Movement and habitat use of introduced, piscivorous largemouth bass in the Sacramento-San Jouquin Delta
Introduction:
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 and the introduction of non-native species.
Studies in the past decade have demonstrated a correlation between a decrease in populations of native fishes and an increase in submerged aquatic vegetation, primarily Brazilian waterweed (Egeria densa), and alien fishes, primarily centrarchids2. The piscivorous largemouth bass (Micropterus salmoides) is among these non-native centrarchids. Largemouth bass (LMB) are generalist predators and due to their voracity and abundance, they are likely to have a large impact on other shallow-water fishes3. Their increasing abundance may intensify predation and competition pressure on declining populations of other fishes, including those identified as POD species (Pelagic Organisms of Decline)4. This project will examine the role of LMB in Delta food-webs, and attempt to clarify the role of LMB predation in the decline of the POD species.
Objective:
The goal of the first year of study (summer 2009) is to obtain baseline information about the movement patterns, habitat use, and home range size of largemouth bass in the Delta. It will also provide an opportunity to test the feasibility of various tracking techniques in weedy, shallow water areas. The second summer of study will expand the data collection to two different habitat types to examine potential behavioral shifts in response to different spatial structuring.
Methods:
We will begin by implanting ten individuals with ultrasonic tags and tracking them in a shallow-water area with both the VEMCO Radio Acoustic Positioning (VRAP) system and VEMCO Positioning System (VPS). The VRAP system is proven as an effective and accurate method for monitoring the movement of aquatic organisms5,6 and will provide a means to assess the performance of the newer VPS system. If VPS proves to be a valuable technique for shallow, weedy Delta habitats, it will be used for the second phase of study to monitor ten fish in each of two sites, differing in their habitat configuration.
By collecting fine-scale movement data and correlating it with SAV density and cover, as well as other environmental variables, this study will begin to provide a detailed description of the habitats occupied by LMB in the Delta. The resulting information will be valuable for understanding the extent of the trophic impact of these predators in the Delta.
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. Other aspects of the project include surveys of fish assemblage throughout the Delta via electrofishing (L. Conrad, K. Smith), the collection of information on LMB abundance, distribution, size, and diet (L. Conrad, K. Smith), 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).
Progress:
Both the VRAP and VPS monitoring arrays have been deployed in Mildred Island, in the central Delta. Assuming the tagged fish remain in the area, the VPS will remain in place until as least March 2010. Vegetation sampling has begun, and will be conducted every six weeks through the course of the year.