Resilience of Splittail in the Sacramento–San Joaquin Estuary

Abstract

Splittail Pogonichthys macrolepidotus, an endemic cyprinid of the Sacramento-San Joaquin estuary, has been proposed for listing as threatened under the U.S. Endangered Species Act. Almost continuous low outflow conditions in the estuary from 1987 to 1994 led to reduced abundance of young splittails, but adult abundance did not decline consistently except in the downstream portion of the species' range. This range had decreased primarily as a result of historical levee and dam construction but did not appear to have changed substantially in the past 20 years. The distribution of young splittails appears to be relatively plastic on an interannual basis. Evidence of the resilience of the species was seen when high freshwater outflows in extremely wet years (such as 1982, 1983. 1986, and 1995) resulted in high numbers of young splittails. Splittail year-class strength was positively related to freshwater outflow during the spawning season. High outflow inundates the floodplain, which provides spawning, rearing, and foraging habitat. The relatively long life span, high reproductive capacity, and broad environmental tolerances of splittails are contrasted with delta smelt Hypomesus transpacifcus and longfin smelt Spirinchus thaleichthys. other native species of special concern in the system.

Factors Affecting Fish Entrainment into Massive Water Diversions in a Tidal Freshwater Estuary: Can Fish Losses be Managed?

“…Similar to Sommer et al (1997), our salvage analyses suggest that entrainment patterns may be affected by year-class strength. Specifically, we found that the number of age-0 striped bass salvaged was well predicted (r ¼ 0.81) by their numbers in the TNS (Delta stations only) conducted by CDFG.…”

“…Few studies have examined patterns and mechanisms explaining fish losses at the CVP and SWP over the years (Stevens and Miller 1983;Stevens et al 1985;Brown et al 1996;Sommer et al 1997;Bennett 2005;Kimmerer 2008), despite the fact that they are two of the largest continuous fish sampling devices in the world. There have also been relatively few studies of the direct effects of water diversions from riverine and tidal ecosystems (Nobriga et al 2004;Moyle and Israel 2005).…”

“…This result was expected since there are simply more age-0 fishes than older age-classes and because smaller fishes are often more vulnerable to entrainment flows (Hadderingh et al 1983;Henderson et al 1984; FACTORS AFFECTING FISH ENTRAINMENT Maes et al 1998). However, in the San Francisco Estuary, the biology of the fishes examined here must be considered to understand the salvage patterns of these species (Sommer et al 1997). For example, adult delta smelt and longfin smelt migrate into freshwater to spawn; thus, they increase their vulnerability to entrainment simply because they move closer to the SWP and CVP.…”

“…Abundance and distribution.-Abundance of fish in the vicinity of the diversions can have a major effect on entrainment (Sommer et al 1997). Hence, we included estimates of abundance near the diversions in our analyses of factors that may affect salvage rates.…”

“…In recent years, these conflicts have increased because many pelagic fishes in the estuary have dropped to record low abundances while demands for water have increased ). Historically, many fishes in the estuary responded favorably to wetter years because high inflows usually improved spawning and rearing conditions in the estuary (Stevens and Miller 1983;Sommer et al 1997;Kimmerer et al 2001;Bennett 2005;Feyrer et al 2007;Rosenfield and Baxter 2007). The strength of these relationships has diminished during the last few years for several possible reasons, including habitat changes, water diversions, food web alterations, and stock-recruitment effects .…”