Dennis E. Cocherell

Unscreened Water-Diversion Pipes Pose an Entrainment Risk to the Threatened Green Sturgeon, Acipenser medirostris

Over 3,300 unscreened agricultural water diversion pipes line the levees and riverbanks of the Sacramento River (California) watershed, where the threatened Southern Distinct Population Segment of green sturgeon, Acipenser medirostris, spawn. The number of sturgeon drawn into (entrained) and killed by these pipes is greatly unknown. We examined avoidance behaviors and entrainment susceptibility of juvenile green sturgeon (35±0.6 cm mean fork length) to entrainment in a large (>500-kl) outdoor flume with a 0.46-m-diameter water-diversion pipe. Fish entrainment was generally high (range: 26–61%), likely due to a lack of avoidance behavior prior to entering inescapable inflow conditions. We estimated that up to 52% of green sturgeon could be entrained after passing within 1.5 m of an active water-diversion pipe three times. These data suggest that green sturgeon are vulnerable to unscreened water-diversion pipes, and that additional research is needed to determine the potential impacts of entrainment mortality on declining sturgeon populations. Data under various hydraulic conditions also suggest that entrainment-related mortality could be decreased by extracting water at lower diversion rates over longer periods of time, balancing agricultural needs with green sturgeon conservation.

A lateral-displacement flume for fish behavior and stranding studies during simulated pulsed flows

In regulated rivers, fluctuating water depths associated with pulsed discharges may strand small fish in side channels and pools. Quantitative assessments of stranded fish are difficult in field studies (e.g., due to unknown effects of avian and terrestrial vertebrate predators). To assess such lateral displacement and stranding on juvenile stream fishes, we designed, constructed, and tested (with three species) a 2 × 1-m, lateral-displacement flume. The flume featured a main channel that never drained and a raised, wide “floodplain” channel that alternately flooded, with a simulated pulse, and became dewatered. The floodplain contained four pools, with different shapes and draining capacities, in which fish could become stranded as the water level subsided. Fish-stranding rates (8%) in this relatively compact laboratory flume, after exposure to simulated pulsed stream flows, were comparable to those observed in past investigations using larger, artificial streams.

High thermal tolerance of a rainbow trout population near its southern range limit suggests local thermal adjustment

California is the southern limit for indigenous rainbow trout. We studied wild-caught, Endangered Species Act (ESA)-listed fish beside their home stream and showed that the thermal aerobic performance capacity of these fish remains at 95% of peak aerobic scope across temperatures of 17.8–24.6°C. This range represents an unusually high temperature tolerance compared with conspecifics and congeneric species from northern latitudes.

Effects of nutritional deprivation on juvenile green sturgeon growth and thermal tolerance

Environ Biol Fish (2015) 99:145–159 DOI 10.1007/s10641-015-0463-8 Effects of nutritional deprivation on juvenile green sturgeon growth and thermal tolerance Christine E. Verhille & Seunghyung Lee & Anne E. Todgham & Dennis E. Cocherell & Silas S. O. Hung & Nann A. Fangue Received: 3 February 2015 / Accepted: 9 November 2015 / Published online: 19 November 2015 # Springer Science+Business Media Dordrecht 2015 Keywords Green sturgeon . Nutrition . Temperature . HSP70 . CTmax . Growth Introduction In the San Francisco Estuary (SFE), declines in the abundance of native fish species have been linked to altered food webs and reduced food availability (Moyle 2002; Feyrer et al. 2003; Kimmerer 2004). This is particularly concerning for protected green sturgeon (Acipenser medirostris Ayres, 1854) populations re- stricted to the SFE during early life stages. Green stur- geon are composed of at least two genetically distinct and protected populations (Israel et al. 2009): the North- ern and Southern Distinct Population Segments (DPS), and spawn only in Oregon and California, USA. Details of the current distribution and spawning locations of these fish can be found in Beamesderfer et al. (2007). Briefly, anadromous adult green sturgeon spend most of their lives in the marine environment, with seasonal migrations between natal freshwater spawning grounds C. E. Verhille : D. E. Cocherell : N. A. Fangue (*) Department of Wildlife, Fish and Conservation Biology, University of California, One Shields Avenue, Davis, CA 95616, USA e-mail: nafangue@ucdavis.edu S. Lee : A. E. Todgham : S. S. O. Hung Department of Animal Science, University of California, One Shields Avenue, Davis, CA 95616, USA and the ocean. Adults of the Northern DPS, which is classified as a species of concern by the National Oce- anic and Atmospheric Administration (NOAA) of the USA, spawn in rivers north of the Eel River of north- west California (Adams et al. 2007). The Southern DPS is classified as threatened under the Endangered Species Act, and all suspected and confirmed spawning loca- tions are within the watershed of the Sacramento and San Joaquin rivers (Adams et al. 2007). As juvenile green sturgeon are intolerant of full strength seawater until they are 0.5 to 1.5 years old (Allen and Cech 2007; Allen et al. 2009, 2011), habitat is restricted to the fresh to brackish water sections of the SFE for early life stages. The SFE is made up of two major Californian rivers, the Sacramento and San Joaquin, and their drainage to the Pacific Ocean through the San Francisco Bay. Since the 1800’s, this watershed has been highly modified through hydraulic gold mining in the Sierra Nevada mountains, followed by consumption and diversion of water to support urbanization and agriculture, introduc- tion of sewage input, and active management through environmental policy (Atwater et al. 1979; Cloern and Jassby 2012). International shipping, climate change and ecosystem alterations have created conditions con- ducive to pervasive invasions by non-native species, which now characterize the entire system (Atwater et al. 1979; Cloern and Jassby 2012). These ecosystem-wide changes are directly attributable to pop- ulation declines of native aquatic biota associated with all trophic levels (Cloern and Jassby 2012), resulting in large shifts in the composition of biological

Fish-protection devices at unscreened water diversions can reduce entrainment: evidence from behavioural laboratory investigations

Water diversion can pose a risk to resident or migratory fishes, but fish-protection devices that reduce the entrainment of native fishes and maintain normal diversion activities exist.

Foraging and metabolic consequences of semi-anadromy for an endangered estuarine fish

Diadromy affords fish access to productive ecosystems, increasing growth and ultimately fitness, but it is unclear whether these advantages persist for species migrating within highly altered habitat. Here, we compared the foraging success of wild Delta Smelt—an endangered, zooplanktivorous, annual, semi-anadromous fish that is endemic to the highly altered San Francisco Estuary (SFE)—collected from freshwater (<0.55 psu) and brackish habitat (≥0.55 psu). Stomach fullness, averaged across three generations of wild Delta Smelt sampled from juvenile through adult life stages (n = 1,318), was 1.5-fold higher in brackish than in freshwater habitat. However, salinity and season interacted, with higher fullness (1.7-fold) in freshwater than in brackish habitat in summer, but far higher fullness in brackish than freshwater habitat during fall/winter and winter/spring (1.8 and 2.0-fold, respectively). To examine potential causes of this interaction we compared mesozooplankton abundance, collected concurrently with the Delta Smelt, in freshwater and brackish habitat during summer and fall/winter, and the metabolic rate of sub-adult Delta Smelt acclimated to salinities of 0.4, 2.0, and 12.0 psu in a laboratory experiment. A seasonal peak in mesozooplankton density coincided with the summer peak in Delta Smelt foraging success in freshwater, and a pronounced decline in freshwater mesozooplankton abundance in the fall coincided with declining stomach fullness, which persisted for the remainder of the year (fall, winter and spring). In brackish habitat, greater foraging ‘efficiency’ (prey items in stomachs/mesozooplankton abundance) led to more prey items per fish and generally higher stomach fullness (i.e., a higher proportion of mesozooplankton detected in concurrent trawls were eaten by fish in brackish habitat). Delta Smelt exhibited no difference in metabolic rate across the three salinities, indicating that metabolic responses to salinity are unlikely to have caused the stomach fullness results. Adult migration and freshwater spawning therefore places young fish in a position to exploit higher densities of prey in freshwater in the late spring/summer, and subsequent movement downstream provides older fish more accessible prey in brackish habitat. Thus, despite endemism to a highly-altered estuary, semi-anadromy provided substantial foraging benefits to Delta Smelt, consistent with other temperate migratory fish.

Inter-population differences in salinity tolerance and osmoregulation of juvenile wild and hatchery-born Sacramento splittail

Genetically-distinct populations of the Sacramento Splittail, a minnow endemic to the San Francisco Estuary, exhibit different physiological responses and tolerances to salinity as juveniles. Population-level responses correspond with known salinity habitat differences suggesting some degree of local adaptation. Further studies are needed to elucidate these differences, but these data suggest a potential need for population-specific conservation and management strategies.

Plastic responses to diel thermal variation in juvenile green sturgeon, Acipenser medirostris

Human-induced thermal variability can disrupt energy balance and performance in ectotherms; however, phenotypic plasticity may play a pivotal protective role. Ectotherm performance can be maintained in thermally heterogeneous habitats by reducing the thermal sensitivity of physiological processes and concomitant performance. We examined the capacity of juvenile green sturgeon (Acipenser medirostris) to respond to daily thermal variation. Juveniles (47 days post-hatch) were exposed to either stable (15 ± 0.5 °C) or variable (narrowly variable: 13-17 °C day-1 or widely variable 11-21 °C day-1) thermoperiod treatments, with equivalent mean temperatures (15 ± 0.5 °C), for 21 days. Growth (relative growth rate, % body mass gain), upper thermal tolerance (critical thermal maxima, CTMax) and the thermal sensitivity of swimming performance (critical swimming speed, Ucrit) were assessed in fish from all treatments. Accelerated growth was observed in fish maintained under widely variable temperatures compared to narrowly variable and stable temperatures. No significant variation in CTMax was observed among thermoperiod treatments, suggesting all treatment groups acclimated to the mean temperature rather than daily maximums. The widely variable treatment induced a plastic response in swimming performance, where Ucrit was insensitive to temperature and performance was maintained across a widened thermal breadth. Maximum Ucrit attained was similar among thermoperiod treatments, but performance was maximised at different test temperatures (stable: 4.62 ± 0.44 BL s-1 at 15 °C; narrowly variable: 4.52 ± 0.23 BL s-1 at 21 °C; widely variable: 3.90 ± 0.24 BL s-1 at 11 °C, mean ± s.e.m.). In combination, these findings suggest juvenile A. medirostris are resilient to daily fluctuations in temperature, within the temperature range tested here.

Native Chinook salmon Oncorhynchus tshawytscha and non-native brook trout Salvelinus fontinalis prefer similar water temperatures

Preferred water temperatures and acute temperature tolerance limits of two salmonids in California were assessed: juvenile Chinook salmon Oncorhynchus tshawytscha, a native anadromous species, and sub-adult brook trout Salvelinus fontinalis, an introduced game species. These two species preferred similar temperatures across an 18 h temperature preference experiment and showed similar critical thermal tolerance limits, suggesting a substantial thermal habitat overlap in the wild.

Assessment of multiple stressors on the growth of larval green sturgeon Acipenser medirostris: implications for recruitment of early life-history stages

Early developmental stages of fishes are particularly sensitive to changes in environmental variables that affect physiological processes such as metabolism and growth. Both temperature and food availability have significant effects on the growth and survival of larval and juvenile fishes. As climate change and anthropogenic disturbances influence sensitive rearing environments of fishes it is unlikely that they will experience changes in temperature or food availability in isolation. Therefore, it is critical that we determine the effects of each of these potential stressors on larval growth and development, as well as understand the additive, synergistic or antagonistic effects of both. We reared threatened green sturgeon Acipenser medirostris (initial age ca. 32 days post hatch) at four temperatures (11, 13, 16 and 19°C) and two food availability rates (100% and 40% of optimal) to assess the effects of these stressors and their interactions on larval growth. We compared the overall size (fork length, total length and mass), growth rates (cm day-1 and g day-1 ) and relative condition factor of these larval and juvenile fish at 3 week intervals for up to 12 weeks. Our results indicated that temperature and food availability both had significant effects on growth and condition and that there was a significant interaction between the two. Fish reared with limited food availability exhibited similar patterns in growth rates to those reared with elevated food rates, but the effects of temperature were greatly attenuated when fish were food-limited. Also, the effects of temperature on condition were reversed when fish were reared with restricted food, such that fish reared at 19°C exhibited the highest relative condition when fed optimally, but the lowest relative condition when food was limited. These data are critical for the development of relevant bioenergetics models, which are needed to link the survival of larval sturgeons with historic environmental regimes, pinpoint temperature ranges for optimal survival and help target future restoration sites that will be important for the recovery of sturgeon populations.