Ontogenetic shift in geotaxis for walleye pollock,Theragra chalcogramma free embryos and larvae: potential role in controlling vertical distribution

Synopsis The behavioral capability of walleye pollock,Theragra chalcogramma free embryos and larvae to control vertical distribution was assessed by examining buoyancy during resting and swimming orientation and activity as they developed in complete darkness from hatching to first feeding readiness (1 to 7 d post hatching at 6° C). Free embryos exhibited positive geotaxis 1 d post hatching, actively swimming through a density gradient to remain in the lower water column. Activity increased with free embryo development and by 7 d post hatching, feeding-ready larvae reversed their vertical orientation, now exhibiting negative geotaxis as they migrated to the upper water column. The results indicate that even at the earliest developmental stages, walleye pollock possess the capability to control vertical distribution. Laboratory results are compared with patterns of vertical distribution observed in the sea.     

The influence of light on egg buoyancy and hatching rate of the walleye pollock, Theragra chalcogramma

Changes in buoyancy in fertilized bathypelagic eggs of the walleye pollock, Theragra chalcogramma , collected from Shelikof Strait in the Gulf of Alaska were measured under controlled laboratory conditions in density gradient columns from 90 h post–fertilization through hatching. Eggs were incubated at 6° C and exposed to either diel light or constant dark. Eggs held under diel light conditions became more dense than eggs under constant dark beginning <10 h after exposure to light and remained so until 12 h before hatching. Eggs held under constant dark then became more dense than those under diel light. Hatching of eggs under both conditions began at the same time but eggs under diel light showed a delayed hatching rate. Light–induced changes in egg density indicate the ability of walleye pollock eggs to respond to external stimuli and thereby alter their position in the water column in an ecologically meaningful way.     

Prey selection by age-0 walleye pollock, Theragra chalcogramma, in nearshore waters of the Gulf of Alaska

Juvenile walleye pollock, Theragra chalcogramma, is the dominant forage fish on the continental shelf of the Gulf of Alaska, yet little is known about the feeding habits of this important interval of pollock life history. The taxonomic composition and size of prey found in the stomachs of age-0 juveniles collected at three nearshore locations in the Gulf of Alaska in September 1990 were compared to the composition and size of zooplankton collected in concurrent plankton tows. The maximum length of prey consumed increased dramatically over the length range of pollock examined (58–110 mm) from approximately 7 mm to 30 mm, due mainly to the consumption of large euphausiids and chaetognaths by the bigger individuals. The maximum width of prey changed little over this size range although there was a general increase in prey width with increasing predator size. The minimum prey length and width did not change with increasing fish size. Juvenile pollock generally selected the larger prey sizes relative to what was available. Juvenile pollock showed a marked preference for adult euphausiids and decapod larvae and an avoidance of copepods and chaetognaths relative to the numbers collected in net tows. These results are discussed relative to the feeding ecology of these juvenile fishes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Assessing the condition of walleye pollock Theragra chalcogramma (Pallas) larvae using muscle-based flow cytometric cell cycle analysis

Flow cytometric cell cycle analysis was used to determine the fraction of muscle cells in the S and G2 phases of the cell cycle, which were used as covariates with temperature and standard length, in a laboratory-developed model to assess the physiological condition of wild walleye pollock, Theragra chalcogramma, larvae. The assay was calibrated to the range of temperatures larvae are likely to encounter in the eastern Bering Sea, and it was sensitive to changes in condition within 3 days of starvation. The S and G2 phases of the cell cycle gave an indication of larval walleye pollock condition. Healthy larvae had a larger fraction of cells in the S phase than G2 phase, and unhealthy larvae had a larger fraction of cells in the G2 phase than the S phase. Validation tests showed that the model classified 75% to 83% of the larvae correctly. The assessment of the condition of walleye pollock larvae collected from the southeastern Bering Sea in 2007 indicated that unhealthy larvae were located on the continental shelf (6%), and this may be due in part to the coldest temperatures occurring there and less abundant prey. In the continental slope/ocean basin waters, where prey levels were higher and temperatures warmest, no larvae in unhealthy condition were found.     

Formation and maintenance of aggregations in walleye pollock,Theragra chalcogramma,larvae under laboratory conditions: role of visual and chemical stimuli

Synopsis Although planktonic marine fish larvae are often distributed in aggregations, the role of behavioral responses to environmental factors in these aggregations is not well understood. This work examines, under laboratory conditions, the influence of visual and chemical stimuli in the formation and maintenance of aggregations in walleye pollock,Theragra chalcogramma, larvae. Larvae were exposed to a horizontal gradient of light (visual stimulus), prey scent (chemical stimuli: squid/copepod and rotifer) or prey density (visual & chemical stimuli: rotifers). While larvae did not respond to prey scent, they did respond to a gradient of light or prey, which resulted in the formation and maintenance of aggregations. Larvae moved into and remained in a zone of higher light intensity (0.56 versus 0.01 mol photons m^-2 s^-1). Once encountering a patch of prey, larvae remained aggregated within the patch to feed. In nature, movement of walleye pollock larvae in response to selected environmental factors (e.g., gravity, light, temperature, turbulence) may serendipitously bring them into contact with prey patches, where they then could remain to feed as long as light intensity remained at or above levels necessary for feeding.     

The effect of early and late hatching on the escape response of walleye pollock (Theragra chalcogramma) larvae

Hatching of fish eggs fertilized at the same time occurs overa period of several days. Differences in the escape responseof fish larvae during the hatching period have not hithertobeen studied. In this study, the escape response of walleyepollock (Theragra chalcogramma) larvae over the hatching periodwas examined. Escape speed, response to multiple touches witha fine probe, response to water currents generated by a predatorand predation by euphausiids (Thysanoessa inermis) and amphipods(Pleusirus secorrus) were measured in the laboratory. Otolithmeasurements of field-collected larvae support a broad hatchingperiod for walleye pollock eggs in the sea similar to that observedin the laboratory. The escape response of walleye pollock larvaewas affected by rank in the order of hatching, thus with respectto predation, hatching order may affect the survival of larvaein the sea. Early hatching larvae were smaller, less sensitiveto tactile stimulation, had a slower, weaker escape responseand higher laboratory rates of predation mortality than thosethat hatched later.     

Behavioral responses to food odor in juvenile marine fish: Acuity varies with species and fish length

This study tested the hypothesis that acuity of behavioral responses to food odor in three commercially important species of marine fish would increase as juvenile length increased. Swimming activity among two size groups of fish was measured in the presence of a series of squid extract dilutions. Increased swimming activity in juvenile Pacific halibut Hippoglossus stenolepis Schmidt, walleye pollock Theragra chalcogramma Pallas, and sablefish Anoplopoma fimbria Pallas was stimulated above threshold concentrations of squid extract, expressed as dilution from full strength. Maximum chemosensory acuity was observed in smaller (8-14 cm total length, TL) Pacific halibut and walleye pollock, while larger sablefish (15-23 cm TL) continued to develop acuity. Response thresholds were highest (10^− 3 dilution) in Pacific halibut, at intermediate levels (10^− 4-10^− 6 dilution) in walleye pollock and smaller sablefish and reached the lowest levels (10^− 13 dilution) in larger sablefish. The widely held view that dissolved free amino acids (DFAA) are the primary chemosensory stimulants for fish food searching may not be valid for sablefish, as they detected squid extract at dilutions containing DFAA that appeared to be far below ambient sea water DFAA concentrations.     

Spatial Match-Mismatch between Juvenile Fish and Prey Provides a Mechanism for Recruitment Variability across Contrasting Climate Conditions in the Eastern Bering Sea

Understanding mechanisms behind variability in early life survival of marine fishes through modeling efforts can improve predictive capabilities for recruitment success under changing climate conditions. Walleye pollock (Theragra chalcogramma) support the largest single-species commercial fishery in the United States and represent an ecologically important component of the Bering Sea ecosystem. Variability in walleye pollock growth and survival is structured in part by climate-driven bottom-up control of zooplankton composition. We used two modeling approaches, informed by observations, to understand the roles of prey quality, prey composition, and water temperature on juvenile walleye pollock growth: (1) a bioenergetics model that included local predator and prey energy densities, and (2) an individual-based model that included a mechanistic feeding component dependent on larval development and behavior, local prey densities and size, and physical oceanographic conditions. Prey composition in late-summer shifted from predominantly smaller copepod species in the warmer 2005 season to larger species in the cooler 2010 season, reflecting differences in zooplankton composition between years. In 2010, the main prey of juvenile walleye pollock were more abundant, had greater biomass, and higher mean energy density, resulting in better growth conditions. Moreover, spatial patterns in prey composition and water temperature lead to areas of enhanced growth, or growth ‘hot spots’, for juvenile walleye pollock and survival may be enhanced when fish overlap with these areas. This study provides evidence that a spatial mismatch between juvenile walleye pollock and growth ‘hot spots’ in 2005 contributed to poor recruitment while a higher degree of overlap in 2010 resulted in improved recruitment. Our results indicate that climate-driven changes in prey quality and composition can impact growth of juvenile walleye pollock, potentially severely affecting recruitment variability.     

Behavioral Responses of Walleye Pollock, Theragra Chalcogramma, Larvae to Experimental Gradients of Sea Water Flow: Implications for Vertical Distribution

Walleye pollock larvae under controlled laboratory conditions were exposed to vertical gradients of sea water flow in low and high light. Whether flow originated from the surface or the bottom, larvae responded by altering depth distribution, showing attraction to low flows, avoidance of higher flows and when flow was above a threshold level, loss of ability to orient, swim and feed. These results demonstrate that walleye pollock have the capability for responding to gradients of flow by adjusting their vertical distribution. Walleye pollock and many other pelagic fish larvae have weak swimming capabilities and are generally unable to directly control horizontal distributions in the sea by swimming in higher flow regimens. However, using vertical migration, larvae may select conditions of flow direction and speed which are favorable for feeding and predator avoidance and which indirectly allow them to control transport, aggregation and dispersion.     

Contrasting behavioral responses to cold temperatures by two marine fish species during their pelagic juvenile interval

Motor activity of juvenile walleye pollock, Theragra chalcogramma, and sablefish, Anoplopoma fimbria, was monitored in the laboratory during high and low light levels under a changing temperature regime over a 5 d period. Water temperatures were ambient (12 °C) for the first 24 h of observation, rapidly lowered to 3 °C for the next 48 h, then raised back to 12 °C for the final 48 h. We hypothesized that the fishes' behavior would either follow a simple bioenergetic response of lowered activity associated with reduced metabolic rates at the colder temperature, or an avoidance response, with increased activity at decreased temperatures. Results for walleye pollock were consistent with a bioenergetic response, with activity decreasing in the presence of cold water under both high and low light levels, then returning to initial levels when temperatures increased. The response of sablefish, in contrast, indicated avoidance of cold temperatures, depending on light level. During high light, when sablefish were typically highly active, cold water induced a slight but insignificant decrease in activity. At low light, however, the presence of cold water caused a marked increase in sablefish movement through the experimental tanks, with a seven fold increase in the measured index of activity. When water temperatures were raised back to 12 °C, sablefish activity at low light returned to its normal, minimal level. The sharp increase in activity of sablefish in cold water, followed by a decrease in activity when the temperature was raised to pre-test levels, is suggestive of an avoidance response. The contrasting responses of the two species to thermal changes are consistent with their separate life history patterns and natural distributions.     

Variability of the Characteristics of the First Scale Annulus and Length of Year-Old Fish in the Bering Sea Walleye Pollock Theragra chalcogramma

Geographic and interannual variability of the number of annuli and the radius of the first scale annulus, as well as the retrospective length of year-old walleye pollock from the western, northern, and eastern parts of the Bering Sea, was examined using data for the years 1995, 1996, 1998, and 1999. The characteristics of the first scale annulus were varying. By these parameters, walleye pollock from the western Bering Sea significantly differs statistically from walleye pollock of the eastern Bering Sea. With respect to number of annuli, the radius of the first ring, and the retrospective length of year-old specimens, the walleye pollock from the Navarinskii region occupies an intermediate position between fish from the western and eastern parts of the Bering Sea. Interannual variability of the three parameters was found for walleye pollock from the Navarinskii region.     

Accessibility of sulfhydryl groups to 5,5'-dithiobis-2-nitrobenzoic acid and acid-base properties of bovine and walleye pollock rhodopsin preparations]

Both the number of exposed SH-groups and the rate of reaction with 5,5'dithiobis-2-nitrobenzoic acid (DTNB) in walleye pollock and bovine rhodopsin depend on a degree of native structure of the preparation to be investigated. The preparations studied can be arranged in the order of increase of these parameters as follows: ROS less than rhodopsin extracted by digitonin less than triton X-100 less than cetyltrimethylammonium bromide (CTAB) less than sodium dodecylsulphate (SDS). After illumination of ROS and digitonin, triton X-100 and CTAB-solubilized rhodopsin, and increase was observed in the number of modified SH-groups. Dark and bleached samples of walleye pollock rhodopsin exhibited a faster rate reaction and a more number of modified SH-groups as compared to bovine preparation. The differences between bovine and walleye pollock preparation disappeared after complete opsin unfolding as a result ROS solubilization in SDS. Six SH-groups per molecule of rhodopsin were modified in both preparation under these conditions. No differences in the number of cysteine residues (10--11), disulfide groups (2), acid (35--40) and base (25--30) titratable groups per rhodopsin molecule were found between bovine and walleye pollock ROS membranes. The isoelectric point of both rhodopsin preparations was within the pH range 5.2--5.6. After proteolysis of ROS with papain, a fragment with molecular weight 24500 +/- 1000 was detected, which contained the same number of SH-groups and cysteine residues as in the case of intact rhodopsin. The results obtained suggest that, in spite of a similar primary structure, the walleye pollock visual pigment has more "loose" and "fluid" space packing in the ROS membrane than the bovine pigment.     

Gut evacuation of walleye pollock larvae in response to feeding conditions

Gut residence times of first-feeding walleye pollock larvae were measured at 6°C in continuous and discontinuous feeding regimes. Larvae fed tagged copepod prey evacuated their guts more quickly in continuous feeding as compared to the discontinuous feeding treatment. The mean gut residence time was estimated to be 5.0 h for larvae feeding continuously. Gut evacuation by larvae fed tagged prey and then isolated without food (discontinuous treatment) was slower and more variable, with an estimated gut residence time exceeding 8.0 h. Field and laboratory observations suggest that the larval fish gut may be modeled as an intermittent plug-flow reactor (PFR) in response to diel feeding patterns. The cyclical nature of gut dynamics has implications for gut content analyses and the estimation of daily food rations.     

Social enhancement of foraging on an ephemeral food source in juvenile walleye pollock,Theragra chalcogramma

Synopsis The foraging effectiveness of walleye pollock juveniles, Theragra chalcogramma, was determined experimentally to test the hypothesis that social cues may facilitate the ability of individuals to exploit ephemeral food patches. Fish were tested when isolated, paired with one other fish, and in a group of six fish. Test fish exploited more food patches while in a group of six than when they were isolated. Patch exploitation by paired fish was intermediate to but not statistically different than isolate or grouped treatments. The number of pellets eaten by test fish in a group and a pair was more than 3.5 times that of when they were isolated, although the overall relationship between the amount of food eaten and group size was not statistically significant. Results support the hypothesis that juvenile walleye pollock exploit ephemeral food patches more effectively in the presence of conspecifics. In planktivores such as walleye pollock, social cues may enhance foraging on transient food sources either by facilitating detection of food patches (local enhancement) or by stimulating foraging activity when a food patch is located (social facilitation).     

Changes in vertebral-column characters and in swimming abilities of the walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> (Pallas) under the pressure of fishing activities

After 30 years of intensive walleye pollock fishing in the Sea of Japan and the Sea of Okhotsk, the average number of vertebrae has increased significantly in the caudal section of the spinal column. The maximum frequency of occurrence of “multivertebrae” phenotype (33 caudal vertebrae) has increased from 3–5 to 76–78%. The number and frequency of occurrence of the abdominal vertebrae did not change. Experimental testing of walleye pollock on its ability to resist the flow justifies our suggestion that the number of vertebrae in the caudal section is an important adaptive feature. We argue that the changes in phenotypic structure of the studied population are the result of intensive fishing.     

Behavioral responses to light gradients,olfactory cues,and prey in larvae of two North Pacific gadids (<Emphasis Type="Italic">Gadus macrocephalus</Emphasis> and <Emphasis Type="Italic">Theragra chalcogramma</Emphasis>)

The growth and survival of larvae can be significantly enhanced through close association with patches of high prey concentration. However, the taxis and kinesis responses used by larvae to locate and maintain residence in micro-patches remains poorly understood. In this study, the behavioral responses of Pacific cod (Gadus macrocephalus) and walleye pollock (Theragra chalcogramma) larvae (45–100 dph) to light, prey scent, and prey were examined. Both species displayed an ontogenetic shift in response to a horizontal light gradient, with small larvae (11–13 mm SL) exhibiting a positive phototaxis and large larvae (23–32 mm SL) exhibiting a negative phototaxis. Whether this reversal is related to ontogenetically appropriate foraging cues or some other aspect of the environment remains to be determined. Neither species displayed significant behavioral responsiveness to the introduction of olfactory prey cues at either size. The aggregating (taxis) response of large larvae to introduction of live prey was stronger than that of small larvae, possibly due to increased reaction distances and encounter rates. In addition, both species exhibited a kinesis response of reducing the frequency of swimming bouts in response to introduction of live prey. These results suggest that the scale of prey patchiness and the physical factors that determine patch encounter rates are a significant determinant of larval growth and survival in the early feeding stages of marine fishes.     

Assessment of the consumption of walleye pollock eggs by nekton and jellyfish in the northern Sea of Okhotsk in the spring

In the northern Sea of Okhotsk, nekton and jellyfish consumed as many as 831 × 10⁹ walleye pollock eggs per day in 2011. The nekton exerted the highest pressure, viz., 98.3% of the overall predation on pollock egg by aquatic animals. Of the entire quantity of consumed eggs, 55.9% were eaten by herring, 35.9% by walleye pollock, 6.5% by Sakhalin sole, and 1.7% by jellyfish. Among jellyfish, scyphomedusae Cyanea capillata and Chrysaora melonaster, as well as the hydromedusa Tima sachalinensis consumed the largest quantities of eggs. The total consumption of pollock egg by aquatic animals in 2011 was estimated at 42.4 × 10¹², or 11.4% of the entire quantity of eggs that were spawned by walleye pollock in the waters of the northern part of the sea. The total amount of pollock eggs that were eaten by herring and pollock together for 51 days in 2011 amounted to 38.9 × 10¹², which was 5.7 times as much as that in 2002. Thus, a significant growth of predation on pollock eggs by their main consumers, viz., herring and walleye pollock, was observed in 2011. This was caused by an increase in the populations of both species during the recent years and also by a higher concentration of pollock eggs.     

Social and reproductive behavior of a captive group of walleye pollock,Theragra chalcogramma

Synopsis The social and reproductive behavior of a group of four male and seven female walleye pollock,Theragra chalcogramma, were observed in a large tank. Pollock spent most of their time swimming in a loose aggregation near the surface. Males descended from the aggregation more often than females to follow and make physical contact with other males as well as with females. The difference between males and females in the frequency of diving in our tank is consistent with the reported pattern of depth segregation of the sexes in natural pollock spawning aggregations. The frequency of social interactions increased when pollock became reproductively active and was higher at night and during twilight when most of the spawning occurred. Male interactions with females most frequently involved physical contact, while male interactions with other males were more often limited to following. There was no indication that male-male interactions result in the formation of stable social dominance relationships that determine priority of access to mates, as has been suggested previously for walleye pollock. Rather, following and contact interactions appear to promote male identification of potential mates and encounters with ripe females. The possible functional significance of male social interactions is discussed in relation to reports on natural walleye pollock spawning aggregations.     

Rate of energy depletion and overwintering mortality of juvenile walleye pollock in cold water

The winter energy deficit and mortality of juvenile walleye pollock at extremely cold temperature were examined by field observations and laboratory experiments. In the Doto area, along the northern coast of Japan, juvenile walleye pollock resided on the continental shelf despite extremely cold temperatures (mean 0·4° C) during the latter half of winter (March to April). Measurements of the rate of energy depletion (equivalent to the routine metabolic rate) revealed that juvenile walleye pollock consumed 37% less energy at 0·5° C than at 2·0° C, suggesting an energetic benefit of residence in cold water (<1·0° C) over the shelf during winter. Prior to the starvation experiments, temperatures and ration level in the holding tanks were adjusted to create two different body condition groups of fish. Under the thermal condition of the latter half of winter (0·5° C), fish with a mean condition factor of 0·6 and 0·5 suffered 19·1 and 74·5% mortality, respectively, at the end of the experiments (after 56 days). The residual analysis of total body energy demonstrated that the cause of mortality was mainly associated with the depletion of energy reserves. When a logistic regression model for mortality derived from the experiments was applied to wild fish collected in March, the estimated overwintering mortality in 2004 and 2005 was 25·4 and <2·3%, respectively, assuming no feeding during the winter. Considering that juvenile walleye pollock feed during winter as shown in previous studies, intense overwintering mortality induced by energy depletion is improbable during the latter half of winter in the Doto area.     

Gut evacuation and absorption efficiency of walleye larvae

Gut evacuation and absorption efficiency in zooplanktivorous walleye Stizostedion vitreum larvae (10–19 mm; 0.53–6.8 mg dry weight) were examined in the laboratory. The decline in dry weight of whole digestive tract contents was examined at 15, 20, and 25) C in discontinuous feeding larvae. The observed relationship between short-term evacuation rate and gut fullness indicated that evacuation approximated most closely an exponential decline. When gut evacuation rate, R , was calculated as the slope of loge gut fullness v . time there were significant effects of temperature and walleye size on R . Gut evacuation rate was higher at 20° C than at 15 or 25° C and declined with increasing walleye dry weight. Absorption efficiency at 20° C was examined by qualitative analysis of food and faeces. Apparent digestibility, D _a, of crustacean zooplankton increased with gut retention time and declined with walleye dry weight. Maximum D _a was estimated to be 79% of organic matter from food retained in the gut for 6 h.