Living in a Dynamic Environment: Variability in Life History Traits of Age-0 Splittail in Tributaries of San Francisco Bay

—Splittail Pogonichthys macrolepidotus is a relatively large cyprinid endemic to the San Francisco Estuary watershed. During late winter and early spring, splittails migrate from the estuary to upstream rivers and floodplains for spawning. During 2002 and 2003, we examined the diet composition, muscle stable isotope signatures (dC and dN), and growth rates of age-0 splittails in the four primary rivers used for spawning. Overall, we found substantial variability in all three traits in spatial and temporal comparisons. Age-0 splittails consumed a variety of prey taxa, consisting almost exclusively of aquatic invertebrates, larval stages of chironomids or copepods generally being the most common. We found that the dN and dC signatures of age-0 splittails significantly varied spatially and temporally (dN range 1⁄4 6.1– 19.6%; dC range 1⁄4 36.3 to 17.5%). Environmental conditions, namely flow and how it manipulates habitat connectivity, appeared to affect dC. Age-0 splittails exhibited substantial variability in growth rate both spatially and temporally. However, this variability was not associated with diet composition or stable isotope signatures, suggesting that food availability and physical habitat conditions were important factors affecting growth rates during our study. A fundamental goal in aquatic ecology is to understand how populations persist in highly variable environments. Spatial and temporal variability in environmental conditions can exert strong influences on life history traits, which may ultimately affect population parameters (Baltz and Moyle 1982; Frazer et al. 1991; Bronikowski et al. 2002). For fish, the quantification of early life history traits is an important step in building population models that can evaluate the relative affects of environmental conditions and other factors on abundance (Houde 1987; Levin and Stunz 2005). This is particularly true for fishes that exhibit periodic life history strategies (Winemiller and Rose 1992). Periodic strategists exhibit variable non– density-dependent recruitment episodes that are typically driven by favorable environmental conditions for early life stages (Winemiller and Rose 1992; Winemiller 2005). Moreover, many species have alternative life histories that may be influenced by habitat use at early life stages (Secor 1999). Thus, information on how early life history traits vary along environmental gradients may provide valuable insights on mechanisms of recruitment for fish living in dynamic environments. The splittail Pogonichthys macrolepidotus is a species of high management concern in California. It was formerly a federally listed threatened species because of presumed long-term declines in abundance and distribution, and remains a species of special concern for the U.S. Fish and Wildlife Service and the California Department of Fish and Game. It is also the last living member of its genus following the extinction of the Clear Lake splittail P. ciscoides during the previous century (Moyle 2002). Splittail exhibits periodic life history attributes that suggest environmental conditions early in life control recruitment. There is no stock–recruit relationship for splittail, and large fluctuations in annual recruitment coincide with wet (high recruitment) or dry (low recruitment) conditions (Daniels and Moyle 1983; Meng and Moyle 1995; Sommer et al. 1997). It is believed that the availability of expansive floodplains and inundated river margins during wet years provide optimal spawning and rearing habitat that leads to high recruitment (Sommer et al. 1997; Moyle et al. 2004; Feyrer et al. 2006a). The splittail’s longevity, large size, high fecundity, and broad environmental tolerances presumably enable it to withstand extended dry periods, such as droughts, between wet years (Daniels and Moyle 1983; Young and Cech 1996; Feyrer and Baxter 1998; Moyle et al. 2004). * Corresponding author: [email protected] Received November 13, 2006; accepted May 5, 2007 Published online September 6, 2007 1393 Transactions of the American Fisheries Society 136:1393–1405, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-253.1 [Article]