Abstract
Pond smelt, Hypomesus nipponensis, inhabit fresh, brackish, and oceanic waters, and support substantial commercial fisheries in Japanese lakes. Pond smelt in Lake Ogawara, northern Japan, display a bimodal body length distribution during the spawning season, despite being O+ fish. Analyses of otolith microstructure and microchemistry were utilized to discriminate anadromous from resident individuals, and revealed that individuals smaller than 60 mm SL were resident, those between 60-80 mm were mixed resident and anadromous, and those larger than 80 mm were anadromous. Intensive research on the reproductive ecology identified spawning localities in the lake and inflowing rivers. Although only anadromous fish spawned in inflowing rivers, spawners in the lake were a mixture of anadromous and resident individuals, suggesting that anadromous and resident spawning groups share a common spawning ground. These fish spawn during almost the same period from mid March to early May. Therefore, reproductive isolation does not appear to occur, and genetic differentiation has not been found through isozyme and mtDNA analyses. The anadromous and resident life history styles appear to be ecological variations within a single population. Lastly, qualitative and quantitative contributions of migratory and non-migratory pond smelts to the next generation were examined and heterogeneity in the life history of this population was discussed.
