2022 Volume 69 Issue 1 Pages 25-33
The sakura-ebi [Lucensosergia lucens (Hansen, 1922)] fishery in Suruga Bay, Japan, sharply declined in 2018 and has not yet recovered (140 tons was reported for the spring catch in 2021). I have proposed two causal hypotheses to explain the slump:1) a decline in the spawning population in the inner area of the bay due to excessive fishing in the spring season and a shift in the spawning peak from early summer to late summer-autumn, and 2) a decline in biological productivity in the inner and western area of the bay after the 1980s due to recent coastal development and environmental deterioration. A delay in the spawning peak may causes a shift in the main spawning area from the inner area to the western area of the bay.
Test the first hypothesis; we performed numerical experiments to investigate the transport processes of eggs and larvae of L. lucens in Suruga Bay, focusing on the physical effects of the current with a different peak in the spawning season and in the area using the particle tracing method. Particles regarded as eggs and early larvae were passively transported in flow fields for 1 month after being deployed at three locations: a) off the mouth of the Fuji River, the inner area, b) off the mouth of the Abe River, the western area, and c) off the mouth of the Oi River, in the southwestern area. The particles were tracked by two ocean circulation models using simplified hydrographic conditions in September 2007 and with predicted and real observations of current flow from 1 June to 1 October, 2020.
In the first experiment, using simplified conditions without considering the effect of the Kuroshio current, approximately half of the particles deployed in the 0–24 m layer off the mouth of the Fuji River remained in the inner area of the bay after 1 month. Nearly 90% of the particles remained in the bay. However, only 22% of the particles deployed off the mouth of the Abe River and less than 10% deployed off the mouth of the Oi River remained in the bay after 1 month. In the case where the particles were deployed in the 24–60 m layer off the mouth of the Fuji River, 100% remained in the bay. In the second experiment with predicted and real observations of the current flow, 54% of the particles that were deployed off the mouth of the Fuji River remained in the bay, whereas only 23% from off the mouth of the Abe River and 15% from off the mouth of the Oi River remained in the bay after 1 month. In Sagami Bay, 76% of the particles that were deployed in the 25–45 m layer off the mouth of the Sakawa River were run out from the bay after 1 month. These results indicate the importance of the retention area in the inner area of Suruga Bay, where the eggs and larvae can stay for a long time.
A good correlation was observed between the spawning peak during 1991 to 1997 and catch per unit effort (CPUE) (catch [kg] per 30 min tow) of the 0-year class shrimp, suggesting that earlier spawning in July causes an increase in the 0-year class shrimp population for the autumn catch.
Thus, I believe that the first hypothesis is correct. Delays in peak spawning time cause a shift in the main spawning area from the inner area to the western and southwestern area of Suruga Bay, and therefore cause a remarkable loss of larvae to outside the bay. To restore the shrimp stock, it is essential to reduce fishing effort during the spring catch to facilitate spawning of larger eggs in June/July in the inner area, and hence many larvae remain and grow in the bay. Distinct differences exist in the hydrographic conditions between Suruga Bay and the neighbouring Sagami Bay. The eggs and larvae in Sagami Bay are easily run out from the bay, and hence the population cannot be maintained to the level of commercial fishery.
