This document provides an update on the abundance and distribution of the splittail, Pogonichthys macrolepidotus, in the Sacramento and San Joaquin rivers, their tributaries and estuary, and other drainages to San Francisco Bay. The splittail is a candidate for "Threatened" status under the federal Endangered Species Act. Historic abundance and distribution information is also provided for reference.

In 1998, high flows and consistent floodplain inundation resulted in record or near record age-0 splittail abundance for abundance indices calculated to date. High abundance was supported by age-0 recruitment from the San Joaquin, Sacramento, Cosumnes, Napa and Petaluma rivers, the Sutter and Yolo bypasses, Suisun Marsh, the delta and numerous small tributaries. For the fourth year in a row, significant numbers of age-0 splittail were produced in the San Joaquin River and its tributaries. Floodplain inundation appears to provide the best explanation for increased abundance in high outflow years .

Long-term indices exhibited several peaks associated with high outflow years, but no trend. Medium-term age-specific indices generally declined or showed no trend. Most of these indices rebounded in 1995 and 1998 (or are expected to when calculated) from low values associated with the 1987-1992 drought. Some of the declining trends are attributable to surveys starting at or near a peak abundance period for splittail in the early 1980s and sampling through a low abundance period during the drought. Reduced young-of-the-year (age-0) abundance in the Delta was expected as a result of reduced incidence of floodplain inundation during the drought. Reduction in adult abundance from successive years of low age-0 abundance during the drought was not sufficient to inhibit a strong reproductive response when favorable outflows returned in the mid- and late 1990s.

The geographic distribution of splittail is broader than previously believed and continues to expand as more information is gathered. Adult foraging and spawning migrations occur in the Sacramento River every year and in the San Joaquin River during years with high freshwater outflow. Changes in the timing, magnitude and duration of high river flows (i.e., floodplain inundation) probably affect when and where adults migrate and, thus, their winter-spring distribution and the initial distribution of their offspring. In 1997 and 1998, splittail were captured 60 km and 17 km upstream of previous range limits (Sommer et al. 1997) in the Sacramento and San Joaquin rivers, respectively. Within these limits, juvenile and adult splittail use both river systems extensively during the winter and spring, and there is evidence some fish of all age groups remain in the Sacramento River year-round. At the western edge of their known range, splittail continue to inhabit the Petaluma River and Marsh as they did previous to and during the drought. Use of the Napa Marsh appears to vary with freshwater outflow. Use of Suisun Marsh also varies with outflow, but not as much as in Napa Marsh. The splittail's distribution is broad and expanding and, with the expansion, some adult fish appear to remain resident in the Sacramento River near Red Bluff. Outflow conditions during winter and spring 1998 continued this trend.