This section describes trends in splittail abundance based upon all data available from each sampling site or survey, henceforth called survey. Unless otherwise indicated, abundance indices were calculated as described in Sommer et al. (1997). Surveys were grouped based upon starting year into those starting in the 1960s, depicting long-term trends, and those starting in the mid-1970s or later depicting medium-term trends (Table 1). In the case of the Summer Townet Survey (TN), which started in 1959, data for species other than striped bass were irregularly recorded before 1962, and not recorded in 1966, 1967 or 1968, so abundance indices for these years were not calculated. When length data were recorded, length-frequency analyses were used to stratify catch into age 0, 1 and 2+ (adult) and abundance indices were calculated for each age group. The Fall Midwater Trawl Survey (FMWT) started in 1967, but splittail lengths were not recorded before 1975, so abundance indices were calculated two ways: 1) indices for the entire period 1967 through 1997 based on total catch and 2) set of indices for 1975 through 1997 based on age-stratified catch. For surveys where no splittail were measured from some tows, catch was age-stratified based on the proportion of each age group in tows made on the same day in the same general area. Beginning in 1997, the USFWS recorded only splittail ³ 25 mm in beach seine and Chipps Island trawl surveys, so indices were recalculated using 25 mm as a minimum length. The CVP and SWP salvage facilities use a 20 mm minimum length. No other surveys employ a minimum length. Each survey samples only a portion of the splittail's range (Table 1); thus shifts in spawning distribution or changes in summer rearing habitat may significantly affect abundance indices.

Both the TN and FMWT surveys provide long-term views important to the interpretation of the validity of trends observed in medium-term surveys (Table 1; Sommer et al. 1997). The TN and FMWT surveys also broadly sample splittail habitat in the estuary.

The TN survey captures only age-0 splittail. TN abundance indices varied without trend and reached a maximum in 1982 (Fig. 1a). Indices were relatively high in some (i.e., 1963, 1978, 1982, 1983, 1986, 1995 and 1998), but not all years (i.e., 1965, 1970, 1971, 1973, 1974, 1980, 1983, 1996 and 1997) with high winter-spring outflow (Fig. 1a, Table 2). Indices were low in most (i.e., 1964, 1976, 1977, 1979, 1981, 1985, 1987, 1990, 1992, 1994), but not all years (i.e., 1988, 1989, 1991) with low winter-spring outflow.

The FMWT captures all age groups of splittail, and all age groups were combined to show abundance from 1967 through 1997. Like TN indices, FMWT abundance indices varied without trend and reached a maximum in 1998 (Fig. 1b). The peak in 1998 resulted primarily from age-0 catch whereas the previous peak in 1983 resulted from the combined catch of strong 1982 and 1983 year-class fish. The buffering effect of combining several age classes in this index masks some effects of freshwater outflow on abundance. While some strong year classes in high-outflow years were apparent (i.e., 1967, 1978, 1982, 1983, 1986, 1995, and 1998), the effect of low flow was often masked (i.e., 1981, 1985, 1987, 1988, 1990 and 1991; Fig. 1b, Table 2).

These two surveys are consistent in showing no long-term trend and in detecting peaks in abundance in 1978, 1982-1983, 1986, 1995 and 1998. The interval between high abundances ranged from 1 to 9 years and indices of approximately equal magnitude occurred both before and after peak abundances in 1982-1983, indicating that the population still capable of produces strong year classes in response to favorable environmental conditions.

In the mid- to late 1970s, surveys began recording length data for all species. By 1980, recording length data for all species was universal, but not always complete (i.e., a representative number of each species may not have been measured from each sample). When length data were sufficient, splittail catch was separated into age classes (age 0, age 1 and age 2+) and abundance indices calculated by age group. This allows estimation and tracking of year-class strength. Surveys used to examine medium-term trends fall into two categories: those with a narrow sampling area and those with a broad one (Table 1). Surveys that have a narrow sampling area include the CVP Salvage and SWP Salvage, the Chipps Island Trawl, and the Suisun Marsh Survey. The Beach Seine, Bay Study Otter and Midwater Trawl, and the FMWT surveys each sample a broad area.

Abundance of age-0 splittail often varied by orders of magnitude annually (Fig. 2). High and moderately high indices were associated with years having high winter-spring outflow (e.g., 1978, 1982, 1983, 1986, 1995, 1998), but high abundance indices were not associated with all such years for all surveys. Age-0 abundance in 1998 has only been calculated for Bay Study and Fall Midwater Trawl surveys. Abundance reached a maximum in 1998 for the Fall Midwater Trawl and ranked second behind 1995 for both Bay Study gear types (Fig. 2). Prior to 1998, maximum age-0 abundance occurred in 1995 in six of eight surveys; exceptions were the Suisun Marsh Survey when the highest abundance occurred in 1979 and the FMWT Survey with a peak index in 1982. High abundance of age 0 fish in 1995 resulted, in large part, from exceptional recruitment of age-0 splittail from the San Joaquin River (Armor et al. 1996). Salvage indices at the federal and state fish facilities for 1995 were double the previous high indices (Fig. 2), and transport of salvaged fish appeared to contribute to age-0 abundance at Chipps Island (Armor et al. 1996).

In 1998, splittail abundance at Mossdale was high, though only about one-eighth that of 1995 (IEP Realtime Monitoring). It was likely that trawling at Mossdale underestimated San Joaquin River splittail abundance in 1998. Unlike previous years, San Joaquin River flows had yet to decline by late June 1998, potentially allowing more age-0 splittail to remain in the river. Thus, substantial numbers of age-0 fish probably emigrated during summer and fall. Most of these fish were not detected at Mossdale because sampling ceased June 19, 1998, however, some were detected by other surveys including CVP and SWP salvage, TN, and FMWT.

In addition to substantial numbers of age-0 splittail emigrating from the San Joaquin River in 1998, thousands also were collected in screw traps in the upper Sutter Bypass (K. Hill, CDFG, Region II, pers. comm.) and the lower Yolo Bypass (T. Sommer, CDWR Environmental Services Office, pers. comm.). Though summary information is not yet available, these areas contributed substantially to the age-0 population in 1998.

Trends in age-0 abundance may reflect a shift in distribution. Two surveys sampling different parts of the age-0 splittail range in the estuary show opposite trends: abundance declined over time in the Suisun Marsh Survey, whereas it increased in the Beach Seine Survey, which samples throughout the Delta (Fig. 2). Changes in the distribution of age-0 splittail are described in the Distribution section.

Only Bay Study age-1 splittail abundance indices have been updated for 1998. Trends in age-1 abundance loosely follow those of age 0. Age-1 abundance often increased in the year following high age-0 abundance, but this was not true for every index (Fig. 3). For example, CVP salvage of age 1 increased in 1981, 1983, 1984, and 1996 following increased age-0 abundance in the previous year, but it did not increase much in 1987. The SWP salvage of age 1 increased in 1984 and 1987 following increased age-0 abundance in 1983 and 1986, but did not increase much in 1981, 1982, or 1996 (Fig. 3). Trends in Bay Study age-1 indices followed those of age 0 very well. However Bay Study age-1 indices showed recruitment for some year classes that age-0 indices did not, specifically in 1990 and 1993 (1989 and 1992 year classes, Figs. 2, 3).

Although the 1995 year-class was a record high at age 0, this was not always the case in 1996 at age 1 (Fig. 3). Salvage indices for age-1 fish in 1996 were well below those of the late 1970s and early 1980s. Conversely, Bay Study MWT and OT age-1 indices reached a maximum in 1996. These high age-1 indices indicate that low salvage probably resulted from low abundance in the south delta rather than low abundance over all.

Chipps Island data for 1978 need to be regarded with some skepticism: 3275 splittail were captured in May and June index period, but only 188 were measured. A typical practice was to measure one of the largest and one of the smallest fish from each tow. In many cases only a single fish was measured. Thus, age assignments are likely to be unreliable.

Age-2+ (adult) abundance did not vary as widely as age 0 or 1, largely because several age classes contributed to each year's index (Fig. 4). Age-2+ abundance trends differed by survey. CVP Salvage and Bay Study Midwater Trawl indices show no trend, but both peaked in 1993 after 6 years of drought. SWP Salvage and Bay Study Otter Trawl indices declined from peak abundance in 1980 and 1985, respectively. Chipps Island and Suisun Marsh survey indices declined from their respective peaks in 1978 and 1980, then increased slightly in 1997. An increase in 1997 was expected, because the 1995 year-class reached age 2. Thus, depending upon the survey, age-2+ abundance declined or remained stable. A decline was expected in 1990 as drought year-classes aged and contributed to the index; most indices declined starting in 1990. The substantial abundance increase in 1993 apparent in four of six indices contradicts the conclusion that little or no recruitment occurs during drought years. The "March miracle" of 1991 apparently provided conditions sufficient for successful spawning and age-0 survival (Fig. 1a). In 1993, increased turbidity and the potential to spawn after 6 dry years probably stimulated movement of age-2+ splittail and increased vulnerability to capture, contributing in a minor way to the high indices.