Individual habitat use and behavior of acoustically-tagged juvenile green sturgeon in the Sacramento-San Joaquin Delta

Southern distinct population segment (sDPS) juvenile Green Sturgeon are thought to rear in the Sacramento-San Joaquin Delta for two to four years. The southern population segment has been listed as threatened under the Endangered Species Act. Habitat loss is cited as a significant driver in declines of the population. Specific to juvenile rearing, much of the historic floodplain habitat has been lost and replaced by leveed channelized waterways. In addition to geomorphic alterations, other direct and indirect activities such as water export, dredging, and the introduction of invasive species may have significant impacts to the viability of the species. The objective of this tracking study was to provide a first of its kind look at the movements and behaviors of wild caught juvenile Green Sturgeon within the putative nursery grounds of the Sacramento-San Joaquin Delta. We acoustically tagged and continuously tracked six yearling juvenile Green Sturgeon with mean total length (TL) of 50.6cm (SD=5.3) and mean mass of 563.3g (SD=151.7). We performed continuous mobile tracking for a period of up to five days per individual while recording GPS coordinates, depths, and water temperature at the sturgeon's location. We used first-passage times, defined as the amount of time spent within a given area, for which a radius was determined post hoc. First passage times were used as a metric for parsing bouts of localized movements from directed movements. Our results indicate that juvenile Green Sturgeon largely oriented at or near the bottom. Depths utilized by all sturgeon ranged from >3m to 20m across all tracks. Individuals exhibited fidelity to the San Joaquin River Channel with only one individual leaving the main channel. Juvenile green sturgeon were shown to exhibit both positive and negative rheotaxis during sustained directed movements under independent ebb tidal cycles. This study provides the first detailed examination into the early life movements and behaviors of a rare species within their putative nursery grounds.