Largemouth bass are native to the Mississippi drainage but were introduced to California in the late 1890s as a game fish and probably expanded into the Delta in the early 1900s. While they have been residing in the Delta for at least 100 years, it is only in the last few decades that they have become extremely abundant, now supporting a renowned recreational fishery.

Largemouth bass are notoriously voracious predators, but their natural history in the Delta (distribution, habitat preferences, food habits) and reasons for their increasing populations have not been well documented. One major hypothesis for their rise in population numbers is that aquatic weeds (such as Brazilian waterweed, Egeria densa) have also expanded in the Delta over the same time period, and might provide excellent habitat for bass. The increases in bass and aquatic weeds are part of major changes in the entire biological community along the shorelines of the Delta that have occurred since the mid 1980s.

Our recent studies have examined the largemouth bass distribution, diet, association with aquatic weeds and other environmental variables in the Delta, as well as investigations of their movement patterns using acoustic telemetry. This work has been done by researchers in the Aquatic Ecology Section of DWR and the University of California, Davis (Dr. Andrew Sih in the Department of Environmental Science and Policy, and Dr. Peter Klimley’s Biotelemetry Laboratory). Resulting analyses will provide information on seasonal movement patterns, patterns of abundance and growth with respect to abiotic and biotic conditions, and diet composition and bioenergetics for the largemouth bass population in the Delta. The results from this research will provide insights that managers can use to assess whether largemouth bass are important predators for native fishes and understand the reason for their recent expansion in the Delta.

Recent Results

• Conrad, J.L., K.L. Weinersmith, M.J. Young, D. de Carion, E. Hestir, M. Santos, P. Crain, S. Ustin, P.B. Moyle, and A. Sih. 2010. More big bass: Understanding the role of largemouth bass as top predators in the littoral zone (PDF, 1 KB). Delta Science Council Conference, Sacramento, CA. September 2010.
• Vella, M., C. Nelson, and J.L. Conrad. 2010. Patterns in largemouth bass diet composition in the Sacramento-San Joaquin Delta (PDF, 1.20 MB). DWR Environmental Scientist Workshop, Sacramento, CA. September 2010.
• Conrad, J.L., K.L. Weinersmith, M. J. Young, D.de Carion, P. Crain, E. L. Hestir, M. J. Santos, S.L. Ustin, P. B. Moyle, and A. Sih 2010. Rising abundance of largemouth bass in the littoral zone of the Sacramento-San Joaquin Delta: the role of Egeria densa (PDF, 1.40 MB). Interagency Ecological Program 2010 Workshop, California State University, Sacramento, May, 2010.
• Conrad, J.L. , K.L. Smith, E. Hestir, M. Santos, A. Sih, and S. Ustin. Largemouth bass abundance and diet in the Sacramento-San Joaquin Delta: investigating temporal and spatial variation with respect to submerged aquatic vegetation. State of the Estuary Conference, Oakland, CA, September 2009.