The Japanese sardine Sardinops melanostictus started to decline after 1989. Recruitment to age 1 population was small in four year-classes from 1988 to 1991. The population decline after 1989 resulted from recruitment failures in 4 consecutive years. Egg production was high in the years of poor recruitment. The recruitment failures were caused not by a reduction in reproductive output but by low survival between egg stage and age 1 recruitment. Abundance of post first-feeding larvae positively correlated with egg and yolksac larval abundance. Mortality at the first-feeding stage was not so variable as to destroy correlations between the abundance of early life stages. The population of age 1 recruits did not correlate with the abundance of post first-feeding larvae. Recruitment of the sardine was not fixed by the end of the first-feeding stage. Cumulative mortality through the early life stages, rather than relatively instantaneous mortality at the first-feeding stage, is thought to be responsible for the recruitment success or failure and eventual population fluctuations of the sardine.

No abstract

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.

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.

In order to establish criteria with which to evaluate the nutritional condition of wild firstfeeding larvae of Japanese anchovy Engraulis japonicus, the whole body RNA:DNA ratio and daily protein growth rate of reared larvae were individually determined. Anchovy larvae were reared for up to 9 days after the first-feeding day at 20∞C and 25∞C under various feeding conditions. Percent survival of the group starved for 2 days after the first-feeding day was extremely lower than that of those fed from the first-feeding day and 1 day starved groups at the two temperatures, even if they were supplied with food thereafter. Therefore it was concluded that the point of no return (PNR) was between 1 and 2 days after the first-feeding day within this temperature range. The RNA:DNA ratio of the PNR criterion was determined. Also, a 0% growth formula was established representing the relationship between the daily growth rate in protein, RNA:DNA ratio and temperature for first-feeding larvae.

With data sampled from 2003 to 2006, this study presents the effects of temperature, food availability (C), and body size on the somatic growth rateĝ (mm day -1 ) and on the size-specific growth ratesĜ (day -1 ) of larval Japanese anchovy Engraulis japonicus caught in Hiuchi-nada, Seto Inland Sea, Japan, a semienclosed narrow sea. C was calculated using an individualbased model, which is an encounter model between copepods and anchovy where the anchovy resulted in a successful capture of the copepod.ĝ decreased with increasing anchovy length, but increased with increasing C. No significant relationship was found betweenĝ and temperature. G decreased with increasing temperature, but anchovy weight, on the other hand, increased with increasing C. In the literature, the mechanism of growth determination for larvae of pelagic fish species is considered to be primarily determined by sea temperature and secondarily by food availability. However,ĝ andĜ were found to be dependent on C rather than temperature.Ĝ was dependent on anchovy weight rather than C and temperature. For forecasting the number of days required for development from an arbitrary pre-recruitment size to the recruitment size, we address not only temperature but also food availability and the size effect when considering the mechanism of growth determination for larvae.

We attempted to estimate the survival rates of larval cohorts (3-15 mm in body length) of the Japanese anchovy Engraulis japonicus population in Hiuchi-nada, Seto Inland Sea, Japan, and examined the relationship to their food availability. The survival rates were directly calculated from the change in larval density during the survey. The estimated daily survival rate ranged between 0 and 89% d )1 , but increased with the increase in the mean concentration of small-sized copepod nauplii (<100 lm in body length) sampled at 10 m depth. When the food concentration was higher than about 5 nauplii L )1 , the daily survival rate of larval anchovy reached an asymptote of approximately 89% day )1 . It might be possible to provide a framework for the forecast for larval abundance immediately prior to recruitment size (10-15 mm SL), based on larval abundance of the monitored size (3-8 mm SL) and the survival rate which is estimated from the concentration of small-sized nauplii.

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.