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.     

Chlorophyll a variation in the Kuroshio Extension revealed with a mixed-layer tracking float: implication on the long-term change of Pacific saury (Cololabis saira)

Variation of chlorophyll a from March 2004 to July 2005 in the formation region of Subtropical Mode Water in the Kuroshio Extension was observed with a mixed‐layer tracking profiling float parking at around 40 m depth. Chlorophyll a concentration in the mixed‐layer is seasonally high from winter to early spring (January–April) even in the deep mixed layer; whereas during winter it tends to be lower for the corresponding deeper mixed layer. The chlorophyll a integrated over the winter mixed‐layer depth (MLD) increases with MLD for MLD <200 m and for MLD > 250 m the integrated chlorophyll a almost disappears, probably because of the large MLD exceeds in the critical depth with light limitation. These results suggest that the wintertime MLD influences the recruitment of Pacific saury (Cololabis saira) in the Kuroshio Extension because the wintertime food environment for larvae and juvenile of the Pacific saury could be better in the shallow wintertime MLD period of the 1950s and 1990s and worse in the deep MLD period of the 1970s and early 1980s.     

Empirical biomass model for the Japanese sardine, Sardinops melanostictus, with sea surface temperature in the Kuroshio Extension

An Empirical Biomass Model for the Japanese sardine, Sardinops melanostictus, was developed on the basis of the relationship between February sea surface temperature (SST) in the Kuroshio Extension (30–35°N, 145–180°E) and the mortality coefficient during the period from egg to age 1, observed in 1979–94, to examine the long‐term variation of biomass. The periods of the good and bad catch, the year of the biomass peak, and the speed of the biomass decline in the period from 1957 to 1994 were successfully reproduced, except for the biomass increase in the early 1970s. When the model also included with a density‐dependent effect, the whole history of the observed catch during 1957–94 was almost perfectly reproduced. These results suggest that the environment in the Kuroshio Extension region, represented by winter–spring SST, is regarded as a leading factor for determining fluctuations of the sardine biomass in the long term, and that the density effect has a secondary contribution.     

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.     

Importance of the Shatsky Rise Area in the Kuroshio Extension as an offshore nursery ground for Japanese anchovy (Engraulis japonicus) and sardine (Sardinops melanostictus)

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.     

Alternating dominance of postlarval sardine and anchovy caught by coastal fishery in relation to the Kuroshio meander in the Enshu-nada Sea

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.     

Relationship between recruitment of Japanese sardine (Sardinops melanostictus) and environment of larval habitat in the low‐stock period (1995–2010)

Stock level of Japanese sardine (Sardinops melanostictus) was high from 1980s to early 1990s and low from late 1990s to 2000s. The warm and cold water masses in the vicinity of the Kuroshio axis from winter to early spring used to be critical for the recruitment in the high‐stock period, because most of the larvae were distributed there. However, the environmental fluctuation might not affect the recruitment in the low‐stock period. Some studies reported that spawning location and spawning season, and hence the larval habitat, differ depending on the stock level. Three points were investigated in this study: (a) how spawning location and spawning season shifted from the late 1990s, (b) confirmation of the distribution area of larvae in the recent low‐stock period and (c) whether the water temperature in the vicinity of the Kuroshio axis was still related to the recruitment in the low‐stock period. The spawning location and spawning season clearly changed after 1995. Consequently, particle tracking experiments suggested that the larvae appeared in the vicinity of the Kuroshio axis from winter to early spring decreased. Nevertheless, only the ambient temperature of larvae that appeared in the vicinity of the Kuroshio axis from winter had a significant negative correlation with an index of the recruitment in the low‐stock period. It is suggested that the warm and cold masses in the vicinity of the Kuroshio axis are critical for the recruitment regardless of the stock level.     

Wind‐induced stock variation of the neon flying squid (Ommastrephes bartramii) winter–spring cohort in the subtropical North Pacific Ocean

Our examination of the neon flying squid (Ommastrephes bartramii) winter–spring cohort catch per unit effort (CPUE, an index of stock) revealed significant positive correlations with the interannual variations of observed chlorophyll‐a (Chl‐a) concentration and autumn–winter mixed layer depth (MLD) in the winter–spring feeding grounds of paralarvae and juveniles (130–170°E, 20–27°N). These correlations suggest the importance of integrated bottom‐up effects by the autumn–winter MLD for the neon flying squid stocks. However, the influence of autumn–winter MLD interannual variation in the forage availability for paralarvae and juveniles, i.e., particulate organic matter and zooplankton, has still been unclear. In this study, we use the lower trophic ecosystem model NEMURO, which uses the physical environmental data from the ocean reanalysis dataset obtained by the four‐dimensional variational (4DVAR) data assimilation method. The model‐based investigation enables us to clarify how the autumn–winter MLD controls the particulate organic matter and zooplankton abundance in the feeding grounds. Further, our investigation of the autumn–winter MLD interannual variation demonstrates that the stronger autumn wind in the feeding grounds develops a deeper mixed layer. Therefore, the deep mixed layer entrains nutrient‐rich water and enhances photosynthesis, which results in good feeding conditions for paralarvae and juveniles. Our results underline that the wind system interannual variation has critical roles on the winter–spring cohort of the neon flying squid stock.     

Transport and survival of Japanese sardine (Sardinops melanostictus) eggs and larvae via particle‐tracking experiments

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).     

A modeling approach to evaluate growth and movement for recruitment success of Japanese sardine (Sardinops melanostictus) in the western Pacific

A two‐dimensional individual‐based fish movement model coupled with fish bioenergetics was developed to simulate the observed migration and growth of Japanese sardine (Sardinops melanostictus) in the western North Pacific. In the model, derived from the observed ocean–environmental data as the driving force, fish movement was adapted as a kinesis behavior. The model successfully simulated the observed transport patterns during the egg and larval stages and the northward migrations during the juvenile stage in 2005, 2006 and 2007. The model results showed that both temperature during the larval stage in the Kuroshio Extension and the prey availability during the early juvenile stage in the Kuroshio–Oyashio transitional area are important factors for growth of Japanese sardine. In autumn, the observed juvenile sardine were mainly distributed in the subarctic water region off the Kuril Islands, which is an area (158–165°E, 43–47°N) with a high chlorophyll‐a (Chl‐a) concentration. The model reproduced the fish distribution, which has a high density in this region. The high Chl‐a concentration area in autumn may contribute to increasing the survival rate of Japanese sardine by cascading up the food chain, from the high primary production, and is an important habitat for recruitment success of Japanese sardine.     

Anomalous conditions in the south-eastern Bering Sea, 1997: nutrients, phytoplankton and zooplankton

Anomalies in the regional weather over the south-eastern Bering Sea during spring and summer of 1997 resulted in significant differences in nutrient availability, phytoplankton species composition, and zooplankton abundance over the continental shelf as compared with measurements in the 1980s. Calm winds and the reduction of cloud cover in spring and summer produced a very shallow mixed layer in which nitrate and silicate were depleted after an April diatom bloom. High submarine light levels allowed subsequent phytoplankton growth below the pycnocline and eventual depletion of nitrate from the water column to depths of 70 m or more. Thus, total new production during 1997 may have exceeded that of previous years when nitrate was not depleted below the pycnocline. A bloom of the coccolithophorid, Emiliania huxleyi , was observed in early July in the warm, nutrient-depleted waters over the middle and inner shelf. Emiliania huxleyi concentrations reached 4.5 × 10⁶ cells L^–1 by September, and the bloom persisted through the autumn. There was evidence for increased abundance of some species of copepods in 1997 as compared with data from the middle domain in June 1981. The abundance of adult and juvenile euphausiids in 1997 was statistically similar to values measured in 1980 and 1981. However, near-surface swarms were rarely observed on the inner shelf in August–September 1997. Lack of euphausiid availability in the upper water column may partially explain the August–September mass mortality of planktivorous short-tailed shearwaters ( Puffinus tenuirostris ) observed on the inner shelf.     

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).     

Variations in food abundance for Japanese sardine larvae related to the Kuroshio meander

Long-term fluctuations of chlorophyll a concentration, and abundance of herbivorous or omnivorous small copepods during the spawning season of the Japanese sardine, Sardinops melanosticta , were examined in relation to the types of Kuroshio meander. The purpose was to clarify the influence of the meander on the production of food organisms for the sardine larvae and their survival in the Pacific coastal region of central Japan. During an A-type meander of the Kuroshio, when the offshore and inshore movements of the Kuroshio path were small except at the beginning of the meander, the surface chlorophyll a concentration in the coastal region was lower than that during a non-A-type meander with frequent and prompt fluctuations of the Kuroshio path. The abundance of small copepods was also low during the A-type meander except in the subsequent spring just after the beginning of an A-type meander, but often high during the non-A-type meander. Prompt fluctuations of the Kuroshio path during the non-A-type meander probably cause local upwellings which stimulate primary and secondary production near the Kuroshio, and may enhance the survival of the sardine larvae. On the contrary, the quasi-stationary state of an A-type meander may be unfavourable for the production of phytoplankton and nauplii of herbivorous or omnivorous small copepods, and therefore the survival of the sardine larvae becomes poor.     

Environmental characteristics of spawning and nursery grounds of Japanese sardine and mackerels in the Kuroshio and Kuroshio Extension area

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.     

Differences in food availability for Japanese sardine larvae between the frontal region and the waters on the offshore side of Kuroshio

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.     

Ecological variations and El Niño effects off the southern coast of the Korean Peninsula during the last three decades

To explore ecosystem response to environmental changes, we investigated interannual and decadal changes in the marine ecosystem using data collected from the southern coast of the Korean Peninsula during the 1960s–1990s. Water properties such as sea surface temperature (SST) and salinity in April did not show large variation during the 1970s, but a relatively cool water mass appeared from the early to mid 1980s and a warm water mass replaced it after 1989. Chl a concentration in April, which was converted from Secchi disc information, was low (mean=1.18 mg m⁻³) during the period 1968–1980 excluding 1974, but there was a period of greater biomass for 7 years from 1981 (mean=2.78 mg m⁻³). A negative correlation ( r =−0.387, P  < 0.05) between SST and chl a in April matched high chl a during the low-SST period in the early and mid 1980s. Annual zooplankton biomass ranged from 37 to 132 mg m⁻³, but abundance was frequently high after 1984 and low during 1972–1982. The most representative pelagic fishes in the South Sea seemed to respond to the increase in planktonic organisms. Anchovy, mackerel and sardine increased in abundance since the mid 1970s. High catches of anchovy and mackerel lasted until the 1990s, whereas sardine decreased after the early 1990s. From the matrix of simple correlation coefficient, the Southern Oscillation Index (SOI) exhibited a high correlation with SST in December in the study area ( P  < 0.05). Catches of anchovy and mackerel were positively correlated with chl a and zooplankton during their early life periods, as well as with SST in December.     

Improvement in recruitment of Japanese sardine with delays of the spring phytoplankton bloom in the Sea of Japan

To evaluate the impact of temporal variation of primary productivity on the recruitment of Japanese sardine (Sardinops melanostictus) in the Sea of Japan, the phenology of sea surface phytoplankton abundance was estimated from 8 day multiple satellite (SeaWiFS, MODIS‐Aqua, MERIS, and VIIRS) derived sea surface chlorophyll (SSChl) a concentrations from January 1998 to December 2015. Because relationships between SSChl a and in situ chlorophyll a concentrations were significantly different among periods based on the satellite combinations used, maximum and minimum SSChl a concentrations of 1 year were relativized as 1 and 0, respectively. Spatio‐temporal variation of relativized SSChl a concentrations was determined by using empirical orthogonal function (EOF) analysis. Scores in the first EOF mode denoted the basin‐scale variations of SSChl a concentrations in the Sea of Japan, and the major peak from the end of February to the end of May displayed the spring bloom. The logarithm of recruitment per spawner (LNRPS) for sardine was positively affected by delays in the start and end dates of the spring phytoplankton bloom. The delay of the date of the lowest sea surface temperature contributed to the delay of the end of the spring bloom during the period 1998–2015 and elevated the LNRPS during the period 1982–2015. Sardine spawns in the southern Sea of Japan from April to May, hence, delays of the spring bloom prolonged its overlap with sardine larval periods, and thus improved the recruitment of Japanese sardine in the Sea of Japan.     

Modelling the dispersal of larval Japanese sardine, Sardinops melanostictus, by the Kuroshio Current in 1993 and 1994

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.     

A spatial model of population dynamics of the early life stages of Japanese sardine, Sardinops melanostictus, off the Pacific coast of Japan

We constructed a numerical model reproducing the transport, survival and individual growth of the early life stages of Japanese sardine, Sardinops melanostictus, off the Pacific coast of Japan during 1978–93. The causes of early life stage mortality, including the influence of the effects of the spatial relationship between the spawning grounds and the Kuroshio on the mortality rate, were investigated. Survival and transport from egg stage to 60 days after spawning were modelled daily in a 1 × 1 degree mesh cell and individual growth in the period was modelled in each region (Kuroshio, Inshore, Offshore and Transition regions). Individual growth and survival from 60 to 180 days after spawning were modelled daily in the Transition region. Environmental data were taken from outside the model system. Our simulation indicates that survival variability in the larval stage (5–25 mm in standard length) is the key factor in determining the year‐class strength. The simulation revealed that strong year classes occurred with good survival in the spawning ground and whilst entrained in the Kuroshio current being transported to the main feeding grounds in the Transition region. The simulation also indicated that survival rates in 1988–93 were low in the Inshore, Kuroshio and Offshore regions, which depressed the year‐class strength during that period.     

Impact of spring upwelling variability off southern‐central Chile on common sardine (Strangomera bentincki) recruitment

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.