Bull trout population assessment and life-history characteristics in association with habitat quality and land use: a template for recovery planning Annual Progress Report for 2003

Research was initiated in 2002 and continued and expanded through 2003. In 2003, we sampled 40% of the South Fork Walla Walla River (SFWW) study area (20% from 2002 reaches, 20% new), 468 fish were tagged, and all associated demographic and project assessment data were collected, analyzed, and synthesized. In addition, we completed detailed habitat assessments in the SFWW in 21 reaches; habitat availability and habitat use were quantified at the microhabitat scale, the mesohabitat (unit) scale, and at the macrohabitat (reach) scale. In 2003, we also expanded the fish population tagging and monitoring into the North Fork Umatilla River (NFUMAT); 20 reaches were sampled, 81 fish were tagged, and all baseline demographic and assessment data were collected, analyzed, and synthesized. Equipment was purchased and permits (US Forest Service) were received to install a passive PIT-tag antennae, (slated for June 2004), at the lowermost reach of the NFUMAT study area.

In 2003, population estimation resulted in an abundance estimate of 8533, a slight, but insignificant increase over 2002. These data suggest there is a large population of bull trout in the SFWW, ranging in size from 50 to 620 mm. Population estimates for the NFUMAT indicate that there are about 2500 fish (but with less certainty); bull trout were generally smaller ranging from 75 to 400 mm. The relationship between redd count-based abundance estimates and population estimates based on mark-recapture, for large fish, indicated that even with the application of a high estimate of fish per redd, redd counts significantly underestimate the population abundance. As observed in the previous year, in 2003, annual population estimation based on mark-resight appeared to be a robust technique, as demonstrated by several different data comparisons and evaluations.

Most fish movement in the SFWW occurred between the months of May to November, at both detection sites, and for all sizes of fish. Fish condition was greatest, and the most variable, for the intermediate adult size class (220 – 330 mm) and declined overall from 2002 to 2003, in the SFWW. Diet analyses demonstrated that bull trout in both systems consume a high proportion of aquatic invertebrates with less fish; a relatively high degree of cannibalism was noted. Future research, monitoring, assessment, and evaluation plans are discussed.