Project: Movement and habitat use of introduced, piscivorous largemouth bass in the Sacramento-San Joaquin Delta



The Sacramento-San Joaquin Delta provides critical goods and services to the people of California (e.g. irrigation and drinking water, agricultural land, and recreational opportunities)1. In addition, it performs vital ecological roles such as nurturing juvenile salmon and other fish, sequestering carbon, and transporting water from the central valley1. In the past century, this habitat has experienced numerous changes, ranging from natural sea level rise to unnatural channelization, altered flow patterns, and the introduction of non-native species.

Studies from the past decade have demonstrated a correlation between decreasing populations of native fishes and increasing abundance of both the submerged aquatic plant community and the populations of alien centrarchids2. The piscivorous largemouth bass (Micropterus salmoides) is among these non-native centrarchids. Due to their abundance and diet breadth, the population of Largemouth bass (LMB) is likely to have a large impact on other shallow-water fishes3. The increasing abundance of adults may intensify predation pressure on populations of other fishes already in decline, including those identified as POD species (Pelagic Organism Decline)4. In order to better understand the role of LMB in the Delta ecosystem, a broad research project was conducted to examine the diet, abundance, and movement of this predator. Within the biotelemetry laboratory we worked to quantify the movement of the largemouth bass, focusing on its interaction with the community of submerged aquatic vegetation.


The primary goals of this study are to examine the relationship between LMB and various densities and types of submerged aquatic vegetation (SAV). We focus on the patterns of movement from SAV beds into open water areas in two major habitat types of the Delta (lake-like and channel-like) to examine behavioral shifts in response to differing flow patterns.


Largemouth bass were tracked using VEMCO acoustic technology. Thirty adult individuals (>300mm FL) were implanted with V13 ultrasonic tags and tracked in a two study areas using a VR2W Positioning System (VPS). This system relies on triangulation techniques to record 2D positions of tagged animals within the array.

The primary study site was in a flooded agricultural tract, Mildred Island. This ‘island’ experienced a levee breach in 1983 and was never reclaimed, resulting in a large lake-like area in the south central Delta. Twenty fish were tracked within the Mildred array and tracked for sixteen months. The secondary study site was in Latham Slough, a channel along the outer edge of the Mildred Island levees with stronger currents and more varied bathymetry. Ten fish were tracked within this array for five months (one summer-fall season). In addition to recording fish movements, we sampled the density and species composition of SAV beds at each study site every six weeks to track changes in SAV throughout the study.

Using these data our analyses will examine how frequently and extensively LMB move from littoral areas into open water areas. We will also consider the influence of changing environmental parameters, such as water temperature, winds, flow patterns, and tidal cycles on the movement patterns of the bass. This research is part of a collaborative effort focused on identifying the role of LMB in both littoral and pelagic communities of the Sacramento-San Joaquin Delta. These large and abundant predators have the potential to influence community structure and ecosystem processes in the near-shore habitats, and could act as an important link between the near shore and pelagic communities.

Other aspects of the collaborative project include surveys of fish assemblage throughout the Delta via electrofishing (L. Conrad, A. Sih), the collection of information on LMB abundance, distribution, size, and diet (L. Conrad, A. Sih), and quantification of the distributions, density, and species composition of submerged aquatic vegetation throughout the Delta via local sampling and remote sensing (E. Hester, M. Santos).


Anna (Stephenson) Steel - lead in LMB movement component

Louise Conrad - Project lead
California Department of Water Resources

Pete Klimley, Ph.D.
Department of Wildlife, Fish, & Conservation Biology, University of California, Davis

Andrew Sih, Ph.D.
Environmental Science and Policy, University of California, Davis

Peter Moyle, Ph.D.
Department of Wildlife, Fish, and Conservation Biology, University of California, Davis

Additional assistance from:

Phil Sandstrom
US Geological Survey, Northern Rocky Mountain Science Center

Mike Thomas
University of California, Davis

Gabe Singer
University of California, Davis

Andan Bailey
Davis Senior High School

Erin Hester
University of California, Davis

Maria Santos
Stanford University
And numerous other wonderful volunteers!

Funding Sources:

This project was generously funded by a grant from the Interagency Ecological Program, grants from the Graduate Group in Ecology (UC Davis), the Marin Rod and Gun Club Scholarship, and the Bob Wisecarver Scholarship (Diablo Valley Fly Fishermen). Support was also received through the CAMEOS GK-12 program, funded by the National Science Foundation, Grant #DGE0841297 to S.L. Williams.


1.   Healey, M. (2008) Science and the Bay-Delta. In The State of Bay-Delta Science (Healey, M., et al., eds), p. 19-35

2.   Brown, L.R., and Michniuk, D. (2007) Littoral fish assemblages of the alien-dominated Sacramento - San Joaquin Delta, California, 1980-1983 and 2001-200

3.   Estuaries Coasts 30, 186-200 3. Norbriga, M.L., and Feyrer, F. (2007) Shallow-water piscivore-prey dynamics in California's Sacramento-San Joaquin Delta. San Francisco Estuary and Watershed Science. 5,

4.   Brown, L.R. (2003) Will Tidal Wetland Restoration Enhance Populations of Native Fishes? San Francisco Estuary and Watershed Science 1,



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