Natural habitat contributes more to estuarine fish production than artificial habitat: an example from inter-river comparison in the Ohta River estuaries

摘要

Production of juvenile sea bass Lateolabrax japonicus cohorts during the period of post-migration into the Ohta River was compared between a drainage channel (DC) and a natural river (NR) in Hiroshima, southwestern Japan. Freshwater discharge during periods of high precipitation through the DC is controlled to minimize discharge into other rivers which run through the urban area. Juveniles in the DC had been expected to be affected by stronger disturbance in physical properties to their habitat due to higher fluctuations of freshwater discharge. In order to test this hypothesis, cohort-specific growth (G, d(-1)) and mortality (M, d(-1)) coefficients and the ratio of G to M (G/M as a proxy of juvenile production) were compared between the two rivers. Juvenile vital rates were estimated through (1) repeated sampling at fine time intervals (6-15 days), (2) application of otolith daily increments for cohort identification, and (3) standardization of abundance at age based on the length-dependent catch efficiency of the sampling gear to estimate M more accurately. G (0.012-0.021) did not significantly differ between the DC and NR. M in the DC (0.184-0.239) was significantly higher than in the NR (0.140-0.148) and average ratio of G/M (0.111) in the NR was higher than in the DC (0.082). High mortality due to physical processes (high variability in salinity) was concluded to contribute to the inter-river difference in juvenile production since the differences in prey availability, and vulnerability to predation between the two rivers were minimal.