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

Mortality 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.     

Walleye pollock recruitment in Shelikof Strait: applied fisheries oceanography

Fisheries-Oceanography Coordinated Investigations (FOCI) is a National Oceanic and Atmospheric Administration (NOAA) research programme seeking to understand recruitment processes of commercially exploited Alaskan fishes. The FOCI is mainly comprised of scientists at the Pacific Marine Environmental Laboratory and the Alaska Fisheries Science Center who study both the biotic and abiotic environment, including processes within larval patches through integrated field, laboratory, and modelling studies. The initial focus of studies was walleye pollock ( Theragra chakogramma ) spawning in Shelikof Strait, Gulf of Alaska. The choice of this population for our research was based on development of a large fishery and the substantial variation in recruitment that was observed in the late 1970s and early 1980s. Also, the early life history of this population is quite predictable and restricted both temporally and spatially. Walleye pollock spawn consistently in a small part of Shelikof Strait in early spring from which a large patch of eggs and later larvae is produced. In most years this concentration of larvae drifts to the south-west through the strait during April and May. Large numbers of larvae are often found in eddies which frequent the area and we have observed improved feeding conditions for larvae, in as opposed to out of eddies. We have found that first-feeding larvae have higher survival rates during calm periods, rather than in storms, and that in many years recruitment is largely set by the end of the larval period, although in some years age-0 juvenile mortality is also important. FOCI now generates information that is being used for management of this resource.     

Predation on walleye pollock (Theragra chakograrnma) eggs and yolk-sac larvae by pelagic crustacean invertebrates in the western Gulf of Alaska

Immunological detection of yolk protein was used to assess predation by pelagic amphipods (gammarid and hyperiid), mysids, and euphausiids on eggs and yolk-sac larvae of walleye pollock Theragra chalcogramma during 1988 and 1989. Consumption estimates were made on the basis of frequency of positive immunoassays, assay detection times (gut clearance time), predator abundance, and spatial overlap of predators and prey. From our results gammarid amphipods and euphausiids were important predators on eggs and yolk-sac larvae, respectively. Gammarid amphipods alone consumed about 14% of the standing stock of pollock eggs in 1989. These results were compared with those from clearance rate experiments of predators feeding on pollock eggs in 300-1 bags. In general, clearance rate estimates of egg consumption were lower than those determined from gut contents.     

A review of the distribution, ecology and population dynamics of age-0 walleye pollock in the Gulf of Alaska

This review paper synthesizes published research and unpublished data on the abundance and distribution patterns, ecology and population dynamics of walleye pollock ( Theragra chalcogramma ) during their first year of life (age-0) in the Gulf of Alaska. Distribution patterns have been described using mainly trawl catches, but recently, acoustic methodology has been employed, especially in examining vertical distributions. Although age-0 pollock are found throughout the Gulf, the highest catches occurred west of Kodiak Island. Pollock are pelagic for at least their first 6 months of life but show an ontogenetic increase in depth distribution superimposed on a pronounced diel vertical migration at a larger size. Daily growth rates are variable depending on year, season and area, and growth generally ceases during the winter. The diet of age-0 pollock shifts from mainly copepods in early juveniles to euphausiids by fall, with epibenthic organisms becoming important during the winter months. Feeding occurs mainly at night in surface waters. Age-0 pollock are most frequently associated with gelatinous zooplankton (medusae) and older pollock. Many predators on age-0 pollock have been identified; the most important are arrowtooth flounder ( Atheresthes stomias ), adult pollock, puffins ( Fratercula spp.), murres ( Una spp.), harbor seals ( Phoca vitulina richardsi ) and Steller sea lions ( Eumetopiasjubatus ). Modelling provides some insight into the population dynamics of these juveniles and environmental conditions which interannually affect their survival. These results are discussed relative to the importance of age-0 pollock in the recruitment of this species and to their role in the pelagic ecosystem.     

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

Interannual 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.     

Behavioural determinants of distribution and survival in early stages of walleye pollock, Theragra chalcogrammai a synthesis of experimental studies

This review summarizes results of an extensive series of laboratory studies on the behavioural responses of early life stages of walleye pollock, Theragra chakogramma to key environmental factors including light, temperature, gravity, turbulence, food availability and predator presence. Experiments focused on vertical distribution of egg through 0-age stages, and social interactions of age-0 juveniles. Key factors were modified either singly or in concert to determine their direct and indirect influence on fish behaviour. The observed results suggest that the behaviour of individual fish depends on the integration of a continually changing hierarchy of both intrinsic and extrinsic factors, with the relative importance of a specific factor varying with ontogeny and over time and space. The broad range of responses displayed by early life stages suggests that behaviour plays an important role in determining the consequences of environmental variability on walleye pollock populations. In general, results from our laboratory experiments are consistent with patterns observed in field studies, supporting the efficacy of using experimental behavioural research to define some of the underlying mechanisms controlling distribution and survival in the field, and eventual recruitment to adult populations.     

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

Mesoscale 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.     

Patterns of foraging and distribution of bluegill sunfish in a Mississippi River backwater: influence of macrophytes and predation

Abstract— We studied the trophic interactions and spatial distributions of bluegills Lepomis macrochirus and largemouth bass Micropterus salmoides in a macrophyte bed in Lake Onalaska, a backwater lake in the upper Mississippi River. The diets of adult and age-0 bluegills were similar and changed seasonally probably in response to changes in life stages of macroinvertebrates (i. e. emergence of winged adults). Diets and diel patterns of abundance of bluegill suggest that age-O and adults were feeding in the vegetated, littoral zone. Predation by age-O largemouth bass appears to influence use of vegetated habitat by age-0 bluegills. In summer, when most age-O bluegills were vulnerable to predation by age-O largemouth bass, bluegill abundance was strongly correlated with vegetation biomass. In October and November, piscivory by age-O largemouth bass was limited by gape. Consequently, the relationship between the abundance of age-O bluegills and vegetation biomass was weakened because predation risk by age-O largemouth bass was reduced.     

Temporal variation of seasonality of egg production and the spawning biomass of Pacific anchovy, Engraulis japonicus, in the southern waters of Korea in 1983–1994

Embryonic mortality, egg production and the spawning stock biomass of Pacific anchovy, Engraulis japonicus , off Southern Korea during 1983–1994, and their biological response to oceanographic features in spring and summer, were analysed. The instantaneous mortality rate (IMR) of embryonic stages decreased in spring and increased in summer, with a range of 0.33–1.23 day^–1 in spring and 0.78–1.69 day^–1 in summer. Egg production in summer was three times that during spring and production was low in the late 1980s. Mean lengths of yolk-sac larvae and adult females were greater in spring than in summer, whereas spawning fraction and spawning stock ratio (spawning biomass:adult biomass) were lower in spring than summer. Estimated mean spawning stock biomass ranged from 141 × 10³ to 380 × 10³ MT in spring and from 221 × 10³ to 557 × 10³ MT in summer. Statistically, the seasonal and long-term trends of embryonic mortality, egg production and spawning stock biomass of Pacific anchovy can be explained largely by spring warming, summer cooling and by less abundant zooplankton in the late 1980s.     

福建三沙湾鱼卵、仔稚鱼种类组成及其丰度时空分布

根据2008年春季(5月5日、5月17日)和秋季(11月14日、11月30日)分别在三沙湾进行4次鱼卵、仔稚鱼调查资料及历史相关资料,对三沙湾鱼卵、仔稚鱼种类组成、丰度分布的变化进行分析和讨论.结果表明:调查海区出现的鱼卵、仔稚鱼有35种(包括2种未定种类),比1990年和2007年分别增加了9种和12种;其中鱼卵数量...     

Springtime abundance of chaetognaths in the shelf region of the northern Gulf of Alaska, with observations on the vertical distribution and feeding of Sagitta elegans

Chaetognatha were among the most dominant macrozooplankton taxa collected in 6 years of springtime collections, both throughout the water column and near-bottom on the continental shelf in the northern Gulf of Alaska. Three species ( Sagitta elegans, S. scrippsae, and Eukrohnia hamata ) were collected in 248 collections, although S. elegans was the numerically dominant species overall during most cruises. Collections taken in epibenthic sleds generally contained about two orders of magnitude more chaetognaths than those that sampled throughout the water column. Two size modes were apparent in the S. elegans size-frequency distributions which were believed to belong to different cohorts. Growth rate was on the order of 2–3 mm per month. The two cohorts showed asynchronous diel vertical distribution patterns, with the smaller individuals found near the surface during the day whereas the larger individuals were near the surface at night. About 7.5% of the S. elegans examined contained food. Copepods made up the majority (∼89% by number) of the diet of both large and small individuals. Euphausiid juveniles and cirripede larvae were also observed, as well as several incidences of cannibalism by large chaetognaths on smaller individuals. Based on the results of two diel series, this species was observed to feed mainly during the night-time.     

Effects of stocking density and size distribution on growth, survival and cannibalism in juvenile fat snook (Centropomus parallelus Poey)

This study was carried out in order to determine the effect of stocking density and size distribution on growth, survival and cannibalism of fat snook (Centropomus parallelus Poey) juveniles. Three stocking densities (1.5, 3 and 6 fish L⁻¹) were tested in two groups of fish: homogeneous [total length (TL)=22.1 mm; coefficient of variation of length (CV)=8%] and heterogeneous (TL=22.7 mm; CV=15%). The experimental design was factorial (3 × 2) using triplicates. Fish were reared for 30 days in 80‐L circular tanks with an open flow‐through system. There was no significant interaction between the two factors (density and size distribution). The highest cannibalism rate (14.2%) was observed in the heterogeneous group, which also had a significantly higher growth rate (TL=45.6 mm) than the homogeneous group (3.2% and TL=40.9 mm). Significant differences were found between the two lowest densities and the highest density (6.2%, 6.3% and 13.4% respectively). Growth was not affected by fish density. Cannibalism was the main cause of mortality. In 90% of the occurrences, whole prey ingestion was observed. According to the results obtained, cannibalism was positively correlated to density, although size grading of fat snook juveniles is recommended to control cannibalism in all the densities tested.     

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.     

Fertilization success and blastomere morphology as predictors of egg and juvenile quality for domesticated Atlantic cod,Gadus morhua,broodstock

We investigated if simplified routines based on blastomere morphology and fertilization success can be used as predictors of egg and embryo viability in Atlantic cod, and abnormal blastomere morphology is related to juvenile deformities. Egg morphology data from 312 families that were produced by hand stripping of gametes were used. Fertilized eggs from all families were separately incubated in 25 L incubators. Blastomere morphology and fertilization success were assessed at 16–32 cell stage. Embryo mortality was measured until hatch. Fertilization success showed a significant positive correlation with the proportion of normal blastomere and 74% of the variation in the incidence of blastomere abnormality was explained by fertilization success. Juvenile deformities were positively correlated with blastomere abnormality and 8% of the variation in juvenile deformities was explained by incidences of blastomere abnormality. To our knowledge, this is the first direct evidence showing that incidences of blastomere abnormalities may be related to cod juvenile deformities. Our results suggest that fertilization success and the occurrence of blastomere abnormalities could be easily adopted by commercial cod hatcheries for the early determination of egg quality.     

On the relationship between low winter temperatures and mortality of juvenile scallops, Pecten maximus L., cultured in western Norway

The mortality of juvenile scallops (Pecten maximus L.) cultured at two densities at 8‐ and 15‐m depth was investigated at 17 trial locations along the western Norwegian coast between latitudes 58 and 65°N. During the winter of 1995/1996, scallops suffered high mortality at many sites. The results show a clear relationship between minimum temperature recorded on the sites and scallop mortality. All the scallops died at sites where minimum temperatures were lower than 2°C, while a mortality rate below 30% was only obtained at locations where the minimum temperature was 4–5°C. At temperatures between 2 and 4°C, mortality was intermediate, ranging from 52% to 100%. There were no mortality differences between culture densities, while a significantly higher mortality at 8‐ than at 15‐m depth was shown for two of the four sites with a total mean mortality lower than 30%. The results are discussed in relation to low‐temperature conditions along the Norwegian coast to suggest risk assessments for site selection in scallop culture.