Christina Swanson

Comparative environmental tolerances of threatened delta smelt (Hypomesus transpacificus) and introduced wakasagi (H. nipponensis) in an altered California estuary

Abstract In California’s Sacramento-San Joaquin estuary, environmental protection and habitat restoration efforts directed at a threatened native osmerid, the delta smelt (Hypomesus transpacificus), are complicated by the presence of a morphologically similar non-native congener, the wakasagi (H. nipponensis), transported to the estuary from upstream reservoirs. In order to better define delta smelt critical habitat and to evaluate the potential for habitat overlap by these two species, we compared the tolerances of the two species to temperature, salinity, and water velocity, environmental factors that vary spatially and temporally within the estuary. For fishes acclimated to 17°C and fresh water (0 ppt), we measured critical thermal maxima and minima, chronic upper salinity tolerance limits, and critical swimming velocities. Wakasagi had higher critical thermal maxima (29.1°C vs. 25.4°C for delta smelt), lower critical thermal minima (2.3°C vs. 7.5°C for delta smelt), higher upper salinity tolerances (26.8 ppt vs. 19.1 ppt for delta smelt), and swam faster (for 6–6.9 cm SL fish, 43.3 cm s–1 vs. 28.2 cm s–1 for delta smelt) than delta smelt. This suggests that the wide seasonal and year-to-year fluctuations in temperature, salinity, and flow typical in the estuary would not exclude wakasagi, although their eggs and larvae may be less tolerant. With respect to these factors, the native delta smelt may be at a physiological disadvantage, particularly in habitats with suboptimal environmental conditions, and may be excluded from shallow-water habitat restoration sites, which are characterized by poor circulation, low flows, and more environmentally extreme conditions. The low abundance of wakasagi in the estuary recorded to date may indicate that factors other than temperature, salinity, and flow determine wakasagi distribution.

Swimming in two-vector flows: Performance and behavior of juvenile Chinook salmon near a simulated screened water diversion

Abstract We used a large, annular flume equipped with a simulated fish screen to assess the swimming and behavioral responses of juvenile Chinook salmon Oncorhynchus tshawytscha to two-vector flows typifying habitats near water diversions, where small fish may become entrained. Groups of 20 fish (4.4–7.9 cm long, at 12°C or 19°C) were tested for 2 h at one of nine experimental flow regimes derived from combinations of three (approach) velocities perpendicular to the screen and three (sweeping) velocities parallel to the screen and a (0-cm/s) control during daytime (lighted) and nighttime (dark) conditions. In the high-velocity (resultant vector) flow regimes, all fish swam at velocities comparable to the critical swimming velocities measured for similar-sized conspecifics, suggesting that exposure to such flow conditions near a water diversion is energetically expensive. Although most fish exhibited strong positive rheotaxis, older (smolt-size) fish acclimated to the warmer temperature exhibited higher ra...

Use of Salts, Anesthetics, and Polymers to Minimize Handling and Transport Mortality in Delta Smelt

Abstract We tested the effects of transport containers and transport water treatments on the survival of field-collected delta smelt Hypomesus transpacificus, a threatened osmerid endemic to the Sacramento–San Joaquin estuary in California. Use of cylindrical polyethylene bags instead of rectangular coolers as transport containers increased survival from 40.7 to 83.6% at 4 h postcollection, from 11.9 to 33.1 % at 48 h, and from 6.9 to 27.9% at 72 h. Addition of NovAqua, a commercial water conditioner containing polymers, to transport water of 8‰ NaCl significantly increased 72-h survival (54.8%) over that of the 8‰ NaCl control (27.9%). Survival of fish lightly anesthetized with MS-222 (tricaine methanesulfonate) during transport was intermediate between the NaCl and NaCl plus NovAqua treatments. Survival of delta smelt in the NaCl plus NovAqua treatment also improved from August through November, as fish increased in size and water temperature decreased. Improved survival of delta smelt treated with NovA...

Close Encounters with a Fish Screen III: Behavior, Performance, Physiological Stress Responses, and Recovery of Adult Delta Smelt Exposed to Two-Vector Flows near a Fish Screen

Abstract The threatened delta smelt Hypomesus transpacificus is vulnerable to more than 2,000 water diversions distributed throughout the Sacramento–San Joaquin Delta system of California. Although fish screen installation should decrease direct losses, the physiological stress associated with exposure to two-vector flows near a fish screen may increase delta smelt mortality. We measured the swimming performance, screen contact rate, and stress responses of adult delta smelt to handling and simulated screen exposure at 12°C. Results from handling stress and flow stress experiments showed that fish collection, crowding, and transport elicited significant stress responses in delta smelt, as did exposure to the two-vector flows and a fish screen. However, delta smelt recovered from the cumulative handling stress within 2 h. Combinations of high approach velocity (10 or 15 cm/s) and high sweeping velocity (31 or 62 cm/s) increased the primary stress response (plasma cortisol concentrations) and the number of ...

Photophase and Illumination Effects on the Swimming Performance and Behavior of Five California Estuarine Fishes

Abstract Photophase and illumination affect many fish activities. In this study, we examined their effects on the critical swimming velocities (Ucrit), swimming gait patterns, and oral grasping behavior of five California estuarine fishes. All species (4–5 cm SL) swam similarly (mean Ucrit range: 30–36 cm/sec) under day/light conditions. However, both nighttime photophase and darkness decreased Delta Smelt (Hypomesus transpacificus) swimming velocities. Congeneric Wakasagi (Hypomesus nipponensis) swimming performance also decreased at night/dark conditions. Regardless of photophase and illumination, Delta Smelt, Wakasagi, and Splittail (Pogonichthys macrolepidotus) exhibited three swimming gaits: intermittent stroke-and-glide at low velocities, continuous stroking at moderate velocities, and intermittent burst-and-glide at high velocities near Ucrit. In contrast, Chinook Salmon (Onchorhynchus tshawytscha) used only two swimming gaits: continuous stroking and burst-and-glide under all conditions. Inland Silversides (Menidia beryllina) used these two gaits under light conditions and all three gaits under dark conditions. Some Wakasagi, Splittail, and Chinook Salmon orally grasped the upstream screen in the flume at moderate to high water velocities. Oral grasping does not require jaw teeth and may represent adaptive behavior in natural habitats. Regarding vulnerability to water diversions that operate in the dark at night, the threatened Delta Smelt and introduced Wakasagi, comparatively, may be more at risk than the other species.

Close encounters with a fish screen II: Delta smelt behavior before and during screen contact

Abstract Fish screens reduce entrainment losses of fish at water diversions, but little is known about the behavior of individual fish near these structures, including the likelihood, consequences, and mechanics of involuntary screen contact. Recent studies using a large, laboratory-based flume, the Fish Treadmill, have shown that such contact can result in injury, stress, and mortality. We investigated key aspects of screen contact events using videotape records of Fish Treadmill experiments. We quantitatively described precontact behavior, impact velocity and angle, contact duration, distance traveled, and velocity of delta smelt Hypomesus transpacificus while in contact with the screen, and related these responses to postexperiment injuries and mortality. Immediately prior to contact, delta smelt generally exhibited positive rheotaxis and moderate swimming velocity (mean ± SD = 20.2 ± 9.7 cm/s). Most fish impacted the screen broadside (within 40° of parallel to the screen) and tail first. Impact veloci...