Integrating Sensory Ecology and Behavior With the Management of Juvenile Green Sturgeon (Acipenser medirostris)

The green sturgeon, Acipenser medirostris, is an anadromous California native fish species that is of particular importance for federal and state conservation and management programs, though little is known about juvenile behavior. Therefore, we examined aspects of the behavioral ecology of this cryptic species during early life history. While the physiological adjustments made by this species as they move among waters with varying salinities have been investigated, little work has been done to determine the behavioral preferences of green sturgeon to waters with different salinities. We examined the behavioral salinity preference of juvenile sturgeon (200--220 days post-hatch [dph]) acclimated to fresh water and full-strength seawater in a rectangular preference flume (maximum salinity gradient: 5--33 ppt, mean salinity of each side 9.3 +/- 0.2 vs. 29.2 +/- 0.4 ppt) and found that fish acclimated to both seawater and to fresh water spent a significantly greater amount of time on the side of the testing area with the highest salinity concentration. These results indicate that juveniles likely enter saline environments within the first year of life, and that migrating fish actively seek waters with increasing salinity. During such outmigrations, juveniles are particularly susceptible to interactions with anthropogenic disturbances such as water diversions, which are prevalent throughout their native watersheds. We evaluated the behavior of green sturgeon (150--192 dph) within a laboratory swimming flume near fish exclusion screens under various environmental conditions (different flow velocities [21.3 vs. 37.6 cm/s], and day or nighttime), and in the presence of commonly used sensory deterrents (a strobe light or vibrations). We then compared their performance to the closely related, semi-anadromous white sturgeon, Acipenser transmontanus (170--192 dph). Behavioral responses, including screen contacts, impingements, and time spent near screens were quantified. Green sturgeon contacted and impinged upon the screens twice as frequently as white sturgeon, and also differed in how their behaviors were altered by water velocities and time of day. This heightened susceptibility to interactions with fish screens lead us to test a sensory deterrent (multiple strobe lights), and two alternative structural modifications at reducing the entrainment of juvenile green sturgeon (128--141 dph) into a large-scale simulated water diversion. We quantified fish passage, fish entrainment, entrainment risk, and entrainment distance, and projected a population entrainment risk for migrating juvenile green sturgeon. We found that while the strobe light was not an effective deterrent, both structural modifications significantly reduced entrainment and entrainment risk of green sturgeon into the water diversion. Because little is known about the mechanisms by which sturgeon guide their outmigration, we assessed the ability of green sturgeon to detect local magnetic stimuli as a potential orientation mechanism. Additionally, due to the limited success of previously tested sensory deterrents to inhibit sturgeon interactions with anthropogenic devices, magnetic stimuli may become a potential option for future deterrent development. Distributions of juvenile green sturgeon (183--254 dph) positions were compared in the presence and absence of magnetic stimuli under low and high light conditions. Our results are informative in developing effective management strategies to balance natural green sturgeon recruitment with other water-related needs, mitigate the impacts of water diversions on sturgeon populations, and improve effective restoration strategies.