Oceanographic factors affecting interannual recruitment variability of Pacific saury (Cololabis saira) in the central and western North Pacific

Pacific saury (Cololabis saira) has a short life span of 2 years and tends to exhibit marked population fluctuations. To examine the importance of sea surface temperature (SST) and mixed layer depth (MLD) as oceanographic factors for interannual variability of saury recruitment in early life history, we analyzed the relationship between abundance index (survey CPUE (catch per unit of effort)) of age‐1 fish and the oceanographic factors in the spawning and nursery grounds of the previous year when they were born, for the period of 1979–2006, in the central and western North Pacific. Applying the mixture of two linear regression models, the variability in the survey CPUE was positively correlated with previous year's winter SST in the Kuroshio Recirculation region (KR) throughout the survey period except 1994–2002. In contrast, the survey CPUE was positively correlated with the previous year's spring MLD (a proxy of spring chlorophyll a (Chl‐a) concentration) in the Kuroshio‐Oyashio Transition and Kuroshio Extension (TKE) during 1994–2002. This period is characterized by unusually deep spring MLD during 1994–1997 and anomalous climate conditions during 1998–2002. We suggest that saury recruitment variability was generally driven by the winter SST in the KR (winter spawning/nursery ground), or by the spring Chl‐a concentration (a proxy of prey for saury larvae) in the TKE (spring spawning/nursery ground). These oceanographic factors could be potentially useful to predict abundance trends of age‐1 saury in the future if the conditions leading to the switch between SST and MLD as the key input variable are elucidated further.     

Migration of albacore, Thunnus alalunga, in the North Pacific Ocean in relation to large oceanic phenomena

On the basis of Japanese long-line fishery data during 1970–1988, anticlockwise migration routes of albacore in the North Pacific are newly proposed. The annual migration route for mature albacore is described as a closed ellipse with a centre at 20°N and 170°E, and is wider in El Niño years than non-El Niño years associated with an appearance of a cold-water region in the central and south-western North Pacific. Immature albacore also have an anticlockwise migration route in winter which extends from 25°N to 35°N and from 130°E to 180°E, when the Kuroshio has a relatively straight path. However, the migration does not persist when the Kuroshio takes a large meander path.     

Purification of serine carboxypeptidase from the hepatopancreas of Japanese common squid Todarodes pacificus and its application for elimination of bitterness from bitter peptides

ABSTRACT:   An acidic serine carboxypeptidase (CPase Tpa) from the hepatopancreas of Japanese common squid Todarodes pacificus was purified. Purified CPase Tpa had a molecular mass of 36 kDa on sodium dodecylsulfate–polyacrylamide gel electrophoresis, and an isoelectric point of 6.0. The optimum pH of CPase Tpa was pH 4.0. In investigating the specificity of CPase Tpa for several peptide substances, it was found that peptides with hydrophobic or bulky amino acid residues at the P₁ position reacted well. The enzymatic activity was almost completely inhibited by p -chloromercuribenzoic acid, monoiodoacetic acid, diisopropylfluorophosphate and HgCl₂. This is the basis for its grouping in the serine carboxypeptidase family (EC 3. 4. 16. 5). The substrate specificity of CPase Tpa can be used to eliminate the bitterness of bitter peptides. In this study, the bitterness-reductive effect using bitter peptides prepared by hydrolyzing soy protein, casein and corn gluten with pepsin or trypsin was tested. The bitterness of soy peptide digested with pepsin was completely eliminated by treatment with CPase Tpa, whereas the bitterness of casein digested with trypsin and corn peptide digested with pepsin were somewhat less efficient. On the basis of these results, it is anticipated that CPase Tpa would be effective in eliminating the bitterness of some bitter peptides.     

Long-term changes in zooplankton and its relationship with squid, Todarodes pacificus, catch in Japan/East Sea

Long‐term change in zooplankton biomass and composition of major zooplankton assemblages was studied with a focus on climate change and squid catch in the Japan/East Sea. This study deals with data sets of zooplankton biomass, abundance of major zooplankton assemblages, and sea surface temperature (SST) obtained during the period from 1965 to 1998, and the catch of squid, Todarodes pacificus, from 1978 to 1998. Based on its physical characteristics, the study area was divided into two subregions, a northern and southern region. The SST during the winter and spring (February and April) steadily increased after the late 1980s in the southern region. On the other hand, the northern region did not have a warm winter and had higher interannual fluctuations. Zooplankton biomass also showed an increasing trend and major zooplankton assemblages have shifted in their composition since the early 1990s. The northern region exhibited a greater increasing trend than the southern region in zooplankton biomass. Macrozooplankton such as chaethognaths, euphausiids and amphipods gradually increased after the early 1990s. In the Japan/East Sea, squid catches increased continuously after the early 1990s. The squid catch for the autumn of year n is significantly associated with the zooplankton biomass collected in October and December of year n ? 1 (r=0.864 in the northern region and r=0.818 in the southern region, P < 0.001) and macrozooplankton, especially euphausiids (r=0.578 in the northern region and r=0.840 in the southern region, P < 0.05) and amphipods (r=0.695 in the northern region and r=0.648 in the southern region, P < 0.05).     

Diel vertical migration of myctophid fishes (Family Myctophidae) in the transitional waters of the western North Pacific

The diel vertical migration patterns of adult myctophid fishes were determined in the transitional waters of the western North Pacific off Japan, using day–night sampling from 20 to 700 m depths with a commercial otter trawl in the summer of 1995. A total of 12 species belonging to 9 genera were collected. Four patterns were recognized in the diel vertical migration of 11 of the 12 species. (1) Migrants showing clear day–night habitat separation with peak abundance above 200 m at night: Symbolophorus californiensis, Tarletonbeania taylori, Notoscopelus japonicus, Diaphus theta, Ceratoscopelus warmingi, and Diaphus gigas. (2) Semi-migrants, in which part of the population often remains in the daytime habitat at night. The distribution depths of migratory and nonmigratory individuals do not overlap: Stenobrachius leucopsarus . (3) Passive-migrants, in which there is no separation of day–night habitats, but the upper limit of daytime distribution depth shifts to a shallower layer at night, probably as the fish follow migratory prey: Lampanyctus jordani . (4) Nonmigrants: Stenobrachius nannochir, Lampanyctus regalis (> 140 mm SL), and Protomyctophum thompsoni . The day–night habitat temperature ranges are also given for the 11 species. No remarkable east–west differences were seen in the vertical migration patterns compared with previous knowledge of eight of these species in the eastern Pacific. The diel migration patterns are newly described for three other species endemic to the western Pacific. The standing stock of myctophids in the study area was conservatively estimated at 18.5 ± 4.7 g m⁻² (avg. ± SD).     

Characterization of 38 kDa and 42 kDa chitinase isozymes from the liver of Japanese common squid Todarodes pacificus

ABSTRACT: Characterization was investigated on the 38 kDa and 42 kDa chitinase (EC3.2.1.14) isozymes from the liver of Japanese common squid Todarodes pacificus . Optimum pH toward colloidal chitin was observed at pH 3.0 for the 38 kDa chitinase, and pH 3.0 and 9.0 for the 42 kDa chitinase. K _m and k _cat of the 38 kDa and 42 kDa chitinases toward a longer substrate, glycol chitin, were 0.071 mg/mL and 1.22/s, and 0.074 mg/mL and 0.196/s, respectively. Alternatively, strong substrate inhibition of both chitinases were observed toward a short substrate, N -acetylchitopentaose (GlcNAc₅). Both chitinases decomposed not only chitin but also chitosan (D. A. 95%). The cleavage pattern and reaction rate were investigated using N -acetylchitooligosaccharides (GlcNAc_n, n  = 2–6). Both chitinases hydrolyzed GlcNAc_n ( n  = 4,5, and 6). The release of GlcNAc was not observed. The speed of the reaction was observed to be in the following order: GlcNAc₄ > GlcNAc₅ > GlcNAc₆ for the 38 kDa chitinase, and GlcNAc₆ > GlcNAc₅ > GlcNAc₄ for the 42 kDa chitinase. Both the chitinases released p -nitrophenol from p -nitrophenyl GlcNAc_n ( n  = 2, 3, and 4). N-terminal amino acid sequences of the 38 kDa and 42 kDa chitinases were YLLSXYFTNWSQYRPGAGKYFPQNI and EYRKVXYYTNWSQYREVPAKFFPEN, respectively.     

Late winter larval mesopelagic fish assemblage in the Kuroshio waters of the western North Pacific

The larval mesopelagic fish assemblage and its distribution patterns were investigated in the Kuroshio region off southern Japan in late winter. A total of 8690 fish larvae was collected, 85.8% of which were mesopelagic fish larvae. Mesopelagic fish larvae were significantly more abundant in the area east of the Kuroshio axis than west of the Kuroshio axis (660.6 versus 194.5 ind 10 m^?2). Sigmops gracile, Bathylagus ochotensis, Notoscopelus japonicus, Diaphus slender type and Myctophum asperum were the five most abundant larvae and accounted for 16.9, 16.4, 15.2, 13.9 and 9.3% of the total catch in numbers, respectively. We conclude that these larvae were transported by the Kuroshio Current to the more productive transition waters, where they spend their juvenile stage from spring to early summer. The possibility of expatriation and southward long‐distance spawning migrations of N. japonicus and B. ochotensis are discussed, based on the geographic distribution patterns of their larvae, juveniles and adults.     

Night-time vertical distribution and abundance of small epipelagic and mesopelagic fishes in the upper 100 m layer of the Kuroshio-Oyashio Transition Zone in Spring

ABSTRACT:   In order to simultaneously compare vertical distributions of small pelagic fishes and mesopelagic fishes during the night in the upper 100 m, one oblique and 11 horizontal tows of large-sized (opening area 480 m²) trawl operations were conducted at different depths in the Kuroshio–Oyashio Transition Zone during 20–22 May 1995. While Japanese sardine Sardinops melanostictus and Japanese anchovy Engraulis japonicus were concentrated in the upper 20 m, mackerels Scomber spp. were collected uniformly in the upper 80 m layer. Myctophid fishes Ceratoscopelus warmingii, Diaphus kuroshio, Diogenichthys atlanticus , Myctophum asperum , Notoscopelus spp. mostly N. resplendens , and Symbolophorus californiensis , a gempylid Nealotus tripes , a microstomatid Lipolagus ochotensis and a phosichthyid Vinciguerria nimbaria were dominant components of mesopelagic fishes that ascended to the upper 100 m layer at night, where these mesopelagic fishes concentrated at depths of 20–80 m. In the upper 20 m layer, Japanese anchovy comprised 60% of the total wet weight of all fishes followed by D. atlanticus (3%), C. warmingii (2%) and S. californiensis (2%). Juvenile myctophids were dominant in the upper 20 m layer compared to deeper layers.     

Mechanism of body cavity temperature regulation of chum salmon (Oncorhynchus keta) during homing migration in the North Pacific Ocean

Vertical movement patterns of five chum salmon (Oncorhynchus keta) during homing migration were examined using archival tags. The standard deviation of the depth and ambient and body cavity temperatures during daytime were larger than those during night‐time. Vertical movements through the thermocline with a periodicity of less than 1 h were observed during daytime in addition to the diel vertical movement patterns in the open ocean. During these periods of frequent short‐term vertical movements, the difference between the body cavity temperature and ambient temperature was large while the variance of the body cavity temperature was less than that of the ambient temperature. From the results of a random simulation, the variation of the body cavity temperature was shown to decrease due to these periodic high frequency movements in comparison with random vertical movements. The whole‐body heat‐transfer coefficient k (s^?1), which was estimated by a heat budget model, was 1.48 × 10^?3. The k of chum salmon was larger than that of bigeye tuna (Thunnus obesus) by about one order of magnitude for the cooling of the body. The k of chum salmon did not change like tuna, which are physiologically adapted to conserve body cavity temperature. This indicates that the regulation of body cavity temperature by chum salmon is dependent on the vertical movements only. The maintenance of the body cavity temperature is concluded to be advantageous for their maturation and growth from the relationship between energy input and output during their homing migration.     

Assemblages of vertical migratory mesopelagic fish in the transitional region of the western North Pacific

Assemblages of vertical migratory mesopelagic fish are described for the nighttime upper 20 m layer of the transitional and adjacent waters in the western North Pacific. Twenty-three mesopelagic fish species belonging to six families and 16 genera were collected during June 1996. Family Myctophidae was most speciose, represented by 17 species, followed by Sternoptychidae (two spp.) and Bathylagidae (one spp.). Six assemblages are recognized based on their species composition: Subarctic, Northern Transition, Southern Transition, Subtropical, Slope Water and Seamount Assemblages. These assemblages are also characterized by size composition and biomass of the catches. The distributions of the Subarctic, Northern Transition, Southern Transition and Subtropical Assemblages are approximately defined by the Kuroshio and Oyashio fronts and the Subarctic Boundary. Slope Water and Seamount Assemblages are closely related to the slope water around the shelf edge off Japan and waters around the Shatsky Rise and the Emperor Seamount chain, respectively. The importance of the Southern Transition Assemblage area between the Kuroshio front and Subarctic Boundary as spawning and nursery grounds of many subarctic, transitional and subtropical fishes is elucidated, based on the geographical distribution patterns of their larvae and juveniles.     

东海太平洋褶柔鱼的数量分布及其与环境的关系

利用1997～2000年在东海4个季节的拖网以及温盐度调查资料,分析了东海太平洋褶柔鱼的数量分布及其与环境条件的关系。结果表明：东海太平洋褶柔鱼分布水深在40～175m,集中分布水深范围在80～150m。春、夏、秋三个季节的平均资源密度最高值都出现在北部外海,冬季的平均资源密度最高值转移到南部外海。四季都有生殖群体的存在,从春季到冬季,生殖群体的重心位置由北部外海向南部外海转移。四季生活海域的表层水温为13．63～28．38℃,生物量高区的表层水温为22～26℃;表层盐度为27．72～34．72,集中分布海域的盐度为32．5～34．7。因此,东海太平洋褶柔鱼属于暖水性外海高温高盐种。     

Variability of the Pacific North Equatorial Current and its implications on Japanese eel (Anguilla japonica) larval migration

The zonal velocity produced by a Regional Ocean Modeling System (ROMS)‐based Pacific Ocean circulation model was validated against in situ measurements along the 137°E longitude. The Pacific model successfully reproduced the position and the shape of the North Equatorial Current (NEC) as well as the latitude of maximum surface velocity in the NEC region (8–17°N). The flow field produced by the Pacific model was then used to conduct numerical Lagrangian experiments, in which passive particles were released along a transect (142.5°E, 12.5–17°N) that traverses the known Japanese eel spawning area, and the effects of NEC strength and bifurcation latitude on the particle advection in the northwest Pacific were studied. Our results suggest that, in the 20‐yr period (1993–2012), the variability of the currents alone can cause interannual variability of one order of magnitude in the Kuroshio Entrance (KE), the percentage of particles entering the Kuroshio, the range of which varies from 43% in 1997 to 6% in 2012. The yearly‐averaged KE is not sensitive to the NEC bifurcation latitude. Instead, it is controlled by the average zonal velocity of a fixed domain (125–143°E, 13.5–17°N) and related to a recently‐developed climate index, the Philippines–Taiwan Oscillation (PTO). During the positive phase of the PTO, the zonal velocity in the domain, hence the yearly‐averaged KE, increases, and the opposite is true in the negative phase of the PTO. Considering only the trajectories, diel vertical migrations (DVM) in the top 400 m do not significantly affect Japanese eel larval transport, as incorporating DVM schemes does not increase the KE.     

Differing body size between the autumn and the winter–spring cohorts of neon flying squid (Ommastrephes bartramii) related to the oceanographic regime in the North Pacific: a hypothesis

The neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter–spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38–46 cm) and the winter–spring cohorts (ML range: 16–28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration habitats of the two different‐sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21–25°C) indicates that the spawning ground occurs farther north during autumn (28–34°N) than winter–spring (20–28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food‐rich nursery ground in winter. The winter–spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter–spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food‐rich habitat from winter through summer, which, we hypothesize, causes its members to grow larger than those in the winter–spring cohort in summer.     

Basin-scale distribution pattern and biomass estimation of Japanese anchovy Engraulis japonicus in the western North Pacific

The distribution pattern and biomass of the Japanese anchovy Engraulis japonicus in the offshore region of the western North Pacific (north of 35°N and west of 170°E) were studied using a quantitative echosounder. This is the first attempt at such a study in this region. Data were collected in summer from 2004 to 2007. The biomass was estimated using data collected at 38?kHz. Species compositions in the backscatterings from pelagic fish were assigned based on the results of trawl hauls taking account of sea surface temperature (SST). Japanese anchovy tended to be high density to the west of 153°E and were distributed in an SST range of 9?C24?°C. Although the temporal and spatial coverage of the survey differed each year, at least 1.5?C3.4 million tons of Japanese anchovy were present in the survey area between 2004 and 2007. To take account of the spatial coverage of the survey each year, the most reliable biomass estimate for this region in the time period was 3.4 million tons (coefficient of variation?0.22).     

Patterns of stable carbon and nitrogen isotopes in the baleen of common minke whale Balaenoptera acutorostrata from the western North Pacific

ABSTRACT:   Stable carbon and nitrogen isotope ratios were determined in the baleen plates of 17 common minke whales Balaenoptera acutorostrata from the north-western Pacific Ocean off Japan, as well as prey species (krill Euphausia pacifica , Japanese anchovy Engraulis japonicus and Pacific saury Cololabis saira ) collected in the stomach contents, to investigate the trophic relationship between the minke whales and their prey. A few δ¹⁵N-depleted peaks occurred along the length of baleen plates for 10 males irrespective of stomach content (anchovies and sauries). Similar δ¹⁵N-depleted peaks were also found for one female and two immature individuals. It was likely that these δ¹⁵N-depleted peaks formed in early summer. The stable nitrogen isotope ratio (δ¹⁵N) values in Pacific saury (9.3 ± 1.4‰) did not differ significantly from that in Japanese anchovy (8.8 ± 0.9‰). In contrast, δ¹⁵N in krill (7.2 ± 0.5‰ in July and 8.0 ± 0.2‰ in September) were significantly lower than in the Pacific saury. Thus, these peaks may reflect the dietary change from krill to fishes in the feeding migration of the whales. Growth rate of the baleen plate was estimated to be 129 mm/y, and it appeared that a dietary record of about 1.4 years remained in the baleen plate. For two immature whales, the maximum value of δ¹⁵N occurred at the tip of baleen. This δ¹⁵N enrichment may possibly be useful for discriminating weanlings and older whales.     

Spatial distribution analysis of the North Pacific spiny dogfish,Squalus suckleyi,in the North Pacific using generalized additive models

The North Pacific spiny dogfish (SPD), Squalus suckleyi, is a commercially exploited shark species that plays an important role in the ecosystem. To elucidate the distribution of the SPD in the North Pacific and to evaluate the effects of sea surface temperature (SST) and prey availability on its distribution, we estimated the probability of SPD presence using a generalized additive model with a binomial error distribution from SPD presence/absence data on 14,824 operations in fishery‐independent gillnet surveys between 1972 and 2011. The habitat model was structured in the east and west to reflect differences in the North Pacific oceanic environments. In the east, a higher probability of SPD presence was identified along the coast from the eastern Gulf of Alaska to Queen Charlotte Sound. In the west, it was identified around northern Japan. The estimated distribution was continuous between the two areas, whereas the probability of SPD presence was relatively low. Although the probability of SPD presence was higher at SSTs between 6°C and 12°C, the SST at the peak probability of SPD presence differed between the west and east. The prey species, Japanese sardine, Sardinops melanostictus, and walleye pollock, Gadus chalcogrammus, in the west and boreal clubhook squid, Onychoteuthis borealijaponica, in the east significantly affected the probability of SPD presence, which was higher if the prey species co‐existed with SPD. Therefore, SPD might adapt their distribution to that of available prey species. SPD stock assessment and management in these two important areas are required for its sustainable utilization.