Evaluation of an Unsuccessful Brook Trout Electrofishing Removal Project in a Small Rocky Mountain Stream

Abstract

Abstract.-In the western United States, exotic brook trout Salvelinus fontinalis frequently have a deleterious effect on native salmonids, and biologists often attempt to remove brook trout from streams by means of electrofishing. Although the success of such projects typically is low, few studies have assessed the underlying mechanisms of failure, especially in terms of compensatory responses. A multiagency watershed advisory group (WAG) conducted a 3-year removal project to reduce brook trout and enhance native salmonids in 7.8 km of a southwestern Idaho stream. We evaluated the costs and success of their project in suppressing brook trout and looked for brook trout compensatory responses, such as decreased natural mortality, increased growth, increased fecundity at length, and earlier maturation. The total number of brook trout removed was 1,401 in 1998, 1,241 in 1999, and 890 in 2000; removal constituted an estimated 88% of the total number of brook trout in the stream in 1999 and 79% in 2000. Although abundance of age-1 and older brook trout declined slightly during and after the removals, abundance of age-0 brook trout increased 789% in the entire stream 2 years after the removals ceased. Total annual survival rate for age-2 and older brook trout did not decrease during the removals, and the removals failed to produce an increase in the abundance of native redband trout Oncorhynchus mykiss gairdneri. Lack of a meaningful decline and unchanged total mortality for older brook trout during the removals suggest that a compensatory response occurred in the brook trout population via reduced natural mortality, which offset the removal of large numbers of brook trout. Although we applaud WAG personnel for their goal of enhancing native salmonids by suppressing brook trout via electrofishing removal, we conclude that their efforts were unsuccessful and suggest that similar future projects elsewhere over such large stream lengths would be costly, quixotic enterprises.

Responses of Aquatic Non-Native Species to Novel Predator Cues and Increased Mortality

“…Of the case studies examined, 16 showed evidence of compensatory recruitment (Table 3. 1). Of case studies showing signs of compensation, the increase in juvenile and overall abundance was sufficient in five cases to suggest overcompensation may have occurred (Smith et al 1996, Meyer et al 2006, Weidel et al 2007, Thuesen et al 2011, Shepard et al 2014. The majority of unsuccessful removal efforts showed evidence of compensation (Fisher's exact p < 0.001; Table 3.2), though compensation appeared to have occurred in a few efforts that resulted in the successful reduction or eradication of the target species.…”

“…Similarly, removal of brook trout, Salvelinus fontinalis, resulted in a 789% increase in individuals < 1 year old, comprising an estimated 74% of the population, up from 16% at the beginning of electrofishing efforts (Meyer et al 2006). If overcompensation occurred in Bodega Harbor with similar impacts, our model would be capable of detecting the ensuing demographic shift.…”

“…Overcompensation occurs when population size increases as a result of the increase in the number of juveniles (Smith et al 1996, Meyer et al 2006, Weidel et al 2007, Thuesen et al 2011. As overall abundances of Carcinus in Bodega Harbor not only failed to increase but in fact declined post-removal, our data do not support the conclusion that overcompensation occurred in Bodega Harbor.…”

“…insects (Nicholson 1954, Postma et al 1994, Moe et al 2002 and fish (Smith et al 1996, Meyer et al 2006, Weidel et al 2007, Ohlberger et al 2011, Thuesen et al 2011). …”

“…For example, a seven year electrofishing effort to remove an invasive population of small mouth bass (Micropterus dolomieu) from Little Moose Lake, New York, resulted in a massive increase in juvenile abundance, causing a large increase in overall bass abundance (Weidel et al 2007, Zipkin et al 2008. Similar efforts to remove other invasive freshwater fish populations also coincided with increased juvenile abundance, causing an overall increase in population size (Smith et al 1996, Meyer et al 2006). Thus, a population's response to management strategies will depend on its potential for overcompensation, along with demographic parameters.…”