Oceanic larval duration and recruitment mechanism of the amphidromous fish Sicyopterus japonicus (Gobioidei: Sicydiinae)

Abstract

Amphidromy is a type of diadromous life history that includes migrations between the ocean and freshwater. The amphidromous fish Sicyopterus japonicus spends its early life in the ocean and then migrates back to estuaries as post-larvae, but little is known about their transition between marine and freshwater habitats. To understand their oceanic larval duration and recruitment mechanism, an intensive three-year survey was conducted in the Ota River, Wakayama, Japan. Their general amphidromous migratory patterns were confirmed using otolith Sr:Ca ratios that all showed an oceanic larval phase. Newly recruited S. japonicus post-larvae were sampled daily in the estuary from March to September 2006, 2007 and 2008. Post-larvae were collected mainly from April to June (spring to early summer), but the number of post-larvae collected varied among years (N=12,766 in 2006, 372 in 2007 and 942 in 2008). No clear relationships were observed between environmental factors, such as moon phase and water temperature, and the number of post-larvae collected each day, but post-larvae were observed to recruit with flood tides (flood tide transport). Standard lengths of collected post-larvae ranged 22.5–34.0 mm, and the oceanic larval durations estimated by their otoliths ranged from 176 to 283 days (mean: 219 days). Although there were no clear trends in length, body weight, condition factor and larval duration during each recruitment season, most post-larvae recruited at a constant size of approximately 27 mm. Our results indicated that amphidromous S. japonicus have a limited recruitment season and a generally constant body size when they recruit. Their recruitment season was shorter than that of similar species inhabiting tropical and subtropical regions, which might be an adaptation to seasonality of the temperate region. It is possible that body size is a more important factor affecting the timing of recruitment of S. japonicus post-larvae than environmental factors.