Larval Pollock Research Articles

Otolith biochronology reveals factors underlying dynamics in marine fish larvae

The environmental factors include the drift out of the Gulf of Alaska (GOA) (Transport), sea surface temperature (SST), surface wind speed in Shelikof Strait (WindS) and the Pacific Decadal Oscillation (PDO). The biological factors consist of the spawning biomass of the pollock in GOA (Fig. S1) and the proportion of spawners that are 8 yr or older (Fig. S2). We only had monthly average SST, so we used linear interpolation of the observed SST to approximate daily SST. Transport and WindS are daily data. The first 5 mo SST and WindS in each year from 1981 to 2001 are shown in Figs. S3 & S4. For the environmental factors SST and WindS, we compute their anomalous daily values by subtracting the daily average from the observed daily values. For each of the covariates: anomalous SST and anomalous WindS, we computed 2 auxiliary variables. One auxiliary variable is calculated as the average covariate over the 30 d prior to a hatchdate, which attempts to quantify the effects of environmental conditions relevant to the spawning pattern and the hatching process. The second auxiliary variable attempts to quantify the environmental conditions on larval pollock survival, which is computed as the average of a given covariate over the period from the hatchdate to the catch date. The auxiliary variables for the anomalous SST from 1981 to 2001 are shown in Figs. S5 & S6, where SSTB measures the average 30 d anomalous SST before a hatchdate, while SSTA measures the average anomalous SST between the hatchdate and the corresponding catch date. As transport is postulated to affect larval survival but not mature fish, we compute the average transport from the hatchdate to the catch date in order to assess the transport effect on larval survival. PDO mainly affects pollock spawning patterns, so the mean anomalous PDO from January to March was computed for each year. Supporting information

Simulation of physically mediated variability in prey resources of a larval fish: a three‐dimensional NPZ model

AbstractA three‐dimensional biophysical nutrient–phytoplankton–zooplankton model was used to investigate the spatial and temporal dynamics of food resources for young walleye pollock in the western Gulf of Alaska, to further understanding of recruitment processes for pollock. We modeled nitrogen, phytoplankton, a large herbivorous grazer parameterized as Neocalanus spp. (the biomass dominant copepod in the Gulf), and the 13 stages (egg, naupliar and copepodite) of Pseudocalanus spp. (a major constituent of the diet of pollock) so that the appropriate size class of food for each size of larval pollock was represented. Model results identified an area between the Semidi and Shumagin Islands that may not be suitable as a nursery area early in the year due to low prey abundance. Modeled mesoscale eddies, previously hypothesized to be important for larval pollock retention in Shelikof Strait, contained higher prey concentrations than the surrounding waters when they were cyclonic. This work also help to understand the consistency of pollock spawning in time and space in Shelikof Strait, by examining the timing and location of prey availability which, along with transport, narrows the window for optimal spawning.

Applied and theoretical considerations for constructing spatially explicit individual-based models of marine larval fish that include multiple trophic levels

Individual-based modelling (IBM) techniques offer many advantages for spatially explicit modelling of marine fish early life history. However, computationally efficient methods are needed for incorporating spatially explicit circulation and prey dynamics into IBMs. Models of nutrient-phytoplankton-zooplankton (NPZ) dynamics have traditionally been formulated in an Eulerian (fixed spatial grid) framework, as opposed to the pseudo-Lagrangian (individual-following) framework of some IBMs. We describe our recent linkage of three models for the western Gulf of Alaska: (1) a three-dimensional, eddy-resolving, wind- and runoff-driven circulation model, (2) a probabilistic IBM of growth and mortality for egg and larval stages of walleye pollock (Theragra chalcogramma), and (3) an Eulerian, stage-structured NPZ model which specifies production of larval pollock prey items. Individual fish in the IBM are tracked through space using daily velocity fields generated from the hydrodynamic model, along with self-directed vertical migrations of pollock appropriate to each life stage. The NPZ dynamics are driven by the same velocity, temperature, and salinity fields as the pollock IBM, and provide spatially and temporally varying prey fields to that model. The resulting prey fields yield greater variance of individual fish attributes (e.g. length), relative to models with spatially uniform prey. Practical issues addressed include the proper time filtering and storage of circulation model output for subsequent use by biological models, and use of different spatial grids for physical and biological dynamics. We demonstrate the feasibility and computational costs of our coupled approach using specific examples from the western Gulf of Alaska.

Importance of spawning location and timing to successful transport to nursery areas: a simulation study of Gulf of Alaska walleye pollock

We conducted a model experiment to examine the hypothesis that the spatial and temporal specificity of spawning of walleye pollock (Theragra chalcogramma )i n Shelikof Strait, Alaska, evolved to optimize physical transport to the juvenile nursery area near the Shumagin Islands some 375 km to the southwest. The alternative hypothesis is that factors other than physical transport alone must also be important in the choice of spawning location and timing. We used a coupled biophysical model consisting of a three-dimensional hydrodynamic model of the currents in the region, an individual-based model of the early life stages of pollock, and a nutrient– phytoplankton–zooplankton model that provides a spatially and temporally dynamic source of food for larval pollock. Results showed that fish spawned to the south of Kodiak Island, or too early (February) or too late (July), did not reach the Shumagin Island nursery area by early September. However, the potential region and time of spawning that did allow successful transport to the nursery area was much broader than the observed spawning region and time. Therefore, factors other than physical transport alone must be considered in explaining the specificity of the location and timing of spawning for this stock.

Development of the escape response in larval walleye pollock ( Theragra chalcogramma )

The development of the escape response of walleye pollock (Theragra chalcogramma) larvae from attacks by macrozooplanktonic and small-fish predators was quantified in laboratory experiments. Behavior was recorded using video cameras with silhouette illumination from infrared-emitting diodes and by visual observation. Laboratory-reared larvae of 1, 3, 8, 10, 12, 18, 22, 27, 42 days post-hatching, ranging in size from 4 mm to 10 mm total length, were used in the experiments. Even the youngest larvae were observed to exhibit a fast startle response. The percentage of successful larval escapes from the different predators increased as the larvae developed. Euphausiids (Thysanoessa raschii) and amphipods (Calliopiella pratti) often touched larvae but the larvae were usually able to escape and no successful captures of larvae over 22 days old were observed. Although successful escape from initial attacks by three-spine sticklebacks (Gasterosteus aculeatus) increased ontogenetically, sticklebacks were able to consume most larvae, even of the oldest age group, by repeated attacks. Day-old larvae had the lowest percent of escapes after encounters with jellyfish (Sarsia sp.), but the percentage of escapes increased dramatically for 3-day-old larvae. Escape speeds after an attack also increased with age, and tended to be higher after stickleback attacks and lower after jellyfish attacks. This study revealed that the escape response of larval pollock to attack by predators improves rapidly with development during the early larval stage.

A synthesis of biological and physical processes affecting the feeding environment of larval walleye pollock (Theragra chalcogramma) in the eastern Bering Sea

Biological and physical phenomena that affect conditions for larval survival and eventual recruitment differ in the oceanic and shelf regions. In the oceanic region, eddies are a common feature. While their genesis is not well known, eddies have unique biophysical characteristics and occur with such regularity that they likely affect larval survival. High concentrations of larval pollock often are associated with eddies. Some eddies are transported onto the shelf, thereby providing larvae to the Outer Shelf Domain. Advection, rather than local production, dominated the observed springtime increase in chlorophyll (often a correlate of larval food) in the oceanic region. Over two‐thirds of the south‐eastern shelf, eddies are absent and other phenomena are important. Sea ice is a feature of the shelf region: its interannual variability (time of arrival, persistence, and areal extent) affects developmental rate of larvae, timing of the phytoplankton bloom (and potentially the match/mismatch of larvae and prey), and abundance and distribution of juvenile pollock. In the oceanic region, interannual variation in food for first‐feeding pollock larvae is determined by advection; in the shelf region, it is the coupled dynamics of the atmosphere–ice–ocean system.

Anomalous transport of walleye pollock larvae linked to ocean and atmospheric patterns in May 1996

Larvae from a large aggregation of walleye pollock spawning in early spring in Shelikof Strait, Gulf of Alaska, are normally transported to the south-west in the vigorous Alaska Coastal Current. In the spring of 1996, anomalous winds resulted in unusually weak transport in the Shelikof Strait sea valley. The main aggregation of larval pollock in the Shelikof region was surveyed four times in 1996 over a period of about 40 days, including finer-scale sampling of the leading south-western edge of the larval distribution. The south-western edge of the larval distribution showed weak transport up the sea valley for a period of about 10 days, corresponding to the observations of currents, after which many larvae were transported over the shelf region to the west. These observations are unique in over 15 years of monitoring larval transport patterns and demonstrate how anomalous weather, and hence current patterns, influence variability in larval transport.

Dependence of survival on growth in larval pollock Pollachius virens and haddock Melanogrammus aeglefinus:a field study based on individual hatchdates

The hatchdate frequency distributions (HFD) of pollock and haddock larvae sampled at monthly intervals west of Sable Island (Scotian Shelf, northwest Atlantic) in 1992 and 1993 were reconstructed for different age intervals (0-20, 21-40, 41-80 d) and corrected for aliasing due to sampling discontinuity and mortality-dispersion. The ratio of the HFD at a given age to the HFD at an earlier age was used as an index of the relative survival of larvae grouped into 5 d hatchdate cohorts. Pollock hatched from November to March and haddock from February to June. In pollock, seasonal variations in relative survival of the cohorts over the 21-40 and 41-80 d age intervals were correlated to Sorong vanations in growth. In haddock, growth varied little over the hatching season and there was no significant link with survival. For pollock, slow growth invariably resulted in low survival but fast growth resulted in either low or high survival, indicating that fast growth is a necessary but not sufficient condition for survival. Increased predation pressure late in the hatching season of both species Could explain the decoupling of growth and survival in cohorts hatched in spring and early summer.

A new method for live-staining protists with DAPI and its application as a tracer of ingestion by walleye pollock ( Theragra chalcogramma (Pallas)) larvae

The efficacy of DAPI (2,4-diamadino-6-phenylindole), a fluorescent DNA-specific stain, to live-stain protists was evaluated for use as tracers in laboratory studies of feeding by larval pollock, Theragra chalcogramma (Pallas). At 4 μg · ml −1, DAPI effectively live-stained all eight species of heterotrophic protists examined. DAPI also stained four species of autotrophic protists although staining was more variable. No toxic effects of DAPI were detected. DAPI did not appear to alter the swimming behavior of stained protists. Further, the growth rates of a ciliate, Euplotes sp. and an autotrophic flagellate, Rhodomonas salina (Wislouch) Hill and Wetherbee, exposed to DAPI, did not differ significantly from growth in controls. When DAPI-stained Euplotes sp. were presented to larval pollock, ingested cells were easily seen in larval guts. This is the first report of ingestion of an aloricate ciliate by larval pollock. The lack of toxicity of DAPI and the easy visualization of ingested DAPI-stained protists, make this method a useful new tool for examining the ingestion of protists by metazoa and other protists.

The plankton of Shelikof Strait, Alaska: standing stock, production, mesoscale variability and their relevance to larval fish survival

ABSTRACTPhysically‐mediated variations in production, standing stock and distribution of plankton have a significant impact on the growth and survival of larval fishes in Shelikof Strait. We integrate descriptions of mechanisms that control the distribution and regional production of planktonic organisms with mechanisms that influence survival of early life history stages of marine fish, especially the locally abundant walleye pollock, Theragra chalcogramma.The timing of the spring phytoplankton bloom is more variable than the regular appearance of pollock larvae, and is affected by variability in the winter to spring storm season transition, stratification in the Alaska Coastal Current (ACC), and cloudiness. The spring bloom occurs first in the ACC, fuelling local production of copepod nauplii, the main prey item of early larval pollock. Shelikof Strait zooplankton standing stock is higher in the ACC than the surrounding Coastal Water (CW) throughout the spring. This is the result of local production as well as regional production which occurs upstream of the strait. Mesoscale features associated with the ACC (fronts, meanders, and eddies) determine the distribution of plankton through physical convergence, and reduced dispersion and transport. Evidence for enhancement of planktonic production in these features is lacking. Thus the ACC plays a strong role in determining plankton production, standing stock, and distribution in the Shelikof region.The strength of the relationship among plankton, planktonic production, larval pollock growth and survival, and fisheries recruitment is variable. The plankton is only one of several key variables that affect eventual recruitment to the pollock fishery.

Acoustic detection of mesoscale biophysical features in the Shelikof sea valley, and their relevance to pollock larvae in the Gulf of Alaska

ABSTRACTMesoscale features such as fronts and eddies can act to retain larval walleye pollock (Theragra chalcogramma) within the continental shelf zone in the western Gulf of Alaska. During two May cruises, we observed unusual patterns of backscattering with a 38 kHz acoustic system. Characteristics of this signal were a strong scattering layer at the surface and in midwater, with the water column between nearly void of sound scattering organisms. This signal appeared in several transects where satellite remote sensing indicated the presence of an eddy. Analysis of concomitant water properties and ADCP (153 kHz) data confirmed the existence of an anomalous physical feature at this location. Biological properties (chlorophyll and zooplankton) showed marked changes across the edge of the feature. Larval pollock densities were estimated to be an order of magnitude higher within this feature compared to those outside. Acoustic backscatter signals can be used to identify and characterize mesoscale biophysical features in the ocean, thereby enabling real‐time studies of these features.

Springtime microprotozoan abundance and biomass in the southeastern Bering Sea and Shelik of Strait, Alaska

We surveyed springtime biomass and abundance of the >20 nm microprotozoa in surface waters of the SE Bering Sea and Shelikof Strait, Alaska. This study was part of the Fisheries Ocean- ography Coordinated Investigations (FOCI) program examining processes which affect the recruit- ment variability of walleye pollock (Theragra chalcogramma). Microprotozoa are a potential prey resource for larval pollock which has not been previously examined. In both areas, the >20 (im micro- protozoa were predominantly dinoflagellates and ciliates. At the time of sampling (May 1990 in Sheli- kof Strait and April 1992 in the SE Bering Sea), the spring diatom bloom was under way in Shelikof Strait, but not in the SE Bering Sea. Heterotrophic dinoflagellates dominated the microprotozoan assemblage in Shelikof Strait, but not in the SE Bering Sea. In the SE Bering Sea. total microprotozoan abundances ranged from 300 to 6233 organisms 1 ' and biomass from 0.58 to 9.73 jig C I1. In Shelikof Strait, abundance and biomass were higher, ranging from 850 to 14 960 organisms 1' and from 1.29 to 70.73 jig C I ', respectively. These biomass levels are comparable to those reported from other coastal and oceanic regions. Microprotozoan biomass levels were sufficient to support the estimated metabolic needs of first-feeding larval walleye pollock. It remains to be shown whether larval pollock use this resource.

Ontogenetic development of digestive enzyme activities in larval walleye pollock, Theragra chalcogramma

Activities of digestive enzymes trypsin, amylase and lipase in laboratory-reared walleye pollock, Theragra chalcogramma, were measured from hatching to Day 39 (just before notochord flexion) in 1993. All measurements were conducted individually or semi-individually (groups of two larvae of the same standard length). Close relationships between digestive enzyme activities and morphological development of digestive organs were observed. Activities of trypsin and lipase were low during the transition period from endogenous to exogenous energy. Amylase activity was constant with large variance during the same period. Specific enzyme activities of trypsin and amylase indicated high values with large variance during the early period. All three enzyme activities increased with age afterthe transition period, and the specific enzyme activities became constant. The existence of two types of lipase was suggested. One lipase showed a peak of specific activity at Day 4 and might be related to yolk-sac absorption. The activity of the other lipase increased with age after Day 14 and might be related to digestion of prey lipid. Our results suggest that digestive enzymes included in food organisms supplement larval pollock digestive enzymes.

Ubiquitous eddies of the eastern Bering Sea and their coincidence with concentrations of larval pollock

ABSTRACTBetween 1988 and 1993, 12 satellite‐tracked buoys were deployed in four eddies in the south‐eastern Bering Sea. Our success in finding eddies resulted from placing buoys in high concentrations of walleye pollock (Them‐gra chalcogramma) larvae. We utilize data from hydro‐graphic surveys, satellite‐tracked buoys and moored current meters to describe the eddies. Small (< 25 km diameter) eddies likely transit along the slope of the eastern Bering Sea every 45–60 days. In previous studies such small features were not observed because their size fell within typical separation of hydrographic stations and the weak sea surface temperature gradients are not resolved by satellite‐borne infrared imagery.

Observation and analysis of fishery processes: larval pollock at Shelikof Strait, Alaska

ABSTRACTConditions affecting distributions of larval walleye pollock (Theragra chalcogramma) were examined at Shelikof Strait, Alaska, during springtime, 1986 and 1987. Abundance and distribution of larval pollock southwest of the Strait's southern entrance was determined with oblique plankton tows taken each year in May. Infrared images of sea surface temperature patterns were derived from AVHRR scenes obtained by NOAA satellites during each April and May. Pattern displacements between 24‐hour‐interval images were used to estimate surface motion. Each spring, measurements were taken by remote weather stations and ships, and a nearsurface current meter record was obtained during 1987. Treated as quasi‐synoptic, spatial relations between sets of surface temperature, surface flow, and larval pollock distributions show coincidences between submesoscale physical and biological features. The highest larval abundances occurred as patches within a cold plume (1986) and an eddy (1987). These confirm that physical features can retain larval pollock on the continental shelf. Observations are examined for evidence of physical and biological events that jointly can cause such coincidences and foster alternatives for survival during transport to nursery grounds. Explanations for presence of cohorts observed within the 1987 eddy are given in terms of spatial and temporal relationships evident between spawning and hatching areas, hatch date distributions, meanders, eddy generation and movement, background flow, and advection times. The observations, analyses, and results are consistent with the concept of a coupled, fluctuating biophysical process that can emulate variations in larval abundance and provide a multiplicity of system pathways for early‐life stages representations.

Temporal and geographic differences in feeding and nutritional condition of walleye pollock larvae Theragra chalcogramma in Shelikof Strait, Gulf of Alaska

Feeding and nutritional condition of first-feeding walleye pollock larvae Theragra chalcogramma were compared to available prey levels measured during early spring (late April- early May) and mid-spring (mid-May) 1989. In early spring, feeding intensity, mean RNNDNA values of larvae, and microzooplankton abundance were higher within a large patch of larvae compared with areas outside the patch. In mid-spring, microzooplankton prey abundance, feedmg levels and RNA/DNA of larvae m and out of the previously defined patch were higher, indicating better overall conditions for growth than in early spring. The results suggest that limiting food densities may occur during spring over spatial and temporal scales that affect feeding and growth of larval pollock.

Variations in Egg Production Rates by Pseudocalanus Spp. In a Subarctic Alaskan Bay During the Onset of Feeding by Larval Fish

Weekly estimates of female abundance and daily egg production rates were made for Pseudocalanusspp. during 1987 and 1988 in Auke Bay, southeastern Alaska. The sampling, which included measurements in primary production, was done during April and May when larval fish initiate feeding on copepod nauplii. The objective of the study was to examine the relative importance of female abundance and daily egg production rates for Pseudocalanus spp. in determining production of nauplii during the study period. In both years primary production rates exceeded 500 mg C·m―2·d―1. At this level of primary production Pseudocalanus spp. should not have been food-limited in either year. The average egg production rates per individual female during the 2 years were identical: 2.6 eggs·female―1· d―1. The average populations of female Pseudocalanus were 4,765 and 7,656 individuals·m―2 in 1987 and 1988, respectively. The larger populations of female Pseudocalanus spp. in 1988 resulted in higher overall egg production and better feeding conditions for larval fish. Thus, during the period when larval pollock began feeding on nauplii, female copepod abundance was more important than temporal differences in egg production rates in determining production of nauplii. A complete understanding of the copepod-larval fish relationship in Auke Bay will require explanation of the causes for interannual variability in abundance of adult female Pseudocalanus spp.

Otolith microstructure of three larval gadids in the Gulf of Maine, with inferences on early life history

A detailed analysis of the assumptions underlying the application of otolith microstructure was undertaken for three gadid species from the Gulf of Maine. Daily increment counts provided a precise and accurate index of age in cod (Gadus morhua) and appeared to do so in haddock (Melanogrammus aeglefinus). However, the widths of increments formed shortly after hatch in cod and haddock otoliths approached the resolution limit of light microscopy, resulting in an underestimation of age by 2–3 d for all fish. Daily increments were also formed in the otoliths of pollock (Pollachius virens) but, perhaps because of more severe resolution problems, the microstructure associated with the early larval stage was unclear. Factors that could confound growth back-calculation efforts included differing otolith length – larval length relationships among samples and an inverse correlation between hatch check diameter and temperature. Estimates of hatch date and growth rate developed in this study were not affected by slight deviations from the underlying assumptions, but might be affected by such deviations under different environmental conditions. Larval pollock growth was comparable to that of cod and haddock, despite a later hatch date. Pollock hatched primarily in November, while peak dates for cod and haddock hatching occurred in March–April and May, respectively. Cod on Georges Bank hatched significantly earlier than those on Browns Bank, although the latter included a less abundant component that hatched in the fall.