Lead CDFW biologist: Steve Parmenter
Formerly found in deep pools and slough-like areas of the Mojave River, this species now only occurs in highly modified refuge sites in San Bernardino County.
The Mohave tui chub is a chunky, large-scaled fish with a small, terminal, slightly oblique mouth. This subspecies has a bright brassy-brown to dark olive back with a bluish-white to silver belly. The average size for adults is four to six inches, while some fish may be as large as nine inches. Mohave tui chub typically spawn from February to October. Females lay approximately 4,000 to 50,000 adhesive eggs over aquatic vegetation. Once hatched, the fry will school in the shallows, while medium-sized tui chub (1 to 3 inches) school in water one to two inches deep. Large chub are typically solitary and found in deeper water. Mohave tui chub feed on insect larvae and detritus.
The Mohave tui chub is the only fish native to the Mojave River. They were historically restricted to the Mojave River, from the confluence of the east and west forks at the base of the San Bernardino Mountains to its terminus at Soda Dry Lake. Habitat modifications, including damming of the headwaters and withdrawals of the river's underflow, were major causes of the decline of the species. In the 1930's, non-native arroyo chub were illegally introduced into the headwater reservoirs of the Mojave River as a baitfish. Arroyo chub are better able to withstand warmer and shallower habitat conditions that prevail in the Mojave river today. As the new species quickly spread throughout the drainage, it replaced native Mohave tui chubs through competition and hybridization.
By 1979, Mohave tui chubs no longer lived in the Mojave River. Other threats include genetic contamination, introduction of other exotic species, habitat alteration, water diversion and pollution. In 1997, a genetics study by UC Davis confirmed that the Mohave tui chub is a distinct subspecies and recommended that it continue to receive protection as an endangered, unique subspecies. The existing, geneticallypure Mohave tui chub populations occur at three sites: Soda Springs (Zzyzx Springs), the Department's Camp Cady Wildlife Area, and China Lake Naval Air Weapons Station (NAWS). At Camp Cady, the population is thriving in two ponds, one of which is threatened by encroaching vegetation. Mosquito fish and Asian tapeworm were discovered at Soda Lake in 2001, with unknown consequences for the chubs. Perhaps the most secure population exists at China Lake, where the US Navy monitors and protects a population numbering in the thousands. China Lake was chosen as a refuge site and the Mohave tui chub were introduced into Lark Seep in 1971.
Since 1995, NAWS has conducted annual mark-recapture surveys to estimate the chub population at China Lake. From 1995 through 2000 the project was conducted in late May or early June, but encountered spawning fish. To reduce inadvertent capture of spawning fish or damage to their eggs, the survey date was changed to November in 2001. No spawning fish were encountered at that time.
In 2002, two 24-hour water quality meters were installed in the Lark Seep system. Both meters monitor pH, dissolved oxygen, temperature and conductivity. Cattail and tamarisk were also removed. To obtain more reliable population estimates, the Lark Seep System was divided into various habitat types. The defining features used for determination of habitat types were depth and width of the channels, as well as the existence of contiguous habitat. Channel banks support pickleweed, cattail, rush, saltgrass, and some tamarisk.
The 2002 survey captured 530 fish although few of those fish represented recapture of fish marked in previous sampling. Using statistical methods, NAWS estimated that population size to be about 6000 fish. A number of dead fish were found in the North Channel of the seep system. Mortality was attributed to low dissolved oxygen in the water. No spawning fish were noted during this mark-recapture survey. A study has been proposed for 2003 to characterize the dissolved oxygen levels at different depths within the water column along the North Channel. The results will also dictate where and when fish traps are placed into the channel for future mark-recapture surveys. No trapping would occur in 2003 until this study is completed and dissolved oxygen level changes are better understood and modifications to trapping locations are addressed as appropriate. These modifications may include changes such as suspension of traps in the upper part of the water column or relocation to shallower sites.
Recapture rates were low again for the 2002 census. NAWS concluded that the low number of recaptured fish indicated that additional trapping days should be added to future mark-recapture surveys. Additional trapping time within each habitat may increase the number of individuals captured and recaptured; data needed to gain a more precise population estimate. Recommendations for future projects include placement of additional 24-hour water meters the Lark Seep System; continuation of tamarisk removal; a complete dissolved oxygen study of the North Channel habitat; a complete topographical study of channel depths; and implementation of a bullfrog eradication program.
(Excerpted from DFG publication, Species Accounts - Fish (PDF)