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1.
Recent findings suggest that recruitment of Japanese anchovy (Engraulis japonicus) and sardine (Sardinops melanostictus) depends on survival during not only the first feeding larval stage in the Japanese coastal waters and the Kuroshio front but also during the post‐larval and juvenile stages in the Kuroshio Extension. Spatial distributions of juvenile anchovy and sardine around the Shatsky Rise area in the Kuroshio Extension region and the Kuroshio–Oyashio transition region are described, based on a field survey in the late spring using a newly developed mid‐water trawl for sampling juveniles. All stages of anchovy from post‐larvae to juveniles were obtained in the northern Shatsky Rise area. The Kuroshio Extension bifurcates west of the Shatsky Rise area and eddies are generated, leading to higher chlorophyll concentrations than in the surrounding regions in April and May. When Japanese anchovy and sardine spawn near the Kuroshio front or the coastal waters south‐east of Japan, their larvae are transported by the Kuroshio Extension and are retained in the Shatsky Rise area, which forms an important offshore nursery ground, especially during periods of high stock abundance.  相似文献   

2.
The availability of food for larvae of the Japanese sardine, Sardinops melanostictus , was investigated in the Kuroshio frontal region and the waters on the offshore side of the Kuroshio, the Pacific coast of central Japan, in March 1990 and 1991, respectively. Food availability was assessed by changes in biomass and production of nauplii and small copepods, and RNA/DNA ratios of the larvae during about 2.5 days (the frontal region) or 3 days (the offshore waters) of tracking a drifter released in a pitch of the larvae. The biomas of the nauplii tended to increase with time in the frontal region and to decrease in the waters on the offshore side of the Kuroshio during the drifter tracking periods. The production of small copepods including nauplii in the waters on the offshore de of the kuroshio was 14% of that in the frontal region. The sum of the mean food requirements of the carnivorous macrozooplankters and sardine larvae was 11% of the production of small copepods including nauplii in the frontal region, compared with 136% in the waters offshore of the Kuroshio. The RNA/DNA ratios of postlarvae smaller than 8 mm in the frontal reqion were significantly higher than those in the waters on the offshore side of the Kuroshio ( P < 0.001) It is considered that the food availability for sardine larvae was relatively high in the frontal region and low in the waters on the offshore side of the Kuroshio. The food availability for the larvae probably deteriorated with the offshore shift of the main spawning ground from the frontal region to the waters on the offshore side of the Kuroshio in the latter half of the 1980s.  相似文献   

3.
Acoustic Doppler current profiler (ADCP) data collected during routine monitoring surveys of the distribution and abundance of Japanese sardine larvae ( Sardinops melanostictus ) off the Pacific coast of Japan in February 1993 and 1994 were used to construct stationary average flowfields for three levels in the upper 100 m in each year. No large-scale meanders in the path of the Kuroshio Current were present in either year, but the axis of the current was closer to the coast in 1993 than in 1994. The flowfields were used to drive a particle-tracking model representing the dispersal of sardine eggs and larvae. Particles were released in accordance with the observed distribution of eggs, and their positions tracked for up to 40 days. In 1993, the model indicated that ≈ 50% of the egg production was carried north-eastwards out of the survey area into the area of the NW Pacific referred to as the Kuroshio Extension Zone. In contrast, only 5% of the egg production was exported to the Extension Zone in 1994, the remainder being retained in Japanese coastal waters. The consequences of the different dispersal patterns are discussed in relation to subsequent recruitment to the sardine stock. Based on commercial catch data, survival of the 1993 year class was 15% of that for the 1994 class. Hence, the results indicate that export of larvae to the Kuroshio Extension cannot in itself lead to successful recruitment.  相似文献   

4.
In the mid 1970s, the fishery catch of postlarval Japanese anchovy (Engraulis japonica) in a shelf region of the Enshu‐nada Sea, off the central Pacific coast of Japan, started to decline corresponding to a rapid increase of postlarval sardine (Sardinops melanostictus). In late 1980s, sardine started to decline, and it was replaced by anchovy in the 1990s. This alternating dominance of postlarval sardine and anchovy corresponded to the alternation in egg abundance of these two species in the spawning habitat of this sea. It was also noteworthy that during the period of sardine decline, sardine spawning occurred in April–May, a delay of two months compared with spawning in the late 1970s. The implication of oceanographic changes in the spawning habitat for the alternating dominance of sardine and anchovy eggs was explored using time‐series data obtained in 1975–1998, focusing on the effect of the Kuroshio meander. Large meanders of the Kuroshio may have enhanced the onshore intrusion of the warm water into the shelf region and contributed to an increase in temperature in the spawning habitat. This might favour sardine, because its egg abundance in the shelf region was more dependent on the temperature in early spring than was that of anchovy. In addition, enhanced onshore intrusion could contribute to transport of sardine larvae from upstream spawning grounds of the Kuroshio region. On the other hand, anchovy egg abundance was more closely related to lower transparency at the shelf edge, which may indicate the prevalence and prolonged residence of the coastal water, and therefore higher food availability, frequently accompanying non‐meandering Kuroshio. The expansion/shrinkage of the spawning habitat of sardine and anchovy in the shelf region, apparently responding to the change in the Kuroshio, possibly makes the alternation in dominance of postlarval sardine and anchovy most prominent in the Enshu‐nada Sea, in combination with changes in the abundance of spawning adults, which occurred almost simultaneously in the overall Kuroshio region. The implication of this rather regional feature for the alternating dominance of sardine and anchovy populations on a larger spatial scale is also discussed.  相似文献   

5.
We have numerically modeled the advection and diffusion of sardine eggs and larvae to investigate the larval transport processes of Japanese sardine from the spawning grounds by the Kuroshio.
The results indicated that the offshore drift current induced by the winter monsoon and the location of the spawning ground have significant effects on the survival of the Japanese sardine. The contribution of the drift current, the distance of the spawning ground from the Kuroshio axis, and the eddy diffusivity to the larval retention in the coastal area is approximately expressed by the following equation: where R is the retention rate in the coastal area, a the variance of initial distribution of eggs, T the time after the eggs were spawned, – V0 the velocity of the wind-induced offshore current, y0 the distance of the center of the spawning area from the Kuroshio axis, and K the coefficient of horizontal eddy diffusivity.
The year-to-year variation in larval survival rates stimulated by the two-dimensional model are consistent with those estimated previously by using field data of egg and larval abundance during 1978–1988.  相似文献   

6.
We examined the effects of climate factors on interannual variations of copepod biomass and gut content composition in early spring in the Kuroshio and the slope water off the Pacific coast of central Japan from 1971 to 1989. The biomass trends were different for large (prosomal length ≥ 1 mm) and small (prosomal length < 1 mm) copepods in both waters. Peaks in biomass of large copepods decreased in magnitude, and the biomass of small copepods was low around 1980. For the large copepods in the Kuroshio, 3-year running mean biomass was related to the Kuroshio meander index. The yearly mean biomass was related to diatom abundance in the gut which, in turn, was related to wind speed and temperature. The 3-year running mean biomass of large copepods in the slope water was positively related to solar radiation in March. The biomass of small copepods in both waters was negatively related to solar radiation in February, and years with high biomass of small copepods corresponded with not only the years with high abundance of larger foods (diatoms and micro-sized foods) in copepod guts, but also with the years with high abundance of the nano-sized foods. We hypothesize that nutrient supply to upper layers regulates interannual variation of biomass of large copepods in the Kuroshio. Thus, climate influences both size composition and biomass of copepods in and near the Kuroshio in early spring.  相似文献   

7.
The occurrence and density of Pacific saury Cololabis saira larvae and juveniles were examined in relation to environmental factors during the winter spawning season in the Kuroshio Current system, based on samples from extensive surveys off the Pacific coast of Japan in 2003–2012. Dense distributions of larvae and juveniles were observed in areas around and on the offshore side of the Kuroshio axis except during a large Kuroshio meander year (2005). The relationships of larval and juvenile occurrence and density given the occurrence to sea surface temperature (SST), salinity (SSS), and chlorophyll‐a concentration (CHL) were examined by generalized additive models for 10‐mm size classes up to 40 mm. In general, the optimal SST for larval and juvenile occurrence and density given the occurrence was consistently observed at 19–20°C. The patterns were more complex for SSS, but a peak in occurrence was observed at 34.75–34.80. In contrast, there were negative relationships of occurrence and density given the occurrence to CHL. These patterns tended to be consistent among different size classes, although the patterns differed for the smallest size class depending on environmental factors. Synthetically, the window for spawning and larval and juvenile occurrence and density seems to be largely determined by physical factors, in particular temperature. The environmental conditions which larvae and juveniles encounter would be maintained while they are transported. The survival success under the physically favorable but food‐poor conditions of the Kuroshio Current system could be key to their recruitment success.  相似文献   

8.
Off southern‐central Chile, the impact of spring upwelling variability on common sardine (Strangomera bentincki) recruitment was examined by analyzing satellite and coastal station winds, satellite chlorophyll, and common sardine recruitment from a stock assessment model. In austral spring, the intensity of wind‐driven upwelling is related to sea surface temperature (SST) from the Niño 3.4 region, being weak during warm periods (El Niño) and strong during cold periods (La Niña). Interannual changes in both spring upwelling intensity and SST from the Niño 3.4 region are related to changes in remotely sensed chlorophyll over the continental shelf. In turn, year‐to‐year changes in coastal chlorophyll are tightly coupled to common sardine recruitment. We propose that, in the period 1991–2004, interannual changes in the intensity of spring upwelling affected the abundance and availability of planktonic food for common sardine, and consequently determined pre‐recruit survival and recruitment strength. However, the importance of density‐dependent factors on the reproductive dynamic cannot be neglected, as a negative association exists between spawning biomass and recruitment‐per‐spawning biomass. Coastal chlorophyll, upwelling intensity, and SST anomalies from the Niño 3.4 region could potentially help to predict common sardine recruitment scenarios under strong spring upwelling and El Niño Southern Oscillation (ENSO)‐related anomalies.  相似文献   

9.
The sustainable use of marine resources requires understanding the surrounding ecosystem and elucidating mechanisms of variation. However, we still lack a comprehensive understanding of environmental variation in the spawning and nursery grounds of important fisheries species Japanese sardine (Sardinops melanostictus) and mackerels (Scomber japonicus and Scomber australasicus) in the northwest Pacific. Here, we investigate detailed physical, chemical, and biological environment variations in the spawning and nursery grounds along the Kuroshio and Kuroshio Extension area from intensive investigation in spawning season (April) of 2013. We found similar water mass property and copepod community in the egg‐rich Kuroshio area and the larvae‐rich downstream Kuroshio Extension area, indicating environmental variability is small during transportation and development processes. The egg‐rich northern Izu Islands region showed high copepod abundance, although low nutrient and chlorophyll concentrations were observed. Eggs were scarce or absent in the second survey 10 days after abundant eggs were observed in the region, along with differences in water property and copepod community. This indicates that not only the location but also the specific water characteristic and copepod community are a determining factor for spawning. Indicator communities of copepod found in our study (indicator community of transportation process from spawning ground, of non‐spawning ground, and of reproductive area in the Kuroshio Extension area) would be a key factor for recruitment prediction.  相似文献   

10.
Particle‐tracking experiments were performed to infer the distribution of larvae of the Japanese sardine (Sardinops melanostictus) and to detect effects of transport environment on sardine recruitment, using the output of a high‐resolution ocean general circulation model and observed data of sardine spawning grounds during 1978–2004. By the 60th day following spawning, approximately 50% of the larvae had been transported to the Kuroshio Extension (KE). Whereas the spawning period and grounds changed markedly in relation to the stock level, the proportion of larvae transported to the KE remained relatively constant and no significant correlations were found between sardine recruitment and the transport proportion. Instead, the recruitment was found to be correlated with physical parameters including the mixed layer depth and the sea surface temperature along several major transport trajectories of sardine larvae. The correlations were most significant for the trajectories in the region 0.5° south to 1° north of the Kuroshio axis (defined as the location of velocity maxima at each longitude) and for larvae spawned in February and March during the high stock period (1978–94), and for larvae spawned in March and April during the low stock period (1995–2004).  相似文献   

11.
Naupliar copepods were distributed at similar concentrations over the waters inshore and offshore of the Kuroshio Current off central Japan in early spring 1993 and 1994, overlapping with the distribution of early feeding larvae of Sardinops melanostictus . Although N, P, Si and chlorophyll a concentrations were higher in the waters inshore of the Kuroshio axis than in the offshore waters, the mean concentrations of nauplii were not statistically different between the two waters. Food availability for larval S. melanostictus did not seem to be different between the two waters in terms of the mean food concentrations. Using the critical food concentration (>9 nauplii L–1) for 25% survival during 3 days after first feeding derived from a rearing experiment, percentages in number of stations or water samples with nauplii concentrations >9 nauplii L–1 were higher in the inshore waters than in the offshore waters in both years examined. Considering that fish larvae may depend on small-scale patchiness of food for their survival, the inshore waters seemed to be more favourable for first-feeding larvae than the offshore waters.  相似文献   

12.
Despite the low productivity that has been thought to characterize plankton communities in the western boundary current of the North Pacific Subtropical Gyre, many migratory fishes risk encountering low food availability during crucial life history stages by reproducing and recruiting in the Kuroshio region (i.e., the Kuroshio Paradox). Here, we report on geographic variability in taxonomic composition, biomass, and productivity of the mesozooplankton community in the Kuroshio Current and neighboring waters in the East China Sea. Calanoid copepods were the most abundant mesozooplankton taxon throughout our sampling stations. Small copepods, which include nauplii and poecilostomatoids, and gelatinous metazoans were the next most abundant. Seasonal variability in mesozooplankton standing stock (i.e., abundance and biomass) and productivity (i.e., production rate and protein synthetase activity) exceeded spatial variability across the stations and regions. The mesozooplankton community was characterized by high biomass and production rates in the summer, as well as high abundance and protein synthetase activity in the fall. No significant differences were found for mesozooplankton standing stock and productivity in the Kuroshio Path relative to those on the continental shelf or on the outside of the Kuroshio Path. Our results indicate that the standing stock and productivity of the mesozooplankton community in the Kuroshio Path are equivalent to those on the continental shelf, and that these communities are supported by small copepods and gelatinous zooplankton. We suggest that the mesozooplankton standing stocks and productivity provide adequate food availability for migratory fishes in the Kuroshio and neighboring waters in the East China Sea.  相似文献   

13.
Many migratory fishes reproduce and recruit around the Kuroshio, and their survival of early life stages is supported by mesozooplankton. Mesozooplankton standing stocks and productivity equivalent to those on the continental shelf have been found in the Kuroshio; however, there is limited information on trophic sources and linkages to support the mesozooplankton community. Here, we evaluate mesozooplankton feeding on protists and their trophodynamics importance by removal bottle experiments in the Kuroshio of the East China (ECS‐Kuroshio). Pico‐ and nano‐autotrophs dominated chlorophyll a concentrations throughout the study sites across the continental shelf, within the Kuroshio path and in adjacent waters. Calanoid and poecilostomatoid copepods comprised more than 85% of mesozooplankton biomass. Significant mesozooplankton ingestion rates were found for nano‐autotrophs based on size‐fractionated chlorophyll; for haptophytes, chrysophytes, chlorophytes, and diatoms from pigment‐based phytoplankton analysis; and for naked ciliates in the microzooplankton. Based on the estimates of individual carbon budgets, nano‐autotrophs and naked ciliates ingested by mesozooplankton composed 39% of their food requirements, suggesting other available prey like nano‐heterotrophs. These results imply that mesozooplankton ingestion in the ECS‐Kuroshio has great impacts on protozoan and phytoplankton communities and their major trophic pathways are from nano‐sized auto‐ and heterotrophs and ciliates to copepods.  相似文献   

14.
Environmental variability and growth‐rate histories from hatching to capture were investigated for larval Japanese sardine (Sardinops melanostictus) and Japanese anchovy (Engraulis japonicus). Larvae collected around the front of the Kuroshio Current were examined using otolith microstructure analysis, and their movement was estimated from numerical particle‐tracking experiments. Sardine larvae collected inshore of the Kuroshio front originated from a coastal area near the sampling site, while those collected in the offshore area originated from an area 500–800 km west‐southwest of the sampling site. Anchovy larvae collected both inshore and offshore had been transported from widely distributed spawning areas located west of the sampling area. At the age of 13–14 days for sardine and 19–20 days for anchovy, the offshore group exhibited significantly higher mean growth rates than did the inshore group. Although the offshore area was generally warmer than the inshore area, temporal variations in growth rate are not attributable solely to fluctuations in environmental temperature. While previous studies have examined the relationship between larval growth rates and environment based solely on data at capture, the methods used in the present study, combining otolith analysis and numerical particle‐tracking experiments, utilize data up until hatching. Although the relationship between growth rate and environment was not fully confirmed, this approach will greatly advance our understanding of fish population dynamics.  相似文献   

15.
Gonosomatic indices and egg and larval densities observed from 1986 to 2001 suggest that the peak spawning season of the Australian anchovy (Engraulis australis) in South Australia occurs during January to March (summer and autumn). This coincides with the spawning season of sardine (Sardinops sagax) and the period when productivity in shelf waters is enhanced by upwelling. Anchovy eggs were abundant throughout gulf and shelf waters, but the highest densities occurred in the northern parts of Spencer Gulf and Gulf St Vincent where sea surface temperatures (SST) were 24–26°C. In contrast, larvae >10 mm total length (TL) were found mainly in shelf waters near upwelling zones where SSTs were relatively low (<20°C) and levels of chlorophyll a (chl a) relatively high. Larvae >15 mm TL were collected only from shelf waters near upwelling zones. The high levels of larval abundance in the upwelling zones may reflect higher levels of recruitment to later stages in these areas compared with the gulfs. The sardine spawns mainly in shelf waters; few eggs and no larvae were collected from the northern gulfs. The abundance of anchovy eggs and larvae in shelf waters increased when sardine abundance was reduced by large‐scale mortality events, and decreased as the sardine numbers subsequently recovered. We hypothesize that the upwelling zones provide optimal conditions for the survival of larval anchovy in South Australia, but that anchovy can only utilize these zones effectively when the sardine population is low. At other times, northern gulf waters of South Australia may provide a refuge for the anchovy that the sardine cannot utilize.  相似文献   

16.
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.  相似文献   

17.
ABSTRACT:   Transport and survival of larvae of pelagic fishes in the Kuroshio system region were studied using Lagrangian drifter data recorded from 1990 to 2003. A large portion of the drifters from the Kuroshio area south of Japan spread around the Kuroshio Extension up to 170°E, while some moved south to the offshore area of the Kuroshio because of a recirculation gyre. The monthly mean eastward movement from areas south of Japan was approximately 800–900 km, which was smaller than previous numerical estimates. The results of a survival model assuming optimal temperatures for larvae suggest that surface waters during the observation period were too warm for larval Japanese sardine, which has an optimal temperature of 16°C, and the adult abundances did not increase during the observation period. In contrast, the spawning ground temperatures and transport conditions from an area south-west of Japan in April–June matched the requirement for the larval Japanese anchovy, which has an optimal temperature of 22°C. The combined effects of temperature variations due to seasonality and water mass mixing are suggested to play an important role in determining the environmental temperatures that occur in an area.  相似文献   

18.
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.  相似文献   

19.
We examined the distribution of sardine larvae relative to environmental conditions with the purpose of identifying and characterizing habitat that encourages high larval growth and survival, based on the 1983–1998 surveys of the California Cooperative Oceanic Fisheries Investigations (CalCOFI). Long-term averages show that sardine 'survivors' (spatially aggregated larvae ≥ 18 days old) were most abundant offshore, whereas sardine egg density, chlorophyll biomass and zooplankton volume were greatest inshore. In contrast, mesoscale eddies, observed in remotely sensed sea surface temperature imagery, were found only in offshore regions. To further examine the link between eddies – which often result in locally elevated chlorophyll and zooplankton – and sardine survival, we compared the distribution of larvae and eddies survey by survey. Sardine survivors were most abundant offshore in only one-quarter of the research surveys, and when they were most abundant offshore they were associated with eddies. This indicates that the offshore eddy habitat produced exceptionally large numbers of survivors, as evidenced by the disproportionate effect on the long-term average.  相似文献   

20.
Growth variability was examined for Pacific saury Cololabis saira larvae under contrasting environments across the Kuroshio axis, based on samples collected during the winter spawning season in 2013 and 2014. The growth rate index (residual of the otolith marginal 3‐day mean increment width from the linear regression on knob length) of larvae was compared among three areas: the inshore side of the Kuroshio axis, the Kuroshio axis, and the offshore side of the Kuroshio axis in relation to sea surface temperature (SST), salinity (SSS) and chlorophyll‐a (CHL) concentration. The larvae were more densely distributed in the Kuroshio axis and offshore areas of higher temperature and salinity and lower chlorophyll‐a concentration than in the inshore areas of lower temperature and salinity and higher chlorophyll‐a concentration. No marked differences in the growth rate index were found among the three areas, even though the larvae in the inshore areas showed slightly higher growth rates in 2013. Despite the broad ranges of environmental factors, no clear relationship between the growth rate index and any environmental factor was detected. The survival potential of Pacific saury larvae was considered to be at least comparable under contrasting environments across the Kuroshio axis. Such a geographical homogeneity is concluded to be attributable to compensable effects of physical and biological factors. We hypothesize that the minority under physically‐unfavorable but biologically‐favorable conditions on the inshore side of the Kuroshio axis could survive equally well as the majority under physically‐favorable but biologically‐unfavorable conditions around the Kuroshio axis and on the offshore side of the Kuroshio axis.  相似文献   

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