Recent spatial distribution of splittail was presented in Table 3 of Sommer et al. (1997). This section updates that table with range extensions in both the Sacramento and San Joaquin rivers and provides information on splittail use of areas within their known range, including evidence of riverine rearing of age-0 and, possibly, age-1 and adult fish.

The summer through fall distribution of adult splittail is primarily limited to tidal fresh and brackish waters of the Sacramento-San Joaquin Delta, Suisun Bay, Suisun, Napa and Petaluma marshes (Baxter 1994, Meng and Moyle 1995, Baxter et al. 1996), yet in the past two years this range has expanded. In the Sacramento River, three adult splittail were collected at the Red Bluff Diversion Dam (rkm 391): one each in April and August 1997 and one (a recent mortality) in October 1998 (C. Martin, USFWS, pers. comm.). Red Bluff is the most upstream location of capture for splittail in recent years, surpassing the Hamilton City area (rkm 331) reported in Sommer et al. (1997). An additional adult splittail was collected August 1997 in a screw trap at the Glenn-Colusa Irrigation intake (rkm 331, Table 3). The splittail from August and October are the first recent captures of adult fish in the Sacramento River upstream of the delta in summer or fall. This indicates that some adult fish now spend the summer in the main-stem Sacramento River instead of migrating back to the estuary after spawning.

Splittail spawn in the Sacramento River upstream of Hamilton City, as evidenced by sporadic collection of adult and age-0 fish at a screw trap near the Glenn-Colusa Fish Screen (rkm 331) (Table 3). Some age-0 fish rear in the river though the late fall, and the presence of an age-1 fish in May 1997 indicates year-round rearing. Adults begin their spawning migration in December (Meng and Moyle 1995, CDFG unpublished data), so collection of adult fish in June, August and October means that adult use of riverine habitat can extent at least 7 to 11 months.

Adult splittail forage and may spawn in tributaries of the Sacramento River upstream of the Feather River. Three adult fish were recently collected in tributaries of Big Chico Creek (ca. rkm 312): two from Mud Creek in March 1996 and one from Kusal Slough (Rock Creek) in March 1997 (Maslin et al. 1997). Although these locations were sampled until late April in both years, the 1/4-inch mesh of the block nets and seines was too large to retain age-0 splittail well and none were caught.

Plankton sampling also provides evidence of splittail spawning relatively high in the Sacramento River system. Johnson Wang identified larval splittail from archived plankton samples collected from 1988 through 1994 by the Striped Bass Egg and Larval Survey. Larval splittail were collected from the most upstream region the Striped Bass Egg and Larva Survey sampled annually (Fig. 5). CPUE (catch/1000m³) in regions upstream of the City of Sacramento (i.e., Verona-Sacramento, Tisdale-Verona) was frequently as high or higher than that of downstream regions. Except for years with some (1989, 1992) or substantial (1993) winter/spring outflows, splittail larvae were uncommon in Sacramento River collections downstream of the City of Sacramento (Fig. 5). Larvae were not collected in Suisun Bay or in Suisun Marsh except in 1993 (Fig. 5). Two interpretations are possible for these data: 1) splittail spawn relatively high in the Sacramento River system every year, but downstream dispersal of larvae increases with higher flows, and 2) some splittail are able to spawn in Suisun Bay or in Suisun Marsh during high flow years because of reduced salinities and increased small stream flooding.

The distribution of age-0 splittail from the Beach Seine Survey provides evidence of Sacramento River spawning in both high- and low-flow years (Fig. 6), and also of extended riverine rearing (Tables 4 and 5). The age-0 splittail distribution from the Beach Seine Survey was similar to that of larvae from the plankton survey: age-0 fish were always collected from the most upstream region the survey sampled (rkm 231-296 for stations in Upper Sacramento River, Fig. 6); sampling did not begin in the Upper, Mid and Lower Sacramento regions until 1981. Splittail mean catch per seine haul (CPUE) was often highest in one of the Sacramento River regions (i.e., Far North Delta (rkm 69-97) - 1979, 1980, 1993, 1995; Lower Sacramento River (rkm 114-129) - 1997; and Upper Sacramento River (rkm 145-222) - 1992, 1994). This pattern of highest CPUE in the Sacramento River was accentuated in fall as the fish grew and became more difficult to catch (Table 4). Except for catches in the west and central Delta in 1995 and the west Delta in 1996, all juveniles caught in fall came from the north Delta and Sacramento River. Although only 109 age-0 splittail were caught by the beach seine during fall months (of which 55 were from the west Delta in 1995), 80 % of the remaining catch came from the far north Delta and upstream. Thus, some age-0 fish spent their first summer in the Sacramento River and not in the delta. Assuming the Beach Seine Survey accurately depicts the distribution of age-0 splittail, then by fall most age-0 fish were in the Sacramento River, outside the sampling area of the Bay Study and FMWT whose index periods include fall months (Sommer et al. 1997).

In winter and spring, the Beach Seine Survey continued to collect age-0 splittail in relatively high numbers in the Sacramento River, though they were also collected frequently in Delta regions (Table 5). The USFWS expands the beach seine sampling in winter, so more data were available for comparison than for fall. CPUE was high in the west Delta in 1996 as a result of the strong 1995 year-class. Nonetheless, about 50 % of the 70 age-0 splittail collected came from the far north Delta and upstream. These data indicate that age-0 splittail are difficult to capture with a beach seine in fall and winter, probably because they are good swimmers and their ability to escape increases rapidly with size (Young and Cech 1996). The low catches in the central, west and south Delta during fall suggests either upstream densities are higher or that age-0 splittail were not as restricted to edgewaters <1.5 m deep in the Delta as they may have been in the river. In either case, some proportion of the population consistently spent its first summer, fall, and winter in the Sacramento River and was not indexed by other surveys (Tables 4 and 5).

Sampling in the Sutter Bypass also provides evidence of spawning and first year rearing in the bypass itself or upstream in Butte Creek. Butte Creek enters the Sacramento River between Colusa and Meridan (river km 224). At high river flows, Butte Creek and the overflow from Moulton and Colusa weirs are diverted down Butte Slough into the Sutter Bypass, which in turn enters the Sacramento River just upstream of Verona (river km 129). In 1995, adult splittail were electrofished from the riparian strip separating the lower Sutter Bypass from the Sacramento River channel and larvae were caught in plankton tows in the Sutter Bypass plume (Baxter et al. 1996). In 1996, 32 adult splittail were caught in the flooded Sutter Bypass, 17 were radio-tagged and seven tracked to potential spawning areas, also in the bypass (IEP Splittail Spawning Investigations, unpublished data). Light traps set in areas where tagged adults congregated caught 63 larvae in the riparian strip between East Canal and the main Sutter Bypass (IEP Splittail Spawning Investigations, unpublished data). In spring 1998, 102 ripening and ripe adult splittail were captured in 44 20-minute gill-net sets in the lower 12 km of the bypass (IEP Splittail Spawning Investigations, unpublished data). Light traps set in areas where adults were captured caught 226 splittail larvae. These data confirm splittail spawn in the lower Sutter Bypass.

Screw trapping in the mid-Sutter Bypass (i.e., south end of Sutter National Wildlife Refuge) from December 1995 to July 1996 collected adult, age-0, and age-1 splittail (Fig. 7). Fyke trapping nearby during the same period provided similar catch and age-group results. The high catch of age-0 fish (several thousand) provides evidence of successful spawning upstream in the bypass or in Butte Creek. A less likely possibility is that the fish entered the Bypass with Sacramento River water over one or more of the weirs. The 30 age-1 fish are evidence that some 1995 year-class fish remained upstream during their first year of life; they may have been in the Bypass, in Butte Creek itself, or may have entered into the Bypass over one of the weirs. It is unlikely that these age-1 fish migrated back upstream from the delta during the winter.

Splittail inhabit the San Joaquin River and the valley portions of some tributaries during high outflow years, but are rarely caught in low outflow years (Sommer et al. 1997; T. Ford, Turlock Irrigation District, pers. comm.; Baxter et al., in press). Age-0 fish emigrate primarily in the late spring and early summer (Armor et al. 1996), so they are rarely collected during the summer or fall (Saiki 1984; Brown and Moyle 1993; Baxter et al., in press). Thus, the seasons sampled can determine whether splittail are collected or not.

The most upstream collection from the San Joaquin River occurred from June 15 to 19, 1998, when a joint USFWS/CDFG crew collected juvenile splittail from Salt Slough in San Luis National Wildlife Refuge upstream of Lander Ave. (State Highway 165) (M. Dunn, CDFG Stockton, pers. comm.). The mouth of Salt Slough is at rkm 208.5 and the collection site is about 10 km upstream. Juvenile splittail were also collected about 8 km from the mouth of Mud Slough. Thus, splittail were able to locate flooded habitat well upstream in the San Joaquin River and spawn when conditions were suitable.

In late June 1995 and in late April 1998, age-0 splittail were collected at Fremont Ford (State Highway 140, rkm 201) on the San Joaquin River (Baxter et al., in press; CDFG unpublished data). In late June 1995, 26 age-0 splittail were captured from a single beach seine haul at Fremont Ford; seven more hauls made over two dates in August caught no splittail (Baxter et al., in press). Additional age-0 fish were caught in 1995 at locations between the Merced and Tuolumne rivers in July and early August, but were absent in 15 seine hauls in late August and two hauls in early September. In late September, two juveniles were collected in one of five beach seine hauls at Turtle Beach (rkm 92) (Baxter et al., in press). These data show splittail spawned high in the San Joaquin River in 1995 and that age-0 fish remained in the river well into the summer when outflow was relatively high.

In 1998, age-0 splittail were again collected at Fremont Ford. High water levels and runoff from Great Valley Grasslands State Park and Kesterson National Wildlife Refuge, immediately to the south, attracted splittail out of the San Joaquin River channel to spawn. One of the samples containing splittail was collected several hundred meters into Great Valley Grasslands State Park and away from the connecting channel to the San Joaquin River.

When river flows create suitable spawning habitat, as occurred annually from 1995 through 1998, the San Joaquin River is used for spawning and can produce substantial numbers of age-0 fish (Real-Time Monitoring Data 1995-1998, http://www.delta.dfg.ca.gov/index.html). The 1995 beach seine data and data from trawling at Mossdale in the lower San Joaquin River (i.e., Real-Time Monitoring) suggest that most age-0 splittail emigrate during the late spring and early summer (Armor et al. 1996).

The Petaluma River and Marsh supported splittail at the end of the 1987-1992 drought as evidenced by 31 adult fish caught in fyke traps (within Petaluma City limits) from February to May 1991 (Levy 1993) and six age-0 fish caught by beach seine in the Petaluma Marsh in May 1992 (CDFG Bay Study, unpublished data). Splittail continue to be collected from the Petaluma River: 142 in 1995, 4 in 1996 and an untallied number of age-0, age-1 and adult fish in 1998 (K. Hieb, CDFG Stockton, pers. comm.). These data indicate that splittail continuously inhabited the Petaluma River from the early 1980's to present and that they successfully spawned in 1992, 1995 and 1998.

In the Napa Marsh, 40 age-0 splittail were collected from six locations sampled in September 21, 1995 (K. Hieb, pers. comm.). In another effort, splittail of all ages (n=42) were collected in Pond 2A of Napa Marsh, July 18 -August 14, 1996 (William Kier and Robert Blizzard 1997, Monitoring the use by fish of the Napa-Sanoma Marsh Wildlife Area). These splittail were collected at salinities ranging from 18 to 21 l , which is well above the range where they are normally collected (Sommer et al. 1997), yet still within their tolerance (Young and Cech 1996).

Historic sampling in the Napa Marsh found splittail present during seasons and years with high freshwater outflow and rare or absent when the outflow was low. From June 1974 through February 1979, four sloughs each in the Napa and Suisun marshes were sampled once each in February, June, and October. The effort expended at each slough included two 10-minute otter trawl tows (15-ft head rope, 1/2-inch stretch mesh cod end) and two gill nets set for 24 hrs each: 1) 250 ft by 12 ft with equal-sized panels of 2.5-, 3-, 3.5- and 4-inch stretch mesh webbing; 2) 75 ft by 6 ft with 1-inch stretch mesh webbing). Splittail were captured in summer and fall of 1974 (a wet year, Table 2), in winter and summer 1975 (an above normal year), but only in winter 1976 (a critically dry year)(Table 6). They were not collected in the marsh again until the severe 1976-1977 drought ended in winter 1978. In the winters of 1978 and 1979, splittail catch was higher than at any time before the drought. In Suisun Marsh, splittail catch declined from June 1976 through October 1977, except in October 1976, but a few fish were caught throughout the drought (Table 6). Similar to Napa Marsh, Suisun Marsh numbers rebounded strongly when the drought broke in 1978. These data suggest that splittail shift their distribution as a result of salinity conditions, and that they are quick to respond and move into an area when conditions become favorable.

Splittail are known to have inhabited Coyote Creek, a tributary to South San Francisco Bay, in the late 1800s, but were thought to be exptirpated early in the 20^th Century (Aceituno et al. 1976). However, in 1983, splittail were again captured in Coyote Creek (Kinnetic Labs Inc. and L.W. Associates 1987). Three age-1 fish were collected, two in May and one in December (M. Stevenson, Kinnetic Labs. Inc., pers. comm.). These fish may have migrated to Coyote Creek during the high flows of winter 1983 that created low salinity conditions in shallow waters throughout San Francisco Bay. The winters of 1995, 1997, and 1998 produced similar low salinity conditions. No other records of splittail from Coyote Creek have been uncovered, but M. Stevenson will again sample it in the summer of 1999.