Probable Pheromonal Attraction of Sexually Mature Brook Trout to Mature Male Conspecifics

—Invasions of nonnative species are a leading cause of the loss of populations of cutthroat trout Oncorhynchus clarki in the western United States, but effective control of invasions of species such as brook trout Salvelinus fontinalis are expensive, labor intensive, and not species specific. Because salmonids respond to chemical cues, we hypothesized that pheromonal attraction could be exploited to capture brook trout. We conducted two tests as brook trout spawning began in 1999 and 2000 in two streams in Wyoming, using hoop nets seeded with sexually mature male brook trout, sexually mature female brook trout, or no fish. Catches of sexually mature fish in both tests were nonrandom (P , 0.001); traps seeded with sexually mature male brook trout resulted in a disproportionately high number of captures. Catches of fish that were not sexually mature were not significantly different from expected values (P . 0.075). In the second test, we estimated that 23% of adult fish were captured in hoop nets in 8 d. Catches were biased toward males by 10:1, and once-captured fish were rarely recaptured. We believe that with further refinements, this approach may be useful for sampling or manipulating brook trout and perhaps other salmonid populations. Invasions of nonnative species are believed to be the leading cause of the loss of populations of many subspecies of native salmonids in the western United States (Behnke 1992). For example, brook trout Salvelinus fontinalis have successfully invaded and replaced many populations of cutthroat trout Oncorhynchus clarki (Young 1995 and chapters therein). To maintain or restore populations of cutthroat trout to streams, biologists often attempt to remove brook trout by applying toxicants, such as rotenone or antimycin (Gresswell 1991), or repeatedly electrofishing stream reaches (Thompson and Rahel 1996). * Corresponding author: [email protected] 1 Present address: U.S. Forest Service, Rocky Mountain Research Station, 800 East Beckwith Avenue, Missoula, Montana 59801, USA. 2 Present address: 525 West 4th Avenue, Denver, Colorado 80223, USA. 3 Present address: 308 Michigan Avenue, Apartment 105, East Lansing, Michigan 48823, USA. Received November 30, 2001; accepted May 20, 2002 Both techniques have shortcomings. Applications of toxicants inconsistently eradicate brook trout from target waters, apparently because these fish occupy habitats that are difficult to treat or are overlooked (Rinne and Turner 1991). Consequently, even repeated treatments have failed to eliminate brook trout from some waters (B. Wengert, Wyoming Game and Fish Department, personal communication). A second concern involves the effects of toxicants on nontarget vertebrate and invertebrate species, including unintended effects downstream of treatment areas (e.g., Demarais et al. 1993). The primary failing of electrofishing is that even multiple passes are unlikely to capture all fish present (Thompson and Rahel 1996), particularly in inaccessible or complex habitats such as beaver ponds or channels with extensive accumulations of woody debris. Rarely has the technique been successful, and then only in narrow, short streams with simple habitats (Kulp and Moore 2000). There are also concerns that nontarget fish will be harmed by repeated exposure to an electrical field (Thompson et al. 1997). Consequently, additional removal techniques are de-