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

In California's Sacramento-San Joaquin estuary, environmental protection an d 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 habit at overlap by these two species, we compared the tolerances of the two spec ies to temperature, salinity, and water velocity, environmental factors tha t vary spatially and temporally within the estuary. For fishes acclimated t o 17 degrees 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 degrees C vs . 25.4 degrees C for delta smelt), lower critical thermal minima (2.3 degre es C vs. 7.5 degrees 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 fi sh, 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 temperatu re, salinity, and flow typical in the estuary would not exclude wakasagi, a lthough their eggs and larvae may be less tolerant. With respect to these f actors, the native delta smelt may be at a physiological disadvantage, part icularly in habitats with suboptimal environmental conditions, and may be e xcluded from shallow-water habitat restoration sites, which are characteriz ed by poor circulation, low flows, and more environmentally extreme conditi ons. The low abundance of wakasagi in the estuary recorded to date may indi cate that factors other than temperature, salinity, and flow determine waka sagi distribution.