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.     

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).     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Analysis of sibling cannibalism among pike,Esox lucius,juveniles reared under semi-natural conditions

Synopsis Sibling cannibalism in pike, Esox lucius, larvae and juveniles living in outdoor rearing ponds was studied using stomach contents analysis. For the two initial densities tested (6 and 18 larvae m^–2, equivalent to 12 and 36 larvae m^–3), cannibalism was non-existent during the larval period (13 to 35 mm total length) and was observed only during the juvenile stages. Initial density of larvae influenced both the date of first detection of cannibalistic individuals and the rate of development of cannibalism in the population. At initial stocking densities of 18 larvae m^–2 (36 larvae m^–3), cannibalism was observed from 21 days after the start of exogenous feeding (mean total length: 60 mm) onwards. At a mean total length of 100 mm and for initial stocking densities of 6 and 18 larvae m^–2, (12 and 36 larvae m^–3), the average proportions of cannibals in the populations of juveniles were 7.8% and 41.3% and the cannibals accounted for 15.5% and 65.9% of the total pike biomass, respectively. In stomachs of cannibals, young pike were the dominant prey in terms of weight. Dry weights of invertebrate-prey were lower in cannibals than in non-cannibals of similar size. Cannibalism among pike juveniles was characterized by the prey being swallowed whole and head first in the vast majority of cases. There was a strong positive correlation between predator and prey size and the mouth size of a cannibal was found to be an important constraint determining maximum victim size. The overall mean ratio of pike prey length to pike cannibal length was 66.2% and the average ratio of prey head depth to predator mouth width amounted to 87.6%. Prey size selection could be demonstrated for several length-groups of cannibals. These results are compared with the characteristics of early cannibalism in other fish species.     

Key fish species, northern fur seals, Callorhinus ursinus, and fisheries interactions involving walleye pollock, Theragra chalcogramma, in the eastern Bering Sea

The recent decline of the northern fur seal population breeding on the Pribilof Islands has not yet been explained. This study estimates the amounts and sizes of walleye pollock (the fur seal's main prey) removed by predatory fish, fur seals and commercial fisheries in the main fur seal feeding area in the eastern Bering Sea during summer 1985. Fur seals relied primarily on age-1 pollock during 1985, while predatory fish consumed pollock mainly of ages 0-1. The fishery took pollock older than age-3. This indicated no direct effect of fishery removal on fur seal prey abundance, at least during the study period. In the short term, the 1985 pollock fishery may have indirectly affected fur seal prey abundance in a positive manner by removing adult pollock that compete with fur seals for age-1 pollock.     

Substrate preference in age-0 red snapper, Lutjanus campechanus

We tested age-0 red snapper, Lutjanus campechanus, for sand or shell substrate preference in a circular tank (1.5 m diameter × 0.6 m deep). The test tank was divided into two equal areas of whole oyster shell or sand substrates. All trials were video taped for 20 to 25 min. Tapes were viewed on a monitor and locations of all fish recorded and timed with respect to substrate. Mean ± SE time on shell was 11.6 ± 0.5 sec (4 fish trial^–1) and 13.8 ± 1.1 sec (1 fish trial^–1). Mean ± SE time on sand was 9.7 ± 0.4 sec (4 fish trial^–1) and 8.9 ± 0.5 sec (1 fish trial^–1). Fish spent significantly more time on shell compared to sand substrate in four-fish trials (paired t-test, p < 0.10) and also in single-fish trials (paired t-test, p < 0.05). Shell substrate may offer increased food and shelter for age-0 red snapper compared to sand substrate. However, a preference for shell substrate in the natural habitat may occur only during the nursery period, because as fish grow (> 100 mm TL) in the late fall they probably outgrow the shell habitat.     

The effects of temperature on feeding in zooplanktivorous walleye,Stizostedion vitreum,larvae

Synopsis We examined the prey capture process in walleye larvae (9.8–18.0 mm mean length) feeding on zooplankton (density 1501^–1) in laboratory aquaria at 15,18.5 and 22°C. Larvae were starved for 8 h prior to the experiment and only allowed to feed for 10 min during the experiment in order to minimize the influence of gut processing or satiation on feeding behaviour. Prey consumption (g min^–1) increased exponentially and prey capture success (%) increased logarithmically with mean walleye length. Prey consumption and attack rate (strikes min^–1) increased significantly with increasing temperature. The effect of temperature on capture success was slight (2% increase from 15 to 22°C) and not significant. Walleye showed positive electivity for medium-sized prey (0.3–0.6 mm body width, mostlyCeriodaphnia quadrangula), negative electivity for small prey (0.3 mm, mostly cyclopoid copepods) and neutral electivity for large prey (0.6 mm, mostlyDaphnia sp.). Neither prey species electivity nor prey size electivity were significantly affected by temperature. However, prey size electivity did show a fairly large effect size with respect to temperature and we suggest that this relationship should be examined further. These results indicate that temperature affects prey capture in walleye larvae primarily by influencing attack rate.     

Physiological variables determined under laboratory conditions may explain the bloom of Aphanizomenon ovalisporum in Lake Kinneret

Response of Aphanizomenon ovalisporum to certain environmental parameters was studied to gain a better understanding of the conditions which may have stimulated its autumnal bloom in Lake Kinneret. Optimal temperature for A. ovalisporum growth was 26–30?°C, resulting in growth rates of 0.2–0.3?day^?1, similar to those observed in the lake. Maximal rate of CO₂ fixation (assimilation numbers of 6–8?μg?C?μg^?1?Chl?h^?1) was obtained at low irradiances (I _k of 40–100?μmol?photons?m^?2?s^?1), 200?μM Pi and low N:Pi ratios. Growth was strongly affected by phosphorus availability, reaching a maximum at Pi concentrations above 40?μM. The high demand for phosphorus was indicated by an increase in alkaline phosphatase activity. The relative abundance of Pi in the cells increased by 4-fold in Pi-rich compared with Pi-limited cultures. Uptake of Pi was faster in Pi-depleted compared with Pi-sufficient cells. Maximal photosynthetic rates and K_1/2(HCO₃ ^?) were 140–220?μmol?O₂?mg^?1?Chl?h^?1 and 10–24?μM, respectively. At pH 7.0 the K _1/2(CO₂) was 2.2 and fell to 0.04?μM at pH 9.0. These data indicated that A. ovalisporum is a HCO₃ ^? user, and can explain its high photosynthetic rates during the bloom, under high pH and low dissolved CO₂ conditions. Na⁺ concentrations of about 5?mM were essential for A. ovalisporum growth at high pH approaching values in the lake.     

Annual changes in the copulatory behavior of male rats maintained under constant laboratory conditions

ABSTRACT

The objective of the present study was to evaluate the sexual behavior of male rats kept under constant laboratory conditions for one entire year. A total of 213 sexually-inexperienced, male Wistar rats were maintained in controlled environmental conditions from birth. Depending the month in which they reached the age 3-month-old, the male rats were divided into 12 groups, one for each month of the year, and their sexual behavior was evaluated. Records of their sexual behavior were made from 09:00 to 11:00 hrs am. The following parameters were recorded: mount (latency and number), intromission (latency and number), ejaculation latency, and intromission rate. During the months of March, June, July and September, the rats showed lower mount and intromission latencies than in January, February, April, May and October-to-December. Similarly, in March, June, July and August they had higher copulatory efficiency than in January, February, April and December. Results suggest that male rats exposed to controlled environmental conditions could have endogenous mechanisms that regulate sexual behavior but are independent of seasonal environmental signals. The annual variability in the sexual behavior of male rats maintained under constant laboratory conditions should be considered when planning research and experiments.