Complete nucleotide sequence and variation of mitochondrial DNA from 10 individuals of walleye pollock, Theragra chalcogramma

ABSTRACT:   The complete mitochondrial genome sequence of 10 walleye pollocks, Theragra chalcogramma , from the Japan Sea and Bering Sea was determined. The 16 568–16 571 bp genome contains the same 37 mitochondrial structural genes (two ribosomal RNA, 22 transfer RNA, and 13 protein-coding genes) as found in all other vertebrates analyzed, in an organization identical to that of other bony fish. The major non-coding region had several conserved sequence features. Nucleotide variations of ND1, ND5, and control region were high, and these regions appear to be good candidates for high-resolution markers in population studies.     

Temperature-dependent stock-recruitment model for walleye pollock (Theragra chalcogramma) around northern Japan

Changes in fish year‐class strength have been attributed to year‐to‐year variability in environmental conditions and spawning stock biomass (SSB). In particular, sea temperature has been shown to be linked to fish recruitment. In the present study, I examined the relationship between sea surface temperature (SST), SSB and recruitment for two stocks of walleye pollock (Theragra chalcogramma) around northern Japan [Japanese Pacific stock (JPS) and northern Japan Sea stock (JSS)] using a temperature‐dependent stock‐recruitment model (TDSRM). The recruitment fluctuation of JPS was successfully reproduced by the TDSRM with February and April SSTs, and February SST was a better environmental predictor than April SST. In addition, the JPS recruitment was positively related to February SST and negatively to April SST. The JSS recruitment modeled by the TDSRM incorporating February SST was also consistent with the observation, whereas the relationship between recruitment and February SST was negative, that is the opposite trend to JPS. These findings suggest that SST in February is important as a predictor of recruitment for both stocks, and that higher and lower SSTs in February act favorably on the recruitment of JPS and JSS respectively. Furthermore, Ricker‐type TDSRM was not selected for either of the stocks, suggesting that the strong density‐dependent effect as in the Ricker model does not exist for JPS and JSS. I formulate hypotheses to explain the links between SST and recruitment, and note that these relationships should be considered in any future attempts to understand the recruitment dynamics of JPS and JSS.     

Spatial and temporal patterns of walleye pollock (Theragra chalcogramma) spawning in the eastern Bering Sea inferred from egg and larval distributions

Walleye pollock Theragra chalcogramma (pollock hereafter) is a key ecological and economic species in the eastern Bering Sea, yet detailed synthesis of the spatial and temporal patterns of pollock ichthyoplankton in this important region is lacking. This knowledge gap is particularly severe considering that egg and larval distribution are essential to reconstructing spawning locations and early life stages drift pathways. We used 19 yr of ichthyoplankton collections to determine the spatial and temporal patterns of egg and larval distribution. Generalized additive models (GAMs) identified two primary temporal pulses of pollock eggs, the first occurring from 20 February to 31 March and the second from 20 April to 20 May; larvae showed similar, but slightly lagged, pulses. Based on generalized cross‐validation and information theory, a GAM model that allowed for different seasonal patterns in egg density within three unique areas outperformed a GAM that assumed a single fixed seasonal pattern across the entire eastern Bering Sea. This ‘area‐dependent’ GAM predicted the highest densities of eggs (i.e., potential spawning locations) in three major areas of the eastern Bering Sea: near Bogoslof Island (February–April), north of Unimak Island and the Alaska Peninsula (March–April), and around the Pribilof Islands (April–August). Unique temporal patterns of egg density were observed for each area, suggesting that pollock spawning may be more spatially and temporally complex than previously assumed. Moreover, this work provides a valuable baseline of pollock spawning to which future changes, such as those resulting from climate variability, may be compared.     

Mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska, 1988–91

Mortality rates of larval walleye pollock Theragra chakogramma were estimated from larval survey data from 1988 to 1991. Mortality estimates were based on cohort-specific losses between occupations of survey grids. Interannually, estimates of early feeding stage larval mortality rates ranged over an order of magnitude, from 0.045–0.43 day^-1, and declined sharply with age. There is some evidence that mortality rates of early feeding larvae tend to be negatively correlated with temperature and postively correlated with wind mixing.     

Predation on walleye pollock (Theragra chakograrnma) eggs and yolk-sac larvae by pelagic crustacean invertebrates in the western Gulf of Alaska

Immunological detection of yolk protein was used to assess predation by pelagic amphipods (gammarid and hyperiid), mysids, and euphausiids on eggs and yolk-sac larvae of walleye pollock Theragra chalcogramma during 1988 and 1989. Consumption estimates were made on the basis of frequency of positive immunoassays, assay detection times (gut clearance time), predator abundance, and spatial overlap of predators and prey. From our results gammarid amphipods and euphausiids were important predators on eggs and yolk-sac larvae, respectively. Gammarid amphipods alone consumed about 14% of the standing stock of pollock eggs in 1989. These results were compared with those from clearance rate experiments of predators feeding on pollock eggs in 300-1 bags. In general, clearance rate estimates of egg consumption were lower than those determined from gut contents.     

Abiotic and biotic factors affecting recruitment variability of walleye pollock (Theragra chalcogramma) off the Pacific coast of Hokkaido,Japan

Abiotic and biotic factors affecting the recruitment variability of the Japanese Pacific stock (JPS) of walleye pollock (Theragra chalcogramma) were examined using a bivariate regression and multivariate combined model. Of the abiotic variables around Funka Bay (spawning ground), February sea surface temperature (SST) and wind direction index showed significant bivariate relationships with recruitment. February SST was positively related to recruitment, suggesting that warmer water temperature in February favors JPS recruitment. On the other hand, the relationship between February wind direction index and recruitment predicts high JPS recruitment under predominant northwest winds in February. For the biotic variables in the Doto area (nursery ground), significant and negative bivariate relationships with recruitment were observed for catch per unit effort of Kamchatka flounder (Atheresthes evermanni), Pacific cod (Gadus macrocephalus), and walleye pollock, implying an important impact of predation by these groundfishes on JPS recruitment. The overall model incorporating these abiotic and biotic factors successfully reproduced the variability in JPS recruitment. Temperature and wind conditions around the spawning ground along with predator condition in the nursery ground appear to play a dominant role in the recruitment dynamics of JPS. Based on these results and prior knowledge, we propose a new hypothesis to explain the processes controlling JPS recruitment.     

Early- to late-summer population growth and prey consumption by age-0 pollock (Theragra chalcogramma), in two years of contrasting pollock abundance near the Pribilof Islands, Bering Sea

Acoustic survey data were used to estimate the abundance and distribution of age-0 walleye pollock and zooplankton near the Pribilof Islands, Bering Sea, nursery area at two time periods in two consecutive years: the beginning of August, and mid-September, of 1996 and 1997. The 1996 pollock year class ultimately produced a large adult cohort in the eastern Bering Sea, while the 1997 year class produced a below-average adult cohort. Acoustic densities of age-0 pollock were significantly lower in August – and declined more strongly from August to September – in 1997 than in 1996, indicating that the trend to adult cohort strength was already set by August. Diet composition analyses revealed that age-0 pollock ate a much higher proportion of euphausiids in 1997 than in 1996, despite lower acoustic abundance of euphausiids in 1997. We infer that in 1996, age-0 pollock experienced greater feeding success by August, with high concentrations of copepods available for smaller fish to consume, and high concentrations of euphausiids available for larger individuals. In 1997, age-0 pollock had lower body condition in August and may have been limited by the availability of small (<2 mm) copepods. Bioenergetic modeling of prey consumption did not indicate a likelihood that age-0 pollock would begin to deplete euphausiids until late August in 1996, and not at all between August and mid-September in 1997.     

Interannual variability in growth of larval and juvenile walleye pollock Theragra chalcogramma in the western Gulf of Alaska, 1983–91

Interannual variability in growth of larval walleye pollock Theragra chalcogramma was examined from 1983 to 1991 and of juveniles from 1985 to 1990. ANCOVA was used to assess differences in population growth rates, and an alternate method was developed to examine variations between annual length-at-age data and average 'expected' values over different age groupings. For larvae, the years 1986, 1987, 1989 and 1990 had higher than average length-at-age, and 1988 and 1991 had lower than average values. Relationships between growth and SST and larval density were not clear. A tentative relationship between copepod nauplii abundance and larval length-at-age was noted. The consequence of larval growth to larval mortality, late larval abundance or recruitment was not clear. We conclude that larval mortality rates are highly variable and tend to mask effects of moderate variability in growth on later abundance. For juveniles, 1987 had significantly lower than average length-at-age and 1988 had higher than average values. Although there are few years of data, they tend to support the importance of juvenile growth in the recruitment process. Conditions for the large 1988 year class are documented and discussed, including warm SST, calm winds, relatively low larval growth rates, low abundances of potential predators on larvae, low larval mortality rates, and high juvenile growth rates.     

Interannual variability in hatching period and early growth of juvenile walleye pollock, Theragra chalcogramma, in the Pacific coastal area of Hokkaido

Juvenile walleye pollock of the Japanese Pacific population were collected from the Funka Bay [spawning ground; 16–64 mm fork length (FL)] in spring and the Doto area (nursery ground; 70–146 mm FL) in summer. Hatch dates were estimated by subtracting the number of otolith daily increments from sampling dates, and their early growth was back‐calculated using otolith radius–somatic length relationships. Interannual change of the hatching period was observed during 2000–02, and the peaks ranged from mid‐February in 2000 to early‐April in 2002. In 2000, when a strong year class occurred, early life history of the surviving juveniles could be characterized by early hatching and slower growth in the larval stage (<22 mm length). Higher growth rate in 2001 and 2002 did not always lead to good survival and recruitment success. Even though their growth was slow in 2000, the larvae hatched early in the season had larger body size on a given date than faster‐growing larvae hatched in later season in 2001 and 2002. Bigger individuals at a certain moment may have advantage for survival. The delay of hatching period may result in higher size‐selective mortality, and as a necessary consequence, back‐calculated growth in 2001 and 2002 could shift towards higher growth rate, although abundance of such a year class would be at the lower level. Variability in spawning period, early growth and their interaction might have a strong relation to larval survival through cumulative predation pressure or ontogenetic changes in food availability.     

Causes of walleye pollock (Theragra chalcogramma) recruitment decline in the northern Sea of Japan: implications for stock management

Recruitment of the northern Japan Sea stock (JSS) of walleye pollock has been decreasing since around 1990. In this study, I analyzed the factors causing this decrease in recruitment by investigating the relationship between recruitment, spawning stock biomass (SSB) and environmental factors using a generalized additive model (GAM). GAM fit to the data showed the importance of SSB, sea surface temperature (SST), ocean current strength (Tsushima Warm Current) and wind intensity (Asian monsoon) in determining the recruitment. Of these, the relationship between SSB and recruitment was positive and not negatively density‐dependent. On the other hand, the recruitment was negatively related to SST and ocean current strength, and a dome‐shaped relationship was observed between wind intensity and recruitment. Since around 1990, the values of SST and ocean current strength have mostly been high and that of wind intensity mostly low. In addition, SSB has been decreasing since the late 1990s. It is likely that the recruitment decline of JSS after approximately 1990 has been caused by warm water temperature, strong Tsushima Warm Current and weak Asian monsoon, and that the recent decrease in SSB has amplified this recruitment decline. According to the model’s estimation, a recruitment recovery due to environmental improvement will be highly restricted as long as SSB remains at its current low level. Significant recovery of SSB is urgently needed for JSS.     

The influence of pelagic habitat selection and interspecific competition on productivity of juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus) in the Gulf of Alaska

Here we investigate processes affecting productivity of capelin and walleye pollock in the Gulf of Alaska. We examine pelagic habitat selection by comparing the distribution of juvenile fish and their prey with oceanographic properties and we evaluate the potential for interspecific competition by comparing diets and measures of foraging. The primary field study was conducted in Barnabus Trough, Kodiak Island, Alaska, during September 2005. The distribution of fish was assessed acoustically and trawls were used to collect individual fish for stomach content analyses. Physical and biological data were collected with conductivity–temperature–depth probes and zooplankton tows. Age‐0 pollock were distributed in cool waters offshore of a mid‐trough front, coincident with the distribution of euphausiids, their preferred prey. In contrast, capelin and their prey (copepods) were distributed throughout the trough. We observed that sympatric capelin (occurring with pollock) often had reduced foraging success compared to allopatric capelin (occurring alone). Results of a bioenergetic model also suggest that the exclusion of capelin from foraging on euphausiids can have negative consequences for capelin growth.     

Effect of ocean conditions on the cross-shelf distribution of walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus)

Acoustic trawl surveys were conducted in 2000 and 2001 in two troughs located off the eastern coast of Kodiak Island in the Gulf of Alaska as part of a multiyear, multidisciplinary experiment to examine the influence of environmental conditions on the spatial distribution of adult and juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus). Continuous underway sea surface temperature samples and water column profiles collected in 2000 and 2001 showed the presence of a sharp shelf‐break front in Chiniak Trough and a mid‐trough front in Barnabas Trough. At distances <22 km from shore, the water column was well mixed, whereas a well‐defined mixed layer was present beyond approximately 22 km from shore. Satellite drifter tracks in Barnabas Trough entered along the upstream edge of the trough and appeared to follow the frontal boundary across the middle portion of the trough. A storm in 2001 weakened stratification and cooled surface water temperature by 1.6–2.1°C. Wind mixing associated with the storm event mixed subsurface chlorophyll a to the surface and enhanced nutrients in the surface waters. The storm event revealed spatial partitioning of summer production in Barnabas Trough, with production concentrated in regions inside the mid‐trough front. In contrast, post‐storm summer production was distributed throughout Chiniak Trough. The spatial distribution of walleye pollock and capelin differed and appeared to be related to differences in habitat characteristics. Acoustic survey data identified four acoustic sign types: age‐1 pollock, adult pollock, capelin, capelin–age‐0 pollock mix. The spatial distribution of these four sign types appears to be influenced by the oceanographic and topographic features of the two troughs. Adult pollock were broadly distributed throughout Chiniak Trough, whereas adult pollock were aggregated on the coastal side of the frontal system in Barnabas Trough. In 2000, capelin occurred with age‐0 pollock. In Chiniak Trough, capelin were most abundant along steep topographic gradients at the edges of the trough and in a deep region near Cape Chiniak, whereas the capelin–age‐0 mix (2000) or capelin (2001) concentrations were observed in slope water intrusions over the outer shelf in Barnabas Trough. Results suggest that habitat selection of walleye pollock and capelin are controlled by different processes. Capelin distributions appear to be limited by oceanographic conditions while other factors appear to be more important for pollock.     

Annual variation in otolith increment widths of walleye pollock (Gadus chalcogrammus) larvae in Funka Bay,Hokkaido, Japan

To clarify relationships between year‐class strength and larval growth of walleye pollock (Gadus chalcogrammus), and oceanographic conditions in the Pacific stock off Hokkaido and Tohoku, Japan, we undertook conductivity/temperature/depth (CTD) observations and investigated larval densities, larval otolith increment widths and larval prey densities (of copepod nauplii) of the 2008, 2009, 2010 and 2011 yr classes in Funka Bay. Oyashio Coastal Water (OCW) flowed into the bay in late February in 2008, 2010 and 2011, and the mean water temperatures decreased to 1.9–3.1 °C in March. OCW was not observed in 2009, and it was warm in late February (≥3.4 °C). Increment widths of lapillar otoliths during the yolk‐sac stage were wide in 2009 and 2011, medium in 2010 and narrow in 2008. Increment widths during the first‐feeding stage tended to become wider as the hatch month progressed, and the annual variation during the first‐feeding stage was larger than that of the yolk‐sac stage. The densities of the primary food for the larvae were high in 2008 when larval increment widths were narrowest, so the effect of prey abundance on larval growth appeared to be small. The ranking of the larval abundance in March was nearly coincident with that of the increment width during the larval stage. We, therefore, suggest that the larval growth rate is associated with the mortality rate and that the growth–mortality hypothesis may be applicable to walleye pollock in Funka Bay. Feeding success under warm water conditions may be an important factor that contributes towards high growth rates.     

Ichthyophonus‐infected walleye pollock Theragra chalcogramma (Pallas) in the eastern Bering Sea: a potential reservoir of infections in the North Pacific

In 2003, the Alaska walleye pollock industry reported product quality issues attributed to an unspecified parasite in fish muscle. Using molecular and histological methods, we identified the parasite in Bering Sea pollock as Ichthyophonus. Infected pollock were identified throughout the study area, and prevalence was greater in adults than in juveniles. This study not only provides the first documented report of Ichthyophonus in any fish species captured in the Bering Sea, but also reveals that the parasite has been present in this region for nearly 20 years and is not a recent introduction. Sequence analysis of 18S rDNA from Ichthyophonus in pollock revealed that consensus sequences were identical to published parasite sequences from Pacific herring and Yukon River Chinook salmon. Results from this study suggest potential for Ichthyophonus exposures from infected pollock via two trophic pathways; feeding on whole fish as prey and scavenging on industry‐discharged offal. Considering the notable Ichthyophonus levels in pollock, the low host specificity of the parasite and the role of this host as a central prey item in the Bering Sea, pollock likely serve as a key Ichthyophonus reservoir for other susceptible hosts in the North Pacific.     

Presence of Anisakis simplex (Rudolphi, 1809 det. Krabbe, 1878) and Hysterothylacium aduncum (Rudolphi, 1802) (Nematoda; Anisakidae) in runts of farmed Atlantic salmon,Salmo salar L

One hundred farmed Atlantic salmon, Salmo salar L., were examined for the presence of nematodes by digestion of tissue in HCl–pepsin solution. All fish were sampled from one cage in a fish farm on the Norwegian south‐west coast. Fifty harvest quality salmon, that is, salmon for human consumption (mean 5.4 kg, variation 3.0–7.6 kg), were sampled at the processing line while 50 salmon runts (mean 1.1 kg, variation 0.4–1.8 kg), discarded due to poor performance, were sampled from the discard bin after the grading station. Runts are individual fish with clear signs of poor performance over time and abnormal appearance and are thus not processed for human consumption. No nematodes were found in the musculature or viscera of the 50 harvest quality salmon. In total, 75 nematodes were found in 10 (20%) of the runts; 53 nematodes in the viscera and 22 in the musculature. Nematodes in the musculature were identified as Anisakis simplex (Rudolphi, 1809 det. Krabbe, 1878), while nematodes in the viscera were identified as A. simplex and Hysterothylacium aduncum (Rudolphi, 1802).