Distribution Of Euphausiids Research Articles

Structure of the macroplankton–pelagic fish–cod trophic complex in a warmer Barents Sea

Long-term data indicate that climatic fluctuations in the Barents Sea can have significant impacts on populations of euphausiids, capelin and cod, but it is unclear how the current warming period in the area has altered trophic interactions among these taxa. The species composition and distribution of euphausiids; the abundance, distribution and feeding habits of capelin; and the feeding of cod on capelin and euphausiids were analysed. Data from the last decade suggest a shift in the spatial patterns and intensity of the euphausiid–capelin link, and a reduction in the volatility of population sizes of these species. Meanwhile, cod feeding has expanded farther to the north in the Barents Sea and may benefit from reduced fluctuations in prey availability. The reliability of advection of euphausiids into the region and the potential interactions with boreal species expanding into the Barents Sea, both of which are tightly coupled to climatic conditions, will determine how the pelagic ecosystem functions in the future.

Abundance patterns and species assemblages of euphausiids associated with the Mid-Atlantic Ridge, North Atlantic

New baseline information is presented on biogeography, abundance and distri-bution of euphausiids from discrete depth samples collected throughout the watercolumn to 3000 m at 42 locations along the Mid-Atlantic Ridge (MAR) betweenIceland and the Azores. Eighteen species were recorded, with Euphausia krohni andThysanoessa longicaudata being most abundant. Eight species had not been recordedin the area previously. The Subpolar Front is a northern boundary to somesouthern species, but not a southern boundary to northern ubiquitous species thatshow submergence. Four major species assemblages were identified and character-ized in terms of spatial distribution and species composition. Numerical abun-dance was highly variable but decreased logarithmically with depth. The greatestaverage abundance and the greatest spatial variation in abundance occurred innorthern waters, over the Reykjanes Ridge. Abundance declined less with depth inthe sub-Arctic Intermediate Water/North Atlantic Current Water than in otherareas and water masses. The near bottom layer (0–200 m from the seabed) hadtypically low density of euphausiids. The MAR is generally deeper than the usualdepth distribution of euphausiids (0–300 m), but many species conduct verticalmigrations to ridge crest depths. Euphausiid distribution and abundance patternsappeared only weakly affected by the MAR.KEYWORDS: zooplankton; Euphausiacea; oceanic; deepwater; biogeography;communities; Mid-Atlantic Ridge

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.

Euphausiid community structure and population structure of Euphausia superba off Adélie Land in the Southern Ocean during austral summer 2003, 2005 and 2008

The distribution and community structure of euphausiids and the population structure of Euphausia superba were investigated mainly along 140°E off Adélie Land in the Indian Ocean sector of the Southern Ocean during the austral summers of 2003, 2005 and 2008. Euphausiids were collected from six discrete depth layers, primarily between 0 and 2000 m, using an RMT 8 net. Euphausia frigida and Euphausia triacantha mainly occurred north of the Southern Boundary of the Antarctic Circumpolar Current (SB) whereas E. superba was distributed south of the SB, and occurred abundantly in the continental slope area where Antarctic Winter Water was observed. E. frigida and E. triacantha occurred in the upper 200 m during the night whereas they were mainly found below 200 m during the daytime. Cluster analysis on stations suggested that the SB is an important biological boundary for euphausiid communities. The population structure of E. superba in 2003 was different from that of 2005 and 2008. While large mature individuals dominated in 2003, small immature krill (juveniles and subadult males) were more abundant in 2005 and 2008. Further sea-ice extension in the preceding winter in 2005 and 2008 likely provided favourable conditions for spawning and survival. A cluster analysis based on similarity of the maturity stages of E. superba revealed that mature males and gravid females (stage IIIC–E) were mainly distributed in the offshore area and mesopelagic zone, while juveniles and subadult males were found in the epipelagic zone of the continental slope area. Therefore, it is considered that E. superba migrates to the offshore area and mesopelagic zone as it grows.

Euphausiids in the Bering Sea: better ecosystem information through acoustics.

Better information on the distribution and abundance of euphausiids (Thysanoessa spp.), a key group of crustacean zooplankton, meets a specific assessment need in the Bering Sea ecosystem. Multifrequency acoustic classification, backscatter modeling, and net capture were recently used to develop, apply, and validate a method of surveying euphausiid distribution and abundance during regular acoustic‐trawl surveys of walleye pollock (Theragra chalcogramma), an important commercial fish stock. These observations of euphausiids have both ecological and management implications. Summer surveys indicate that pollock predation may control euphausiid abundance: the stocks are inversely correlated in space and time, and estimated predation by pollock is sufficient to influence the euphausiid standing stock. Spring observations show that euphausiids and pollock are spatially segregated by ice cover and water temperature, which might mediate predation by pollock. This information on euphausiid abundance and distribut...

The influence of pelagic habitat selection and interspecific competition on productivity of juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus) in the Gulf of Alaska

AbstractHere we investigate processes affecting productivity of capelin and walleye pollock in the Gulf of Alaska. We examine pelagic habitat selection by comparing the distribution of juvenile fish and their prey with oceanographic properties and we evaluate the potential for interspecific competition by comparing diets and measures of foraging. The primary field study was conducted in Barnabus Trough, Kodiak Island, Alaska, during September 2005. The distribution of fish was assessed acoustically and trawls were used to collect individual fish for stomach content analyses. Physical and biological data were collected with conductivity–temperature–depth probes and zooplankton tows. Age‐0 pollock were distributed in cool waters offshore of a mid‐trough front, coincident with the distribution of euphausiids, their preferred prey. In contrast, capelin and their prey (copepods) were distributed throughout the trough. We observed that sympatric capelin (occurring with pollock) often had reduced foraging success compared to allopatric capelin (occurring alone). Results of a bioenergetic model also suggest that the exclusion of capelin from foraging on euphausiids can have negative consequences for capelin growth.

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.

Through a glass less darkly—New approaches for studying the distribution, abundance and biology of Euphausiids

Euphausiids are vital components of many oceanic foodwebs. Due to their large size, they are key prey species for many vertebrates, and several species are subject to commercial fisheries. The best studied euphausiid species is Antarctic krill ( Euphausia superba ), and a considerable literature exists documenting its ecology. The abundance of Antarctic krill varies enormously over multiple time and space scales in the Southern Ocean. This variability is driven partly by physical processes, but also by the behaviour of the animals as individuals and as groups. It is likely that such variability also affects the populations of other large euphausiid species. In order to understand this variability, particularly in the context of rapid environmental change and expanding fisheries interests, we need to shift emphasis from conventional point- and transect-sampling conducted from brief research cruises—usually in summer—to basin-scale observations over extended time periods. Future studies of the large-scale biology of krill of all species will also need to consider how their growth, reproduction and behaviour might alter in response to predicted environmental changes such as decline in sea ice cover, rising temperature and acidification of the ocean. Paradoxically, to answer many outstanding questions in krill biology, there also needs to be a shift towards examining the three-dimensional dynamics of krill aggregations on very small space and timescales. This paper outlines some of the approaches that might enable us to make progress in understanding these key organisms at multiple spatial and temporal scales, approaches that in turn may leave us better able to manage krill populations into the future. Whilst the paper’s focus is primarily on Antarctic krill, the findings are applicable generally to other large, schooling euphausiid species.

Summer distribution of euphausiids in the entrance of the Sea of Cortés in relation to hydrography

Samples obtained in the entrance of the Sea of Cortés were used to study the distribution of euphausiids. This area has a complex hydrography with mixing of the California Current water from the north with Gulf of California and tropical waters arriving from the south probably from the warm pool in southern México, as shown by satellite images and hydrography. The origin of the different water masses is also reflected in their species composition. Three sectors can be recognized according to hydrographic evidence: North, South, and East, with each showing differences in species composition. The dominant species was the neritic low- oxygen-affinity species Euphausia lamelligera, followed by Euphausia eximia, Euphausia tenera, and Euphausia distinguenda. Significant differences exist in postlarval abundance between the three sectors. Early larval stages were found everywhere, but frontal areas of the southern open-ocean sector showed higher abundances, implying greater reproductive activity. E. lamelligera was dominant near the coast in the east sector, and E. eximia, E. distinguenda and E. tenera were dominant in the northern and southern open-ocean sectors. E. tenera was the most abundant species in the western open-ocean transect, but was found mixed with other non-neritic species. Higher diversity indices were found in the northern and southern (open-ocean) sectors; however, diversity indexes were higher at night. This mixing of species was presumably caused by horizontal mixing induced from a large eddy that covers both sectors as shown by the geostrophic currents derived from altimeter data. Complex summer species distributions reflect the complex hydrographic conditions of the area, with water masses mixing, and, consequently, mixing of species with different water mass affinities.

Drivers of euphausiid species abundance and numerical abundance in the Atlantic Ocean

Mid-ocean ridges are common features of the world’s oceans but there is a lack of understanding as to how their presence affects overlying pelagic biota. The Mid-Atlantic Ridge (MAR) is a dominant feature of the Atlantic Ocean. Here, we examined data on euphausiid distribution and abundance arising from several international research programmes and from the continuous plankton recorder. We used a generalized additive model (GAM) framework to explore spatial patterns of variability in euphausiid distribution on, and at either side of, the MAR from 60°N to 55°S in conjunction with variability in a suite of biological, physical and environmental parameters. Euphausiid species abundance peaked in mid-latitudes and was significantly higher on the ridge than in adjacent waters, but the ridge did not influence numerical abundance significantly. Sea surface temperature (SST) was the most important single factor influencing both euphausiid numerical abundance and species abundance. Increases in sea surface height variance, a proxy for mixing, increased the numerical abundance of euphausiids. GAM predictions of variability in species abundance as a function of SST and depth of the mixed layer were consistent with present theories, which suggest that pelagic niche availability is related to the thermal structure of the near surface water: more deeply-mixed water contained higher euphausiid biodiversity. In addition to exposing present distributional patterns, the GAM framework enables responses to potential future and past environmental variability including temperature change to be explored.

Distribution, egg production and growth of euphausiids in the vicinity of the Pribilof Islands, southeastern Bering Sea, August 2004

Spatial distribution, egg production and growth of the predominant euphausiids in the vicinity of the Pribilof Islands, Bering Sea, were studied during August 2004. Thysanoessa longipes was abundant in the oceanic water, aggregations of Thysanoessa inermis were observed both in the oceanic region and on the shelf near the Pribilofs, while Thysanoessa raschii was found near the islands. Euphausia pacifica and Thysanoessa spinifera occurred in much smaller quantities in the oceanic water. Euphausiid abundances on the middle shelf were several orders of magnitude lower than in the cold summer of 1999. The instantaneous growth rate (IGR) technique was applied to measure somatic growth. Juvenile T. inermis showed the most intensive growth (� 8% of dry body weight per day). In contrast, the somatic growth of mature specimens of both T. inermis and T. raschii was slow (o1% of dry body weight per day) suggesting the investment of resources into seasonal lipid deposition and reproduction. Egg release corresponded to an average of 1.1% of female dry body weight per day for T. raschii, indicating that enough resources were available for the animals to maintain reproductive activity. Our data suggest that the area near the Pribilofs, known for its high biological productivity, may provide a refuge for the shelf euphausiid population during warmer summers, when conditions on the shelf are unfavorable for their growth and reproduction.

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.

Vertical distribution and diel migration of euphausiids from Oyashio Current to Kuroshio area off northeastern Japan

The vertical distribution of dominant euphausiid species was investigated in the offshore waters of north- eastern Japan in April and September 1998 and November 1999 using MOCNESS-I net samples. In total, 32 eu- phausiid species in eight genera were identified in the study area. Subarctic Euphausia pacifica and Thysanoessa in- spinata occurred over a wide range, although mainly in the Oyashio and cooler transitional areas, and the former made a relative contribution of 78% to the total numerical abundance of euphausiids. At night, these species tended to stay near the surface in the Oyashio area and in deeper layers in warmer areas. During the daytime, they tended to stay in deeper layers in the warmer areas, although E. pacifica stayed in deeper layers even in the Oyashio area where the chlorophyll a concentration was low. T. inspinata did not perform clear diel vertical migration. Except for these subarc- tic species, five warm-water species of Euphausia were abundant among the epipelagic euphausiids. At night, these species tended to stay at a wide range of depths from the surface to subsurface layers in the warmer Kuroshio area, but were concentrated near the surface in the transitional area. During the daytime, they stayed in deeper layers in the Kuroshio area. Diel vertical migration of euphausiids seems to be strongly affected by water temperature, but other fac- tors such as phytoplankton abundance may also be important. The possible effects of diel vertical migration of eu- phausiids on euphausiid fisheries are discussed.

Euphausiid distribution along the Western Antarctic Peninsula—Part A: Development of robust multi-frequency acoustic techniques to identify euphausiid aggregations and quantify euphausiid size, abundance, and biomass

Abstract Methods were refined and tested for identifying the aggregations of Antarctic euphausiids (Euphausia spp.) and then estimating euphausiid size, abundance, and biomass, based on multi-frequency acoustic survey data. A threshold level of volume backscattering strength for distinguishing euphausiid aggregations from other zooplankton was derived on the basis of published measurements of euphausiid visual acuity and estimates of the minimum density of animals over which an individual can maintain visual contact with its nearest neighbor. Differences in mean volume backscattering strength at 120 and 43 kHz further served to distinguish euphausiids from other sources of scattering. An inversion method was then developed to estimate simultaneously the mean length and density of euphausiids in these acoustically identified aggregations based on measurements of mean volume backscattering strength at four frequencies (43, 120, 200, and 420 kHz). The methods were tested at certain locations within an acoustically surveyed continental shelf region in and around Marguerite Bay, west of the Antarctic Peninsula, where independent evidence was also available from net and video systems. Inversion results at these test sites were similar to net samples for estimated length, but acoustic estimates of euphausiid density exceeded those from nets by one to two orders of magnitude, likely due primarily to avoidance and to a lesser extent to differences in the volumes sampled by the two systems. In a companion study, these methods were applied to the full acoustic survey data in order to examine the distribution of euphausiids in relation to aspects of the physical and biological environment [Lawson, G.L., Wiebe, P.H., Ashjian, C.J., Stanton, T.K., 2008. Euphausiid distribution along the Western Antarctic Peninsula—Part B: Distribution of euphausiid aggregations and biomass, and associations with environmental features. Deep-Sea Research II, this issue [doi:10.1016/j.dsr2.2007.11.014]].

Euphausiid distribution along the Western Antarctic Peninsula—Part B: Distribution of euphausiid aggregations and biomass, and associations with environmental features

Abstract The distribution of euphausiids ( Euphausia spp.) in the continental shelf region in and around Marguerite Bay, west of the Antarctic Peninsula, was studied in relation to aspects of the physical and biological environment. Acoustic, video, net, and environmental data were collected during surveys in the austral falls and winters of 2001 and 2002. Application of the analysis methods described in Lawson et al. [2008. Euphausiid distribution along the Western Antarctic Peninsula—Part A: Development of robust multi-frequency acoustic techniques to identify euphausiid aggregations and quantify euphausiid size, abundance, and biomass. Deep-Sea Research II, this issue] to acoustic survey data demonstrated strong seasonal, inter-annual, and spatial variability in krill distribution. The distribution of krill aggregations was characterized by many small aggregations closely spaced relative to one another, punctuated by much fewer aggregations of very large size that accounted for the majority of overall biomass in the region. The greatest number of aggregations was found at depths less than 100 m, but biomass was usually greatest at deeper depths. Similar biomass levels were estimated for both the fall surveys and the winter of 2002 survey, while biomass during the winter of 2001 was an order of magnitude smaller. During fall, biomass estimates vertically integrated over the sampled portion of the water column were negatively associated with chlorophyll a concentrations, and some suggestion was observed of highest biomass in regions of lowest current magnitude and horizontal current gradient. Generalized additive models indicated that highest krill biomass was consistently associated with regions close to land where the temperature maximum below 200 m depth was cooler than what was available over the shelf as a whole. Krill were thus not associated with regions where intrusions of warm and nutrient-rich circumpolar deep water onto the shelf were present, despite such intrusions being thought to be an important driver of primary productivity.

Variations in age-0 pollock distribution among eastern Bering Sea nursery areas: A comparative study through acoustic indices

Abstract Cohort abundance of walleye pollock (Theragra chalcogramma) is subject to strong interannual variation in the eastern Bering Sea, and this variation is known to be determined largely at the age-0 stage. We estimated the spatial distributions and densities of age-0 walleye pollock in five nursery areas around the eastern Bering shelf in three successive years (1997–1999) from acoustic survey data. Concurrently, we calculated estimates of the spatial distribution of euphausiids, a major prey of age-0 walleye pollock, and estimates of spatial overlap of groundfish predators with the age-0 walleye pollock. The analyses showed that all nursery areas had low densities of age-0 walleye pollock in 1997, which ultimately produced the weakest adult year-class. In the intermediate year of 1998, age-0 densities were low to medium, and in 1999, which produced the strongest of the three adult year-classes, all nursery areas had medium to high age-0 walleye pollock densities. Euphausiid distributions had a consistently positive spatial relationship with age-0 walleye pollock. Groundfish predator density ratios were positively related to age-0 walleye pollock density when age-0 walleye pollock were displaced relatively northward. Our results suggest that abundance of age-0 walleye pollock, and hence of adult cohorts in the eastern Bering Sea, can be predictable from a concise set of indicators: the densities of age-0 walleye pollock at nursery areas in mid- to late-summer, their spatial relationship to euphausiids and groundfish predators, and the latitudinal trend of their distributions. The 3 years 1997–1999 had significant differences of physical conditions in the eastern Bering Sea, and represent an advantageous framework for testing these hypotheses.

Distribution patterns of pelagic euphausiids in the East China Sea

Distribution patterns and abundance of the euphausiids were examined in the East China Sea (23°30′ –33°00′N, 118°30′ –128°00′ E) in relation to temperature and salinity. The data were collected in 4 surveys from 1997 to 2000. The density or yield density model was used to predict optimum temperature and salinity of water for euphausiid distribution, and thereafter distribution patterns of euphausiids were determined based on the predicted parameters. Of 23 species, Euphausia pacifica, E. nana, Pseudeuphausia sinica and P. latifrons were numerically dominant. The analyses indicate that Euphausia pacifica is an offshore temperate water species, E. nana is an offshore temperate warm water species, P. sinica is a coastal subtropical water species and P. latifrons is an oceanic tropical water species. The 4 species occupied 4 different water masses, respectively, namely, cold water mass, cold and warm water mixed masses in winter and spring, cold and warm water mixed masses in summer and autumn, and warm water mass, which could be the good designators of individual water masses, respectively. The predicated optimal temperatures for E. tenera, S. carinatum, E. diomedeae, Stylocheiron affine, Nematoscelis sp., N. gracilis, N. atlantica, Stylocheiron sp. and S. suhmii are all > 25°. These species are mainly distributed in southern Kuroshio in winter and spring, Kuroshio, the Taiwan Warm Current and Tsushima Current in summer and autumn, the equatorial waters of Pacific Ocean and the eastern waters of the Taiwan Strait. They are called as oceanic tropical water species. Nematoscelis tenella and T. tricuspidata are referred to as offshore subtropical water species according to their geographic distributions even if they are halobionts. Euphausia sanzoi is considered as a typical offshore subtropical water species, which inhabited waters below 25°. Stylocheiron microphthalma, occupying warm current waters where temperature and salinity are nearly 25° and 34 in summer and autumn, belongs to oceanic tropical water species. In the same way, E. similes, E. mutica, Euphausia sp., E. brevis and E. recurva are classified into offshore subtropical water species in accordance with the optimum temperature and salinity of waters as well as locations and seasons of their occurrence. Optimum temperature, rather than salinity, is a better parameter in determining the distribution patterns of euphausiids.

Seasonal changes in distribution and abundance of euphausiids in the coastal area of north-eastern Japan.

ABSTRACT: Seasonal changes in distribution and abundance of euphausiids off south‐eastern Hokkaido (41°−43°N), Sanriku (38°−41°N), and Joban (36°−38°N) were investigated using cylindrical‐conical nets every two months from March 1997 to February 1998. Twenty‐six species of seven genera of euphausiids occurred during the survey. Among them, subarctic‐transitional Euphausia pacifica was the most abundant throughout the year in coastal waters, as their relative contribution to the total abundance of euphausiids was 89–92%. This species occurred in each coastal water throughout the survey and was abundant from winter to early summer (February–June) off Sanriku and Joban and in autumn in south‐eastern Hokkaido. Thysanoessa inspinata occurred off south‐eastern Hokkaido and Sanriku throughout the survey, mainly in spring (April) but rarely occurred off Joban. Three other subarctic Thysanoessa species occurred mainly off south‐eastern Hokkaido from winter to spring. Conversely, warm‐ and transitional‐water epipelagic species occurred exclusively off Sanriku and Joban in autumn. The characteristics of seasonal distributional patterns of euphausiids are discussed in relation to the spatial and temporal changes of oceanographic conditions and several predators off north‐eastern Japan.

Temporal changes in euphausiid distribution and abundance in North Atlantic cold-core rings in relation to the surrounding waters

The species composition of euphausiids was investigated in relationship to the hydrographic conditions in the North Atlantic cold-core rings (CCR) and adjacent waters to elucidate species succession in evolving water masses. Using data, dating back to the 1970s, from as many CCRs as possible and selecting typical cases where no major physical perturbations occurred, a general pattern of euphausiid succession and change in vertical distribution in rings with time was obtained. This pattern was related to the general distribution of euphausiids in the northwestern North Atlantic Ocean, aiming at providing basic information on probable response of North Atlantic marine ecosystem to global warming. Of the 34 euphausiid species identified, 5 were cold-water species, 17 were warm-water species, 6 were wide-ranging warm-water species, 1 was transitional, 4 were cosmopolitan and the remaining was Thysanoessa parva. Among cold-water species, Euphausia krohni and Nematoscelis megalops were dominant in CCRs. E. krohni became rare in rings older than 6 months, whereas N. megalops survived longer, being abundant in some rings of 9 months or older, by staying within its preferred temperature range as the CCR elevated isotherms sank to depths where they are normally found in the Sargasso Sea and because it is an omnivore–carnivore. Among warm-water species, epipelagic species appeared first in rings, corresponding to the physical change occurring most rapidly in the surface layers. Mesopelagic species appeared later. Cold-water species made up 65–85% of the total euphausiid population in number in younger rings (1–5 months old), while warm-water species contributed only 2–7%. Wide-ranging warm-water species made up about up to one fourth of the total in rings 5 and 7 months old. Warm-water species, mainly E. brevis, increased in older rings (9 months old or older) and made up 50% of the total in the oldest ring. The contribution of cold-water species decreased to 14% in older rings. T. parva made up 26–38% of the total in rings 6 months or older. CCR populations can be characterized by high species number, but intermediate evenness between the Slope Water and northern Sargasso Sea. In CCRs, only a limited number of species were dominant even if there were more species present in rings as old as 9–12 months than in the northern Sargasso Sea. In rings older than 9 months, euphausiids showed two peaks in their vertical distribution: a shallow daytime peak at about 400 m and a nighttime peak in the upper 100 m consisting of warm-water species (mainly E. brevis) and a deeper persistent peak at 800 m or deeper consisting of the species N. megalops and T. parva. This shallow peak in CCRs is shallower than that in the surrounding northern Sargasso Sea, and the deep peak is rarely observed in these waters.