With recent better water years beginning in 1993, but showing population effects beginning in 1994, the sunfish declined to zero and smallmouths have become less dominant, while populations of other species seem to be approaching a loose equilibrium, with a variety of year classes present and roughly similar numbers from year to year. It appears clear that wetter years depressed smallmouth populations while favoring natives. There is, unfortunately, no easy way to determine whether the mechanism involved was reduced predation by smallmouths or simply more favorable conditions for the natives. Clearly there are far more young-of-the-year riffle sculpin after the big 1997 flood than have been recorded before. Squawfish and hardheads also show pretty good 1997 year classes. In 1997, for the first time, a California roach was captured at the detox site. Since they have always been abundant only about a km upstream, where smallmouths have been absent, their relationship with smallmouths seems obvious.
At the two sites above Iron Canyon, re-establishment of the original fish community has been even slower. The California Roach, with its high fecundity and short generation time, has re-established populations equal or above those present before treatment. While all the other original species have been observed either at or between the sites, only the riffle sculpin has rebuilt a population even approaching what was there before treatment and that only at the Ponderosa Way site. The 1997 year class of riffle sculpins at Ponderosa Way, like that at the detox site, is the best ever. The reworking of the alluvium by the flood probably provided more space under rocks for them to fasten their egg clusters and may have depressed populations of some unknown egg predator.
Populations of rainbow trout generally increased slightly at all three sites over the study period, although numbers at Ponderosa Way varied sporadically up to 1990 due to the sometimes planting of large numbers of steelhead fry. The general increase seems to be largely independent of other fish populations, being observed at Upper Bidwell where there were no competing/predating species and at the Detox site, where populations of both trout and non-game species increased simultaneously despite the presence of a dense smallmouth bass population. The 1997 year class of rainbows is relatively large at both upstream sites, perhaps favored in some way by the January flood or by reduced predation from brown trout (see below). Unfortunately it is not possible to distinguish anadromous steelhead from resident rainbows without more elaborate tests than we have available.
The exotic brown trout, found pre-treatment in small numbers only at the upper site, increased substantially there and extended its range to include both lower sites during the dry years. The brown is a fall spawner, so eggs or very young fry would have been present during the high water events of 1995 and 1997. No year class was observed for either of those years, probably explaining the substantial decline in number of browns in the last three years.
In terms of biodiversity, the continued failure of native fish to re-establish populations in the middle zone of the creek must be considered a negative consequence of the treatment.
The fluctuations in range and populations of exotic species seems to be closely related to stochastic events, particularly to timing and magnitude of high water. The native fish seem, if anything, to be favored by high water events. (The same can be said for some other native species. We observed more riparian tree seedling establishment on gravel bars and more newly-metamorphosed yellow legged frogs in 1997 than any other study year.) The increase in native fish reproduction may be related in part to the decline in predation and competition by the exotics, but certainly other factors, such as cleaning of gravel and even stimulation of reproductive urges also play a part.
DFG's stated goal of increasing anadromous populations by reducing competition/predation seems not to have been achieved. No increase in anadromous runs was observed until 1995, when, for the first time in many years a substantial run of spring chinook adults occurred in Big Chico Creek. This is far too late to correspond with either the removal of competitors in 1986 or the planting (at Ponderosa Way) of 1.5 million Feather River chinook fry between 1987 and 1991. More likely the good 1995 run was just part of the phenomenon of good runs everywhere in the Central Valley resulting from favorable weather conditions and management changes forced by the federal listing of the winter chinook as threatened in 1990 and endangered in 1994. (The 1996 run was only a fraction of the one in 1995, but was still better than any oher time in the previous 20 years.) In 1997, despite the January flood, spring flow was insufficient to permit entry of many salmon and only two were recorded in DFG's summer count. Steelhead, which migrate up the streams earlier, may have taken advantage of the high January flow, possibly explaining the large 1997 year class of "rainbows".