Annual changes in macrobenthos abundance in Funka Bay,Japan

The temporal variation in macrobenthos abundance was studied in Funka Bay in the periods 2001–2004 and 2007–2010. Dissolved oxygen just above the sea floor of the basin was lower and ignition loss at the level of the sea floor was higher in 2007–2010 than in 2001–2004. Macrobenthic species were categorized into three groups: long-lived species, sedental polychaeta Notomastus latericeus, and short-lived species. Ignition loss was the most influential factor affecting macrobenthos abundance as this variable restricts the recruitment of macrobenthos. The abundance of both long-lived species and N. latericeus was lower between 2007 and 2010 than between 2001 and 2004. Supplementation of oxygen from outside the bay by the Tsugaru Warm Current may be an important factor impacting the increase in the abundance of long-lived species. The abundance of N. latericeus barely increased between 2007 and 2010, possibly because dissolved organic matter, which is their food item, may have become limited due to a narrow chink between sediments with eutrophication. The abundances of short-lived species did not differ between the periods 2001–2004 and 2007–2010 due to the organisms having a low tolerance to hypoxia. The current conditions in Funka Bay are on the verge of turning the bay into an azoic zone.     

Annual variation in otolith increment widths of walleye pollock (Gadus chalcogrammus) larvae in Funka Bay,Hokkaido, Japan

To clarify relationships between year‐class strength and larval growth of walleye pollock (Gadus chalcogrammus), and oceanographic conditions in the Pacific stock off Hokkaido and Tohoku, Japan, we undertook conductivity/temperature/depth (CTD) observations and investigated larval densities, larval otolith increment widths and larval prey densities (of copepod nauplii) of the 2008, 2009, 2010 and 2011 yr classes in Funka Bay. Oyashio Coastal Water (OCW) flowed into the bay in late February in 2008, 2010 and 2011, and the mean water temperatures decreased to 1.9–3.1 °C in March. OCW was not observed in 2009, and it was warm in late February (≥3.4 °C). Increment widths of lapillar otoliths during the yolk‐sac stage were wide in 2009 and 2011, medium in 2010 and narrow in 2008. Increment widths during the first‐feeding stage tended to become wider as the hatch month progressed, and the annual variation during the first‐feeding stage was larger than that of the yolk‐sac stage. The densities of the primary food for the larvae were high in 2008 when larval increment widths were narrowest, so the effect of prey abundance on larval growth appeared to be small. The ranking of the larval abundance in March was nearly coincident with that of the increment width during the larval stage. We, therefore, suggest that the larval growth rate is associated with the mortality rate and that the growth–mortality hypothesis may be applicable to walleye pollock in Funka Bay. Feeding success under warm water conditions may be an important factor that contributes towards high growth rates.     

Improved growth of flathead flounder Hippoglossoides dubius in hypoxic waters in Funka Bay,Japan

To identify the factors that influence the growth rate of flathead flounder Hippoglossoides dubius in Funka Bay, Japan, we studied temporal changes in the growth of this species and its prey from 1989 to 2012. The lowest growth rate of H. dubius was recorded in the 1995 year-class, which had experienced severe hypoxia between 1995 and 1997. However, the highest growth rate was recorded in the 2007 year-class, which had experienced hypoxia between 2009 and 2011. In 2000 and 2001, small-sized flounder (<200 mm TL) from the 1995 year-class were feeding on mysidaceans, whereas those from other year-classes fed on small Ophiura spp. (<9 mm in disc diameter) in 2009, 2011, and 2012. Small Ophiura spp. were more abundant in 2007 and 2011 than in 2002 and 2003. Successful recruitment of Ophiura spp. took place between 2007 and 2011 and cohorts from these years could tolerate hypoxia. This suggests that food availability improved under hypoxic conditions, which led to improved growth in small flounders. Large flounders (≥200 mm TL) from all year-classes fed on prickleback fishes (Stichaeidae) and walleye pollock Gadus chalcogrammus juveniles. The growth of large flounders was enhanced by an increase in the abundance of Stichaeidae fishes and G. chalcogrammus juveniles.     

Biophysical models for Japanese scallop, Mizuhopecten yessoensis, aquaculture site selection in Funka Bay, Hokkaido, Japan, using remotely sensed data and geographic information system

Scallop aquaculture has grown to become the most successful marine shellfish farming venture in Japan. To ensure long-term sustainability of scallop production, suitable sites need to be selected. This study was conducted to identify the most suitable sites for hanging culture of Japanese scallop (Mizuhopecten yessoensis) based on measured biophysical parameters and modeled using geographic information system (GIS) models. Four biophysical parameters that influence Japanese scallop aquaculture in Funka Bay were selected: sea temperature, food availability (chlorophyll-a), suspended solid, and bathymetry. The SeaWiFS and MODIS images were used to extract most of these parameters. A series of GIS models was developed to identify suitable sites for scallop culture using a multi-criteria evaluation (MCE) known as weighted linear combination. Suitability scores were ranked on a scale from 1 (least suitable) to 8 (most suitable), and about 83% of the total potential area with bottom depths less than 60 m had scores of 7 and 8. The final model outputs were compared with field verification data and found to be consistent. Local sensitivity analysis was used to test the robustness of the model output. Based on this analysis, the order of importance of the variables affecting the model was as follows: suspended solid > chlorophyll-a > sea temperature.     

Effects of hypoxia on principal prey and growth of flathead flounder Hippoglossoides dubius in Funka Bay, Japan

ABSTRACT:   Effects of hypoxia on the principal prey and growth of flathead flounder Hippoglossoides dubius were studied in Funka Bay. Of the three dominant year-classes that occurred in recent years, the 1995 year-class was small in total length at age ≥ 3 and low in condition factor at age ≥ 2. Ophiuroids (almost Ophiura sarsi ), which were the dominant prey in the 1980s, were of little importance and instead, crustaceans such as mysids, natant decapods and pelagic amphipods, bivalves and fish were important prey items for H. dubius in 2000–2001. In addition, the feeding intensity of H. dubius in 2000–2001 was lower than that in the 1980s. These facts are closely related to a reduction of prey abundance, particularly ophiuroids. It seems that the hypoxia that occurred in the central part of the bay during the summer and autumn of 1995–1997 caused the poor food supply and low growth rate at ages 2–4 of the 1995 year-class.     

Growth and ontogenetic migration of greeneye Chlorophthalmus albatrossis in Tosa Bay, Pacific Coast of South-Western Japan

ABSTRACT:   Growth and ontogenetic migration of greeneye Chlorophthalmus albatrossis were studied in the area from 100–400 m depths in Tosa Bay, from April 1996 to March 1997, using samples collected by an otter trawl net with a 1.3 mm mesh cod end. In the monthly changes of the standard length (SL) composition, four year classes were recognized, although the third- and fourth-year classes were not clearly separated in all the monthly samples. Individuals of 40–50 mm SL settled on the bottom of the 150–200 m depth zone from August to February. By May and June, they grew to 75–80 mm SL modal size, and migrated to 200–300 m depth. In one year after settlement, they grew to 125–130 mm SL, and migrated to 300–350 m depth. They grew to a maximum of 160 mm SL and were not sampled in the winter three years after settlement, suggesting either migration or mortality. Seasonal changes in the gonadosomatic index for combined samples of males and females showed that greeneye did not mature in Tosa Bay. It is suggested that the planktonic larvae are transported from the spawning grounds, the location of which remains unknown.     

Age and growth of Sebastes vulpes in the coastal waters of western Hokkaido, Japan

ABSTRACT:   Age and growth of Sebastes vulpes collected from the coastal waters of western Hokkaido were studied. Observation of the otolith margin verified that annuli (outer margins of the opaque zone) were produced chiefly from July to August. This period was associated with parturition and the birth season. The maximum age estimated by the surface method was 12 years but the oldest fish was aged at 35 years by the cross-section method. The surface method was inadequate for aging of S. vulpes older than 6 years because of the underestimation of age. No significant difference was found in the parameters of the growth equations between both sexes. The von Bertalanffy growth curve combined for both sexes was as follows: SL _t  = 358.6(1 − exp^{−0.156( t +0.820)}), where SL _t is standard length (mm) at age t (after parturition in years). It seems likely that S. vulpes grows slowly and lives longer than previously thought.     

Growth and habitat of spotted halibut Verasper variegatus in the shallow coastal nursery area, Shimabara Peninsula in Ariake Bay, Japan

ABSTRACT:   To determine the growth performances and habitat characteristics of spotted halibut Verasper variegatus , successive samplings were conducted in the south-eastern coastal area of Shimabara Peninsula in Ariake Bay. The push-net samplings were performed at Kamaga beach from May 2003 to April 2004 to determine the horizontal distribution of larvae and juveniles, while the gill net samplings were done in the coastal waters of Tatsuishi beach from July 2003 to July 2004 to collect the larger juveniles and adults. A total of 478 individuals from metamorphosing larvae to 2-year-old fish (15.2–447.0 mm total length [TL]) were sampled. In particular, the 2003-year class fish were successively collected from May 2003 to June 2004 (15.2–350.0 mm TL; n  = 418) and the mean total length in March, June, September, December 2003 and May 2004 was 22.4, 82.5, 172.5, 203.9 and 296.4 mm, respectively. The present study found that metamorphosing larvae migrated into the estuarine tidal flats during March 2003; newly settled juveniles inhabited the shallow intertidal areas; and juveniles had a remarkably high growth in shallow coastal areas. It is suggested that young fish around 300 mm TL migrated from the nursery grounds in Ariake Bay to deeper waters around Tachibana Bay during May–June 2004.     

EMG telemetry studies on upstream migration of chum salmon in the Toyohira river,Hokkaido, Japan

The movements of 28 adult chum salmon, Oncorhynchus keta (Walbaum) tagged with electromyogram (EMG) transmitters were tracked along the Toyohira river, Hokkaido, Japan, in October of 2007 and 2008 to investigate and evaluate the upstream migratory behavior through the protection bed and fishway of ground sills. The approach time of fish that ascended successfully through the protection bed and fishway was shorter than that of unsuccessful fish. The unsuccessful fish were observed to swim in currents with high water velocity and shallow water depth at swimming speeds that exceeded their critical swimming speed (U _crit) during the approach to these structures. In consequence, unsuccessful fish frequently alternated between burst and maximum sustained speeds without ever ascending the fishway, and eventually became exhausted. It is important that fishway are constructed to enable chum salmon to find a passage way easily, so that they can migrate upstream rapidly without wasting excessive energy.     

A spatial linkage between dams and non-native fish species in Hokkaido, Japan

Abstract –  The native and non-native freshwater fish species richness as well as the proportion of native fish species to overall fish species were examined to test a hypothesis of a spatial linkage between the distributions of dams and non-native fishes based on survey data collected in rivers and streams in Hokkaido, Japan. Non-native fish species have steadily increased in Hokkaido since the mid 1970s, as indicated by the decreasing proportion of native fish species per fish survey. Non-native species richness was significantly higher in areas above dams (i.e., reservoirs and their inlet streams) compared to areas without dams. As a result, the proportion of native fish species was lower above dams. This proportion was lower in developed and agricultural areas compared with forest areas, although both native and non-native species richness were higher in areas with development and agriculture. Non-native cyprinids, catfish and snakehead occurred primarily in a particular river basin with the largest drainage area, the longest history of reclamation and highest human population in the island. Non-native salmonids on the other hand, occurred in forest areas at higher elevations and their occurrence was associated with the existence of dams. There was a spatial linkage between the distribution of dams and non-native species, although it was applicable only to salmonids. The management of dams and reservoirs needs a strategy to prevent further expansion of exotic salmonids such as rainbow and brown trout. These species can be a significant threat to native Japanese salmonids as a result of various biological mechanisms.     

Comparison of the nutritional transition date distributions of marbled sole larvae and juveniles in Hakodate Bay, Hokkaido

To examine survival processes in marbled sole Pseudopleuronectes yokohamae larvae, nutritional transition date distributions (NTDDs) were compared between larvae and large juveniles (LJs) in Hakodate Bay from 2001 to 2003. NTDDs were used instead of hatch date distributions because the onset of increment formation coincides with the timing of the transition from endogenous to exogenous nutritional sources for marbled sole. LJs were defined as being large enough to be safe from predation by the sand shrimp Crangon uritai. In 2001 and 2002, the ranges of the NTDDs of LJs largely overlapped with those of the larvae. However, in 2003, the early cohort (those whose otolith rings formed before 17 March) disappeared from the NTDDs during and after April, and the NTDDs of LJs were heavily biased toward a later period compared to those of the larvae. In 2003, Oyashio coastal water (<3°C and ≤33.0 salinity) unexpectedly intruded into the bay in late March and lowered the water temperature from 6.3°C on 6 March to 4.1°C on 25 March. One possible cause of mortality in the early cohort of 2003 may be feeding failure caused by the extremely low water temperature.     

Habitat niche separation of the nonnative rainbow trout and native masu salmon in the Atsuta River,Hokkaido, Japan

The mechanisms by which nonnative species establish populations can be classified into two broad categories: they usurp the niches of native species through interspecific competition, or they avoid this intense interspecific competition by making use of minimal niche overlap with the native species. In this study, we considered how a nonnative salmonid species, the rainbow trout Oncorhynchus mykiss, established a population in the presence of the native salmonid species, the masu salmon O. masou, in Hokkaido, Japan. Circumstantial field evidence shows that the masu salmon exceeds the rainbow trout in abundance and suggests that these species use different types of cover habitat (rainbow trout abundance increases with increasing abundance of large woody debris aggregates, whereas masu salmon abundance increases with increasing abundance of undercut banks). These results imply that the rainbow trout established a population due to minimal niche overlap with the masu salmon, and not by competitive exclusion of the native species.     

Interdecadal decrease in potential fishing areas for Pacific saury off the southeastern coast of Hokkaido,Japan

We examined interdecadal variations in potential fishing grounds for the Pacific saury, Cololabis saira, off the southeastern coast of Hokkaido, Japan, for the early fishing season of August–September. We applied linear trend analysis to a time series of several oceanographic variables for 1993–2014. Trends in the appearance frequency of sea surface temperatures (SST) of 12–18 °C during August–September indicated an interdecadal reduction in the potential fishing area off the Hokkaido coast. There were localized significant increases of both SST and the sea level anomaly along the Kuril–Kamchatka Trench off the Hokkaido coast. The localized trends coincided with decadal decreases of satellite‐based chlorophyll concentrations after 2002. Analysis of mesoscale eddies revealed that the localized trends were related to clockwise (counterclockwise) eddies that appeared more frequently (less frequently) near the trench in recent years. Moreover, the Oyashio transport on the slope between the coast and the trench decreased significantly and completely disappeared in some recent summers. Interactions between the Oyashio and mesoscale eddies resulted in an interdecadal shift in the cold‐water intrusion along the main stream of the Oyashio from the along‐slope direction to the offshore direction in the upstream region of the Hokkaido coast, and large areas of favorable potential fishing grounds near the Hokkaido coast have disappeared in recent years.     

Characteristics of a Pacific herring Clupea pallasii spawning bed off Minedomari, Hokkaido, Japan

ABSTRACT:   The spawning bed selection of herring Clupea pallasii off Minedomari, Atsuta, on the west coast of Hokkaido, Japan, is discussed in relation to the topography of the rocky shore and wave conditions during the spawning season. From 1998 to 2003, herring have spawned their eggs mainly on the leaves of the seagrass Phyllospadix iwatensis Makino in almost the same site on the rocky shore off Minedomari in the Atsuta area. The site is connected to a valley-like feature offshore. Wave conditions were surveyed at Minedomari during the spawning season in 2000 and the wave height was estimated from 1998 to 2003, except for in 2000, using the correlation of wave height between Minedomari and Ishikari Bay New Port, approximately 18 km south-west of Minedomari. Herring spawned under calm conditions, during which the wave height was approximately 0.5 m off Atusta and Aoshima and was 0.18–1.28 m off Minedomari. The distribution of water particle velocity on the sea bottom surface as a result of ocean waves off the Minedomari area, which was estimated based on the wave height and the topography of the coast, suggested herring could swim easily into the shallower area along the valley-like feature off Minedomari. Therefore, topographical features are thought to be one of the reasons why herring have used Minedomari as a spawning bed. Additionally, seepage of freshwater from the bottom, which was observed in this area, could also be the reason why herring spawn in Minedomari repeatedly.     

Variability in timing and magnitude of spring bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan

The spring bloom of phytoplankton is a well-established, regular, seasonal event in the western subarctic Pacific and is considered one of the most important conditions of massive production of pelagic fishes. A series of 12 cruises was conducted from 1990 to 1992 to examine the timing and magnitude of the spring phytoplankton bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. An interannual variability in the bloom events was also analysed. On the basis of hydrographical characteristics, the study area was divided into three water masses: the Oyashio Water Mass, the Mixed Water Mass, and the Coastal Water Mass. Spring blooms were observed first in April in the Oyashio and the Coastal Water Masses, and continued to May in 1991 and 1992. However, no bloom was recorded in the Mixed Water Mass. High nutrient supply into the surface mixed layer during winter is likely to be one of the factors supporting an intense spring bloom in the Oyashio Water Mass. A significant positive relationship between log-transformed surface chlorophyll a concentration and maximum density gradient (MDG) within the euphotic layer was obtained in April, indicating the importance of vertical stability of the water column in the initiation of spring blooms in the Oyashio and the Coastal Water Masses. The spring blooms in 1991 were much more extensive and lasted longer than in 1990. It is suggested that meteorological conditions and abundance of grazers were responsible for this interannual difference.