Relative contributions from exposed inshore and estuarine nursery grounds to the recruitment of stone flounder, Platichthys bicoloratus, estimated using otolith Sr : Ca ratios

In Sendai Bay, stone flounder larvae settle and spend their juvenile period in either shallow exposed inshore nursery grounds or estuarine nursery gro unds. The purpose of this study is to examine the relative contributions of these two kinds of nursery grounds to the flounder population using otolit h strontium:calcium ratios. Stone flounder juveniles were collected from bo th nursery grounds, and one- and two-year-old flounder were caught deeper i n Sendai Bay. Sr and Ca content in the otoliths were measured by electron p robe micro analysis, The Sr:Ca ratios in the otolith section corresponding to the early postsettlement period ranged from 3.06 to 3.85 for the exposed inshore areas with stable low temperature and high salinity conditions, an d from 3.81 to 5.32 in brackish estuaries with high temperature and low sal inity conditions hut with large diel and tidal cyclical fluctuations. Value s from an estuarine site with stable salinity ranged from 3.58 to 4.15 over lapping with both the above ranges. Rearing experiments supported our infer ence that the high otolith Sr:Ca ratios of juveniles inhabiting estuarine n ursery grounds are attributable to higher temperature and physiological str ess caused by the large diel temperature and salinity fluctuations within t he estuaries. Estimation of the Sr:Ca ratio of recruited fish using the oto lith section formed while in the nursery area showed that at least 20 out o f 42 individuals examined originated from estuarine nursery grounds. The pr esent study indicates that estuaries play an important role as nursery grou nds for stone flounder, producing about half of the stock in spite of the s mall and restricted area compared with the wide expanse of the exposed insh ore area.