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 vs. 28.2 cm s 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.

Open Water Processes of the San Francisco Estuary: From Physical Forcing to Biological Responses

“…These results suggest the value of flow regimes approximating natural conditions for restoring native fishes (Marchetti and Moyle 2001). Conversely, the introduced wakasagi had broader tolerances than its close relative, the native delta smelt, suggesting caution in assuming that flow regimes alone could benefit natives over introduced species (Swanson et al 2000).…”

Conservation physiology

“…Nestlings of an area-sensitive passerine, the Eurasian tree creeper Certhia familiaris, had elevated corticosterone in small, dense patches of forest relative to larger, old-growth patches, indicating the importance of habitat characteristics on physiology and fitness of this species, especially while young are in the nest [38,39]. Comparative environmental tolerance analyses of endemic and exotic species can be useful for evaluating crucial environmental conditions for survival and problems of habitat overlap [40][41][42].…”

Linking molecular biomarkers with higher level condition indicators to identify effects of copper exposures on the endangered delta smelt (Hypomesus transpacificus)

“…Delta smelt are restricted to the Sacramento-San Joaquin estuary, spawning in late winter and early spring, at limited freshwater sites both known and speculated, based on sediment type and other physicochemical properties such as slope, vegetation, depth, temperature, and salinity [7]. A major site of delta smelt spawning is in the Lower Sacramento River, from which hatched larvae are transported with the flow to brackish waters downstream, where they mature, during a short-lived one-year cycle [8].…”