Interactions between rainbow trout and bridgelip sucker spawning in a small Washington stream

We investigated the interactions between two sympatric native fishes (rainbow trout and bridgelip sucker) spawning in Umtanum Creek, a tributary of the Yakima River in central Washington. We used redd surveys to determine spawn location and timing of rainbow trout and bridgelip sucker. We determined microhabitat characteristics of rainbow trout and bridgelip sucker spawning areas and measured the degree of disturbance of rainbow trout redds by spawning bridgelip sucker as well other rainbow trout. Emergence traps were used to compare the mean survival to emergence (STE) rate of experimentally protected rainbow trout redds to those left unprotected. The first behavioral observations of bridgelip sucker spawning are also reported here. Both temporal and spatial overlap in spawning of rainbow trout and bridgelip sucker were observed. No differences were found in the water depth and water velocity used by rainbow trout and bridgelip sucker for spawning. Spawning behavior of bridgelip sucker entailed extensive substrate modification (i.e., digging) prior to the release of gametes. During spawning surveys, bridgelip sucker were observed spawning in 3 of 21 rainbow trout redds in 1994 and 5 of 15 rainbow trout redds in 1995. Estimated mean STE of protected rainbow trout redds (11.2%) was significantly greater (P<0.02) than rainbow trout redds that were unprotected (2.6%). Of those unprotected rainbow trout redds, the mean STE of rainbow trout redds disturbed by bridgelip sucker and other rainbow trout was 0.4% and 2.8%, respectively. When spawning habitat is severely limited, emergence rates of earlier spawning rainbow trout may be reduced due to redd superimposition by bridgelip sucker.