DISTRIBUTION OF 2 CONGENERIC CHARRS IN STREAMS OF HOKKAIDO ISLAND JAPAN - CONSIDERING MULTIPLE FACTORS ACROSS SCALES

Salvelinus leucomaenis (white-spotted charr) and S. malma (Dolly Varde n) are distributed throughout Hokkaido Island, Japan, but sites where they occur in sympatry are rare. In general, S. malma inhabit upstream reaches and S. leucomaenis extend downstream to the ocean. Factors in fluencing their distribution were analyzed at four spatial scales rang ing from the whole island to individual stream pools. At the island sc ale, S. leucomaenis were found in the warmer south-west region and at lower altitudes elsewhere, whereas S. malma were found in the colder n orth-east and at higher altitudes. At a regional scale, the downstream limit of S. malma and upstream limit of S. leucomaenis shifted to low er altitude from south-west to north-east across the island, coinciden t with the decrease in temperature. Further analysis showed that trans ition points from S. leucomaenis or sympatry to S. malma in individual watersheds were closely related to an index of cumulative mean monthl y temperatures exceeding 5-degrees-C. However, at the scale of a singl e watershed, the transition occurred at different altitudes, gradients , and temperatures in two tributaries, apparently because stream disch arge, habitat, and disturbances from floods interacted with these abio tic factors to limit distribution. The two charr species developed int erspecific dominance hierarchies in individual pools, and there was st rong complementary density compensation among stream pools that could be explained by intespecific competition but not by differences in hab itat. However, patterns at watershed and regional scales suggested tha t interspecific competition interacts with temperature in complex ways . We conclude that the importance of various abiotic and biotic factor s in shaping Hokkaido charr distributions depends on the scale at whic h they are viewed.