Salmonid Field Protocols Handbook: Techniques for Assessing Status and Trends in Salmon and Trout

Rotary Screw Traps and Inclined Plane Screen Traps

Gregory C. Volkhardt, Steven L. Johnson, Bruce A. Miller, Thomas E. Nickelson, and David E. Seiler

doi: https://doi.org/10.47886/9781888569926.ch13

Inclined plane screen traps and rotary screw traps have long been used by biologists to capture downstream migrating juvenile anadromous salmonids from medium- and large-sized streams (Schoeneman et al. 1961; Seiler et al. 1981; Kennen et al. 1994) and from small tributary streams (Solazzi et al. 2000). In its original fixed screen design, the floating inclined plane screen (scoop) trap has been used to capture juvenile migrants for more than 40 years (Schoeneman et al. 1961). William Humphreys replaced the fixed screen with a traveling screen powered by a paddle wheel and added a debris drum at the back of the live well (Humphreys trap) in 1966 (McLemore et al. 1989). The rotary screw (screw) trap was developed and patented by two biologists from the Oregon Department of Fish and Wildlife (ODFW) in the late 1980s. All these traps are anchored at a fixed point in the stream channel and intercept a portion of the juvenile salmonids or smolts migrating downstream.

Traditionally, fishery managers have relied on escapement estimates to monitor anadromous salmonid population status and management effectiveness (Ames and Phinney 1977; Beidler and Nickelson 1980; Hilborn et al. 1999); however, estimation of population abundances at earlier life stages enables partitioning survival among life stages and developing hypotheses for restoration actions (Moussalli and Hilborn 1986; Mobrand et al. 1997). Juvenile fish traps have often been used to estimate the abundance (Tsumura and Hume 1986; Baranski 1989; Orciari et al. 1994; Thedinga et al. 1994; Letcher et al. 2002; Johnson et al. 2005), timing (Wagner et al. 1963; Hartman et al. 1982), size (Orciari et al. 1994; Olsson et al. 2001), survival (Schoeneman et al. 1961; Wagner et al. 1963; Tsumura and Hume 1986; Olsson et al. 2001; Letcher et al. 2002), and behavior (Brown and Hartman 1988; Roper and Scarnecchia 1996) of downstream migrant anadromous salmonids. In many salmon-bearing systems, population abundance is only monitored during the adult (spawner) stage. Additional monitoring of smolt abundance is a particularly powerful tool because it enables partitioning mortality between the freshwater life stages (egg-to-smolt) and marine life stages (smolt-to-adult).

While estimating smolt abundance is the most common reason for operating an inclined plane screen trap or screw trap, the capture of downstream migrants has wide utility. Traps can be used to monitor the effects of river management on wild stocks, such as the effectiveness of diversion, lock, and dam management. They are powerful tools for validating assumptions regarding the effects of watershed restoration programs and land-use policies on fish populations (Solazzi et al. 2000; IMW SOC 2004; Johnson et al. 2005). They can also be used to assess survival between life stages, such as egg-to-smolt survival or parr-to-smolt overwinter survival (Solazzi et al. 2000; Seiler et al. 2003; Johnson et al. 2005). Smolt-to-adult survival estimates can be developed for wild populations by coded wire tagging smolts that are captured in inclined plane screen traps and screw traps and estimating the escapement and fishery impacts on the tagged population.