An Evaluation of Redd Counts as a Measure of Bull Trout Population Size and Trend

Abstract

The use of redd counts to monitor abundance and trend of bull trout Salvelinus confluentus has been clouded by uncertainties concerning measurement error, life history variation, and correspondence of redd counts to adult population size. We compared census redd counts with population estimates of mature females for a migratory fluvial population of bull trout (primarily ≥ 300 mm fork length) and for a population of small (<200 mm), likely resident, bull trout. We also compared the measurement error of the experienced surveyors who conducted the redd counts to that of a group of inexperienced surveyors. Although the regression of redd counts on adult females for the migratory population was statistically significant, a large proportion of the variation in the relationship was unexplained (r 2 = 0.47). Despite that variation, redd counts accurately reflected a greater than 50% decline in the population over 10 years; however, 5-year trends in redd counts could be misleading. Power analysis parameterized by using the variation in the number of females per redd and measurement error of experienced surveyors indicated that minimum declines of 44-56% or increases of 78-118% over 10-15 years would be necessary for detection using traditional statistical criteria. Geometric mean abundance of migratory adults derived from redd counts and adults-per-redd values from the present study and published averages were similar to measured adult numbers in most cases. For both migratory and resident populations, redd counts by experienced surveyors were substantially more accurate and precise than those by inexperienced surveyors. Counts of migratory bull trout redds were more accurate and precise than counts of resident bull trout redds, which were significantly smaller and consistently underestimated. Thus, bull trout redd counts can be used to estimate abundance levels and to detect substantial longer-term changes in abundance, particularly for migratory populations. However, the reliability of the counts depends on the skill of the surveyors.

Modelling individual variability in growth of bull trout in the Walla Walla River Basin using a hierarchical von Bertalanffy growth model

“…Upstream migrating adults (≥270 mm FL) were collected in a hoop net as they exited a fish ladder at a diversion dam at the downstream limit of the spawning area; downstream migrating fish (mostly subadults, <270 mm FL) were collected in a rotary screw trap located 0.5 km upstream of this dam. Upstream migrants were scanned by ultrasound to identify sex and maturity (Howell & Sankovich, ). The trap for upstream migrants was operated May to October in all years, while the trap for downstream migrating fish was operated March to October in some years and year‐round in other years (Howell et al., ).…”

Population Changes after 14 Years of Harvest Closure on a Migratory Population of Bull Trout in Idaho

“…The use of redd counts for determining trends, however, has been questioned by various authors (Rieman and McIntyre 1996;Dunham et al 2001;Al-Chokhachy and Budy 2005). For Bull Trout, this requires several years of data collected by experienced personnel (Howell and Sankovich 2012). The accuracy of redd-count trend data for analysis should be critically examined because it has major impacts on carrying capacity estimates and, hence, recruitment estimates (from egg to age 0) for the age-structured models we tested.…”

A Simulation Assessment of Model‐Assisted Estimation of Steelhead Redd Abundance

“…Although redd removal bias can potentially be corrected using the model-assisted approach, there is still potential for actual detection error (i.e., redds that are present but not observed). Several authors suggested that false identification of the redds of Bull Trout Salvelinus confluentus could occur (Dunham et al 2001;Muhlfeld et al 2006;Howell and Sankovich 2012). Gallagher and Gallagher (2005) suggested that false redd identifications (i.e., false positive) were not likely in steelhead redd surveys conducted by experienced surveyors in California; thus, most of the observation error likely arises from removal bias.…”

“…Steelhead generally begin spawning in late winter and continue through late spring (Susac and Jacobs 1999;Zimmerman and Reeves 2000;McMillan et al 2007) when flows are often high and variable. Thus, the potential for removal is high compared with other salmonids that spawn in the fall when flows are more stable (Rieman and Myers 1997;Howell and Sankovich 2012).…”