Silver Creek is a tributary to the Little Wood River in Blaine County, Idaho. Silver Creek originates at the confluence of two main spring creek tributaries, Stalker Creek and Grove Creek on the Nature Conservancy’s Silver Creek Preserve. Silver Creek and its tributaries provide a popular destination fishery for rainbow and brown trout. Several regulation scenarios exist throughout the Silver Creek drainage allowing multiple angling opportunities including fly fishing only, catch and release; no bait, barbless hook, catch and release; bait allowed, none between 12 inches and 16 inches; and general rules. The Silver Creek fishery, including it’s tributaries, has been the focus of several studies over the past 10 years including; monitoring brown trout and rainbow trout movements (Young et al. 1997), describing the fish community structure (Wilkison 1996), analyzing genetic population structure (Williams et al. 2000), and evaluating whirling disease presence (Spall et al. 1996). Standard IDFG population monitoring transects and survey protocols were defined in 2004. In 2007 a standard population monitoring survey was completed to evaluate long-term trends in population abundance and structure.

The Silver Creek system was sampled in 2007 at three locations encompassing Silver Creek and Stalker Creek to evaluate trends in population abundance and structure. Sampled segments included: lower Stalker Creek; Silver Creek, cabin site; and Silver Creek, Martin Bridge (Appendix C). Sample efforts were conducted using a drift boat electrofishing setup (Appendix A). Fish were sampled on two passes separated by seven days. Sampling was conducted during daylight hours on the lower Stalker Creek reach and during dark hours on the remaining two reaches. Fish were identified, measured (TL), weighed (g), marked, and released during the first sampling pass - marking run. Weights were taken only during the marking run. Caudal fin clips were used to mark rainbow and brown trout equal or greater than 100 mm for identification in the recapture run. Other collected species were not marked. Rainbow and brown trout were counted, measured, and observed for marks in the second (recapture run) electrofishing pass. Estimates of rainbow trout and brown trout abundance were made using a modified-Peterson mark-recapture estimator (Ricker 1975). Calculations were completed in FA+ (Fisheries Analysis Software, Version 1.0.8). Estimates were calculated in 100 mm increments for fish equal or greater than 100 mm total length. A minimum of five recaptures was required to complete estimates. Length groups were pooled when less than five recaptures were made within an individual length group. Estimates of rainbow and brown trout equal or greater than 100 mm were reported for evaluation of long term trends.

Marking run data were used to describe the sampled fish community and estimate population parameters. Estimated population parameters included relative stock density and relative weight. Relative stock densities (RSD – 400) were determined for rainbow trout and brown trout collected in each transect to describe the available preferred component of the fishery. RSD-400 was calculated as the number of fish ≥ 400 mm divided by the number of fish ≥ 200 mm (Ney 1993). Relative weight was calculated for rainbow trout and brown trout as a measure of fish condition and reported as mean relative weight by 100 mm length groups (Anderson and Neumann 1996, Simpkins and Hubert 1996). Population estimates and relative stock densities (RSD-400) from 2007 were compared to population estimates and RSD-400 values generated from 2001 and 2004 data (Warren et al. 2001, Ryan and Megargle 2004). Standard locations were sampled among all years. Comparisons of population estimates by species were for all fish equal or greater than 100 mm.

Mean length at age and annual mortality of rainbow and brown trout were estimated from collected otoliths. Otoliths were collected from representative groups of rainbow trout and brown trout. One sample group for each species was collected from a combination of all sampled transects. Age was estimated from otoliths by cleaning and reading in whole view using a dissecting microscope at 30X-40X magnification. Otoliths were cut using a rotary cutting tool or broken and read in cross-section if the whole view was not considered to be clear. Estimated ages were applied to an age-length key, extrapolated to all sampled fish in the marking pass, and used to determine mean length at age (Devries and Frie 1996). Mean length at age was determined for all age classes sampled. Mean length at age four was reported as a benchmark of growth over time. Catch curves were used to estimate mortality and survival. (Van Den Avyle 1993).

Catch Curves were generated in FAST (Fisheries Analysis and Simulation Tools, Version 2.1). Habitat data was collected on a separate date following electrofishing efforts. Transect lengths and widths were measured with an electronic rangefinder (Leica, LRF 900 Rangemaster) and or measuring tape at set intervals. Interval distance was chosen randomly prior to conducting measurements. Transects waypoints were marked for future replication using a Magellan Sporttrack Topo Global Positioning System (GPS) (Appendix C).

Silver Creek mark and recapture electrofishing samples were completed from 19 to 21 June, 2007 and 26 to 28 June, 2007, respectively. Habitat data was collected on 7 July, 2007. Transect length at the lower Stalker Creek; Silver Creek, Cabin site; and Silver Creek, Martin Bridge locations were 795 m, 1190 m, and 840 m, respectively. Mean transect widths at the lower Stalker Creek, Silver Creek - cabin site, and Silver Creek - Martin Bridge locations were 9.00 m, 29.74 m, and 18.12 m, respectively.

Fish sampled in the Stalker Creek transect included wild rainbow trout (39%), brown trout (43%), bridgelip sucker (13%), longnose dace (3%), and speckled dace (3%) (Table SILVERSTALKER 07 CATCH SUMMARY).