Spawning ecology of Japanese surf smelt, Hypomesus pretiosus japonicus (Osmeridae), in Otsuchi Bay, northeastern Japan

The spawning habits of Japanese surf smelt, Hypomesus pretiosus japonicus (Japanese name chika), were studied from 1992 to 1995 at Akaiso Beach in Otsuchi Bay off the Pacific coast of northern Honshu. Investigations were conducted in relation to time, tidal rhythm, sand grain size on the spawning ground, and sex and age of spawners. Spawning occurred from late March to early May with its peak at the spring tide period during full moon in April. Before dark, fish gathered to school from 1–3 m depth, and 10–20 m off the shoreline of the spawning beach. Just before dark, they repeatedly approached the shoreline and stayed near the shoreline to spawn just after dark. In the spawning process, three intervals, i.e. aggregation, approach, and spawning, were recognized and these intervals are thought to be functionally connected. Mating occurred in a unit or group formed by one female and several males. During 1992–1995, except for 1994, spawning took place in a restricted region of the beach where backwash was stronger than other areas, and most sand grains were more than 1 mm in diameter and coarser than those of other areas. Although spawning was observed near the shoreline (34 to 120 cm tide level), most of the eggs were dispersed by backwash and each became attached to a few sand grains close to a step formed at 1–2 m depth off the spawning site. Spawners were composed mainly of age 0+ fish and some 1+ fish. Males usually outnumbered females on the spawning ground. Their spawning styles are discussed in relation to their ecology and habitat.