No abstract

Habitat use by juvenile bull trout (Salvelinus confluentus) and cutthroat trout (Oncorhynchus clarki) in Trestle Creek, Idaho, changed seasonally and dielly. Both cutthroat and bull trout selected pools over riffles in both summer and winter. Both species used a wide range of depths at night but were absent from shallow water (<15 cm) during the day in summer and winter. During summer, juveniles of both species occupied areas of lower velocity water at night than during the day. Both species also occupied lower velocity water during winter days than summer days. During winter days, juvenile bull trout were located below or directly on cobble substrate, whereas cutthroat trout often formed aggregations suspended in the water column of large pools. Both species were more closely associated with cover during the day, and made the greatest use of cover during winter days. Land management activities resulting in decreased pool habitat, instream cover, and stream-bed stability may be especially detrimental to bull trout and cutthroat trout in winter.

It is hypothesized that slow, shallow water habitats benefit larval pallid sturgeon Scaphirhynchus albus; however, testing this hypothesis is difficult, given the low number of larval pallid sturgeon present in large rivers. In contrast, relatively large numbers of age-0 shovelnose sturgeon Scaphirhynchus platorynchus have been sampled, providing a potentially useful baseline to assess the importance of slow, shallow water to age-0 sturgeon of both species (hereafter age-0 sturgeon) in the lower Missouri River. Thus, we investigated the potential relationships between the prevalence of shallow water <1.5 m and the age-0 sturgeon catch rates at multiple scales. Age-0 sturgeon were usually sampled in water >1.5 m, and catch rates were usually highest in the upper half [i.e. river kilometre (RKM) 400 to 800] of the lower Missouri River study area, whereas the availability of water <1.5 m was usually highest in the lower half (i.e. RKM 0 to 400). Similarly, there was no relationship between age-0 sturgeon mean catch-per-unit effort and ha/km of water <1.5 m at any studied scale. Our results may suggest that shallow water, as currently defined, may not be a suitable surrogate for assessing efforts to address pallid sturgeon population declines. However, it is still unknown if lack of appropriate habitat is currently limiting pallid sturgeon. Published 2015.

Validation is critical when considering potential surrogates for endangered species research. The pallid sturgeon Scaphirhynchus albus is federally endangered in the USA, whereas the congeneric shovelnose sturgeon S. platorynchus is common. Inadequate food availability during early life history is a hypothesized factor limiting recruitment of pallid sturgeon in the lower Missouri River, USA, and an established surrogate relationship with shovelnose sturgeon would dramatically expedite research efforts related to potential food limitation. During 2018, 4 exogenously feeding pallid sturgeon larvae were captured concurrently with multiple shovelnose sturgeon, providing a critical opportunity to evaluate diet surrogacy using individuals captured from the same locations. We found that both species primarily consumed ephemeropterans but chironomids were also frequent prey. Over 90% of the pallid sturgeon prey items belonged to taxa also consumed by shovelnose sturgeon. Additionally, we often observed high diet similarity between these congeners. As for total prey weight, pallid sturgeon consumption was similar or higher compared to similarly sized shovelnose sturgeon. Although the sample size was limited, this study supports the hypothesis that larval pallid sturgeon are capable of finding food in the lower Missouri River and exhibiting diet similarity with shovelnose sturgeon. We also provide the first quantification of prey weight for wild-produced larval pallid sturgeon, which consumed relatively large amounts of prey in the wild during the critical transition from endogenous to exogenous feeding.

Insufficient food during early life could limit the population growth of endangered Pallid Sturgeon Scaphirhynchus albus in the lower Missouri River. Shallow-water habitat restoration is intended to provide nursery benefits, including food, for young sturgeon, but the effect of shallow-water habitat on their diet is unknown. Age-0 Pallid Sturgeon are rare, providing little opportunity for direct evaluation; however, studying the closely related and abundant Shovelnose Sturgeon S. platorynchus may provide valuable information to guide habitat restoration efforts. We compared diet, body condition (lipid content), and change in body weight (24-h bioenergetics simulation) for postdrift, age-0 sturgeon among five reaches ranging widely in shallow-water habitat availability. Lipid content of satiated and emaciated laboratory-reared individuals were compared with that of wild-caught fish. In general, shallow-water habitat availability appeared to have little effect on the variables examined. Regardless of reach, wild-caught fish primarily consumed chironomids, and empty stomachs were rare. Additionally, differences in prey weight, lipid content, or the modeled change in body weight did not usually correspond to differences in shallow-water habitat availability. Instead, we found annual differences, as prey weight consumed and the percentage of fish with modeled weight gain was often higher in 2015 than 2014, while the opposite was true for the percentage of fish with lipid content values that were comparable with the emaciated laboratory standard. Overall, our findings complement recent suggestions that shallow-water habitat restoration efforts, as previously implemented, may not benefit sturgeon populations. Our results coupled with previous research suggest that the lower Missouri River prey base can support a stable Shovelnose Sturgeon population; however, additional research is needed to determine whether this applies to Pallid Sturgeon.