Feasibility of Fish Passage at Alameda Creek Diversion Dam

6.0 Additional Considerations and AnalysisFor purposes of this technical memorandum, a sustainable steelhead population in the Upper AlamedaCreek Basin is defined as having both a positive spawner replacement ratio and a minimum viablepopulation size. The spawner replacement ratio is an estimation of the number of adult progeny thatsuccessfully return and spawn compared to the number of spawners that were used to create them. Ifmore adult fish return in subsequent generations than were used to create them, then the replacementratio is positive, and allowing fish passage has contributed to an overall increase in the population. Ifadult returns are smaller than the population used to create them, there is a net negative effect on thepopulation and a negative contribution to overall basin production. When the replacement ratio is1:1, the population is in equilibrium. Spawner replacement ratio is expected to vary from year to yearbased on various life stage survival rates. For example, exceptionally dry years could negativelyimpact juvenile survival and El Nino events would be expected to decrease ocean survival. Similarly,wet years could enhance juvenile survival and the ability for adults to successfully immigrate.Nevertheless, when averaged across years, a positive long-term spawner replacement ratio would berequired for success.In addition to natural fluctuations in productivity of the population, as described above, potentialreductions in fish production are also associated with fish passage components. These reductions inproductivity may result from reduced capture efficiencies or increased stress-related mortalitiesassociated with the handling and transport of fish.NMFS policy regarding recovery of listed anadromous salmonids requires use of the concept ofViable Salmonid Population (VSP), which requires establishment of abundance and productivitygoals, including a long-term spawner replacement ratio of at least 1:1, as well as a minimum viablepopulation size (NMFS, 2000 and 2008b). Shaffer (1981) states “a minimum viable population forany given species in any given habitat is the smallest isolated population having a 99 percent chanceof remaining extant for 1,000 years despite the foreseeable effects of demographic, environmental,and genetic stochasticity, and natural catastrophes.” A review of the fisheries literature suggests that aminimum viable population size for Pacific salmon, including steelhead, is comprised of at least 100breeding pairs. Emlen (1993) reports that a complete run failure for Chinook salmon occurs when thepopulation falls below 100 breeding females. A self-sustaining population of rainbow trout in areservoir system in British Columbia is being created with a “seed” of 100 spawning pairs of fish,based on a literature review of rainbow trout populations by Langston and Zemlak (1998).Facilitating fish passage at the ACDD could potentially produce a minimum viable population of 100spawning pairs if sufficient adult spawning and juvenile rearing habitat is available to accommodatethese fish (see Appendix C for details of this estimate).The quantity and quality of steelhead habitat available upstream of the ACDD have not beendetermined. Due to the intermittent hydrology of Alameda Creek above ACDD, future steelheadnumbers above ACDD may be more limited by rearing habitat than by spawning habitat. Based onlimited ground surveys, topographical maps, and aerial photography, it is estimated here thatapproximately 4 miles (Entrix, 2003) to 10 miles (assuming an extremely wet year and suitable habitatis present in some portion of the un-surveyed tributaries of Alameda Creek above ACDD) of potentialrearing habitat is available (see Section 6.1). For the purposes of this preliminary analysis it isestimated that there are between 4 and 10 miles of potential steelhead habitat (spawning and rearinghabitat) above ACDD.The number of steelhead that may be expected to spawn in the 4 to 10 miles of steelhead habitatpotentially available above ACDD was approximated by evaluating the spawning densities in placeswhere more extensive surveys have been conducted. Lagunitas Creek in Marin County provideshigh-quality habitat for salmonids in the San Francisco Bay area. For the past 12 years, salmon andsteelhead spawning surveys have been conducted in the Lagunitas Creek watershed, which containsACDD Passage June 2009 Page 6-5