Bull Trout Trends in Abundance and Probabilities of Persistence in Idaho

Abstract

We estimated the population growth rates of Bull Trout Salvelinus confluentus and used population growth models to evaluate observation error and estimate Bull Trout persistence probabilities by using 25 data sets (averaging 19 years of record) that indexed abundance across Idaho. These data sets were derived from a variety of fish sampling techniques, including weirs, screw traps, redd counts, daytime snorkeling, electrofishing, and angler creel. Bull Trout populations in Idaho were relatively stable prior to 1994, but after 1994, substantially more population growth rates trended statistically upward (n = 14) than downward (n = 3). Average intrinsic rates of population change were 0.01 (SE = 0.03) prior to 1994 and 0.07 (SE = 0.02) after 1994; across all years of data, the rate of change averaged 0.07 (SE = 0.02). Fifty‐nine percent of the data sets had zero to minimal observation error according to Gompertz state‐space model estimates; observation error was least common in data from screw traps and redd counts and most common in snorkel data. Gompertz‐type density‐dependent models were most often the best‐fitting models for Bull Trout population growth. Moreover, few of the most reliable modeling results (i.e., those from data sets estimated to have zero to minimal observation error) contained a period effect or time (i.e., year) effect, suggesting that carrying capacity generally did not differ between the pre‐1994 and post‐1994 periods and generally was not trending positively or negatively through time. Parametric bootstraps predicted that the mean probability of falling below the quasi‐extinction level of 20 adults in the next 30 years was 9.8% (median = 4.7%) for data sets estimated to have zero to minimal observation error. The weight of evidence from our modeling results suggests that for most Bull Trout populations in Idaho, abundance is stable or increasing and the risk of extirpation in the foreseeable future is low.Received March 12, 2013; accepted November 18, 2013Published online February 10, 2014

Evidence of Climate-Induced Range Contractions in Bull Trout Salvelinus confluentus in a Rocky Mountain Watershed, U.S.A

“…In addition, such trends may be superimposed on long-term variation in abundance dictated by climate cycles such as the Pacific Decadal Oscillation [60]. Nevertheless, correlations between bull trout abundance and broad-scale climate cycles (or this species' abundance and that of other salmonids) are weak [61], [62], [63]. Analyses that rely on occupancy, rather than abundance, may be less vulnerable to temporal fluctuations.…”

New Insights into the Ecology of Adfluvial Bull Trout and the Population Response to the Endangered Species Act in the North Fork Lewis River, Washington

“…), while others remain stable or are potentially increasing (Erhardt and Scarnecchia ; Meyer et al. ). For example, 61% of the core populations in Alberta are considered to be declining, while 39% are stable or increasing (Alberta Sustainable Resource Development ).…”

“…Today, Bull Trout are listed as "Threatened" in the United States under the Endangered Species Act (ESA) and ranked "Of Special Concern or Threatened" across much of their range by the Committee on the Status of Endangered Wildlife in Canada. Population trends appear to vary considerably across regions, with large numbers of migratory and resident populations exhibiting significant declines in adult abundance Kovach et al 2018), while others remain stable or are potentially increasing (Erhardt and Scarnecchia 2014;Meyer et al 2014). For example, 61% of the core populations in Alberta are considered to be declining, while 39% are stable or increasing (Alberta Sustainable Resource Development 2012).…”

Demographic Characteristics of an Adfluvial Bull Trout Population in Lake Pend Oreille, Idaho

“…First, the Bull Trout population declined in size and age between 1998 and 2008, which is consistent with lower mortality from either no-kill harvest regulations (High et al 2008) or favorable stream discharge (Copeland and Meyer 2011). Increased recruitment was indicated by increasing redd counts (Hansen et al 2010), although redd counts did not continue to increase after 2006 (Meyer et al 2014). Second, growth of Bull Trout at ages 1-4 increased while growth of ages 4-9 Bull Trout declined between 1998 and 2008, likely because of favorable stream conditions for small (young) Bull Trout (Meyer et al 2014), whereas kokanee prey supply for large (old) Bull Trout declined as competitor abundance (Lake Trout) increased (Hansen et al 2008(Hansen et al , 2010.…”

“…Increased recruitment was indicated by increasing redd counts (Hansen et al 2010), although redd counts did not continue to increase after 2006 (Meyer et al 2014). Second, growth of Bull Trout at ages 1-4 increased while growth of ages 4-9 Bull Trout declined between 1998 and 2008, likely because of favorable stream conditions for small (young) Bull Trout (Meyer et al 2014), whereas kokanee prey supply for large (old) Bull Trout declined as competitor abundance (Lake Trout) increased (Hansen et al 2008(Hansen et al , 2010. Last, Bull Trout abundance did not change between 1998 and 2008 but the number of fish shorter than 400 mm increased as a fraction of total abundance, as recruitment (and redd counts) increased (Hansen et al 2010).…”