Larval Walleye Pollock Theragra Research Articles

Rearing Trials on Larval Walleye Pollock Theragra chalcogramma Using Nutrient-enriched Rotifers as an Early Live Food

Rearing Trials on Larval Walleye Pollock Theragra chalcogramma Using Nutrient-enriched Rotifers as an Early Live Food

Patterns in diet reveal foraging site fidelity of short-tailed shearwaters in the southeastern Bering Sea

The short-tailed shearwater Puffinus tenuirostris is an apex predator in the southeastern Bering Sea ecosystem. During 1997 to 1999, a period of great variability in the Bering Sea, we used a multi-pronged approach to study transfer of carbon and nitrogen to short-tailed shearwaters through analysis of stomach contents of birds collected while foraging, and stable isotope and fatty acid com- position of tissues from shearwaters and their prey. Two conclusions result from these 3 analyses of feeding history. First, short-tailed shearwaters demonstrated localized differences in diet with respect to sampling location and season, indicating that shearwaters feed in discrete locations long enough (several weeks) to reflect regional differences in prey availability. Second, elevated δ 15 N levels (~1 to 2‰) in the liver of shearwaters in the fall of 1997 and 1998 were likely a response to nutritional stress and overturn of nutrients in the tissues of birds or elevated isotope levels in tissues of prey, rather than an increase in the trophic level of the diet. Over the 3 yr period, shearwater diets switched from adult euphausiids Thysanoessa raschii and T. inermis (usually females with spermatophores) taken in spring to an increase in the amount of fish (Pacific sandlance Ammodytes hexapterus), juvenile Gadi- dae fishes and larval walleye pollock Theragra chalcograma particularly in fall (1997 excepted). For each location and year, there was a consistent trend in isotopic values, with an increase of 3 to 5‰ for δ 15 N and 1 to 3‰ for δ 13 C between shearwaters and their primary prey. The resolution of our sam- pling techniques indicates little movement of shearwater flocks among sample locations for periods of up to several weeks.

Environmental factors influencing larval walleye pollock Theragra chalcogramma feeding in Alaskan waters

This study examines potential interactions among the environmental variables likely to affect larval walleye pollock Theragra chalcogramma feeding in the sea. Walleye pollock larvae were sampled from Shelikof Strait, Gulf of Alaska, and from the eastern Bering Sea, with corresponding environmental data. Variables used in our study were time spent feeding, seawater temperature, light, prey density, wind speed and standard length of the larvae. We applied an additivity test to detect the presence of potential interactions among these variables and adopted an expansion of gen- eralized additive models (GAMs) that allowed the inclusion of interaction terms in a non-parametric regression analysis. After testing all possible 2-way interactions among these variables, we found 4 significant terms: (1) time feeding-standard length, (2) temperature-light, (3) light-wind speed and (4) prey density-standard length. The most influential interaction term was the light-wind speed interaction, which caused a decrease of the model-generalized cross-validation (GCV, whereby lower values indicate more parsimonious models) from 0.0402 (for a model with all variables but no interaction term) to 0.0290 (for a model with all variables and this interaction), and an increase of the model explained variance by 7% (R 2 = 0.89 versus 0.82). This result indicates that the effect of wind speed (turbulence) on larval walleye pollock feeding is dependent upon the amount of light avail- able. This may be due to vertical movements in the water column by both walleye pollock larvae and their prey in response to turbulence.

Consequences of a superabundance of larval walleye pollock Theragra chalcogramma in the Gulf of Alaska in 1981

Abundances of larval walleye pollock in Shelikof Strait, Gulf of Alaska, in 1981 were far greater than any recorded estimates before that time or since (some patch estimates exceeded 100000 larvae per 10 m 2 ). In spite of this extraordinary input, the ensuing 1981 year class was relatively poor. An examination of the feeding habits of larvae collected from inside and outside dense larval patches revealed that in 1981, larval walleye pollock consumed significantly more invertebrate eggs (x = 66.7% of the total diet) and fewer copepod nauplii (x = 13.6%) inside of larval patches relative to outside (x = 11.4 and 63.2%, respectively). These observations suggest that density dependent com- petition inside patches may have locally depleted the primary food source, copepod nauplii, prompting a diet switch to a lower quality but more abundant prey resource, invertebrate eggs. Results from a bioenergetics simulation support this theory, indicating that dense patches of walleye pollock larvae in 1981 were capable of exhausting naupliiar prey resources in Shelikof Strait in a relatively short period of time (14 to 40 d). Our observations contrast with other studies that suggest that ichthyoplankton exert little or no influence on microzooplankton standing stocks. Rather, we present evidence that, in certain unusual circumstances, particularly dense aggregations of larvae are capable of locally de- pleting the prey community. Such events may weaken larval condition and exacerbate natural death rates, which may have contributed to the poor recruitment success of the 1981 year class.

Interannual variability in growth of larval and juvenile walleye pollock Theragra chalcogramma in the western Gulf of Alaska, 1983–91

ABSTRACTInterannual variability in growth of larval walleye pollock Theragra chalcogramma was examined from 1983 to 1991 and of juveniles from 1985 to 1990. ANCOVA was used to assess differences in population growth rates, and an alternate method was developed to examine variations between annual length‐at‐age data and average ‘expected’ values over different age groupings. For larvae, the years 1986, 1987, 1989 and 1990 had higher than average length‐at‐age, and 1988 and 1991 had lower than average values. Relationships between growth and SST and larval density were not clear. A tentative relationship between copepod nauplii abundance and larval length‐at‐age was noted. The consequence of larval growth to larval mortality, late larval abundance or recruitment was not clear. We conclude that larval mortality rates are highly variable and tend to mask effects of moderate variability in growth on later abundance. For juveniles, 1987 had significantly lower than average length‐at‐age and 1988 had higher than average values. Although there are few years of data, they tend to support the importance of juvenile growth in the recruitment process. Conditions for the large 1988 year class are documented and discussed, including warm SST, calm winds, relatively low larval growth rates, low abundances of potential predators on larvae, low larval mortality rates, and high juvenile growth rates.

Mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska, 1988–91

ABSTRACTMortality rates of larval walleye pollock Theragra chakogramma were estimated from larval survey data from 1988 to 1991. Mortality estimates were based on cohort‐specific losses between occupations of survey grids. Interannually, estimates of early feeding stage larval mortality rates ranged over an order of magnitude, from 0.045–0.43 day‐1, and declined sharply with age. There is some evidence that mortality rates of early feeding larvae tend to be negatively correlated with temperature and postively correlated with wind mixing.

Feeding selectivity of larval walleye pollock Theragra chalcogramma in the oceanic domain of the Bering Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 120:1-10 (1995) - doi:10.3354/meps120001 Feeding selectivity of larval walleye pollock Theragra chalcogramma in the oceanic domain of the Bering Sea Hillgruber, N., Haldorson, L. J., Paul, A. J. Feeding of larval walleye pollock was examined with respect to density and distribution of micro- and mesozooplanktonic prey (<500 um) at 6 depths in the oceanic domain of the Bering Sea to determine if first-feeding larvae select among available prey and to assess their foraging environment in comparison to other locations where walleye pollock spawn. At 30 m depth, walleye pollock larvae and copepod nauplii occurred at maximum densities of 106.6 ind. 100m-3 and 26.3 ind. l-1, respectively. First-feeding walleye pollock larvae (3.5 to 6.5 mm) fed exclusively on copepod nauplii and copepod eggs. Among copepod nauplii, larvae selected for Metridia sp. and Microcalanus sp. but against Oithona similis, even though the latter was the most abundant prey taxon in the study area. Of the nauplii ingested by larval walleye pollock, there was evidence of selection for larger nauplii within Metridia sp., Microcalanus sp., and O.similis. In addition, larvae preferred Stages III to V of calanoid nauplii. Larvae at 30 m depth had the highest incidence of feeding (97.5%) and number of prey items (4.6 larva-1) ingested. Although the 30 m depth stratum provided best physical and foraging conditions, the overall low percentage of feeding larvae and low numbers of prey consumed suggest that foraging conditions for larval walleye pollock at the time of sampling were below saturation feeding levels. Walleye pollock larvae . Feeding . Nauplii . Prey selection . Bering Sea Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 120. Publication date: April 20, 1995 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1995 Inter-Research.

Contrasting years of prey levels, feeding conditions and mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska

Contrasting years of prey levels, feeding conditions and mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska

Analysis of patterns in larval walleye pollock Theragra chalcogramma survival and wind mixing events in Shelikof Strait, Gulf of Alaska

Analysis of patterns in larval walleye pollock Theragra chalcogramma survival and wind mixing events in Shelikof Strait, Gulf of Alaska

Occurrence of a check in otoliths of reared and sea-caught larval walleye pollock Theragra chalcogramma (Pallas) and its relationship to events in early-life history

In frontal sections of postlarval otoliths of walleye pollock Theragra chalcogramma (Pallas), some increments were observed to be fused, forming a band structure (check) on the outer distal side. Backward counts of otolith daily growth increments showed the check was formed at different dates for different individuals, beginning from early to late in March and terminating from late in March to mid-April. A comparison between samples from various areas around Hokkaido revealed that the check was formed at almost the same body length (8–11 mm), independent of their age in days and growth patterns. Otoliths of larvae reared under constant environmental conditions also showed the band structure, appearing at the same body length as for wild fish. Comparing these observations with those published in the literature, morphological or physiological changes seem to be responsible for the check formation that the fish experience at 8–11 mm body length.

Age-specific mortality and transport of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska

A field study was conducted in May 1988 to estimate mortality rates of individual cohorts of larval walleye pollock Theragra chalcogramma in Shelikof Strait, Gulf of Alaska. Two surveys of the area of larval distribution were done 12 d apart and daily mortality rates estimated from changes in abundance of each cohort. This study was designed to minimize potential sources of bias to mortality estimates, such as larval extrusion or escapement from sampling gear, advective or diffusive gain or loss of larvae into or out of the sampling area, the uncertainty incurred by the use of length as a proxy for age, and the often erroneous assumption of constant production and mortality rates. The age- specific mortality rates from this study were low compared to rates previously estimated for larval wall- eye pollock. Four significant mortality estimates, ranging from 0.041 to 0.0f6 d-', came from among the youngest cohorts. The low larval mortality rates in 1988 coinaded with very calm ocean conditions. Larval drift rates estimated in this study were coherent with the movement of a satellite-tracked drift buoy and averaged 3.1 to 3.9 an s-', which is comparable to rates reported from other studies.

Electron transport system(ETS) activity as a possible index of respiration for larval walleye pollock Theragra chalcogramma.

Electron transport system (ETS) activity was measured for larvae of the walleye pollock Theragra chalcogramma, with the objective of assessing this method for estimating the respiration of fish larvae in situ. The ETS activity decreased minimally after 50 days' storage at -80°C. The ETS activity increased linearly with larval dry weight. The average dry weight-specific ETS activity of feeding stage larvae was 9.75±2.25 (S.D.) μlO2/mg•h, which was significantly higher than 5.82±0.88μlO2/mg•h for pre-feeding stage larvae. The average ratio of respiration to ETS activity was 0.47±0.16. As in laboratory raised larvae, the ETS activity in wild larvae also increased linearly with larval dry weight. The regression coefficient of laboratory raised and wild larvae were not significantly different. The average dry weight-specific ETS activity for wild larvae was 8.60±2.99μlO2/mg•h. The comparison of the ETS activity in laboratory raise and wild larvae indicates that the ETS activity method has potential to assess respiratory activity of pollock larvae in situ. Further experiments are needed to evaluate the impact of various environmental conditions to the respiration to ETS activity ratio, which is necessary to convert ETS activity data to respiration.

Effects of physical factors on the vertical distribution of larval walleye pollock Theragra chalcogramma under controlled laboratory conditions

Effects of physical factors on the vertical distribution of larval walleye pollock Theragra chalcogramma under controlled laboratory conditions

The effects of feeding periodicity and ration on the rate of increment formation in otoliths of larval walleye pollock Theragra chalcogramma (Pallas)

The effect of feeding conditions (periodicity and ration level provided) on otolith increment counts of laboratory reared larval walleye pollock Theragra chalcogramma (Pallas) was tested. Increments were deposited with daily periodicity for larvae growing under optimal feeding conditions (high, continuous ration). Increment counts determined by light microscopy were less than expected, assuming a rate of formation of 1 · day −1 for larvae grown at low rations. Increment deposition and length (and therefore growth rates) were positively correlated. Daily increment resolution was disrupted after 4 days of starvation. Evidence is discussed indicating that increments continue to be deposited on otoliths of slow-growing and starved larvae, but are too narrowly spaced for resolution by light microscopy. Feeding larvae more than once daily had no effect on increment deposition rates. For up to 38 days, increment counts from the sagitta and lapillus were equivalent, but for the first 20 days increment counts were easier from the lapillus, which was larger. After 20 days, the sagitta was larger and overall easier to read.