other species. A comparison of previous knowledge about JPS to that about JSS revealed various differences in the potential mechanisms driving recruitment. For example, a warmer temperature seems to induce higher recruitment for JPS but lower recruitment for JSS. Cannibalism pressure appears to be significant for JPS recruitment but negligible for JSS recruitment. Moreover, members of the cohort who play important roles in the recruitment dynamics are likely to be early-hatching individuals for JPS but late-hatching individuals for JSS. These differences are thought to be induced by the interaction between local environmental conditions (e.g., water temperature) and species traits of pollock (e.g., optimum growth temperature). Therefore, comparisons throughout the species range are expected to result in more comprehensive information about the species traits of pollock. We believe that these comparisons will also result in a better understanding of the recruitment fluctuation mechanisms for each regional pollock stock. Acknowledgments We would like to thank Keizo Yabuki, Tomonori Azumaya, Tomonori Hamatsu, Masayuki Chimura, Yuho Yamashita, Hiroshige Tanaka, Kouji Kooka, Tokihiro Kono, Kazushi Miyashita, Toshikuni Nakatani, Jun Yamamoto, and Hiroya Miyake for their help and discussions during this study. We also appreciate constructive comments by Reiji Masuda and three anonymous reviewers. This study was partly supported by the Fisheries Agency of Japan under the projects of ‘‘Assessment of Fisheries Stocks in the Waters Around Japan’’ and ‘‘Dynamics of Commercial Fish Stock.’’
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