The fecundity of the walleye pollock, Theragra chalcogramma (Pallas), spawning in Canadian waters

The ovaries of 113 walleye pollock from a resident stock in the Strait of Georgia, British Columbia were examined for determination of fecundity. Oocytes were sized and counted in 20 μm intervals of diameter. Without exception, ovaries contained a pronounced bimodal distribution of oocyte diameters with peaks at 100 and 400–600 μm. Oocytes ≥ 180 μm diameter were undergoing trophoplasmic growth leading to hydration. 'Apparent' fecundity is defined as the estimated number of yolked oocytes ≥ 180 μm diameter, regardless of potential resorbtion. Previous workers have not shown that significant resorbtion takes place in the post-spawned ovary. Total oocyte complement (≥40μm diameter) was best expressed by a linear model where F_t = 33004 f.l. – 869627, where f.l. = fork length in cm and r = 0.86. Estimates of F_t , ranged from 117700 to 1394 100 oocytes ≥40μm. Age was weakly related to fecundity, reflecting large individual differences in annual growth after age 4 years. Apparent fecundity best suited a linear model where F_a = 23522 f.l. – 599713 and r = 0.91. Estimates of F_a fell within the range 58 379–1 151 527. Relative fecundity (eggs g⁻¹) decreased over most of the length range encountered in the sample. The average-sized female in Georgia Strait is twice as fecund as her counterparts in the north-western Pacific Ocean, containing some 390 000 to 420 000 oocytes 7ge;180 μm diameter compared to about 200 000 oocytes in a north-western female of comparable length.     

Diel patterns of behavior in juvenile walleye pollock,Theragra chalcogramma

Synopsis In laboratory experiments, we contrasted the behavior of juvenile walleye pollock under dark and light conditions, comparing group cohesion, activity levels, vertical distribution, and movement into cold water when the water column was thermally stratified. At night the fish displayed a tendency for movement into the upper water column and were less active, with groups more dispersed than during the day. Time spent in cold water under stratified conditions did not differ between day and night. These results are interpreted relative to the potential ecological benefits and costs of particular behaviors, and their application in designing effective sampling surveys of juvenile abundances.     

Electron microprobe analysis of juvenile walleye pollock,Theragra chalcogramma,otoliths from Alaska: a pilot stock separation study

Synopsis The incorporation of dissolved oceanic constituents in the otoliths of fish has potential as a chemical tracer for reconstructing the early life history of marine fish. Wavelength dispersive spectrometers on an electron microprobe were used to measure Na, Mg, P, S, Cl, K, Ca, and Sr concentrations on the outer margins of 57 juvenile walleye pollock, Theragra chalcogramma, otoliths from five locations in the Gulf of Alaska and Bering Sea. Discriminant analyses that used various combinations of Na, P, K, Sr, and fish standard length and/or age showed that 60–80% of the samples could be assigned to the correct capture locality. While the concentrations of some of the measured elements correlated with standard length or age of the fish, there are measurable differences among localities when concentrations are length or age corrected, mainly due to differences in Na and K concentrations. Elemental composition of otoliths potentially could be used to assign fish from a mixed stock fishery to original stocks, information that is greatly needed for the effective management of fish stocks.     

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.     

Climate change causing phase transitions of walleye pollock (Theragra chalcogramma) recruitment dynamics

In 1976 the North Pacific climate shifted, resulting in an average increase of the water temperature. In the Gulf of Alaska the climate shift was followed (i.e. early 1980s) by a gradual but dramatic increase in the abundance of groundfish species that typically prey on pre-recruitment stages of walleye pollock. In the present study we used a previously parameterized model to investigate the effect of these climate and biological changes on the recruitment dynamics of walleye pollock in the Gulf of Alaska. Simulations covered the 1970-2000 time frame and emphasized the medium-to-long temporal scale (i.e. about 5-10 years) of environmental variability. Results showed that during periods characterized by high sea surface temperature and high predation on juvenile pollock stages, recruitment variability and magnitude were below average, and recruitment control was delayed to stages older than the 0-group. Opposite dynamics (i.e. high abundance and variability, and early recruitment control) occurred during periods characterized by low temperature and predation. These results are in general agreement with empirical observations, and allowed us to formulate causal explanations for their occurrence. We interpreted the delay of recruitment control and the reduction of variability as an effect of increased constraint on the abundance of post age-0 stages, in turn imposed by high density dependence and predation mortality. On the other hand, low density-dependence and predation favoured post age-0 survival, and allowed for an unconstrained link between larval and recruitment abundance. Our findings demonstrate that the dominant mechanisms of pollock survival change over contrasting climate regimes. Such changes may in turn cause a phase transition of recruitment dynamics with profound implications for the management of the entire stock.     

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.     

Temperature and prey quality effects on growth of juvenile walleye pollock Theragra chalcogramma (Pallas): a spatially explicit bioenergetics approach

A bioenergetics model for juvenile age‐0 year walleye pollock Theragra chalcogramma was applied to a spatially distinct grid of samples in the western Gulf of Alaska to investigate the influence of temperature and prey quality on size‐specific growth. Daily growth estimates for 50, 70 and 90 mm standard length (L_S) walleye pollock during September 2000 were generated using the bioenergetics model with a fixed ration size. Similarities in independent estimates of prey consumption generated from the bioenergetics model and a gastric evacuation model corroborated the performance of the bioenergetics model, concordance correlation (r_c) = 0·945, lower 95% CL (transformed) (L₁) = 0·834, upper 95% CL (transformed) (L₂) = 0·982, P < 0·001. A mean squared error analysis (M_SE) was also used to partition the sources of error between both model estimates of consumption into a mean component (M_C), slope component (S_C), and random component (R_C). Differences between estimates of daily consumption were largely due to differences in the means of estimates (M_C= 0·45) and random sources (R_C= 0·49) of error, and not differences in slopes (S_C= 0·06). Similarly, daily growth estimates of 0·031–0·167 g day^?1 generated from the bioenergetics model was within the range of growth estimates of 0·026–0·190 g day^?1 obtained from otolith analysis of juvenile walleye pollock. Temperature and prey quality alone accounted for 66% of the observed variation between bioenergetics and otolith growth estimates across all sizes of juvenile walleye pollock. These results suggest that the bioenergetics model for juvenile walleye pollock is a useful tool for evaluating the influence of spatially variable habitat conditions on the growth potential of juvenile walleye pollock.     

Biogeographic evidence for selection on mitochondrial DNA in North Pacific walleye pollock Theragra chalcogramma

Three major mitochondrial DNA (mtDNA) haplogroups were identified in 5 data sets for North Pacific and Bering Sea walleye pollock. The common haplogroup A showed mirror-image clines on both sides of the North Pacific with high frequencies in southern areas (P(A) > 0.84) and low frequencies in the Bering Sea (P(A) < 0.36). Two additional haplogroups showed complimentary, but weaker, clines in the opposite direction. These clines are unlikely to have arisen by chance during postglacial colonizations of coastal waters in the North Pacific and Bering Sea, and they do not appear to reflect isolation by distance. Contrary to these trends, pollock at the western end of the Aleutian Island Archipelago were genetically more similar to Asian than to North American pollock, a pattern likely reflecting postglacial colonization. Haplogroup F(ST) values for a given haplotype diversity were significantly larger than expected under the island model of migration and random drift, a result implicating natural selection. Frequencies of haplogroup A were highly correlated with sea surface temperature (r > 0.91), whereas frequencies of groups B and C showed negative correlations with temperature. Selection may be operating directly on mtDNA variability or may be mediated through cytonuclear interactions. This biogeographic evidence adds to a growing body of literature indicating that selection may play a greater role in sculpting mtDNA variability than previously thought.     

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).     

Winter lipid depletion of juvenile walleye pollock Theragra chalcogramma in the Doto area, northern Japan

Seasonal variation in body size and nutritional condition of juvenile walleye pollock Theragra chalcogramma was examined to elucidate the mechanism underlying their first-winter survival on the continental shelf of the Doto area, northern Japan, based on monthly samples collected over 2 years. Stored lipid mass was highest during autumn, but 93% (2004) and 80% (2005) of lipids were exhausted by the onset of winter. Lipid levels in the winter of 2004 remained low (7–14% of the autumnal maximum), and there was reduced growth rate until the spring, whereas in 2005 lipid levels were higher and more variable (10–46% of the maximum) and some growth occurred. An analysis of the allometric relationships between body size and stored energy showed that larger individuals accumulated disproportionately more energy in the autumn, but the advantage disappeared prior to the winter. In January 2004, stored lipid energy was low throughout the Doto continental shelf relative to the continental slope area. These results suggest that winter feeding opportunities on the shelf are severely limited but not completely absent. Previous studies have shown that winter temperatures on the shelf are lower than those in the slope area. It is possible that juvenile T. chalcogramma survive winter on the shelf without a high level of pre-winter lipid storage because the occasional feeding in the cold shelf water benefits energy conservation.     

Effects of size and light on respiration and activity of walleye pollock (Theragra chalcogramma) larvae

The respiration rate and swimming activity of walleye pollock (Theragra chalcogramma) larvae were measured in the laboratory to determine how these were affected by body size (measured as dry weight), and amount of light. Size influenced respiration rates, but not activity. Activity increased with increased light, and as walleye pollock larvae developed, light had an increasingly important effect on respiration rate. For older larvae, light is an important factor affecting respiration rate and this may be due to an increased sensitivity to light. Thus, in addition to size, light plays an important role in the energetics of walleye pollock larvae.     

Evidence for positive selection at the pantophysin (Pan I) locus in walleye pollock, Theragra chalcogramma

Nucleotide polymorphism at the pantophysin (Pan I) locus in walleye pollock, Theragra chalcogramma, was examined using DNA sequence data. Two distinct allelic lineages were detected in pollock, resulting from three amino acid replacement mutations in the first intravesicular domain of the protein. The common Pan I allelic group, comprising 94% of the samples, was less polymorphic (pi = 0.005) than the uncommon group (pi = 0.008), and nucleotide diversity in both was higher than for two allelic lineages in the related Atlantic cod, Gadus morhua. Phylogenetic analyses of Pan I sequences from these two species did not clearly resolve orthology among allelic groups, in part because of recombination that has occurred between the two pollock lineages. Conventional tests of neutrality comparing polymorphisms within and between homologous regions of the Pan I locus in walleye pollock and Atlantic cod did not detect the effects of selection. This result is likely attributed to low levels of synonymous divergence among allelic lineages and a lack of mutation-drift equilibrium inferred from nucleotide mismatch frequency distributions. However, the ratio of nonsynonymous to synonymous substitutions per site (dN/dS) exceeded unity in two intravesicular domains of the protein and the influence of positive selection at multiple codon sites was strongly inferred through the use of maximum-likelihood analyses. In addition, the frequency spectrum of linked neutral variation showed indirect effects of adaptive hitchhiking in pollock resulting from a selective sweep of the common allelic lineage. Recombination between the two allelic classes may have prevented complete loss of the older, more polymorphic lineage. The results suggest that recurrent sweeps driven by positive selection is the principle mode of evolution at the Pan I locus in gadid fishes.     

Interannual differences in growth and hatch date distributions of age-0 year walleye pollock Theragra chalcogramma (Pallas) sampled from the Shumagin Islands region of the Gulf of Alaska, 1985 – 2001

Daily increments of age-0 year walleye pollock Theragra chalcogramma otoliths from 1985 to 2001 were examined for interannual variability in growth and hatch dates. Fish were collected in summer and autumn surveys near the Shumagin Islands in the western Gulf of Alaska (GOA). Hatch date distributions of these fish were compared with hatch dates of larvae from spawning aggregations located in Shelikof Strait and the Shumagin Islands based on their spawning times. The hatch date distributions of age-0 year fish captured from the Shumagin Islands area were similar to those observed for larvae from the Shelikof Strait spawning group. Age-0 year fish whose hatch dates corresponded to the Shumagin Islands spawning, which occurred earlier in January and February, were not found. Sea surface temperature was associated with variability in hatch date distribution and growth.     

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.     

Phototactic responses of unfed walleye pollock,Theragra chalcogramma larvae: comparisons with other measures of condition

Synopsis The capability of unfed walleye pollock, Theragra chalcogramma, larvae to swim horizontally towards light was used as a sensitive, sublethal measure of larval condition. At 9°C, positive phototaxis and swimming ability of larvae was fully developed by 4–6 d after hatching, then decreased steadily until death by 12 d after hatching. This measure of larval condition corresponded closely with previously established benchmarks of larval condition, including first feeding, yolksac absorption, point of no return and death by starvation. The presence and timing of behavioral deficits associated with starvation, such as decreased ability to swim, feed and avoid predators, may have significant effects on the ability of larvae to vertically migrate, avoid predators and find and capture food.     

Carbon, nitrogen and caloric content of eggs, larvae, and juveniles of the walleye pollock, Theragra chalcogramma

Measurements of dry weight (wt), carbon (C), nitrogen (N) and calories were made on walleye pollock eggs (0.24 mg, 35.3% C, 8.3% N, and 4.6 kcal g⁻¹ dry wt), larvae (0.16 g, 42.9% C, 11.1% N and 5.1 kcal g⁻¹ dry wt) and juveniles (22.4 g, 47.2% C, 9.0% N and 5.6 kcal g⁻¹ dry wt). For juvenile fish (9–360 g wet wt) the measured values were related to dry weight and Fulton's condition factor index (CFI) by regression models. The CFI was a better predictor of body composition than dry weight. As CFI improved from a minimum starvation level of 0.42 to a maximum of 1.16, body caloric content, percentage C, and the C/N ratio increased (kcal g⁻¹ dry wt = 4.4 CFI + 1.7, percentage carbon = 49.7 CFI^0.5, C/N ratio = 5.0 CFI + 0.9), while percentage N and percentage ash decreased (percentage N =−3.5 CFI + 12.1; percentage ash = 9.1 CFI^−1.4). The results of this study suggest that seasonal C, N and caloric content of young pollock can be estimated from measurements of Fulton's condition factor index.     

Coccidia, X-cell pseudotumors and Ichthyophonus sp. infections in walleye pollock (Theregra chalcogramma) from Auke Bay, Alaska

One hundred twenty-five walleye pollock (Theragra chalcogramma) were collected from Auke Bay, Alaska (USA) in 1985 and examined for histologic evidence of disease-causing infectious agents in 1987. A Goussia sp.-like coccidium was found in the kidney tubules of 75% and an Eimeria sp.-like coccidium was found in the intestine of 18% of the fish examined. The kidney coccidium was associated with sloughing of the tubular epithelium, peritubular fibrosis and granuloma formation. The intestinal coccidium was associated with severe tissue displacement and inflammation. In addition, X-cell pseudotumors were observed in the pseudobranchs (4%), and the fungus Ichthyophonus sp. was observed in the kidney, intestine or brain of 2% of the pollock.     

The influence of predator presence on utilization of artificial seagrass habitats by juvenile walleye pollock,Theragra chalcogramma

Synopsis Behavioral preference for a structured habitat (artificial seagrass) by juvenile walleye pollock,Theragra chalcogramma, was tested in controlled laboratory experiments. We monitored position of fish in 2000 1 tanks with and without artificial seagrass present in one half of the tank. In addition, we exposed walleye pollock to a predator model, assessing their response when a grass plot was available or unavailable as a potential refuge. In the absence of predators, the fish avoided the artificial seagrass, displaying a preference for the open water side of the experimental tanks. In the presence of a predator model, however, juvenile walleye pollock readily entered the artificial seagrass plots. In addition, they often remained in the grass canopy in proximity to the predator instead of moving out of the grass to avoid the predator (when no grass was present they consistently moved to the opposite side of the tank from the predator). The behavioral choices exhibited in this study suggest that juvenile walleye pollock modify habitat selection in response to perceived predation risk, and recognize the structure provided by artificial seagrass as a potential refuge.     

The potential for intracohort cannibalism in age-0 walleye pollock,Theragra chalcogramma,as determined under laboratory conditions

Synopsis In laboratory experiments, we tested the capability of larger age-0 walleye pollock to consume smaller members of their cohort. In separate aquaria, 81 pairs of juveniles covering a wide range of size differences (total lengths differing by 12 to 61 mm) were held and monitored over a 4 day period. Complete consumption, in which a smaller fish was swallowed whole by a larger fish, occurred 11% of the time. In 36% of the pairs, attacks by the larger fish resulted in mortality of the smaller fish. The mouth width:body depth ratio between the larger and smaller fish of a pair differed significantly depending on whether the smaller fish survived, was killed but not consumed, or was ingested whole by the larger fish. Cannibalistic individuals could consume fish close to the maximum size physically possible under gape limitation; at this size the length of the cannibal was approximately 1.7 times the length of the prey. Length-frequency distributions of age-0 pollock in field concentrations suggested that, at least in some geographical areas, potential cannibals and prey commonly co-occur. Unsuccessful predatory attacks by larger individuals may have additional detrimental effects on smaller pollock in natural populations.     

The influence of hunger and predation risk on group cohesion in a pelagic fish, walleye pollock Theragra chalcogramma

Variation in the intensity of schooling behavior in fishes suggests that the benefits of aggregation are balanced by certain costs. We examined the proximity of group members to each other in juvenile walleye pollock, Theragra chalcogramma, under different environmental conditions. Food availability, simulated by a gradient of six ration treatments, had a major influence on group cohesion, with increasing dispersion as food level decreased. Group cohesion also decreased at night relative to daytime levels. Small juveniles (x=53 mm TL) maintained on high rations were highly responsive to the potential threat of a predator, with groups becoming more cohesive and remaining so for up to an hour after the initial threat. A chronic threat (continual presence of predators) resulted in tighter group cohesion than an acute threat (single simulated attack). Small juveniles maintained on low rations were less responsive to predation threats and recovered quickly, supporting the hypothesis that hunger induces risk-taking behavior. Large juveniles (x=149 mm TL) did not change their degree of aggregation in response to either type of predation threat. An overall plasticity in the degree of cohesiveness among group members indicates that walleye pollock are capable of gradually modifying their schooling behavior according to the environmental context.