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.     

Use of satellite data to identify critical periods for early life survival of northern shrimp in the Gulf of Maine

The northern shrimp Pandalus borealis is at its southern limit in the Gulf of Maine (GOM), and recruitment success is higher in years with relatively cool water temperature. However, the mechanisms for the temperature effect are not clear. We used rolling window analysis of daily satellite data to identify critical periods for early life survival of the 1998–2012 northern shrimp year‐classes and to investigate the importance of the phenology of the hatch and bloom. Survival was negatively correlated with sea surface temperature (SST) during a 6‐week period around the time of larval emergence (late winter) and during a 4‐week period in late summer when SST and stratification reached annual maxima. Survival was negatively correlated with chlorophyll‐a concentration (chl‐a) during two 5‐week periods centered approximately a month before the hatch midpoint and around the time of settlement to the benthos. A small‐magnitude winter bloom occurred around the time of the hatch in many years, but our results did not reveal a link between survival and bloom‐hatch phenology. The timing of winter and spring blooms were correlated with SST during the preceding 10 months. A survival model including SST and chl‐a during the critical periods explained 73% of the variance in survival. Summer SST increased significantly during the study period; the other critical variables showed no trend. The rolling windows approach revealed sensitive periods in early life history that may not have otherwise been hypothesized, providing a foundation for research towards a greater understanding of processes affecting recruitment.     

Variability of oceanographic and meteorological conditions in the northern Alboran Sea at seasonal,inter‐annual and long‐term time scales and their influence on sardine (Sardina pilchardus Walbaum 1792) landings

Time series of European sardine (Sardina pilchardus) landings from 1962 and environmental variables from 1978 in the northern Alboran Sea are analysed. European sardine spawns in the northern Alboran Sea from mid‐autumn to late winter at a temperature range slightly higher than the one observed in the nearby Eastern North Atlantic and the North Western Mediterranean. Individuals hatched during autumn and winter are incorporated to the fishery during the following summer and autumn producing the maximum annual landings. These landings show both a decreasing long‐term trend and a strong inter‐annual variability. Although further research is needed, the warming trend of sea surface temperature and the decrease in upwelling intensity inferred from empirical orthogonal function (EOF) analyses could have some influence on the negative trends of sardine landings. The inter‐annual variability of sardine abundance seems to be related to the wind intensity at a local scale, the second principal component of the chlorophyll concentration and the sardine abundance during the preceding year. If the inter‐annual variability is considered, a linear model including these three variables with a one‐year time lag allows to explain 79% of the sardine landings variance. If the negative linear trend is also considered, the model explains 86% of the variance. These results indicate that the body condition of spawners, linked to the food availability during the preceding year, is the main factor controlling the recruitment success. The possibility of predicting sardine landings 1 year in advance could have important implications for fishery management.     

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.     

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.     

The importance of episodic weather events to the ecosystem of the Bering Sea shelf

Climate variability on decadal time scales is generally recognized to influence high‐latitude marine populations. Our recent work in studying air–sea interactions in the Bering Sea suggests that interannual to decadal climate variability is important through its modulation of the frequencies and magnitudes of weather events on intraseasonal time scales. We hypothesize that it is these weather events that directly impact the marine ecosystem of the Bering Sea shelf. The linkages between the event‐scale weather and the ecosystem are illustrated with three examples: walleye pollock (Theragra chalcogramma), Tanner crabs (Chionoecetes bairdi), and coccolithophorid phytoplankton (Emiliania huxleyi). We hypothesize that the strong recruitment of walleye pollock that occurred in 1978, 1982, and 1996 can be attributed in part due to the seasonably strong storms that occurred in the early summer of those years. These storms caused greater than normal mixing of nutrients into the euphotic zone which presumably led to sustained primary productivity after the spring bloom and, possibly, enhanced prey concentrations for pollock larvae and their competitors. Recruitment of Tanner crab was particularly strong for the 1981 and 1984 year‐classes. These years had periods of prominent east wind anomalies along the Alaska Peninsula during the previous winter. Such winds promote flow through Unimak Pass, and hence an enhanced flux of nutrient‐rich water onto the shelf. This mechanism may have ultimately resulted in favorable feeding conditions for Tanner crab larvae. Finally, an unprecedented coccolithophorid bloom occurred over the Bering Sea shelf in the summer of 1997. This summer featured lighter winds and greater insolation than usual after a spring that included a very strong May storm. This combination brought about a warm, nutrient‐poor upper mixed layer by mid‐summer. This provided a competitive advantage for coccolithophorid phytoplankton in 1997 and to a lesser extent in 1998. Unusually high concentrations of coccolithophores persisted for the following two years although physical environmental conditions did not remain favorable. While slow variations in the overall aspects of the physical environment may be important for setting the stage, we propose that the significant multi‐year adjustments in the marine ecosystem of the Bering Sea shelf are more directly caused by major air–sea interaction events on intraseasonal time scales.     

Repeated reaching of the harmful algal bloom of Karenia spp. around the Pacific shoreline of Kushiro,eastern Hokkaido,Japan, during autumn 2021

Unprecedented large-scale algal blooms were observed during autumn 2021 around the Katsurakoi fishing port, Kushiro, eastern Hokkaido, Japan. Monitoring of shoreline water showed that chlorophyll a (Chl a) concentrations and the cell density of Karenia spp., dominated by Karenia selliformis, repeatedly increased synchronously between September and November 2021. These increases were associated with a southerly wind-driven current, which transported offshore water on the shelf towards the shoreline at the sea surface. The blooms were prolonged as a result of algal accumulation in the semi-closed fishing port. The maximum Chl a concentration and cell density exceeded 50 µg Chl a/L and 10⁴ cells/mL, respectively. During the autumn bloom of Karenia spp., the nitrate?+?nitrite and phosphate concentrations in the water were lower than those in 2019 and 2020, and the silicate concentration was comparable. The ammonium concentration during the bloom was notably higher than before the bloom period, reaching 15 µM. Mass mortality of several fish species and echinoderms that were cultured using rearing water intake from the same shoreline occurred synchronously with the increase in Karenia spp.

     

Spawning habitat and daily egg production of sardine (Sardina pilchardus) in the eastern Mediterranean

Spawning habitats of two eastern Mediterranean sardine, Sardina pilchardus (Walbaum, 1792), stocks (coastal waters of central Aegean and Ionian Seas) are characterized from daily egg production method (DEPM) surveys conducted during the peak of the spawning period. The latter occurs earlier in the Aegean Sea (December) than in the less‐productive Ionian Sea (February). Single‐parameter quotient analysis showed that the preferred bottom depth for spawning was 40–90 m in both areas but sardine selected sites of increased zooplankton in the Aegean Sea during December and increased fluorescence in the Ionian Sea during February. Estimates of daily egg production (P) and spawning stock biomass (B) were about four times lower for the Ionian Sea (P = 7.81 eggs m^?2, B = 3652 tonnes) than the Aegean Sea (P = 27.52 eggs m^?2, B = 16 174 tonnes). We suggest that zooplankton biomass might not be sufficient to support sardine reproduction in the highly oligotrophic Ionian Sea where the very small sardine stock may rely on the late‐winter phytoplankton bloom. Actively selecting sites with increased zooplankton or phytoplankton and feeding plasticity (the well‐known switching from selective particle feeding to non‐selective filter feeding in sardines) are interpreted as adaptations to grow and reproduce optimally at varying prey conditions. Despite differences in temperature and productivity regimes, reproductive performance of sardine in the Ionian Sea was very similar to that in the Aegean Sea during the peak of the spawning period. In comparing adult parameters from DEPM applications to Sardina and Sardinops stocks around the world, a highly significant linear relation emerged between mean batch fecundity (F) and mean weight of mature female (W, g) (F = 0.364W, r² = 0.98). The latter implies that, during the peak of the spawning period, mean relative batch fecundity (eggs g^?1) of sardine is fairly constant in contrasting ecosystems around the world.     

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.     

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.     

Gonad development, larval settlement and growth of Mytilus edulis L. in a suspended population in Hvalfjördur, south-west Iceland

Annual cycles of gamete development in Mytilus edulis L. in south-west Iceland were investigated during 1986 to 1987. Histological preparations of the gonads showed that all individuals were fully mature in the beginning of June in both years and one spawning period was recorded each year at a sea temperature of 10-12^oC. The main spawning took place from the middle of July to the middle of August, and from the middle of June to the end of July in 1986 and 1987, respectively. Redevelopment of the gonads after spawning was observed in February when both food availability, measured as chlorophyll a concentration, and temperature were low. Rapid gonad maturation took place during the spring phytoplankton bloom in March-April. Larval settlement was heavy on the artificial collectors used. The length of the larval period was estimated as being 4-5 weeks and peak settlement occurred in the middle of September. The size of the early plantigrades settling on the collectors during the study period indicated direct settlement of mussel larvae from the plankton onto the collectors. One year after settlement the spat averaged 24.6 mm ± 6.0 (SD) shell length and two years after settlement a market size of 50.8 mm ± 5.7 (SD) was reached. Growth was correlated with food availability, measured as chlorophyll a concentration. The growth season lasted from March to October with the greatest shell growth in late summer. The results showed that the growth of M. edulis was markedly increased by suspending the animals in a more favourable environment than that found on the natural mussel beds.     

Reproductive success of the Argentine anchovy,Engraulis anchoita,in relation to environmental variability at a mid‐shelf front (Southwestern Atlantic Ocean)

The mid‐shelf front (MSF) of the Buenos Aires province continental shelf in the Southwestern Atlantic Ocean plays a central role in the pelagic ecosystem of the region acting as the main spring reproductive area for the northern population of the Argentine anchovy Engraulis anchoita and supporting high concentrations of chlorophyll as well as zooplankton, the main food of anchovy. To investigate the influence of environmental variability on the reproductive success of E. anchoita, we analyzed a 13‐yr time series (1997–2009) of environmental data at MSF including chlorophyll dynamics, as well as zooplankton composition and abundance, ichthyoplankton distributions, and recruitment of E. anchoita. Spring chlorophyll concentrations showed high interannual variability and were mainly influenced by changes in water temperature and vertical stratification, which in turn control nutrient supply to the surface. Chlorophyll dynamics (magnitude, timing, and duration of the spring bloom) explained most of the variability observed in E. anchoita recruitment, most likely via fluctuations in the availability of adequate food for the larvae. Our results suggest that satellite ocean color products can be valuable tools for understanding variability in ecosystem dynamics and its effects on the recruitment of fish.     

Long-term stock assessment and growth changes of the Japanese sardine (Sardinops melanostictus) in the Sea of Japan and East China Sea from 1953 to 2006

We estimated the stock size of Japanese sardine ( Sardinops melanostictus ) in the Sea of Japan and East China Sea since 1953 using cohort analysis based on the changes of growth patterns. Growth of Japanese sardine, estimated by using annual rings on archived scales since 1961, showed that body lengths were extremely stunted in the 1980–1987 year-classes. The body length at age 3 from February to April in the 1980–1987 year-classes, a period when the stock size exceeded 4 million tons, was 180.0 ± 2.6 mm (mean ± SD), and in the other year-classes was 195.1 ± 7.6 mm. The body length at age 3 and wet weight of zooplankton in August in the offshore area of the Sea of Japan had a significantly positive correlation. We assumed three scenarios for maturation ratios, and estimated Ricker's spawner–recruitment relationships. We analyzed the correlations between logarithmic recruitment residuals (LNRR) and environmental factors in winter, represented by the North Pacific index (NPI), Aleutian low pressure index (ALPI), Pacific decadal oscillation (PDO), monsoon index (MOI), Arctic oscillation (AO) and Southern oscillation index (SOI). Significant correlations were observed between MOI and LNRR and between AO and LNRR. A combination of strong MOI and weak AO would increase the biomass of phytoplankton and zooplankton and subsequently increase the recruitment of Japanese sardine.     

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.     

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.     

Influence of environmental conditions on the reproductive success and recruitment of the Argentine hake Merluccius hubbsi (southwestern Atlantic Ocean)

The Argentine hake Merluccius hubbsi represents the dominant demersal fish and one of the main fisheries in the Argentine Sea. We analyzed over 17 years (September 1997–February 2015) of satellite surface chlorophyll concentration (CHL) and sea surface temperature anomaly (SSTA) data in the main spawning (SUB) and nursery (SJG) areas of the Patagonian stock of M. hubbsi. The variability observed in these environmental conditions was related to interannual differences in the reproductive success of hake. CHL values were maximum during austral spring (October–November) and minimum during winter (June–August). Blooms started in late September and lasted on average 128 and 110 days at SUB and SJG, respectively. At SUB, average CHL at the time of reproduction, the day of occurrence of the maximum chlorophyll concentration, and fall SST anomalies were significantly related to recruitment (R_VPA) a year later and to two Larval Survival Indices (LSI_SSB and LSI_TEP) considered. At SJG, R_VPA, LSI_SSB, and LSI_TEP showed negative correlations with fall (March–May) SSTA. Total egg production was not related to fall SSTA or spring phytoplankton dynamics at SUB or SJG. Significant positive trends were observed in the time series of CHL and SSTA in the study area. The trends observed in CHL represent average increases of 40.7% and 35.7% since 1997 at SUB and SJG, respectively. The potential implications of long‐term changes in environmental conditions for hake reproductive success are discussed.     

Effects of Chaetomorpha valida bloom development on the structure and succession of the phytoplankton community in Apostichopus japonicus culture ponds

Phytoplankton is very important to aquaculture ecosystem and is vulnerable to ambient conditions. In recent years, Chaetomorpha valida, an invasive filamentous green alga, has been blooming in Apostichopus japonicus culture ponds. Here, we conducted a 5‐month investigation, examining whether that bloom affects the structure and succession of the pond phytoplankton community. Differences in dissolved oxygen, light and nutrient concentrations in both bloom and normal (non‐bloom) areas varied monthly. The species and populations of phytoplankton communities in both the bloom and normal areas showed no significant differences when C. valida biomass was low, but through time, differences became increasingly significant. Species in normal areas remained relatively stable as the numbers and species of the dominant species changed little and the diversity and evenness indexes increased monthly. In bloom areas, species abundance decreased gradually with most of the decrease affecting Bacillariophyta. Here, the number of dominant species remained stable from May to July but decreased significantly in August and September. Diversity and evenness indexes also decreased significantly, and the differences between the 2 areas increased rapidly. Results showed that C. valida bloom in A. japonicus culture ponds influenced both the structure and succession of the phytoplankton community, contributing to comprehensive assessment of the effects of C. valida bloom on aquaculture ecology.     

Recruitment of catarina scallop (Argopecten ventricosus) larvae on artificial collectors off the NE coast of the Gulf of California

We evaluated recruitment of larvae of catarina scallop, Argopecten ventricosus, in the area of Puerto Peñasco, NE Gulf of California. We moored artificial collectors in six sites from June 2007 to August 2008 and replaced them every 2 months. We used monthly (July 2002–September 2011) sea surface temperature (SST, °C) and surface chlorophyll‐a concentration (SSChl, mg m^?3) Aqua/MODIS satellite data to describe seasonal environmental behaviour study area. Also, we recorded bottom temperature at each site every 4 h, and every 2 months measured sea surface salinity, temperature and dissolved oxygen. We used a repeated measures anova to evaluate differences in the number of recruited spat between main factors, and analysed the presence of multimodal spat shell size frequency distributions. Overall, spat recruitment was negatively correlated with seawater temperature and showed higher spat recruitment abundances throughout winter, which is the season with the highest surface chlorophyll a concentration. We estimated multimodal shell size frequency distributions characterized by more than one modal size. The natural collection of A. ventricosus spat on artificial collectors in the area can be successfully performed over a protracted period (November–December to May–June). Our results extend the area where collection of A. ventricosus spat can be successful.     

Effects of intensive mariculture on the sediment environment as revealed by phytoplankton pigments in a semi‐enclosed bay,South China Sea

Phytoplankton pigment concentrations in the surface sediments of a fish and oyster farm in Dapeng Cove, South China Sea, were measured monthly to investigate the effects of mariculture on the environment from December 2011 to December 2012. The highest contents of the identified phytopigments, sulphides and total organic carbon in the surface sediments were all found at the fish farm, followed by the oyster farm; the lowest contents were detected at the control site. Fucoxanthin in the surface sediments was the most abundant carotenoid and was significantly correlated with chlorophyll a (Chl a; r = 0.922, P < 0.01). This finding suggested that the deposited diatoms could be the main source of the Chl a concentration. Besides a differential sedimentation of phytoplankton (i.e. higher downflux at fish and oyster site than that at control site), we assume that the extent of pigment diminution from the fish site to control site was also controlled by the chemical conditions of the surface sediments. The fish farm provided the most optimum preservation conditions for the phytopigments and was a good sink for phytodetritus among the three sites because of its anoxic and reductive characteristics. Phytoplankton pigments could be useful biomarkers to evaluate the influence of mariculture on sediment environment.     

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.