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.     

Prey selection of common minke (Balaenoptera acutorostrata) and Bryde's (Balaenoptera edeni) whales in the western North Pacific in 2000 and 2001

A study of common minke and Bryde's whales was conducted in the western North Pacific in the 2000 and 2001 summer seasons to estimate prey selection of cetaceans as this is an important parameter in ecosystem models. Whale sighting and sampling surveys and prey surveys using quantitative echosounder and mid‐water trawl were carried out concurrently in the study. Biomasses of Japanese anchovy, walleye pollock and krill, which were major prey species of common minke and Bryde's whales, were estimated using an echosounder. The results suggested that common minke whale showed prey selection for Japanese anchovy while they seemed to avoid krill in both the offshore and coastal regions and walleye pollock in the continental shelf region. Selection for shoaling pelagic fish was similar to that in the eastern North Atlantic. Bryde's whale showed selection for Japanese anchovy in August 2000 and July 2001, while it showed prey selection for krill in May and June in 2001.     

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.     

Evaluation of stock management procedures for walleye pollock in northern waters of the Sea of Japan using a simulation

To evaluate walleye pollock stock management procedures in the northern waters of the Sea of Japan, 30-year population dynamics, including uncertainties, were forecast. Errors in current stock size estimation, variability in future recruitment and changes in future fishing mortalities were incorporated. Results of virtual population analysis (VPA) from resampled catch-at-age data with bootstrap methods was used as the current stock size estimation with uncertainty. Performances of each scenario were evaluated using conservation, utilization, stability and reliability factors. Twenty-two management scenarios and continuing the current fishing mortality were evaluated. Scenarios with minor regulation changes and continuing the current fishing mortality showed poor stock conservation performances. Scenarios with minor regulation changes produced good short-term but poor long-term utilization. Stabilities were poor in continuing the current fishing mortality and fishing ban scenarios. Reliability in all scenarios after 30 years was smaller than in continuing the current fishing mortality; however, small differences among scenarios were observed. The simulation results indicated that multilateral assessment is needed to evaluate the management candidates. Uncertainty caused by recruitment variability mostly affected future population dynamics. The role of simulations in the production of effective scientific advice is discussed.     

A review of the distribution, ecology and population dynamics of age-0 walleye pollock in the Gulf of Alaska

This review paper synthesizes published research and unpublished data on the abundance and distribution patterns, ecology and population dynamics of walleye pollock ( Theragra chalcogramma ) during their first year of life (age-0) in the Gulf of Alaska. Distribution patterns have been described using mainly trawl catches, but recently, acoustic methodology has been employed, especially in examining vertical distributions. Although age-0 pollock are found throughout the Gulf, the highest catches occurred west of Kodiak Island. Pollock are pelagic for at least their first 6 months of life but show an ontogenetic increase in depth distribution superimposed on a pronounced diel vertical migration at a larger size. Daily growth rates are variable depending on year, season and area, and growth generally ceases during the winter. The diet of age-0 pollock shifts from mainly copepods in early juveniles to euphausiids by fall, with epibenthic organisms becoming important during the winter months. Feeding occurs mainly at night in surface waters. Age-0 pollock are most frequently associated with gelatinous zooplankton (medusae) and older pollock. Many predators on age-0 pollock have been identified; the most important are arrowtooth flounder ( Atheresthes stomias ), adult pollock, puffins ( Fratercula spp.), murres ( Una spp.), harbor seals ( Phoca vitulina richardsi ) and Steller sea lions ( Eumetopiasjubatus ). Modelling provides some insight into the population dynamics of these juveniles and environmental conditions which interannually affect their survival. These results are discussed relative to the importance of age-0 pollock in the recruitment of this species and to their role in the pelagic ecosystem.     

Density effects on length at maturity of walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> off the pacific coast of northern Japan in the 1990s

To determine how stock abundance fluctuations of walleye pollock Theragra chalcogramma are related to variations in reproductive characteristics, the length at 50% maturity of the fish sampled off the Pacific coast of northern Japan from 1990 to 1999 was examined. In both sexes, the fish density increased, and the body length of age-3-5 fish decreased, but the condition factor and the age at 50% maturity showed no clear trend during this period. Male length at 50% maturity decreased, while value for females showed no clear trend. In both sexes, significant negative correlations were found between density index (combined age-3-5) and each body length at age-3-5. Significant positive correlations were seen between each body length at age-3-5 and length at 50% maturity in males. Positive correlation between body length at age-5 and length at 50% maturity was significant in females from 1990 to 1999, except for the samples in 1998. We suggest that the fish density might affect the length at maturity in males and possibly also in females.     

Endocrine and gonadal changes during the annual reproductive cycle of the freshwater teleost,Stizostedion vitreum

The annual reproductive cycle of walleye (Stizostedion vitreum) was characterized by documenting changes in gonadal development and serum levels of estradiol-17β (E₂), testosterone (T), 17α,20β-dihydroxy-4-pregnen-3-one (17,20-P), and 11-ketotestosterone (11-KT) in wild fish captured from upper midwestern lakes and rivers throughout the year. Fish from the populations used in this study spawn annually in early- to mid-April. Walleye showed group synchronous ovarian development with exogenous vitellogenesis beginning in autumn. Oocyte diameters increased rapidly from ∼ 200 μm in October to ∼ 1,000 μm in November, and reached a maximum of 1,500 μm just prior to spawning. Changes in gonadosomatic indices (GSIs) paralleled changes in oocyte diameters. Serum E₂ levels in females increased rapidly from low values in October (< 0.1 ng ml⁻¹) to peak levels of 3.7 ng ml⁻¹ in November, coinciding with the period of the most rapid ovarian growth. Subsequently, E₂ levels decreased from December through spawning. Serum T levels exhibited a bimodal pattern, increasing to 1.6 ng ml⁻¹ in November, and peaking again at 3.3 ng ml⁻¹ just prior to spawning. We detected 11-KT in the serum of some females at concentrations up to 5.6 ng ml⁻¹, but no seasonal pattern was apparent. In this study (unlike our results in a related study) 17,20-P was not detected. In males, differentiation of spermatogonia began in late August, and by January the testes were filled (> 95% of germ cells) with spermatozoa. Mature spermatozoa could be expressed from males from January through April. GSIs ranged from 0.2% (post-spawn) to 3.2% (pre-spawn). Serum T levels rose from undetectable levels in post-spawn males to 1.6 ng ml⁻¹ by November, remained elevated throughout the winter, and peaked at 2.8 ng ml⁻¹ I prior to spawning. Levels of 11-KT in males remained low (< 10 ng ml⁻¹, from post-spawning through January, then increased significantly by March and peaked just prior to spawning at 39.7 ng ml⁻¹. Our results indicate that vitellogenesis and spermatogenesis are complete or nearly so, in walleye by early winter, and suggest that it may be possible to induce spawning in this species several months prior to the normal spawning season by subjecting fish to relatively simple environmental and hormonal treatments.     

Utilization of free amino acids, yolk proteins and lipids in developing eggs and yolk-sac larvae of walleye pollock Theragra chalcogramma

ABSTRACT:   To elucidate the utilization of the major yolk nutrient stocks in eggs and larvae of walleye pollock Theragra chalcogramma , the contents of free amino acids (FAA), the major yolk protein (180 kDa lipovitellin originated from vitellogenin B in ovulated eggs: oLv B), and lipids were measured. Most eggs hatched 18 days after fertilization at 5°C, and all larvae absorbed almost all their yolk mass by 28 days. The total FAA content showed no change during the first 6 days, and then decreased to 28% of the initial level by 18 days. The oLv B contents, measured by an enzyme-linked immunosorbent assay using a specific antiserum against oLv B, gradually decreased from 6 to 18 days, followed by a rapid decline. The content of phospholipids (PL) and triacylglycerols (TG) showed no marked change until hatching, and then decreased until disappearance of yolk sac. From these results, it is proposed that there are two main periods for nutrient utilization in embryos and larvae of walleye pollock. In the first period, FAA was mainly utilized until 18 days after fertilization. Active utilization of oLv B and lipids (PL and TG) instead of FAA occurred during the second period from 18 to 28 days.     

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.