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.     

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.     

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.     

Cloudy with a chance of sardines: forecasting sardine distributions using regional climate models

Despite the significant advances in making monthly or seasonal forecasts of weather, ocean hypoxia, harmful algal blooms and marine pathogens, few such forecasting efforts have extended to the ecology of upper trophic level marine species. Here, we test our ability to use short‐term (up to 9 months) predictions of ocean conditions to create a novel forecast of the spatial distribution of Pacific sardine, Sardinops sagax. Predictions of ocean conditions are derived using the output from the Climate Forecast System (CFS) model downscaled through the Regional Ocean Modeling System (ROMS). Using generalized additive models (GAMs), we estimated significant relationships between sardine presence in a test year (2009) and salinity and temperature. The model, fitted to 2009 data, had a moderate skill [area under the curve (AUC) = 0.67] in predicting 2009 sardine distributions, 5–8 months in advance. Preliminary tests indicate that the model also had the skill to predict sardine presence in August 2013 (AUC = 0.85) and August 2014 (AUC = 0.96), 4–5 months in advance. The approach could be used to provide fishery managers with an early warning of distributional shifts of this species, which migrates from the U.S.–Mexico border to as far north as British Columbia, Canada, in summers with warm water and other favorable ocean conditions. We expect seasonal and monthly forecasts of ocean conditions to be broadly useful for predicting spatial distributions of other pelagic and midwater species.     

Long-term climate forcing of European herring and sardine populations

Records of the herring, Clupea harengus, fishery off the Swedish coast of Bohuslän, in the Skagerrak, date back to the 10th century. Nine periods, each lasting several decades, are known during which large quantities of herring were caught close to the shore. In the 1895–96 season, more than 200 000 tonnes were landed. During the `interim' periods, which stretched over 50 or more years, the herring fishery played little role in the economy of this region. Several other herring fisheries in European waters overlap with recent Bohuslän periods whereas the Norwegian spring-spawning herring and some sardine, Sardina pilchardus, fisheries exhibit alternating periods. A study of the climatological/hydrographic scenario of all Bohuslän periods and those of herring in the English Channel and the Bay of Biscay showed that, on a decadal scale, they coincided with times when there was a strong ice cover off Iceland, severe winters in western Europe with extremely cold air and water temperatures, a reduction of westerly winds as indicated by negative anomalies in the North Atlantic Oscillation (NAO) index and a minimum of south-westerly winds over England in response to meridional migrations of the belt of westerly winds. Periods of the Norwegian spring-spawning herring and sardines in the English Channel coincided with inverse climatological/hydrographic situations. It is concluded that climate variation governed the alternating herring and sardine periods.     

Naupliar copepod concentrations in the spawning grounds of Japanese sardine, Sardinops melanostictus, along the Kuroshio Current

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.     

Climate change and abundance of the Atlantic-Iberian sardine (Sardina pilchardus)

Climatic warming is affecting oceanic circulation patterns in coastal upwelling areas, but the impact of this climatic change on pelagic fish populations remains unclear. From juvenile landings collected over 38 years, the thresholds of environmental factors were determined that limited the optimal environmental window (OEW) for sardine (Sardina pilchardus recruitment success in the northwestern Iberian peninsula. The environmental factors considered were: water column stability in February, offshore water transport in March–April (Q_xMA), upwelling intensity in the preceding year from May to August (Q_xMJJA), and the winter North Atlantic Oscillation (NAO) index. From 1875 to the mid‐1920s, the mean number of years within the OEW was relatively constant. However, since the mid‐1920s, there have been oscillations and alternating decades with high and low number of years within the OEW, which were related to oscillations in sardine landings. From 1906 to 2000, there were four record, low sardine catches in the 1920s, 1950s, 1970s and 1990s, related to a high number of successive years with prevailing conditions out of the OEW. From 1875 to the present, a high year‐to‐year variation of the NAO, Q_xMJJA and water stability in February was observed, although with mean values usually within the OEW. The collapse in the 1950s was related, partly, to successive years with low Q_xMJJA. Successive years with high NAO values may be related to the collapse of the sardine fishery in the 1990s. Q_xMA has been the most significant factor controlling SRS in this area, being the factor related to the low catches observed in the 1920s, 1950s and 1970s. Water stability was not responsible for any of the collapses observed, but since the 1920s, there has been a significant trend toward decreasing water column stability before the onset of the spring bloom.     

Variability in the spawning habitat of Pacific sardine (Sardinops sagax) off southern and central California

The spatial pattern of sardine spawning as revealed by the presence of sardine eggs is examined in relation to sea surface temperature (SST) and mean volume backscatter strength (MVBS) measured by a 150 kHz acoustic Doppler current profiler (ADCP) during four spring surveys off central and southern California in 1996–99. Studies in other regions have shown that MVBS provides an excellent measure of zooplankton distribution and density. Zooplankton biomass as measured by survey net tows correlates well with concurrently measured MVBS. The high along‐track resolution of egg counts provided by the Continuous Underway Fish Egg Sampler (CUFES) is a good match to the ADCP‐based data. Large interannual differences in the pattern and density of sardine eggs are clearly related to the concurrently observed patterns of surface temperature and MVBS. The strong spatial relationship between sardine eggs and MVBS is particularly evident because of the large contrast in zooplankton biomass between the 1998 El Niño and 1999 La Niña. The inshore distribution of sardine spawning appears to be limited by the low temperatures of freshly upwelled waters, although the value of the limiting temperature varies between years. Often there is an abrupt offshore decrease in MVBS that is coincident with the offshore boundary of sardine eggs. Possible reasons for this association of sardine eggs and high zooplankton biomass include an evolved strategy that promotes improved opportunity of an adequate food supply for subsequent larval development, and/or adult nutrient requirements for serial spawning. Hence, the distribution of these parameters can be used as an aid for delineating the boundaries of sardine spawning habitat.     

The effect of vertical and horizontal distribution on retention of sardine (Sardinops sagax) larvae in the Northern Benguela – observations and modelling

In the present study, a modelling experiment is conducted to simulate the transport of sardine (Sardinops sagax) eggs and larvae in the Northern Benguela. Based on historical and newly obtained data, different scenarios of vertical and horizontal distribution are applied and the effects on retention are discussed. The simulations showed that vertical and horizontal distribution were important for retention of sardine larvae in the Northern Benguela. By using age‐dependent data on vertical distribution, it was shown that retention of particles in the simulation was substantially enhanced compared with a scenario where particles were distributed in the offshore moving Ekman layer. Retention was lowest during October–December (when upwelling intensity is high) and highest during February–April (when upwelling intensity is somewhat lower). When different spawning areas were considered, highest retention was observed in an area near Walvis Bay. It is concluded that the behaviour of sardine larvae is adapted to the circulation system in the Northern Benguela in a way that promotes retention of the larvae in inshore nursery areas.     

Mesoscale eddies and survival of late stage Pacific sardine (Sardinops sagax) larvae

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.     

Survival and growth of sardine larvae in the offshore side of the Kuroshio

After 1984 the major spawning of the Japanese sardine, Sardinops melanostictus, has been observed to occur in the offshore waters, where the survival of early-stage larvae (4–10 mm in length) is questionable. The main objective of this study is to estimate the growth and survival rates of the early-stage larval cohort in the offshore side of the Kuroshio Current. A radar-reflecting buoy with a surface drogue was launched to tag a patch of larvae, and the patch was traced for 3 days in March 1991. The survival rate of the early-stage larval cohort was calculated from the change in density during the survey. The range of the instantaneous mortality rate was from 0.83 to 1.11 day, a survival rate of32-44%day-¹.The mean density of the smaller larvae (4–6 mm in length) decreased more rapidly than that of the larger larvae (6–10 mm in length) during the sampling period. The survival rate of the first-feeding larvae in the offshore region seemed to be lower than that of the post-flrst-feeding larvae. However, the growth rates of the first-feeding larvae's survivors in the region were higher than those of the post-first-feeding larvae's survivors. The first-feeding larvae in the offshore region seemed to survive when the growth rate of the larvae was high.     

Modeling the transport and survival of Japanese sardine larvae in and around the Kuroshio Current

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.     

Evidence of density-dependent effects on population variation of Japanese sardine (Sardinops melanosticta) off Korea

Interannual variation of some biological parameters for the Japanese sardine, Sardinops melanosticta, population in Korean waters was investigated using scientific surveys and fisheries information since the late 1970s. The abundance and geographical coverage of sardine eggs were high (peak in 1986) and broad when spawning biomasses were high in the mid 1980s, and vice versa in the late 1970s and the early 1990s. Also, feeding and spawning areas based on fisheries information exhibited the same pattern of expansion/contraction as seen in ichthyoplankton surveys. Annual Gonadal Somatic Index (GSI) in spawning season (February to April) and the size at age 1 of sardine were reduced during the high abundance period. It is suggested that density‐dependent effects on the reproduction and growth of the sardine population in Korean waters existed.     

Variations in spawning ground area and egg density of the Japanese sardine in Pacific coastal and oceanic waters

The spawning ground of the Japanese sardine, Sardinops melanostictus (Schlegel), was distributed over the oceanic waters as well as the coastal waters along the Pacific coasts of western and eastern Japan during 1978–1992. The area of the spawning ground in the coastal waters on the continental shelf has ranged from 95 000 km² in 1992 to 143 000 km² in 1988, constituting 44–77% of the total area of the spawning ground. The area of the coastal spawning ground was relatively constant in spite of the large fluctuations in egg abundance, i.e. size of the spawning population, from 88 trillion (1987) to 668 trillion (1989) in the waters. Spawning adults seemed to extend over the coastal waters irrespective of the size of the spawning population. In contrast to the coastal waters, the spawning area in the oceanic waters offshore of the continental shelf increased from 31 000 km² in 1978 to 183 000 km² in 1988 and then shrank to 83 000 km² in 1992, as a function of the spawning population size. The egg distribution density in the coastal waters stayed less than 6000 m⁻² mo⁻¹, but it reached as high as 27 400 m⁻² mo⁻¹ in the expanded spawning ground in the oceanic waters. The oceanic waters seemed to function as a reserve spawning ground for the sardine in years of extremely high spawning population.     

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.     

Growth effect on the otolith and somatic size relationship in Japanese anchovy and sardine larvae

ABSTRACT:   Relationships between otolith and somatic sizes were examined for Japanese anchovy Engraulis japonicus and sardine Sardinops melanostictus larvae collected broadly in the western North Pacific, based on a substantial data set derived from a previous paper. Allometric formulae showed close fits to the relationships between otolith radius and standard length, and the formulae differed between anchovy and sardine larvae. Despite the high correlations, the effect of somatic growth rate on the otolith and somatic size relationship (the 'growth effect') was significantly detected for both anchovy and sardine larvae. Slower growing larvae tended to have larger otoliths than faster growing conspecifics at the same somatic size. This growth effect was more obvious for sardine larvae than for anchovy larvae, probably because of their differential responses of somatic growth to temperature shifts. The growth effect could lead to the possibility of biases in the back-calculation and size estimation processes. As the growth effect is considered to be a general phenomenon and its extent to be species-specific, the relationship between otolith and somatic size and its uncoupling should be scrutinized before application of techniques based on the otolith and somatic size correlation.     

Global climate change and ocean upwelling

The influence of global greenhouse warming on the ocean's biological productivity may be more complicated and weaker than that proposed by Bakun (1990). A doubled carbon dioxide simulation made with the Canadian Climate Centre atmospheric general circulation model coupled to a simplified mixed layer ocean model suggests that the midlatitude continents do not all follow the Bakun scenario in developing anomalous low pressure in summer and enhancing coastal winds favorable to upwelling. In the open ocean the equatorial and subpolar zonal upwelling bands and the subtropical downwelling bands generally weaken as winds diminish owing to the weakening of the equator-to-pole temperature gradient in the lower troposphere under global warming. With a weakening of open ocean upwelling and an absence of enhanced coastal upwelling, the overall effect of global warming could be to decrease the global biological productivity.     

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.     

Entrainment and advection of larval sardine,Sardinops sagax,by the East Australian Current and retention in the western Tasman Front

The poleward flowing East Australian Current (EAC) drives sporadic upwelling, entrains coastal water and forms the western Tasman Front (wTF), creating a mosaic of water types and larval transport routes along south eastern Australia. The spatial distribution, otolith chemistry and growth rates of larval sardine (Sardinops sagax) were examined to infer spawning location and larval transport. A gradient of increasing larval size from north to south along the shelf was not detected but was evident between the shelf and offshore in the wTF. Here larvae were larger and older. Based on the occurrence of newly hatched larvae, spawning by S. sagax between southern Queensland and mid New South Wales (NSW) was more extensive than previously reported. The otolith chemistry from two wTF larval size classes differed, implying different origins. The otolith chemistry of wTF post‐flexion larvae was similar to larvae from northern NSW, whereas wTF flexion larvae were similar to larvae observed nearby from mid‐NSW. Two possible larval transport routes, direct and indirect, are inferred from otolith chemistry, current velocities and a previously published particle tracking study. Either larvae from northern NSW were advected south and entrained with younger larvae directly into the wTF, or larvae from a range of shelf regions were advected around the southern edge of an anticyclonic eddy, to join younger larvae directly entrained into the wTF. Based on the co‐occurrence of larval ages and sizes in the wTF and their advection routes, the wTF appears to be an important larval retention zone.