MARIONI, N. K.*; TRACY, C. R.; ZIMMERMAN, L. C.; Univeristy of Nevada, Reno; Univiversity of Nevada, Reno; University of Nevada, Reno: Effects of declining lake levels on fish populations: Lahontan cutthroat trout and tui chub in Walker Lake, NV

Prior to the 1960�s, Walker Lake, NV was inhabited by five fish species. Extensive water consumption for agriculture, which has reduced water inflow from the Walker River, and high rates of evaporative water loss have contributed to declining lake levels for decades. Consequently, the concentration of dissolved solids in the lake has been increasing. This change in water chemistry has caused the extirpation of three species of fish, and today, only Lahontan cutthroat trout Oncorhynchus clarki henshawi and tui chub Gila bicolor remain in the lake. Lahontan cutthroat trout (LCT) apparently persist because individuals have a notable ability to survive in water high in salinity (Sigler et al. 1983, Beutel and Horne 1997) Here, we address the question of the distribution and abundance of fish (LCT and tui chub) as a function of the lake characteristics. Water was characterized in terms of depth, temperature, dissolved oxygen, conductivity, and total dissolved solids (TDS) measured bi-weekly between May 2003 and April 2005. Fish were enumerated with respect to depth using a Bottomline Tournament Master 5300 fish finder along 63, one kilometer transects. Water data were collected at each fish survey location. The average depth of the lake has declined by approximately 0.5 meters per year in 2004 and 2005, and water conductivity has increased steadily as a result. Distribution and abundance of fish varied seasonally, and it appears that fish are occasionally forced into areas outside of their optimal thermal and aerobic range. Preservation of Walker Lake warrants continued research to investigate approaches to maintaining and increasing the number available fish refuges.