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.     

Large medusae in surface waters of the Northern California Current: variability in relation to environmental conditions

Blooms of jellyfish around the world have been correlated with climatic variables related to environmental causes. Sizeable populations of large medusae, primarily Chrysaora fuscescens and Aequorea sp., appear annually in shelf waters of the Northeast Pacific Ocean. Previous research has shown that C. fuscescens is abundant seasonally in the inner shelf and exhibits high feeding rates on zooplankton. We examined medusae caught in surface trawls over an 8-year period (2000–2007) using (1) mesoscale surveys sampling 8–10 transects in May, June, and September, and (2) biweekly surveys along two transects from April to August, relating abundance to environmental parameters. C. fuscescens abundances generally peaked in late summer, whereas Aequorea sp. peaked in May or June. General additive models of the mesoscale data indicated that station catches for both species correlated with latitude, temperature, salinity, and distance from shore (and chlorophyll a for Aequorea sp.). Analysis of interannual variability revealed that highest catches of medusae correlated with cool spring–summer conditions, or negative anomalies of the Pacific Decadal Oscillation, and low winter–summer runoff from the Columbia River. Results confirmed our hypothesis of connections between jellyfish populations and regional climate conditions in a region known for strong physical forcing of ecosystem processes.     

Planktonic cnidarian distribution and feeding of <Emphasis Type="Italic">Pelagia noctiluca</Emphasis> in the NW Mediterranean Sea

Pelagic cnidarians are important consumers of zooplankton and ichthyoplankton in the world’s oceans, and thus harm fisheries as competitors and predators of fish. This study examined the inshore-offshore distribution of pelagic cnidarians and the trophic ecology of Pelagia noctiluca ephyrae (<12 mm diameter) and larger medusae in late spring 1995 in the NW Mediterranean Sea. The distribution of pelagic cnidarians was closely related to the presence of the shelf-slope front with most species mainly concentrated close to the front. Meroplanktonic antho- and leptomedusae predominated in coastal waters and more holoplanktonic trachy- and narcomedusae occurred both in shelf and open sea waters. P. noctiluca was more abundant than other medusae, including hydromedusae. Siphonophores, particularly Muggiaea atlantica, outnumbered medusae at most stations. The diet of P. noctiluca ephyrae contained mainly copepods, but ~12% of the prey were fish larvae. P. noctiluca exhibited positive prey selection for chaetognaths and mollusc larvae in day and night samples, but fish larvae were positively selected only at night. These differences may be related to the diel vertical distributions of P. noctiluca and their prey. Most of the ingested fish larvae belonged to the family Myctophidae, but anchovy and sparid larvae also were found in the gastric pouches. The size of ingested fish larvae increased as ephyra diameter increased; however, in the larger medusae (>12 mm) the number of prey increased with medusa size rather than the size of the larvae. The temporal and spatial co-occurrence of P. noctiluca with early life stages of fish suggests that P. noctiluca may be an important predator on summer ichthyoplankton.     

Distribution,quantitative composition,and feeding of jellyfish in the Western Bering Sea in summer and fall

Materials from six complex trawl surveys conducted by the TINRO-Center in the upper epipelagic zone of the Western Bering Sea during the summer-fall seasons from 2002 through 2006 were used as the basis of this paper. The overall biomass (and abundance) of jellyfish increased from 0.6–0.8 million t (1.1–1.8 billion individuals) in the summer up to 0.9–1.7 million t (4.4–4.8 billion individuals) in fall. Scyphomedusa Chrysaora melanaster and the Hydromedusa Aequorea forskalea made up the major part of the jellyfish biomass (67–97%). Their distribution varied significantly. A. forskalea aggregated mainly in the deepwater part of the Bering Sea. Ch. melanaster was widely dispersed across the entire area of the investigations and formed the most considerable aggregations in the Anadyr-Navarin area. In the fall of 2006 the diet of jellyfish generally consisted of plankton organisms (copepods, euphausiids, amphipods, pteropods, chaetognaths, ostracods, and larval decapods). However, Ch. melanaster had a greater portion of fish and larval squid in its diet.     

The zooplankton community structure in relation to its biological and physical environment on the Faroe shelf, 1989-1997

The Faroe shelf water is separated from the offshore water by a persistent tidal front, which surrounds the islands. This shelf water contains a neritic zooplankton community, which, regarding species composition, production, seasonal development and environmental conditions, is quite different from that in the surrounding ocean. While during spring and summer the zooplankton in the oceanic environment are dominated by the copepod Calanus finmarchicus, the zooplankton in the shelf water are largely dominated by neritic copepods, mainly Acartia longiremis and Temora longicornis. Calanus finmarchicus occurs in interannually highly variable abundance in the Faroe shelf ecosystem. Meroplanktonic larvae, mainly Balanus spp, and decapod larvae, are also common in the shelf water during spring and summer. During the period presented (1989-1997), the Faroe shelf ecosystem has undergone very large changes in abundance of different zooplankton species. The midsummer abundance of C.finmarchicus, which originally is advected into the shelf from the open ocean, fluctuated from 400 copepods m^-3 in 1989 to 25 copepods m^-3 in 1994, and at the same time the neritic zooplankton increased from 120 m^-3 in 1989 to 450 m^-3 in 1994. Consequently, the midsummer biomass in the shelf fluctuated by a factor of 10 during the same period. It is presumed that this variability between oceanic- and neritic-dominated zooplankton, their sizes and their biomass has greatly affected the entire pelagic ecosystem.      

Spawning of walleye pollock (<Emphasis Type="Italic">Theragra chalcogramma</Emphasis>) off the Southwestern coast of Kamchatka

The results of ichthyoplankton surveys conducted in 2007–2008 showed that waters off Southwest Kamchatka and North Kuril Islands were areas of mass spawning of walleye pollock. The peak spawning occurred during the last 10 days of April and in early May, which was much later than the peak at the main spawning site off West Kamchatka. The spawning activity of walleye pollock near the southwestern shores of Kamchatka is a regular event, as the analysis of archive materials shows. This gave us grounds to suspect the existence of a southern site that coincided well with the mass spawning in the Shelikhov Gulf in its timing and scale, but was missed during standard ichthyoplankton surveys conducted in early April. After analyzing the growth rates of spawners, the assumption was made that the southern spawning was performed by the East Kamchatkan walleye pollock population, whose mass spawning usually occurs in late April-early May. According to the data of 2008, the estimated biomass of walleye pollock spawning in the area of the Ozernovskaya basin in late April was nearly 600000 tons. Regular monitoring of the southern spawning is proposed by means of additional ichthyoplankton surveys south of 53° N, including the Okhotsk Sea waters of the North Kuril Islands, in late April—early May.     

Reproductive conditions of walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> (Gadidae) off the northeastern coast of Sakhalin Island,Sea of Okhotsk

The spatial distribution of eggs and larvae of the walleye pollock Theragra chalcogramma is considered in respect to dynamics of oceanologic processes, nutrients, chlorophyll а and zooplankton off the northeastern coast of Sakhalin Island in spring 2012. It is shown that the effect of severe temperature regime in the near-bottom horizons in the western Sea of Okhotsk during the spawning period of walleye pollock becomes milder due to specific features of water dynamics. The egg distribution is determined by mesoscale eddies in the region. The species survival depends on the effect of such environmental factors as freshwater discharge from the Amur River, eddy structure in waters of the Sea of Okhotsk, and dynamics of phytoplankton and zooplankton development.     

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.     

The dynamics of the abundance of the walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> (Pallas, 1814) (gadiformes: gadidae) in the Sea of Japan

The biomass of the walleye pollock (Theragra chalcogramma) stock in waters of Primorskii krai, Sea of Japan, during the 1976–2015 observation period ranged from 48000 to 373000 tons; their number ranged from 99 to 1115 million fish. Four very strong year-classes born in 1975, 1981, 1997, and 2006 have been identified. It has been shown that the duration of the sexual-maturity period is determined by the periodicity of the dynamics of the year-class strength close to the 9-year cycle. According to the results of numerical modeling, an increase in the walleye pollock stock in Primorye is expected from 2017 to 2020.     

Prey selection and diel feeding of the freshwater jellyfish, Craspedacusta sowerbyi

1. The diet of the invasive freshwater jellyfish, Craspedacusta sowerbyi (Lankester), was assessed by analysing its stomach contents. 2. The medusae ingested various zooplankton and benthic prey in the 0.1–3.0 mm size range. The selectivity indices for prey showed that larger zooplankton (0.4–1.4 mm) and active prey such as copepods were preferred; small potential prey, and the loricate rotifer Keratella cochlearis in particular, were almost never consumed. Measurements suggest that spacing between the tentacle branches could determine prey selection. Another possibility is that some prey are too small or slow to activate nematocysts after collision with the predator. Prey > 1.4 mm can probably escape or are otherwise too large to be handled. 3. The medusae of Craspedacusta medusae stayed in deep water during daytime and migrated upwards at night. The greatest number of prey in freshwater jellyfish stomachs was detected at night. 4. Calculation of consumption rates, based on mean stomach contents and digestion times of 4–5 h, showed that, on average, one medusa takes 190 zooplankton prey day^??1 at a mean prey density of about 100 L^??1. At the observed medusae density of 1 m^??3, the cropping rate was about 0.2 zooplankton prey L^??1 day^??1.     

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.     

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.     

Development and application of a bioenergetics model for juvenile walleye Pollock

A bioenergetics model was parameterized for age-0 walleye pollock, Theragra chalcogramma , based on a synthesis of literature data. The sensitivity of the new parameters was tested by individual parameter perturbation (IPP) analysis. The model was applied to estimate individual and total cohort food consumption of age-0 pollock in two areas of high pollock density in the Gulf of Alaska during the summer of 1990. Total cohort consumption was compared with zooplankton biomass and production estimates for the same areas and times of the year. The model was also used to examine the bioenergetic implications of age-0 pollock diel vertical migration through a thermal gradient. During a 1-month bioenergetics simulation, individual daily consumption decreased from 16·0 to 6·0% of wet body weight. Daily ration estimates corresponded well with independent field estimates of daily ration for the same areas and time of the year. Comparison of total cohort consumption with prey availability (production and biomass) indicated minimal potential for food limitation. Bioenergetic optimization of growth can be a potential benefit of diel vertical migration to age-0 pollock, however more information on prey density and distribution is needed to test this hypothesis thoroughly.     

Acoustic survey of a jellyfish-dominated ecosystem (Mljet Island,Croatia)

Acoustic techniques have been proposed as a new tool to assess jellyfish populations. However, the presence of mixed echoes from jellyfish and other organisms that share their distribution often prevent accurate estimates of their abundance and distribution being obtained. The isolated population of Aurelia inhabiting the Veliko Jezero (Big Lake-BL) of Mljet Island, in the South Adriatic Sea, offered a good opportunity to employ acoustic techniques to assess an entire jellyfish population. During October 2–5, 2006, combined video and acoustic methods were used in BL to determine the vertical distribution of medusae. Two synoptic acoustic surveys were performed during the day and night. In the daylight echograms, medusae were clearly discernible from the acoustic data, and their presence verified by video camera images, as forming a layer of varying density at and below the thermocline (15–30 m). The depth of the jellyfish layer also coincided with the depth of maximum dissolved oxygen concentration. The echointegration of these daylight data enabled quantification the Aurelia population, at a frequency of 120 kHz. In the night echograms, the acoustic signals of Aurelia were at least partially masked by pelagic and demersal fish, which disaggregated from schools and formed a layer associated with a strong thermocline at 15 m. An average target strength (TS) of −76.4 dB was obtained in situ corresponding to a mean length of 10.8 cm and a mean wet weight of 134 g measured from sampled medusae. These results were combined with echo-integration values to provide an estimate of 4,238,602 individuals and a biomass of 568 tons of Aurelia in BL. This study provided a synoptic view of Mljet Lake and illustrated the potential of acoustic surveys of jellyfish populations to contribute to ecosystems studies. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Distribution patterns of mesozooplankton biomass in the North Sea

During spring 1986 and winter 1987, zooplankton samples were collected over the entire North Sea by means of a multi-closing net-system. Before taxonomic treatment, wet weight estimates and carbon content conversions were carried out. From this data set, 4 962 522 tons zooplankton biomass (dry weight) were estimated for the whole North Sea during the spring survey. High biomasses (more than 100 mg C/m³) were located in areas between the Orkneys and the Shetlands, off the mouth of the Firth of Forth, the Channel and the river Rhine. Considerable zooplankton biomass was also found parallel to the Danish west coast. Furthermore, a narrow tongue of high biomass (partly greater than 200 mg C/m³) intruded from the north, between 1 °E and 4 °E, into the northern North Sea, turning to the east at 56°N, and continuing into deeper water layers to form a left turning “helix” of high biomass in the central part of the North Sea. During the winter survey the carbon content of the zooplankton stock was a factor 10 lower than in summer. Altogether, 519340 tons of zooplankton biomass (dry weight) were estimated in winter. Centres of relatively high biomass were located off the mouth of the rivers Rhine, Weser and Elbe and off the British east coast moving in a cyclic way across the Dogger Bank into the central North Sea. A further maximum of zooplankton abundance was found in the Skagerrak region. However, an intrusion of zooplankton from the shelf edge into the North Sea was not observed in winter. A qualitative analysis of species composition showed that small copepods dominated the zooplankton in the southern and eastern North Sea. The “eddy” of high biomass in the northern North Sea observed in spring, however, was mostly shaped by the large copepodCalanus finmarchicus (70–90%). The distribution of zooplankton biomass in the North Sea is discussed in relation to the hydrographic conditions and to the biology of the dominant species.     

The predator-prey relationship of perch,Perca fluviatilis,larvae and zooplankton in two Scottish lochs

Synopsis Food consumption of perch larvae and the impact of this on zooplankton were examined in two adjacent shallow Scottish lochs. Maximum annual abundance of zooplankton occurred in mid-May at L. Kinord with minimum values in mid-June. Copepods were prominent in spring but were followed by a multi-species community of cladocerans and rotifers in summer. At L. Davan zooplankton biomass remained high through summer with cladocerans dominating andDaphnia longispina the most frequent species. Availability of food items was a principal factor governing feeding behaviour of larvae. Copepodite stages were initially the most common item in the diet in L. Kinord in 1976 and 1977 and rotifers the principal food in June 1977, reflecting the dominance of these items in the zooplankton. Cladocerans were dominant in the plankton community in L. Davan and constituted the greater part of food intake. Overlying this general pattern there was an increase in the size of food items taken by larvae with time and also a definite pattern of food selection for copepods, with initially selection for smaller copepodite stages and later for larger stages and adults. On most occasions larvae selected forCyclops strenuus abyssorum andPolphemus pediculus and selected againstDaphnia longispina. The reduction in the total zooplankton biomass attributed to perch larvae was minimal, with the exception of mid-June at L. Kinord in 1976. However, predation on particular species and copepodite stages was occasionally intense and may have impacted the zooplankton populations.     

Biomass of scyphozoan jellyfish, and its spatial association with 0-group fish in the Barents Sea

An 0-group fish survey is conducted annually in the Barents Sea in order to estimate fish population abundance. Data on jellyfish by-catch have been recorded since 1980, although this dataset has never been analysed. In recent years, however, the ecological importance of jellyfish medusae has become widely recognized. In this paper the biomass of jellyfish (medusae) in 0–60 m depths is calculated for the period 1980–2010. During this period the climate changed from cold to warm, and changes in zooplankton and fish distribution and abundance were observed. This paper discusses the less well known ecosystem component; jellyfish medusae within the Phylum Cnidaria, and their spatial and temporal variation. The long term average was ca. 9×10⁸ kg, with some years showing biomasses in excess of 5×10⁹ kg. The biomasses were low during 1980s, increased during 1990s, and were highest in early 2000s with a subsequent decline. The bulk of the jellyfish were observed in the central parts of the Barents Sea, which is a core area for most 0-group fishes. Jellyfish were associated with haddock in the western area, with haddock and herring in the central and coastal area, and with capelin in the northern area of the Barents Sea. The jellyfish were present in the temperature interval 1°C<T<10°C, with peak densities at ca. 5.5°C, and the greatest proportion of the jellyfish occurring between 4.0–7.0°C. It seems that the ongoing warming trend may be favourable for Barents Sea jellyfish medusae; however their biomass has showed a recent moderate decline during years with record high temperatures in the Barents Sea. Jellyfish are undoubtedly an important component of the Barents Sea ecosystem, and the data presented here represent the best summary of jellyfish biomass and distribution yet published for the region.     

Winter energy allocation and deficit of juvenile walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> in the Doto area,northern Japan

Seasonal energy allocation and deficits of marine juvenile fishes have considerable effects on their survival. To explore the winter survival mechanism of marine fishes with low lipid reserves in their early life, juvenile walleye pollock Theragra chalcogramma were collected along the continental shelf of northern Japan over a 2-year period, and energy allocation and deficit patterns were compared between wild and laboratory-starved fish. Contrary to expectations, wild fish generally continued to accumulate protein mass and concurrently tended to reduce lipid mass from late autumn through winter. The most plausible explanation for the continuous structural growth is that juvenile pollock give priority to reducing mortality risk from size-selective predators under quasi-prey-limited conditions. Exceptionally, inshore small fish reduced both constituents during a winter. The inshore fish consumed 2.5 times more lipid energy than protein energy in November–December, but protein was more important than lipids as a source of energy in December–January and in February–March. However, dependence upon protein reserves was lower for the wild fish than for the laboratory-starved fish, suggesting milder nutritional stress of the wild fish than that observed in the starvation experiment. Moreover, the lipid contents of mortalities in the starvation experiment were mostly <1%, whereas few wild fish had such lipid contents in the field. These results suggest that juvenile pollock are able to avoid both starvation and predation by accumulating protein reserves.     

Body mass and composition responses to short-term low energy intake are seasonally dependent in Steller sea lions (Eumetopias jubatus)

Steller sea lions (Eumetopias jubatus) were fed restricted iso-caloric amounts of Pacific herring (Clupea pallasi) or walleye pollock (Theragra chalcogramma) for 8–9 days, four times over the course of a year to investigate effects of season and prey composition on sea lion physiology. At these levels, the sea lions lost body mass at a significantly higher rate during winter (1.6 ± 0.14 kg day⁻¹), and at a lower rate during summer (1.2 ± 0.32 kg day⁻¹). Decreases in body fat mass and standard metabolic rates during the trials were similar throughout the seasons and for both diet types. The majority of the body mass that was lost when eating pollock derived from decreases in lipid mass, while a greater proportion of the mass lost when eating herring derived from decreases in lean tissue, except in the summer when the pattern was reversed. Metabolic depression was not observed during all trials despite the constant loss of body mass. Our study supports the hypothesis that restricted energy intake may be more critical to Steller sea lions in the winter months, and that the type of prey consumed (e.g., herring or pollock) may have seasonally specific effects on body mass and composition.     

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.