Spawning and swim-up habit of Biwa salmon Oncorhynchus sp. in an inlet stream of Lake Biwa, central Japan

Biwa salmon (Oncorhynchus sp.), a species endemic to Lake Biwa in central Japan, has been stocked in Lake Biwa and its inlets for fisheries purposes since the Meiji period. In recent years, however, there has been a growing demand for ecosystem-based fishery resource management. I investigated spawning and swim-up habit of Biwa salmon in the Chinai River, an inlet of Lake Biwa, to provide fundamental knowledge on ecosystembased management for this species. Spawning redds of Biwa salmon were found from October to December, with the peak in November, suggesting that spawning period of Biwa salmon is longer and later than that of cherry (O. masou masou) and satsukimasu salmon (O. masou ishikawae) . Water depth and current velocity at spawning redds were 10-44 cm and 16-98 cm/s, respectively, with substrates dominated by pebbles (17-64 mm) . The number of egg pocket per redd was one or two, suggesting that Biwa salmon tended to create multiple small redds with a small number of eggs, rather than a single large redd containing a large number of eggs. Biwa salmon fry at the swim-up stage were caught from January to May. This swim-up period is also longer than the swim-up period of cherry salmon. Further, beginnings of the swim-up period (January) recorded by the present study (2011-12) were much earlier than those reported in this study river in the past (mid March in 1970-80's) . Considering increases in river water temperatures due to global warming, it would be better to protect the late spawning stock in December than the stock in October, because the latter is easily influenced by high water temperatures.