USE OF DIVE COUNTS TO ESTIMATE FISH POPULATION ABUNDANCE IN THE ROCK CREEK – CRESTA REACHES OF THE NORTH FORK FEATHER RIVER, CALIFORNIA. Prepared For: Technical & Ecological Services Pacific Gas & Electric Company 3400 Crow Canyon Road San Ramon, CA 94583 Prepared By: Thomas R. Payne & Associates 890 L Street Arcata, CA 95521 707-822-8478 Contributors: Mark Allen, Thomas Gast February 7, 2007 ABSTRACT Direct observation dive counts were conducted by a team of divers in individual habitat units in the Cresta Reach and Rock Creek Reach of the North Fork Feather River in 2002 (a pilot study), 2004, 2005, and 2006, under base flow conditions elevated from historical levels. Habitat units were selected for diving using a stratified random design that allowed estimation of indexes of fish abundance within species, size class, habitat type, and reach strata. Annual comparisons of index abundance were possible due to consistency in sampling designs and field methodologies, and because stream conditions (e.g., streamflow, water temperature, and water visibility) remained relatively similar in each year of study. Statistically significant differences between annual estimates of abundance were determined through assessment of index confidence intervals or by explicitly estimating the difference in abundance between occasions (2005-2006 only). Differences in fish index densities between reaches were minor for rainbow trout, but were consistently higher in the Rock Creek Reach for adult-sized (>10 inches) hardhead, pikeminnows, and suckers. Smallmouth bass, in contrast, were consistently more abundant in the Cresta Reach. Index densities of fry (<5 inches) were not consistently different between reaches, but juvenile (5-10 inches) non-trout species were notably absent from the Cresta Reach in each year of study. Differences in index abundance between habitat types are difficult to assess due to expected differences in fish detectability, but abundance trends clearly showed higher densities of rainbow trout (especially fry and juveniles) in run and riffle habitats, whereas all size classes of hardhead and bass were more common in pool habitats. Pikeminnows and suckers occurred more evenly among habitat types, but few large adults were observed in riffles. Annual trends were dominated by strong increases in fry of all species in 2005 and 2006 (bass only in 2005). Juvenile adult trout showed a general decline in abundance from 2004 to 2006, with 2006 densities less than 50% of the 2004 maxima. Juvenile and adult size classes of all non-salmonid species showed little consistency in annual trends, but densities in 2006 were typically higher than in 2004. The temporal effects of changes in base flows and annual differences in high winter and spring flows, and the spatial effects of reach and habitat location on observed indexes of abundance are discussed. High spring flows during 2005 and 2006 may be associated with high recruitment of trout and non-salmonid species during those years, but the mechanisms by which recruitment appeared enhanced remain unknown. The absence of juvenile hardhead, pikeminnows, and suckers in the Cresta Reach is considered in relation to potential predation by bass and differential recruitment from above the study area. The observed index densities of trout in the study area were compared to data from other California locations. Index densities in the North Fork Feather River were similar to estimated densities in other large, low elevation mainstem rivers in the Sierra Nevada, but densities in higher elevation or “blue ribbon” trout streams (including upper reaches of the North Fork) were typically five to ten times higher than densities in the Cresta and Rock Creek reaches. Rock Creek-Cresta Dive Count 2006 Report TRPA, 7 February 2007 ________ _____________________________________________________________________________________________________________________________________ INTRODUCTION Thomas R. Payne and Associates (TRPA) was contracted by Pacific Gas & Electric Company (PG&E) to assess trends of fish populations in the Rock Creek Reach and Cresta Reach of the North Fork Feather River, California using direct observation methodologies. PG&E has entered into an agreement with the Ecological Resources Committee (ERC) to conduct staged increases in flows released into the Rock Creek and Cresta reaches over a fifteen year period. PG&E and the ERC are interested in determining what effect the staged increases in flows will have on the fish populations resident within the affected reaches. Part of the assessment of effects on fish populations involves conducting electrofishing surveys in shallow portions of the study area following short-term periods of significantly reduced flows (Salamunovich 2005a, 2005b, 2007). A previous component of the assessment involved electrofishing deeper water pool habitats using boat electrofishing equipment (Stillwater Sciences 2005). Because conventional bank electrofishing is only effective in shallower areas after a significant reduction in flow, and because boat electrofishing is also highly restricted in the depths and habitat types that can be sampled, PG&E and the ERC wanted to also use direct observation methodologies to assess fish population characteristics. STUDY OBJECTIVES This study was conducted with the following principal objective: To produce quantitative estimates of fish abundance and density that will contribute to an assessment of population changes over the period of this study (2004-2006) and with future snorkeling efforts to be conducted during the second and third test flow periods through Year 15 of the License (2016). STUDY AREA This study was conducted in the Rock Creek Reach and Cresta Reach of the North Fork Feather River in Plumas and Butte Counties, California (Figure 1). The Rock Creek Reach is approximately 8.5 mi long, and extends from the Rock Creek Diversion Dam downstream to the Rock Creek Powerhouse. The Rock Figure 1. Area map of North Fork Feather River, California. 1 Rock Creek-Cresta Dive Count 2006 Report TRPA, 7 February 2007 ________ _____________________________________________________________________________________________________________________________________ Creek Reach includes a 1.0 mi area between Bucks Powerhouse and Rock Creek Powerhouse that is subject to higher flows and greater daily fluctuations in flows than the rest of study area. Dive counts were not conducted in the one mile portion affected by Bucks Powerhouse. The Cresta Reach is approximately 4.7 mi long and extends from the Cresta Diversion Dam to the Cresta Powerhouse. In general, both reaches have areas with relatively steep gradient (1.2-2.2%) and confined channel, as well as relatively low gradient (0.4%) areas with less-confined channel. Boulder and cobble are the dominant substrates in both reaches, with many bedrock-formed deep pools. Finer substrate particles, including spawning-sized gravels, are relatively rare in both reaches. The Rock Creek Reach has more riparian vegetation, consisting of willows and alder, than the Cresta Reach. SAMPLING DESIGN Use of Direct Observation to Assess Fish Population Abundance Direct observation methodologies, including snorkeling and SCUBA diving, have been used to study riverine fish populations for many years, particularly in regards to fish distribution, species composition, relative abundance, and microhabitat requirements. Data based on direct observation surveys have frequently been misunderstood in a variety of ways, however. For example, it must be clearly understood that most direct observation surveys produce an index of abundance, but do not actually provide an estimate of total abundance. Except under the most ideal of situations (e.g., sculpin counts in precisely defined quadrats, TRPA 2005a), divers can only be expected to see and count a fraction of the total number of fish that are actually present in a habitat area. Restrictions due to poor underwater visibility, the abundance of instream cover, deep water or swift currents, and the rapid swimming speed and avoidance behavior of most fish species all combine to constrain dive counts to a proportion less than the total abundance. Consequently, dive counts should typically be viewed as a conservative (e.g., minimum) estimate of abundance. In small streams, dive counts can be successfully calibrated to estimate true abundance using a more exhaustive methodology such as intensive electrofishing (Hankin and Reeves 1988) or, in extreme cases, poisoning or explosives (Northcote and Wilkie 1963). Such methods for converting index estimates of abundance to estimates of true abundance can only be applied where the exhaustive methodology is highly efficient (such as in smaller streams) or highly destructive (such as poisoning). The use of less accurate and less efficient estimators to calibrate dive counts have been attempted and even proposed to be transferable to other systems (Hillman et al. 1992, Hagen and Baxter 2005), but the validity of such practices is highly questionable (Dave Hankin, personal communication). For a stream the size of the North Fork Feather River, it is highly untenable that any capture methodology, aside from poisoning with intensive collection, would be capable of producing a sufficiently accurate estimate of total abundance to adequately calibrate a dive count, or to accurately represent total fish abundance throughout the entire range of 2 Rock Creek-Cresta Dive Count 2006 Report TRPA, 7 February 2007 ________ _____________________________________________________________________________________________________________________________________