Fine Scale Movement and Habitat Utilization of Adult Green Sturgeon in the Upper Sacramento River

The green sturgeon, Acipenser medirostris, is one of two acipenserid fishes native to the Central Valley.  There are two Distinct Population Segment (DPS). The northern DPS includes the spawning populations from the Rogue and Klamath Rivers.  On April 7, 2006 the Southern DPS, which constitutes the spawning population of the Sacramento River, was listed as “threatened” under the Endangered Species Act (ESA).   Recent interest in green sturgeon behavior, habitat preferences, and migratory path ways has led us to develop a telemetric study to identify potential spawning and aggregate locations, spawning behavior, and habitat utilization.  Five adult green sturgeon were acoustically tagged with Vemco V16 continuous transmitters with depth and pressure sensors. Each individual was tracked by boat for a period of up to five days, while simultaneously recording GPS coordinates (Vemco VR100 portable receiver), depth, and water temperature at the sturgeon’s location.  Environmental conditions (pH, dissolved oxygen, turbidity, and water temperature) were recorded using a shipboard water quality sonde. We performed a First- Passage Time Analysis (FPT) on the movements of all five individuals. High FPT values indicated areas of greater relative utilization of habitat units. Categorical and Regression Trees analysis were used to determine which explanatory variables best described high FPT values. Contrary to previous thought, individuals were highly mobile, exhibiting movements between aggregate locations over great distances(> 10 Km). All individuals showed differences in relative habitat. While results of the CART analysis varied between individuals there is some evidence to suggest that depth of habitat may be a key variable in explaining differences in first passage times. These findings support the recent management decision to leave the operations gates open at the Red Bluff Diversion Dam, thus maintaining the migratory corridor to upper sites and preventing the potential for fragmentation of the spawning population.  Lastly, we show this method of tracking, typically conducted in the open ocean environment, to be a useful tool for filling the spatial-temporal gap typically captured by either stationary course grain monitor deployment or fine-scale localized positioning systems.