The drift dynamics of larval shovelnose sturgeon Scaphirhynchus platorynchus (1, 2, 6, and 10 d posthatch (dph)) and pallid sturgeon S. albus (1, 2, 5, 9, 11, and 17 dph) were examined in a natural side channel of the Missouri River to quantify the vertical drift location of larvae in the water column, determine the drift velocity of larvae relative to water velocity, and simulate the cumulative distance (km) drifted by larvae during ontogenetic development. Larvae were released at the side‐channel inlet and sampled at points 100, 500, 900, and 1,300 m downstream. Larvae drifted primarily near the riverbed, as 58–79% of recaptured shovelnose sturgeon and 63–89% of recaptured pallid sturgeon were sampled in the lower 0.5 m of the water column. The transition from the drifting to the benthic life stage was initiated at 6 dph (mean length, 15.6 mm) for shovelnose sturgeon and at 11–17 dph (mean length, 18.1–20.3 mm) for pallid sturgeon. Across ages, the drift rates of larval shovelnose sturgeon averaged 0.09–0.16 m/s slower than the mean water column velocity. The drift rates of pallid sturgeon were similar to or slightly slower (0.03–0.07 m/s) than the mean water column velocity for 1–11‐dph larvae. Conversely, 17‐dph larval pallid sturgeon dispersed downstream at a much slower rate (mean, 0.20 m/s slower than the mean water column velocity) owing to their transition to benthic habitats. Drift simulations indicated that the average larval shovelnose sturgeon may drift from 94 to 250 km and the average larval pallid sturgeon may drift from 245 to 530 km, depending on water velocity. Differences in larval drift dynamics between species provide a possible explanation for differences in recruitment between shovelnose sturgeon and pallid sturgeon in the upper Missouri River.

The collapse of the European and Asian caviar industry has raised concern about the overexploitation of shovelnose sturgeon Scaphirhynchus platorynchus in the Missouri River. Unfortunately, little is known about the potential effects of harvest on the population dynamics of this species. Therefore, this study was conducted to describe the population characteristics (e.g., growth, longevity, and mortality) and to determine the influence of exploitation and harvest regulations (minimum length limits) on the yield, size structure, and egg production of shovelnose sturgeon from three sites in the Missouri River using a Beverton-Holt equilibrium model. Despite differences in the population characteristics (e.g., growth and longevity) of shovelnose sturgeon among sites, all populations responded similarly to harvest at the conditional natural mortality rates (death rate in the absence of harvest) used in our simulations (i.e., 5% and 20%). Our simulations of yield indicated that growth overfishing (i.e., shovelnose sturgeon being harvested before reaching their full growth potential) occurred with and without length limits at low conditional natural mortality rates in all populations. At a higher conditional natural mortality rate, only a 508-mm (fork length) minimum length limit prevented growth overfishing. Size structure (relative stock density of preferredlength fish [RSD-P]; Ն510 mm) was highly sensitive to exploitation and was reduced up to 87% in simulations without a harvest restriction or with a 406-mm length limit. A 508-mm length limit prevented RSD-P from declining more than 18% in all simulations. As with size structure, maximum lifetime egg production was reduced up to 74% at low exploitation rates (Յ20%), indicating the * Corresponding

Free embryos of wild pallid sturgeon Scaphirhynchus albus were released in the Missouri River and captured at downstream sites through a 180-km reach of the river to examine ontogenetic drift and dispersal processes. Free embryos drifted primarily in the fastest portion of the river channel, and initial drift velocities for all age groups (mean=0.66-0.70 ms −1 ) were only slightly slower than mean water column velocity (0.72 ms −1 ). During the multi-day longdistance drift period, drift velocities of all age groups declined an average of 9.7% day −1 . Younger free embryos remained in the drift upon termination of the study; whereas, older age groups transitioned from drifting to settling during the study. Models based on growth of free embryos, drift behavior, size-related variations in drift rates, and channel hydraulic characteristics were developed to estimate cumulative distance drifted during ontogenetic development through a range of simulated water temperatures and velocity conditions. Those models indicated that the average free embryo would be expected to drift several hundred km during ontogenetic development. Empirical data and model results highlight the long-duration, long-distance drift and dispersal processes for pallid sturgeon early life stages. In addition, results provide a likely mechanism for lack of pallid sturgeon recruitment in fragmented river reaches where dams and reservoirs reduce the length of free-flowing river available for pallid sturgeon free embryos during ontogenetic development.

No abstract

Obtaining food following the transition from endogenous to exogenous feeding and during the first year of life is a critical event that strongly influences growth and survival of youngof-year fishes. For shovelnose sturgeon Scaphirhynchus platorynchus, limited information is available on food habits during the first year of life. The objective of this study was to quantify diet components of shovelnose sturgeon during the transition from endogenous to exogenous feeding and during the youngof-year life stage in the North Dakota and Montana portions of the Missouri River. Young-of-year shovelnose sturgeon were sampled between early August and early September 2003. Shovelnose sturgeon initiated exogenous feeding by 16 mm, and individuals 16-140 mm fed exclusively on two macroinvertebrate orders (Diptera and Ephemeroptera). Youngof-year shovelnose sturgeon exhibited an apparently high feeding success as 99 of 100 individuals contained food in the gut. The number of organisms in the gut increased exponentially with fish length for larval Diptera (r 2 ¼ 0.73, P < 0.0001) and linearly (r 2 ¼ 0.12, P ¼ 0.0006) for larval Ephemeroptera, but the number of Diptera pupae in the gut was not significantly related (P ¼ 0.55) to length of young-ofyear shovelnose sturgeon. The length of ingested prey was linearly related to fish length for Diptera larvae (r 2 ¼ 0.20, P ¼ 0.002), whereas the relationship between lengths of ingested Ephemeroptera larvae and lengths of young-of-year shovelnose sturgeon was best described by a power function (r 2 ¼ 0.50, P < 0.0001). These results provide the first quantification of feeding dynamics for young-of-year shovelnose sturgeon in a natural river environment.

Summary The Pallid Sturgeon is listed as federally endangered under the Endangered Species Act in the United States. When the species was listed in 1990 it was considered extremely rare and was poorly understood. Habitat alteration, commercial harvest, environmental contaminants, and other factors were identified as threats. Today our scientific understanding of the species and its life history requirements have increased greatly as summarized below.

Populations of two short-lived species (emerald shiner Notropis atherinoides and sicklefin chub Macrhybopsis meeki) and three long-lived species (freshwater drum Aplodinotus grunniens, river carpsucker Carpiodes carpio, and sauger Stizostedion canadense) were studied in the Missouri River to examine spatial variations in life history characteristics across a latitudinal and thermal gradient (38Њ47ЈN to 48Њ03ЈN). The life history characteristics included longevity (maximum age), the rate at which asymptotic length was approached (K from the von Bertalanffy growth equation), the mean back-calculated length at age, and growth rates during the first year of life (mm/degree-day and mm/d). The mean water temperature and number of days in the growing season averaged 1.3 times greater in the southern than in the northern latitudes, while degree-days averaged twice as great. The longevity of all species except freshwater drum increased significantly from south to north, but the relationships between maximum age and latitude were curvilinear for short-lived species and linear for long-lived species. The von Bertalanffy growth coefficient for river carpsuckers and saugers increased from north to south, as indicated by significant negative relationships between K and latitude. Mean back-calculated length at age was negatively related to latitude for freshwater drums (Նage 4) and saugers (ages 1-5) but positively related to latitude for river carpsuckers (Նage 6). One of the growth rates examined (mm/degree-day) increased significantly from low to high latitudes for emerald shiners, sicklefin chubs, freshwater drums, and river carpsuckers during the first growing season. The other growth rate (mm/d) increased significantly from low to high latitudes for emerald shiners but was inversely related to latitude for saugers. These results suggest that the thermal regime related to latitude influences the life history characteristics of fishes in the Missouri River.

No abstract