Recruitment processes of Japanese sea bass in the Chikugo estuary, Japan: Shift from density-independence to density-dependence during the early life stages

Abstract

Japanese sea bass larvae migrate from Ariake Bay, southwestern Japan, into the Chikugo Estuary and inhabit there throughout the juvenile period. Abundance of Japanese sea bass larvae and juveniles in the estuary shows a 40-fold fluctuation, higher than that in adult stock (2-fold). It is likely that a density-independent mechanism seems to influence the survival of larval (pre-immigration) period and that a density-dependent mechanism regulates the survival during the juvenile (post-immigration) period. We tested the hypotheses that physical conditions such as freshwater discharge and temperature influence pre-immigration survival through affecting larval growth rate and that a density-dependent mechanism regulates post-immigration survival of the Japanese sea bass. Mean larval growth rate (G15, mm d-1) during the pre-immigration period (<15 mm), weight-specific growth coefficient (Gw, d-1), mortality coefficient (M, d-1), and relative recruitment potential (Gw/M) during the post-immigration period (15-20 mm) were estimated from 1990-2000. Freshwater discharge through the Chikugo River had a significant effect on the temperature of the upper Ariake Bay. The G15 varied between 0.04 and 0.10 mm d-1 and was significantly correlated with the temperature experienced by the larvae. The G15 had a significant effect on the larval and juvenile sea bass abundance, with higher abundance in years with high G15. Freshwater discharge seems to be a primary factor for the density-independent control of larval Japanese sea bass survival, influencing water temperature in the upper Ariake Bay and the larval growth. The Gw/M was significantly affected by the Japanese sea bass abundance at 15 mm: lower Gw/M in years of high abundance. Density-dependent regulation seems to operate on the post-immigration Japanese sea bass in the Chikugo Estuary.