Population Fluctuations of Coastal Cutthroat Trout in Irely Creek, Washington: a Decade-Long ‘Natural Experiment' of Drought Cycles
Tuesday, September 6, 2011: 11:15 AM
4C-3 (Washington State Convention Center)
In the pristine Irely Creek watershed (upper Quinault River drainage) within Olympic National Park, coastal cutthroat coexists (as a native-adfluvial run) with anadromous coho salmon and two resident-fish and several amphibian species. During 2001-2002, cutthroat redds and fry were abundant in the mainstem, particularly in its middle segment, with escapement estimates (twice the redd count) being 48-106. More recently, the population has declined by an order of magnitude, reflecting summer/fall droughts that have often dried up adult habitat in Irely Lake. Although redd counts have risen when summer/fall seasons have been wetter during 2003-2010, they haven’t reached 2001-2002 counts via regular lake dry-outs, including two consecutive dry-outs during 2002-2003. Hence, the population is showing an overall downward trend with some smaller-scale oscillations coupled with escapement estimates during 2003-2010 ranging from 4 to 32. During 2002-2003, the population also spawned in a headwater tributary, but (a) hydraulic drops created by logs in the upper mainstem and/or (b) reduced total-run size hindered tributary spawning in other years. Population extirpation hasn’t occurred, however, given the consistent presence of cutthroat fry and juveniles in this watershed as assessed by stream-walk, seining, and snorkeling observations. A recent (2010-2011) upsurge in the coho population from changing oceanic/harvest conditions off northern Washington is expected to benefit (via enhanced growth from carcass nutrients) the cutthroat run during 2011, because the lake didn’t dry out during 2010. We hope to evaluate this hypothesis with one last spring-redd count and summer/fall seining work to end our long-term study. We’re examining hydrologic trends in relation to lake size via (a) flow comparisons with nearby watersheds and (b) historical, aerial photographs of Irely Lake.
Although we developed hydraulic and substratum suitability models for cutthroat spawners during 2001-2003, we’ve also examined these habitat variables more recently to compare habitat preferences for population boom vs. crunch (declining) periods. We’ve also examined alternative spawning-habitat criteria via larger-scale, more flow-invariant parameters at and near redds, including pool/run (PR) shape, residual-PR depth, relative (to PR length) distance to the hydraulic control, and bed slope. The results show that cutthroat spawn mostly in runs (with few redds in pools and riffles), but the runs are often relatively narrow with the redds being near the downstream hydraulic controls. Hence, pool-tail and between-riffle runs (the latter without intervening pools) were more commonly spawned in than pool-head runs, with the redds often being on upsloping beds where downwelling flow is likely.