Euphausiid Biomass Research Articles

Acoustic surveys of euphausiids and models of baleen whale distribution in the Barents Sea

As in many high-latitude ecosystems, euphausiids (order Euphausiacea, 'krill') play a key role in the Barents Sea by channeling energy from primary producers to fish and other zoo- plankton predators. We used multifrequency acoustic data from several recent multidisciplinary surveys to describe the spatial distribution of backscatter likely to be from euphausiids. Spatial patterns in euphausiid backscatter observed in 2010, 2011, and 2012 were correlated with verti- cally integrated euphausiid biomass collected with plankton nets, and were also broadly consis- tent with the distribution of euphausiids expected from the literature. We used the high-resolution and broad-spatial coverage of our euphausiid backscatter data to update multiple regression models of baleen (fin, humpback, and minke) whale distribution to test the hypothesis that these animals aggregated where euphausiids were abundant. After controlling for physical environ- mental factors and the densities of capelin and several other potential prey taxa, we found that fin whale densities were positively and linearly associated with euphausiid backscatter, and higher than average densities of humpback whales were found in areas with high euphausiid back - scatter. No association was found between minke whales and euphausiids. Densities of all 3 whale species were also positively associated with capelin. For fin and humpback whales, the effects of capelin and euphausiids on whale densities appeared to be principally separate and additive, although there was some evidence for a stronger effect of euphausiids at low capelin densities. In terms of their preference for euphausiids and capelin, these whale species appeared to be flexible, opportunistic predators.

Trophic ecology of Lampanyctus crocodilus on north-west Mediterranean Sea slopes in relation to reproductive cycle and environmental variables.

This study examined the population structure, reproductive cycle and feeding pattern of the lanternfish Lampanyctus crocodilus in the Balearic Basin (north-west Mediterranean Sea) from a depth of 450 to 1800 m and at a seasonal scale. Juveniles were mainly located at shallower depths, but also at deepest stations in autumn, while adults mostly inhabited intermediate depths with their centre of population density (CPD) located at 800-1000 m of depth. The migration of adults to deeper depths was detected in late summer to autumn, probably linked to the occurrence of nepheloid layers at c. 1200 m, which in turn enhances the biomass of the zooplankton prey. The diet was mainly based on euphausiids and mysids, with marked seasonal variations both on the upper (450-800 m) and lower (1000-1800 m), where suprabenthic gammariids and pelagic decapods were also dominant. Stomach fullness increased from winter to autumn on the US, while it had a maximum in spring on the LS, in parallel with high consumption of gelatinous zooplankton, which is probably more available after the phytoplankton bloom in late winter. Reproduction occurred in winter, confirmed by the higher percentage of mature females and high gonadosomatic indices (I(G)) at both depth ranges. Hepatosomatic indices (I(H)) showed an inverse trend to I(G) on the US, except in autumn, and was almost parallel on the LS, probably attributable to the migration of adults, which determined different temporal schemes in energy use and storage for reproduction on the US v. LS. Consistent with the different patterns observed at the two depth ranges, environmental drivers of fullness (i.e. feeding intensity) and I(G) (as a proxy of reproductive cycle) differed on the US and LS. The biomass of mysids and euphausiids was the greatest explanatory variables of fullness on the US and LS, pointing to the increasing feeding intensity when a resource was more available. I(H) also explained fullness, suggesting that greater feeding intensity in pre-reproductive periods enabled energy storage in the liver. I(G) was linked directly (i.e. mysids) or indirectly (i.e. surface primary production recorded 2 months before sampling) to food availability, implying a rapid response to vertical food inputs by deep-sea predators. Also, I(G) in L. crocodilus was related to population density, which suggests aggregations for reproduction. Estimates of L. crocodilus trophic levels, and of other accompanying mesopelagic fishes, indicated that the species feed through a continuum spanning the third trophic level, confirming the key role of mesopelagic fishes in transferring organic carbon between trophic levels. Trophic niche segregation among mesopelagic species was pronounced and non-overlapping groups could be distinguished because of the different vertical distribution and migratory behaviour. The study highlights the important role of the benthic boundary layer in sustaining benthopelagic communities in the deep Mediterranean Sea and the need to study the biology of a species throughout its whole depth range and not just at exploited depths (i.e. fishing grounds).

Zooplankton communities in the Strait of Georgia, British Columbia, track large-scale climate forcing over the Pacific Ocean

Many studies have reported a correlation between offshore biological changes and large-scale climate forcing. However, it remains unclear if the biological changes in spatially confined estuaries are associated more with large-scale climate forcing or local environmental changes. The Strait of Georgia (SoG) in British Columbia, Canada, was the location used to investigate this question. Based on night zooplankton samples from the surface 20m, collected between 1990 and 2007, we identified 10 major taxa and investigated interannual changes for September and June. A combination of cluster analysis, Non-metric Multi-Dimensional Scaling and Principal Component Analysis (PCA) showed an abrupt shift in the September community composition in 1998/1999 associated with a decline in the biomass of euphausiids and calanoid copepods in particular. Although the June samples did not have complete coverage throughout the time period of interest, they confirmed a similar pattern of community change. The PCA analysis showed a phase shift in the large-scale climate indices in 1999. The best single indicator of zooplankton community change was the spring extratropical-based Southern Oscillation Index (SOIx), with a 1-year lag. The best single local indicator was the day of the peak flow of the Fraser River. However, in general, local environmental factors were weakly correlated with zooplankton community changes in the Strait, highlighting this as an important area for future research to understand long-term ecosystem changes. As secondary production plays a key role in food webs, these large changes in the zooplankton community may have had a substantial impact on higher trophic levels and the ecosystem as a whole.

Climate effects on Barents Sea ecosystem dynamics

AbstractDalpadado, P., Ingvaldsen, R. B., Stige, L. C., Bogstad, B., Knutsen, T., Ottersen, G., and Ellertsen, B. 2012. Climate effects on Barents Sea ecosystem dynamics. – ICES Journal of Marine Science, 69: . Effects of climate variability and change on sea temperature, currents, and water mass distribution are likely to affect the productivity and structure of high-latitude ecosystems. This paper focuses on the Barents Sea (BS), a productive Arcto–boreal shelf ecosystem sustaining several ecologically and economically important fish species. The water masses in the region are classified as Atlantic, Arctic, and mixed, each having a distinct ecological signature. The pronounced increase in temperature and a reduction in the area covered by Arctic water that has taken place during the past decade have affected the ecology of the region. An increase in biomass of lipid-rich euphausiids in recent years, possibly linked to the temperature increase, has apparently provided good feeding and growth conditions for several species, including capelin and young cod. The observed reduction in Arctic zooplankton may on the other hand have negative implications for polar cod and other zooplankton predators linked to the Arctic foodweb. Despite these changes, the BS at present seems to maintain relatively stable levels of boreal zooplankton biomass and production, with no significant changes in the abundances of Calanus finmarchicus or the episodic immigrant C. helgolandicus.

SPATIAL ESTIMATION OF EUPHAUSIID BIOMASS ALONG THE PACIFIC COAST OF EASTERN HOKKAIDO, JAPAN IN EARLY SUMMER OF 2008

This study focused on euphausiid distribution and biomass estimation over the coastal shelf of the Pacific coast of Hokkaido Japan in early summer 2008. Acoustic data were obtained in both day (6:00-18:00) and night (18:00-6:00) using a quantitative echosounder (EK60, Simrad) with two frequencies (120 and 38 kHz). Echoes of euphausiids were specified based on the ΔMVBS, calculated with seasonal theoretical target strength of Euphausia pacifica. The best-fit kriging models were used to quantitatively interpolate euphausiid distribution. The detected euphausiid biomass reached 38,278 t (> 15 gWWm-2), suggesting abundant prey for large fishes over study area. On the other hand, the distribution was spatially heterogeneous in produced GIS map, suggesting the spatially limited available prey. Observed day-night difference of estimated biomass was caused by spatial variability of diel movement of euphausiids. Acoustic dead zones and behavioral change in euphausiids may strongly influence biomass estimation in night in coastal shelf environment.

Diet of auklet chicks in the Aleutian Islands, Alaska: similarity among islands, interspecies overlap, and relationships to ocean climate

Seabirds are effective samplers of the marine environment, and can be used to measure resource partitioning among species and sites via food loads destined for chicks. We examined the composition, overlap, and relationships to changing climate and oceanography of 3,216 food loads from Least, Crested, and Whiskered Auklets (Aethia pusilla, A. cristatella, A. pygmaea) breeding in Alaska during 1994–2006. Meals comprised calanoid copepods (Neocalanus spp.) and euphausiids (Thysanoessa spp.) that reflect secondary marine productivity, with no difference among Buldir, Kiska, and Kasatochi islands across 585 km of the Aleutian Islands. Meals were very similar among species (mean Least–Crested Auklet overlap C = 0.68; Least–Whiskered Auklet overlap C = 0.96) and among sites, indicating limited partitioning of prey resources for auklets feeding chicks. The biomass of copepods and euphausiids in Least and Crested Auklet food loads was related negatively to the summer (June–July–August) North Pacific Gyre Oscillation, while in Whiskered Auklet food loads, this was negatively related to the winter (December–January–February) Pacific Decadal Oscillation, both of which track basin-wide sea-surface temperature (SST) anomalies. We found a significant quadratic relationship between the biomass of calanoid copepods in Least Auklet food loads at all three study sites and summer (June–July) SST, with maximal copepod biomass between 3–6°C (r 2 = 0.71). Outside this temperature range, zooplankton becomes less available to auklets through delayed development. Overall, our results suggest that auklets are able to buffer climate-mediated bottom-up forcing of demographic parameters like productivity, as the composition of chick meals has remained constant over the course of our study.

Distribution and community structure of euphausiids in the Southern Ocean along the 140°E meridian during the austral summer 2001/2002

Abstract The abundance, biomass, diversity and community structure of euphausiids in the Southern Ocean along the 140°E meridian from latitudes 47°S–65°S were examined by RMT8 from 0 to ca. 1000 m during the 2001/2002 austral summer. Both total abundance and biomass of euphausiids were much higher in the south of the southern branch of the Polar Front (PF-S) located at 60°S–61°S than in the northern part, corresponding with higher chlorophyll a concentrations. Eleven species of euphausiids were collected from the research areas: 2 species of Thysanoessa, 6 of Euphausia, and a single species each of Thysanopoda, Bentheuphausia and Nematoselis. In Euphausia and Thysanoessa, clear horizontal distribution patterns enabled the classification of each species into one of 3 different types: species occurring only at stations north of the PF, species occurring in a broad distribution from south of the PF-S to the south of the Southern Boundary (SB) of the Antarctic Circumpolar Current, and species occurring only south of the SB. The latitudinal zonations occurrence and peak abundance were evident for all species. Hierarchical cluster analysis by species and station resulted in the identification of 2 distinct communities of euphausiids: community I comprised the three stations north of the PF where both euphausiid abundance and biomass were relatively low, and 2 species, Thysanopoda acutifrons and Thysanoessa gregaria dominated, community II comprised the southern five stations where abundance and biomass were high and Euphausia frigida, E. superba and Thysanoessa macrura were numerically dominant. Among the five oceanographic fronts within the research area, the PF (either south or north) appears to be the important biogeographical boundary separating abundance, biomass, and the community structure of euphausiids.

Zooplankton species composition, abundance and biomass on the eastern Bering Sea shelf during summer: The potential role of water-column stability and nutrients in structuring the zooplankton community

The southeastern Bering Sea sustains one of the largest fisheries in the United States, as well as wildlife resources that support valuable tourist and subsistence economies. The fish and wildlife populations in turn are sustained by a food web linking primary producers to apex predators through the zooplankton community. Recent shifts in climate toward warmer conditions may threaten these resources by altering productivity and trophic relationships in the ecosystem on the southeastern Bering Sea shelf. We examined the zooplankton community near the Pribilof Islands and on the middle shelf of the southeastern Bering Sea in summer of 1999 and 2004 to document differences and similarities in species composition, abundance and biomass by region and year. Between August 1999 and August 2004, the summer zooplankton community of the middle shelf shifted from large to small species. Significant declines were observed in the biomass of large scyphozoans (Chrysaora melanaster), large copepods (Calanus marshallae), arrow worms (Sagitta elegans) and euphausiids (Thysanoessa raschii, T. inermis) between 1999 and 2004. In contrast, significantly higher densities of the small copepods (Pseudocalanus spp., Oithona similis) and small hydromedusae (Euphysa flammea) were observed in 2004 relative to 1999. Stomach analyses of young-of-the-year (age 0) pollock (Theragra chalcogramma) from the middle shelf indicated a dietary shift from large to small copepods in 2004 relative to 1999. The shift in the zooplankton community was accompanied by a 3-fold increase in water-column stability in 2004 relative to 1999, primarily due to warmer water above the thermocline, with a mean temperature of 7.3 °C in 1999 and 12.6 °C in 2004. The elevated water-column stability and warmer conditions may have influenced the zooplankton composition by lowering summer primary production and selecting for species more tolerant of a warm, oligotrophic environment. A time series of temperature from the middle shelf indicates that the warmer conditions in 2004 are part of a trend rather than an expression of interannual variability. These results suggest that if climate on the Bering Sea shelf continues to warm, the zooplankton community may shift from large to small taxa which could strongly impact apex predators and the economies they support.

Trophic relationships and oceanography on and around a small offshore bank

Small offshore banks may be sites of intense feeding by upper trophic level predators. We studied the distribution of cetaceans, seabirds, pelagic fish, euphausiids and zooplankton over a 9 × 15 km bank to determine the conditions and processes that concentrated prey there and to exam- ine the relative importance of bottom-up or top-down controls. Euphausiids were the primary prey during most foraging activity. While these were widespread in subsurface waters, foraging was con- centrated on dense surface swarms that formed during daylight hours over 2 small crests. Internal wave passage resulted in upward movement and concentration of euphausiids in these areas through a coupling of physical processes and euphausiid behavior, resulting in surface swarms. Thus, inter- nal waves appear to provide a critical mechanism enhancing trophic energy transfer. The formation of dense, localized and accessible prey concentrations was more important to foraging than was the overall available prey biomass. The estimated maximum daily consumption of euphausiids by cetaceans, seabirds and herring combined was <0.4% of the estimated instantaneous euphausiid biomass, and top-down control was unlikely to have substantially influenced euphausiid biomass at this site. Some predator species that do not prey extensively on euphausiids or herring were more prevalent in off-bank waters. The scales of predictability and the temporal dynamics of such features determine the manner in which populations of upper trophic level organisms utilize a variable envi- ronment.

Horizontal and vertical distribution and demography of euphausiids in the Ross Sea and its adjacent waters in 2004/2005

The horizontal and vertical distribution and population structure of euphausiids in the Ross Sea and its adjacent waters were investigated during the summers of 2004/2005 using stratified towed samples. Nine species of euphausiids occurred in the survey area. Among them, Euphausia triacantha was dominant in biomass north of the southern boundary of the Antarctic circumpolar current (SB). Thysanoessa spp. was widely distributed north of the continental slope, while E. superba was distributed from the SB to the slope, where it showed the highest biomass. Juvenile E. superba was distributed offshore near the SB and remained at the surface, but gravid females were dominant in the slope and mainly occurred in the middle layers (400–600 m). Adult and juvenile E. crystallorophias were found at 200–300 m in the colder water of the continental shelf. In general, the peak biomass of euphausiids was found in the mid layers of the Ross Sea area. The life span and the number of spawns for major species are also discussed.

Euphausiid population structure and grazing in the Antarctic Polar Frontal Zone — austral autumn 2004

The euphausiid community structure and grazing dynamics were investigated in the West Indian sector of the Polar Frontal Zone during the austral autumn 2004. Subsurface (200m) temperature profiles indicated that an intense frontal feature, formed by the convergence of the Subantarctic Front and the Antarctic Polar Front bisected the survey area into two distinct zones, the Subantarctic Zone (SAZ) and the Antarctic Zone (AAZ). Total integrated chlorophyll a (Chl a) biomass was typical for the region (<25mg Chl a m−2), and was dominated by picophytoplankton. Total euphausiid abundance and biomass ranged from 0.1 m−3 to 3.1 m−3 and from 0.1mg dry weight m−3 to 8.1mg dry weight m−3 respectively, and did not differ significantly between the stations occupied in the SAZ and AAZ (p > 0.05). A multivariate analysis identified two interacting mechanisms controlling the distribution patterns, abundance and biomass of the various euphausiid species, namely (1) diel changes in abundance and biomass, and (2) restricted distribution patterns associated with the different water masses. Ingestion rates were determined for five euphausiid species. Euphausia triacantha had the highest daily ingestion rate, ranging from 1 226.1ng pigment (pigm) ind−1 day−1 to 6 029.1ng pigm ind−1 day−1, whereas the lowest daily ingestion rates were observed in the juvenile Thysanoessa species (6.4–943.0ng pigm ind−1 day−1). The total grazing impact of selected euphausiids ranged from <0.1μg pigm m−2 day−1 to 20.1μg pigm m−2 day−1, corresponding to <0.15% of the areal Chl a biomass. The daily ration estimates of autotrophic carbon for the euphausiids suggest that phytoplankton represent a minor component in their diets, with only the sub-adult E. vallentini consuming sufficient phytoplankton to meet their daily carbon requirements.

Acoustic and satellite remote sensing of blue whale seasonality and habitat in the Northeast Pacific

Northeast Pacific blue whales seasonally migrate, ranging from the waters off Central America to the Gulf of Alaska. Using acoustic and satellite remote sensing, we have continuously monitored the acoustic activity and habitat of blue whales during 1994–2000. Calling blue whales primarily aggregate off the coast of southern and central California in the late summer, coinciding with the timing of the peak euphausiid biomass, their preferred prey. The northward bloom of primary production along the coast and subsequent northbound movements of the blue whales are apparent in the satellite and acoustic records, respectively, with the calling blue whales moving north along the Oregon and Washington coasts to a secondary foraging area with high primary productivity off Vancouver Island in the late fall. El Ni ~ no conditions, indicated by elevated sea-surface temperature and depressed regional chlorophyll-a concentrations, are apparent in the satellite records, particularly in the Southern California Bight during 1997/1998. These conditions disrupt biological production and alter the presence of calling blue whales in primary feeding locations. Remote sensing using acoustics is well suited to characterizing the seasonal movements and relative abundance of the northeast Pacific blue whales, and remote sensing using satellites allows for monitoring their habitat. These technologies are invaluable because of their ability to provide continuous large-scale spatial and temporal coverage of the blue whale migration. r 2004 Elsevier Ltd. All rights reserved.

Acoustic and satellite remote sensing of blue whale seasonality and habitat in the Northeast Pacific

Northeast Pacific blue whales seasonally migrate, ranging from the waters off Central America to the Gulf of Alaska. Using acoustic and satellite remote sensing, we have continuously monitored the acoustic activity and habitat of blue whales during 1994–2000. Calling blue whales primarily aggregate off the coast of southern and central California in the late summer, coinciding with the timing of the peak euphausiid biomass, their preferred prey. The northward bloom of primary production along the coast and subsequent northbound movements of the blue whales are apparent in the satellite and acoustic records, respectively, with the calling blue whales moving north along the Oregon and Washington coasts to a secondary foraging area with high primary productivity off Vancouver Island in the late fall. El Ni n ˜ o conditions, indicated by elevated sea-surface temperature and depressed regional chlorophyll- a concentrations, are apparent in the satellite records, particularly in the Southern California Bight during 1997/1998. These conditions disrupt biological production and alter the presence of calling blue whales in primary feeding locations. Remote sensing using acoustics is well suited to characterizing the seasonal movements and relative abundance of the northeast Pacific blue whales, and remote sensing using satellites allows for monitoring their habitat. These technologies are invaluable because of their ability to provide continuous large-scale spatial and temporal coverage of the blue whale migration.

Mesozooplankton interactions with the shelf around the sub-Antarctic Prince Edward Islands archipelago

Mesozooplankton surveys were conducted in April/May for four consecutive years (1996–1999) in the vicinity of the Prince Edward Islands (PEIs), Southern Ocean. The PEIs are located in the Polar Frontal Zone, directly in the path of the east-flowing Antarctic Circumpolar Current. Zooplankton were collected by oblique tows using a Bongo net fitted with 300 μm mesh. The abundance, biomass and average size of the mesozooplankton in the upstream (USR), inter-island (IIR) and downstream (DSR) regions indicated that some groups and species were significantly affected by their interaction with the shallow shelf waters of the PEIs. Total mesozooplankton abundance and biomass were typically highest in the DSR, but no consistent pattern was evident in the USR and IIR. Copepods, euphausiids and fish were generally of a low average size in the IIR. This small size was largely attributed to the reduced abundance, or complete absence, of mesopelagic species from the shelf region. Of total biomass, the mesopelagic species Euphausia longirostris, Euphausia similis, Pleuromamma abdominalis, Paraeuchaeta biloba and Oncaea antarctica together contributed an average of 16% to the USR, 2% to the IIR and 15% to the DSR. Conversely, epipelagic species showed no consistent pattern of abundance and biomass distribution between regions. The low incidence of mesopelagic species over the island shelf was attributed mainly to reduced advection of deep water into the shelf region (average depth = 200 m), rather than predation, particularly during the through-flow mode between the islands. This resulted in substantial regional differences in euphausiid community structure. The epipelagic species Euphausia vallentini and Thysanoessa vicina completely dominated the IIR, comprising on average 89% of total euphausiid biomass in this region. However, predation may be important during the water-trapping mode between the islands. Advection of zooplankton into the IIR appeared to be affected by the proximity of the Subantarctic Front (SAF). In 1996, when the SAF was far north of the PEIs, reduced current velocities resulted in some degree of water retention over the shelf and an increased predation impact. Conversely, when the SAF was close to the PEIs in 1999, more large plankton were transported over the island shelf. High current velocities and productivity associated with the SAF appear to increase the biomass and size of allochthonous zooplankton/nekton advected into the IIR, and consequently may have increased the availability of prey to land-based predators. The long-term southward movement of the SAF recently observed in the vicinity of the PEIs may therefore have important implications for the ecosystem of these islands.

Mass mortality of krill caused by parasitoid ciliates.

Euphausiids, commonly known as krill, often dominate pelagic crustacean productivity of the world’s oceans. The ecology of these crustaceans has been extensively studied, but sources of mortality remain poorly understood. Predation and starvation are traditionally assumed to be the most important causes of death; however, little attention has been given to the identification of other sources of natural mortality (1). Here we report on a ciliate parasite with significant infection rates in euphausiid populations that causes death of the euphausiids, thus reducing productivity. During six summer oceanographic cruises off the coast of Oregon, in the United States, we collected 71 euphausiids belonging to three species, Euphausia pacifica, Thysanoessa spinifera, and Thysanoessa gregaria, which were infected by an apostomatid ciliate of the genus Collinia. Endoparasitized adult euphausiids were collected at 7% of 313 stations sampled, mostly near the continental shelf break where euphausiids are usually most abundant (fig. S1). Shipboard incubations indicate that the infection, recognized by an orange and swollen cephalothorax (fig. S1), develops rapidly ( 24 hours) and that the parasite undergoes obligatory polymorphic stages of infection, growth, proliferation, and encystment that correspond to specific life-history stages (2). The feeding stage (trophont) consumes all the organs including the lipid-rich gonads (Fig. 1A). The reproductive stage (tomont) divides rapidly by palintomy to form transmission cells (tomites), which rupture the cephalothorax, usually killing the host within 40 hours of infection. This ciliate species is a parasitoid because it must kill the host to achieve transmission and complete the life cycle. For euphausiids, swarming confers benefits such as improving capture of motile prey, increasing the probability of mating and decreasing predation risk. However, it may also increase susceptibility to parasitism and disease. Dense euphausiid aggregations may increase parasitoid transmission after an infected animal bursts and sinks through the aggregation, distributing infective stages. This phenomenon may lead to large-scale euphausiid mortality. We observed such a mass mortality event (June 2001) at the base of Astoria Canyon, Oregon (Fig. 1B). Using a remotely operated vehicle (ROV), we discovered high densities of dead E. pacifica on the sea floor between 550 and 220 m depth along almost 1.5 km of ROV transect (Fig. 1C). Euphausiid swarms and weak or dead adult euphausiids were observed close to the sea floor. Carcasses collected by the ROV contained the same parasitic ciliates and exhibited similar infection signs (partially empty and broken cephalothorax) as infected euphausiids from shipboard incubations. Infection of euphausiids by a similar endoparasitic apostome ciliate Collinia beringensis has been observed in the Bering Sea with prevalences of 68 to 98% (3, 4 ). These reports were based on preserved specimens, and the appearance of infected live euphausiids and the effects of the ciliate were unknown. The Collinia species responsible for euphausiid mortality off Oregon is currently undescribed. It is morphologically distinct from Collinia beringensis having 15 to 20 rows of cilia compared with 24 to 80 in the corresponding stages of C. beringensis. In the Northern California current, E. pacifica and T. spinifera are the dominant euphausiids ( 90% of euphausiid biomass). Our surveys suggest that Collinia sp. endoparasitism is widespread within the euphausiid population, which indicates that the large-scale mortality event we observed is unlikely to be an isolated case. Parasites and pathogens are known to control animal populations in marine and terrestrial environments (5), and members of the genus Collinia are certainly among the most virulent euphausiid parasites known (2). The epizootic character of the euphausiid mortality observed in Astoria Canyon challenges the widely held notion that mortality in pelagic organisms can be primarily attributed to predation and starvation.

The abundance and distribution of euphausiids and zero-age pollock on the inner shelf of the southeast Bering Sea near the Inner Front in 1997–1999

Abstract Acoustic data and net samples were collected during late spring and early fall 1997–1999 to assess zooplankton and micronekton abundance and distribution relative to the Inner Front at three sampling grids (Port Moller, Cape Newenham and Nunivak Island) on the inner shelf of the southeast Bering Sea. Epibenthic scattering layers were observed during May–June and August–September in all three years. Acoustic data were scaled to euphausiid biomass using target strength models. Mean euphausiid biomass determined acoustically for each transect line was 0.7– 21 g m −2 , with most values below 5 g m −2 . There was no consistent relationship between the distribution and biomass of euphausiids and the location of the Inner Front. Zero age pollock were observed on the inner shelf in August–September during all years, but were confined primarily to the stratified side of the Inner Front and to the frontal regime. The acoustic data for pollock were scaled to biomass using laboratory measurements of gas bladder dimensions and target strength models. Acoustic determinations of mean transect biomass for euphausiids did not differ from literature values for the inner shelf of the southeast Bering Sea, and pollock biomass on the inner shelf did not differ from that around the Pribilof Islands. Despite recent anomalies in climate and oceanographic conditions on the inner shelf, and high mortality of shorttail shearwaters during 1997, we found no evidence of significant interannual differences in the biomass of euphausiids or zero-age pollock on the inner shelf of the southeast Bering Sea.

Comparison of euphausiid population size estimates obtained using replicated acoustic surveys of coastal inlets and block average vs. geostatistical spatial interpolation methods

Because of their intense patchiness, euphausiid spatial distributions and stock sizes are often assessed using echosounder surveys. However, statistical error bars appropriate for individual survey results are not well known. We quantified these by examining the statistical repeatability of acoustic estimates of total euphausiid biomass within two enclosed fjords adjoining the Strait of Georgia, British Columbia. Within each inlet, paired and replicated `mirror image' zig‐zag survey tracks provided sets of closely spaced backscatter profiles along the survey lines. Local stock density (biomass per unit area) was calculated by vertical integration across the euphausiid scattering layer. Total inlet population size was then estimated by horizontal interpolation and integration of the local measurements, both by block averaging and by geostatistical interpolation (kriging). By assuming no change in true population biomass over the short time interval separating replicate surveys, we could then estimate statistical precision by analysis‐of‐variance comparison among replicate survey grids. For the partial surveys (one or the other half of the mirror‐image paired grid) multiplicative error bars were about a factor of 1.5 for Jervis Inlet and 1.7 for Saanich Inlet. Use of the full surveys (both parts of the mirror‐image pair, roughly doubling the number of measurements in each estimate) reduced the error to about factor of 1.35 for Jervis but only to about 1.65 for Saanich. Statistical precision was similar for the block average vs. kriging interpolation and integration methods, however, kriging provided additional useful information about spatial pattern within the inlets.

Implications of interannual variability in euphausiid population biology for fish production along the south-west coast of Vancouver Island: a synthesis

This is a synthesis of published and unpublished research on euphausiid and fish populations using the south-west coast of Vancouver Island. Overall, the studies covered 1985–98, when there were two ENSO events and considerable variation in upwelling. The population biology of the dominant euphausiids (Thysanoessa spinifera, Euphausiapacifica) was monitored during 1991–98. The species abundance trends differed. Results of simple correlation analyses suggested that variations in temperature, salinity and upwelling do not explain variations in the abundance of larval or adult euphausiids, or in the abundance of portions of euphausiid populations on which fish feed. I found significant interannual variations in daily ration of the dominant planktivorous fish species, but euphausiids remained the most important prey. Pacific hake (Merluccius productus), the dominant planktivore, fed on larger (>17 mm) T. spinifera, even though the biomass of this part of the euphausiid biomass decreased by 75% between 1991 and 1997, but Pacific herring (Clupea pallasi) may have begun feeding on smaller E. pacifica. Therefore, any study of the relationship between fish production and krill biology must consider that part of the euphausiid biomass exploited by fish. In addition, some fish species and/or life history stages appeared to adapt to changes in euphausiid availability, while others did not. Such variation in adaptations also has to be described and considered to understand how changes in euphausiid biology affect fish productivity.

Feeding ecology of five fishes from the mid-slope micronekton community off southern Tasmania, Australia

We examine the feeding ecology of characteristic micronekton species inhabiting the mid-slope region (~700 to 1,500 m depths) off southern Tasmania. Five fishes, Diaphus danae, Hygophum hanseni, Lampanyctus australis (Myctophidae), Phosichthys argenteus (Phosichthyidae) and Chauliodus sloani (Chauliodontidae), were sampled by depth-stratified midwater trawling on a diel and seasonal basis. Overall, 74 prey taxa were identified from 2,132 stomachs. Euphausiids (mostly Euphausia spinifera and E. similis) and calanoid copepods (mostly Pleuromamma species) were the main prey of the three myctophids; P. argenteus ate fishes and decapods in addition to the euphausiids, while C. sloani ate only fishes. Copepods were less important in the diets of larger D. danae, L. australis and P. argenteus and were replaced by euphausiids in the myctophids and by fishes in P. argenteus. In autumn, when euphausiid biomass increased six-fold, all three myctophids and P. argenteus fed most intensively and consumed a high proportion of euphausiids. The three myctophids appeared to feed nocturnally. Differences in the timing and duration of feeding corresponded to differences in their spatio-temporal overlap with key prey. Daily rations of H. hanseni (1.93% dry-weight biomass) and L. australis (1.43%), estimated from data on stomach fullness, were typical for temperate myctophids and higher than that of the non-migratory P. argenteus (0.43%). The vertical flux of near-surface plankton production to the mesopelagic food web is based primarily on diel feeding in the upper water column (<500 m) rather than consumption of species that migrate seasonally into the deeper mesopelagic zone. Because species such as P. argenteus and C. sloani feed above the third trophic level, their predators, including the commercially important orange roughy, are feeding between levels four and five.

Biomass, Abundance, and Vertical Distribution of Micronekton and Large Gelatinous Zooplankton in the Subarctic Pacific and the Bering Sea during the Summer of 1997

The biomass, abundance, and vertical distribution of micronekton, including enidarians, mysids, euphausiids, decapods, thaliaceans, and fishes, were studied on the basis of samples collected with an 8-m2 opening-closing rectangular midwater trawl (RMT-8, mesh size: 4.5 mm) at three stations in the subarctic Pacific (the western subarctic gyre, the central Subarctic, and the Gulf of Alaska) and one station in the oceanic Bering Sea. The total biomass in the 0–1000 m water column ranged from 2.9 to 5.1 gDW m−2. Except for primary consumers that showed highly variable biomass (thaliaceans and euphausiids), biomass was highest in the oceanic Bering Sea followed by the central (boundary between eastern and western gyres), western gyre, and eastern Gulf of Alaska. The biomass compositions by higher taxa were basically similar between regions: fishes were most dominant, followed by enidarians at all stations, except for the marked predominance of thaliaceans in the Gulf of Alaska. High biomasses of gelatinous animals (31% of overall dry weight), occasionally comparable to those of fishes and crustaceans, suggest their potential importance in the subarctic Pacific. Characteristics in vertical patterns of micronekton biomass common in all stations were: (1) a mesopelagic peak around 500–600 m both day and night, (2) a layer of low biomass in the cold intermediate water and/or in the upper mesopelagic zone, (3) a nighttime shift of biomass to upper layers, and (4) an highly variable biomass of epipelagic/interzonal migrants (euphausiids and thaliaceans).