Japanese sardine (Sardinops melanostictus) mortality in relation to the winter mixed layer depth in the Kuroshio Extension region

A drastic population change in Japanese sardine (Sardinops melanostictus) has been noted as being related to winter sea surface temperature (SST) in the Kuroshio Extension region. The former studies suggest two possible explanations. One is that temperature itself affects sardine. The other is that SST represents the environmental change of the Kuroshio Extension region and other causes directly affecting sardine. In this study, we found that sardine mortality from post‐larva to age 1 negatively correlated with the winter mixed layer depth (MLD) in the Kuroshio Extension region from 1979 to 1993. During the period of a deep winter mixed layer (during the early 1980s), sardine mortality was low, whereas mortality was high when the winter mixed layer was shallow (during the late 1980s to early 1990s). By using a lower trophic‐level ecosystem model forced by the observed time series of MLD, SST, light intensity and nutrient data, we found that the estimated spring zooplankton density drastically varies from year to year and has a significant negative correlation with sardine mortality. The inter‐annual variation of spring zooplankton density is caused by the winter MLD variation. During the deep winter mixed layer years, a phytoplankton bloom occurs in spring, whereas during the shallow winter mixed layer years, the bloom occurs in winter. The results of our study suggest that the decline in the Japanese sardine population during the late 1980s to early 1990s was due to an insufficient spring food supply in the Kuroshio Extension region where sardine larvae and juvenile are transported.     

TGF-β and TNF-α stimulate MMP-9 production in murine epidermal keratinocytes

Matrix metalloproteinases 2 and 9 (MMP‐2 and ‐9) are zinc‐dependent metalloenzymes and have gelatin‐degrading activity. Both MMP are known to be secreted by many types of cells and play important roles in several biological changes including tissue remodeling and wound healing. In the present study, a primary culture of murine epidermal keratinocytes was prepared and effects of transforming growth factor‐β (TGF‐β), tumor necrosis factor‐α (TNF‐α) and interferon‐γ (IFN‐γ) on expression of MMP‐2 and MMP‐9 by the keratinocytes was examined. Gelatin zymography revealed that murine epidermal keratinocytes secreted proenzyme forms of MMP‐2 and MMP‐9, but the active forms of both MMP were hardly detectable, indicating that in vitro autoactivation of these proenzymes did not occur. Both TGF‐β and TNF‐α stimulated MMP‐9 production in a dose‐dependent manner, but the MMP‐2 level was not changed. Interferon‐γ hardly affected production of MMP‐2 or MMP‐9. Ribonuclease protection assay demonstrated that TNF‐α increased the level of MMP‐9 mRNA 6‐fold compared to the control, whereas TGF‐β slightly up‐regulated it. These results suggest that expression of MMP‐9 could be regulated by several cytokines in murine epidermal keratinocytes.     

On the relationship between the Oyashio front and saury fishing grounds in the north-western Pacific: a forecasting method for fishing ground locations

The relationships between fishing ground locations of Pacific saury (Cololabis saira) and the two Oyashio fronts, the offshore front between 146^oE and 155^oE and the coastal Oyashio intrusion, before the saury fishing season, were examined using data from 1971 to 1991. Interannual geographical shifts of both the offshore Oyashio front and saury fishing grounds had a dominant interdecadal fluctuation. In years when the offshore front shifted north (south), the fishing grounds were formed relatively nearshore (offshore). When the offshore front shifted north, the fishing grounds were formed further nearshore in years the coastal Oyashio intrusion extended south. When the offshore front shifted south, on the other hand, the southward extension of the coastal intrusion did not necessarily cause formation of coastal fishing grounds. These results showed that locations of the fishing grounds depend not only on local and instantaneous oceanographic environments around the fishing grounds, but also on oceanographic conditions over an extensive range of the Oyashio area. This might indicate that the ecology of the saury's northward migration, through mixed water regions between the Kuroshio and Oyashio fronts in spring and summer, has a close relation to the shift of the offshore Oyashio front over a time range of months. A practical forecasting method for locations of saury fishing grounds is proposed based on the oceanography before the fishing season.     

Variation in life history parameters of the cardinalfish Apogonlineatus

ABSTRACT:   Life history parameters (growth pattern, the length of the spawningseason and age at sexual maturity) of Apogon lineatus wereinvestigated from three localities in Japan. The maximum ages werefound to be 3 years in Tokyo Bay, 2 years in Osaka Bay and 5 yearsoff Niigata Prefecture. Both sexes from Tokyo Bay grew to largersizes between the ages of 1 and 2 years than both sexes from otherlocalities, food availability being suggested as the major factorfor differences in growth. Spawning seasons were during 4 months(July–October) in Tokyo Bay and Osaka Bay, but were foundto only occur during the two months of July and August off NiigataPrefecture. Factors that could limit the spawning season off NiigataPrefecture were not determined. Age at maturity for both sexes inall localities appeared to be 1 year. We could not elucidate whetherthese variations in life history parameters arose from genetic differencesor from phenotypic plasticity. In either case, the trade-off betweenthe maximum age and the length of the spawning season in the offNiigata Prefecture population and any other population may be interpretedas an adaptation to local conditions, such as with the off NiigataPrefecture population.     

Acoustic identification of isada krill, Euphausia pacifica Hansen, off the Sanriku coast, north-eastern Japan

In this study, the feasibility of identification and abundance estimation of isada krill, Euphausia pacifica , was examined using a two-frequency (38 and 120 kHz) method. The acoustic survey and midwater trawling were conducted off the Sanriku coast, northern part of Honshu, Japan. In one pattern, the SV difference between the two frequencies was large, and in the other, the SV difference was small. These results were interpreted by scattering models, and showed that discrimination between isada krill and walleye pollock was possible.     

Impact of winter‐to‐spring environmental variability along the Kuroshio jet on the recruitment of Japanese sardine (Sardinops melanostictus)

Winter‐to‐spring variability in sea surface temperature (SST) and mixed layer depth (MLD) around the Kuroshio current system and its relationship to the survival rate (ln [recruit per spawning stock biomass], LNRPS) of Japanese sardine (Sardinops melanostictus) were investigated based on a correlation analysis of data from 1980 to 1995. The data were from a high‐resolution ocean general circulation model using the ‘Kuroshio axis coordinates’, in which the meridional positions are relocated to a latitude relative to the Kuroshio axis at each longitude, rather than the geographically fixed coordinates. A significant positive (negative) correlation between LNRPS and winter MLD (winter–spring SST) was detected near the Kuroshio axis from areas south of Japan (where eggs are spawned) to the Kuroshio Extension (where larvae are transported). This result is in contrast to previous studies using geographically fixed coordinates, which showed a significant correlation predominantly in the area south of the Kuroshio Extension in winter, where at this time few larvae have been found. From the late 1980s to early 1990s, when the survival rate was remarkably low, MLD around the axis was shallow and SST was high. Although MLD and SST show a significant correlation, significant partial correlations were also observed between February MLD and LNRPS when the contribution of SST was excluded, and between March SST and LNRPS when the contribution of MLD was excluded. We presume that MLD shoaling reduced the nutrient supply from deep layers, resulting in less productivity in the spring, and SST warming could have a negative influence on larval growth.     

Transport and environmental temperature variability of eggs and larvae of the Japanese anchovy (Engraulis japonicus) and Japanese sardine (Sardinops melanostictus) in the western North Pacific estimated via numerical particle-tracking experiments

Numerical particle-tracking experiments were performed to investigate the transport and variability in environmental temperature experienced by eggs and larvae of Pacific stocks of the Japanese anchovy ( Engraulis japonicus ) and Japanese sardine ( Sardinops melanostictus ) using high-resolution outputs of the Ocean General Circulation Model for the Earth Simulator (OFES) and the observed distributions of eggs collected from 1978 to 2004. The modeled anchovy individuals tend to be trapped in coastal waters or transported to the Kuroshio–Oyashio transition region. In contrast, a large proportion of the sardines are transported to the Kuroshio Extension. The egg density-weighted mean environmental temperature until day 30 of the experiment was 20–24°C for the anchovy and 17–20°C for the sardine, which can be explained by spawning areas and seasons, and interannual oceanic variability. Regression analyses revealed that the contribution of environmental temperature to the logarithm of recruitment per spawning (expected to have a negative relationship with the mean mortality coefficient) was significant for both the anchovy and sardine, especially until day 30, which can be regarded as the initial stages of their life cycles. The relationship was quadratic for the anchovy, with an optimal temperature of 21–22°C, and linear for the sardine, with a negative coefficient. Differences in habitat areas and temperature responses between the sardine and anchovy are suggested to be important factors in controlling the dramatic out-of-phase fluctuations of these species.     

Transport of subarctic large copepods from the Oyashio area to the mixed water region by the coastal Oyashio intrusion

The lateral transport of organic carbon in large grazing copepods ( Neocalanus cristatus, Neocalanus flemingeri, Neocalanus plumchrus and Eucalanus bungii ) from the Oyashio area to the mixed water region (MWR) by the coastal Oyashio intrusion was estimated using the data of VMPS (vertical multiple plankton sampler) and 1500 dbar-referred geostrophic transport from the CTD (conductivity temperature depth sensor) data of five cruises during June 2001 to April 2002 on a repeat observation section OICE (Oyashio Intensive observation line off Cape Erimo), which extends southeastward from Hokkaido Island, Japan. The transport to MWR by the coastal Oyashio intrusion was estimated to be 5.3 × 10¹¹ g C for the four species. Data from profiling floats also indicated that the copepods were advected from OICE to MWR by the coastal Oyashio intrusion within about 2 months. This transport is considered to be one of the significant sources of organic carbon in MWR as it is larger than the amount of large zooplankton consumed by Pacific saury ( Cololabis saira ) in MWR, one of the dominant copepod predators in this region.     

Environmental determinants of growth rates for larval Japanese anchovy Engraulis japonicus in different waters

Do disparate mechanisms determine growth rates of fish larvae in the different regions? The relationship between growth rates and environmental factors (sea temperature and food availability) was examined for larval Japanese anchovy Engraulis japonicus in geographically and environmentally different waters, through sagittal otolith microstructure analysis. Recent 3‐day mean growth rates directly before capture were positively related with sea‐surface temperature (SST) but not with food availability (plankton density) for the larvae in the Kuroshio Extension and Kuroshio–Oyashio transition regions of the western North Pacific. On the contrary, variations in recent growth rates were attributed to food availability (plankton density) as well as SST for the larvae in the East China Sea. In the shirasu fishing ground in Sagami Bay, larval growth rates were variable under the influences of both SST and food availability (feeding incidence). On the surface, the growth–environment relationships seemed to differ among regions. However, a definite general pattern of the dome‐shaped relationship between recent growth rates and SST was observed when all the regions were combined. Growth rates were similar even among clearly different regions if at the same SST. Overall, growth rates roughly increased with SST until they reached the maximum at SST of 21–22°C (i.e. optimal growth temperature), and declined when SST went over 21–22°C. On the contrary, no clear relationship was observed between growth rate and plankton density or between SST and plankton density. Therefore, the apparent among‐region differences would be firstly caused by the differences in regional SST range. The systematic mechanism of growth determination for widespread pelagic fish species larvae would be run by primarily sea temperature and secondarily food availability, at the species level.     

Acoustic observations of fish schools and scattering layers in a Kuroshio warm-core ring and its environs

Biological sound scattering characteristics in a frontal region between warm and cold waters were examined at the southern edge of a Kuroshio warm-core ring off Sanriku, the northeast coast of Japan. The scattering strength was low in the surrounding cold water and high in the warm-core ring. The depth and thickness of sound-scattering layers changed in accordance with the vertical structure of the water mass during the day or at night. Intense, thick scattering layers were observed on the warm-water side of the front and farther inside the warm-core ring. Fish schools were distributed in the mixed layer warmer than 17°C at depths of 10-30m above the scattering layers on the warm-water side of the front.