Studies on stock fluctuation and the ecological changes in eggs and larvae of the Japanese sardine [Sardinops melanostictus] along the Pacific coast of Japan

Pelagic fish species such as sardine have fluctuated dramatically in biomass size and geographical distribution. Especially, the spawning grounds of Japanese sardine are known to expand with stock abundance increases. In this study, clarification and consideration of the effects of the recruitment process by the expansion of spawning area in relation to the stock fluctuation of the Japanese sardine along the Pacific coast of Japan was made. Moreover, I have tried to clarify the relationship between the sardine spawning area and the spawning biomass off the Pacific coast of Japan, to estimate biomass with the spawning area data. The contents of this paper are outlined as follows. 1. Change of spawning grounds The sardine spawning is concentrated into 2 months; February and March, and is independent of the stock abundance. Therefore, the spawning area was calculated by summing up the areas of 1 deg longitude * 1 deg latitude squares where eggs were collected during February to March. The spawning area expanded steadily from 144,000 square km in 1978 to 327,000 square km in 1988. The area decreased to 208,000 square km in 1989, peaked at 349,000 square km in 1990, and then shrank to 82,000 square km in 1995. The analysis of the accumulated data from the spawning surveys revealed that the distribution densities of eggs and larvae of sardine had been higher in the waters on the offshore side of the Kuroshio axis than the inshore side in the waters east of Cape Shionomisaki since 1984. 2. Changes in the amount of lipid of early larval sardine in the main Kuroshio current and its offshore waters off eastern Japan (1) In this study, I described that the levels of triglyceride (TG) reserves were dependent on the nutritional condition of the laboratory-reared sardine larvae. TG of the fed larvae increased proportionally with body length. While TG for larvae starved for a longer period was less than those for the fed and larvae starved for a shorter period at a same body length. A TG value of 0 indicated a severely starved larva which is unlikely to recover. This result suggests that TG is an index of the nutritional condition or starvation tolerance of sardine larvae. (2) The TG measured in this study showed that all of the larvae from the main Kuroshio current and its offshore waters off eastern Japan were not in a starving condition. However, the TG of the 8 - 9mm body size-class in the offshore region was higher than that in the main Kuroshio current. To test whether the TG for each larval sardine in the 8 - 9mm body size-class could be correlated with variables (temperature, chlorophyll a and distribution density of the sardine larvae) measured at the sampling stations, correlation analyses were performed. A highly significant negative correlation between TG and the distribution density of the sardine larvae was found. A density-dependent effect seemed to influence the fluctuation of the larval storage energy component for short term needs. 3. Survival and growth of sardine larvae on the offshore side of the Kuroshio In order to estimate the mortality and growth rate of a cohort on the offshore side of the Kuroshio, a radar reflecting buoy with a surface drogue to tag and pursue a patch of sardine larvae was used. The mortality rates were estimated from the difference in mean distribution density of each larval cohort during successive days. The growth rates were estimated by using the Biological Intercept Method. (1) In this study, the survival rate of early stage larvae in the offshore region was very low (32 - 44%/day). (2) The faster growth larvae at the first feeding periods were able to survive in the offshore region. (3) A simple selection model was presented to show the potential influence of survival on the growth or nutritional index within a cohort of larvae. The results show that mean in growth index will increase as the survival rate decrease. 4. Development of a model to estimate biomass with spawning area data. Firstly, development of a mathematical model for the spawning biomass and spawning area was made. The model was based on a stochastic description of the aggregation of eggs using negative binomial distribution. Secondly, to clarify recruitment process in relation to. the spawning area expansion, analysis of a relationship between sardine spawning area and the recruitment was made. (1) An estimate of sardine spawning biomass can be obtained by using the relationship between the spawning biomass (B) and spawning area (A) (B = 0.8029A sup(1.8077)). (2) Since 1988, there was a negative relationship between the recruitment (RB) per spawning biomass and spawning area (RB/B = 0.1397 exp (0.0172 A)). The fluctuation of spawning area seems to be related a stock regulation mechanism of sardine along the Pacific coast of Japan.