Robert A. Hrabik

Efficacy of a Benthic Trawl for Sampling Small-Bodied Fishes in Large River Systems

Abstract We conducted a study from 1998 to 2001 to determine the efficacy of a benthic trawl designed to increase species detection and reduce the incidence of zero catches of small-bodied fishes. We modified a standard two-seam slingshot balloon trawl by covering the entire trawl with a small-mesh cover. After completing 281 hauls with the modified (Missouri) trawl, we discovered that most fish passed through the body of the standard trawl and were captured in the cover. Logistic regression indicated no noticeable effect of the cover on the catch entering the standard portion of the modified trawl. However, some fishes (e.g., larval sturgeons Scaphirhynchus spp. and pallid sturgeon S. albus) were exclusively captured in the small-mesh cover, while the catch of small-bodied adult fish (e.g., chubs Macrhybopsis spp.) was significantly improved by use of the small-mesh cover design. The Missouri trawl significantly increased the number and species of small-bodied fishes captured over previously used designs...

Relationship among fish assemblages and main-channel-border physical habitats in the unimpounded Upper Mississippi River

Abstract Large rivers worldwide have been altered by the construction and maintenance of navigation channels, which include extensive bank revetments, wing dikes, and levees. Using 7 years of Long-Term Resource Monitoring Program (LTRMP) data collected from the unimpounded upper Mississippi River, we investigated assemblages in two main-channel-border physical habitats—those with wing dikes and those without wing dikes. Fishes were captured using daytime electrofishing, mini-fyke netting, large hoop netting, and small hoop netting. Our objectives were to (1) assess associations among fish species richness, physical measurements, and main-channel-border physical habitats using stepwise multiple regression and indicator variables; (2) identify abundant adult and young-of-year (age-0) families in both physical habitats to further investigate assemblage composition; and (3) calculate standardized species richness estimates within each physical habitat for adult and age-0 fishes to provide additional informati...

Maturation and Reproduction of Shovelnose Sturgeon in the Middle Mississippi River

Abstract Shovelnose sturgeon Scaphirhynchus platorynchus in the middle Mississippi River provide one of the last commercially viable sturgeon fisheries in the world, yet their maturation and reproduction have not been linked. During 2005 and 2006, we sampled adult and age-0 shovelnose sturgeon to link age at maturation, the timing and periodicity of spawning, age-0 sturgeon production, and the resulting age-0 growth rates. Age at maturity was later than previous estimates, the minimum age of first maturation being 8 years for males and 9 years for females. Total egg count was slightly lower than previously reported (mean = 29,573 per female; SE = 2,472). Males and females typically spawned every 2 and 3 years, respectively. Peaks in mature fish coincided with rising river stages and water temperatures at which shovelnose sturgeon probably spawn. Peaks in spent adults followed. Age-0 shovelnose sturgeon occurred during June and July 2005 and May and June 2006, confirming successful spawning. Age-0 sturgeon...

Habitat Use during Early Life History Infers Recovery Needs for Shovelnose Sturgeon and Pallid Sturgeon in the Middle Mississippi River

Abstract Habitat use by wild age-0 shovelnose sturgeon Scaphirhynchus platorynchus and pallid sturgeon S. albus was quantified by trawling macrohabitats (i.e., main channel, main-channel border, island, and artificial structures [e.g., wing dams]) in the middle Mississippi River during 1998–2007 (1,702 trawls; n = 1,507 age-0 sturgeon captured). Age-0 sturgeon catch rates were highest around artificial structures and island areas, while main-channel habitat provided the lowest catch rates. Moreover, size-selective macrohabitat analyses indicated that small age-0 sturgeon (i.e., majority < 50 mm total length) were located in channel-border dike and island side-channel macrohabitats, while all other macrohabitats were occupied by small and large age-0 sturgeon. Furthermore, our analyses indicated that age-0 sturgeon mesohabitat use within macrohabitats was influenced by depth, substrate, and bottom velocity. Low velocity (i.e., ∼0.1 m/s), moderate depths (i.e., 2–5 m), and sand substrate were positively rel...

The Mini-Missouri Trawl: A Useful Methodology for Sampling Small-Bodied Fishes in Small and Large River Systems

ABSTRACT Sampling has been conducted in small to large rivers of the Midwest and northeastern United States to determine the usefulness of a modified Missouri trawl designed to increase species detection of small-bodied fish species. We modified the Missouri trawl, which is a 4.8 m standard two-seam slingshot balloon trawl with a small mesh cover, by reducing the size (to a 2.44 m) and cover. The modified Missouri trawl (a.k.a. the mini-Missouri trawl) increased the number of small-bodied aquatic species of concern captured over that of previously used gears. For instance, shoal chub (Macrhybopsis hyostoma) had only been captured in the St. Croix River of Minnesota at three locations since 1960, yet we sampled it at 14 new locations in September, 2004 employing this methodology. Because of our success in these diverse and numerous systems, we believe this is a useful methodology for sampling the benthic fish community in many aquatic systems when other sampling methods are difficult to use because of water depths and/or velocities.

The Bighead Carp (Hypophthalmichthys nobilis) in the Mississippi River

ABSTRACT We report collections of the bighead carp, Hypophthalmichthys nobilis (Richardson, 1845) in the Mississippi River in Missouri and Illinois between 1991 and 1994. In all, we collected 48 specimens ranging from 18 to 790 mm total length. Young-of-the-year fish were caught in 1992 and 1994, which suggests that the species is able to reproduce in the Mississippi River and may become established. Because H. nobilis is a low level filter feeder, its presence may affect other filter feeding fishes such as the paddlefish (Polyodon spathula), bigmouth buffalo (Ictiobus cyprinellus), and gizzard shad (Dorosoma cepedianum). Further research is needed to confirm that the species is established and what its biological impact will be.

Water Temperature and River Stage Influence Mortality and Abundance of Naturally Occurring Mississippi River Scaphirhynchus Sturgeon

Abstract Several studies have investigated the demographics of shovelnose sturgeon Scaphirhynchus platorynchus and pallid sturgeon S. albus in the Mississippi River through assessment of adult populations; however, comparatively few studies have examined the early life history of these species. Here, we describe a comprehensive 4-year study that examined the effects of water temperature and river stage on the mortality, abundance, hatch timing, and growth rates of age-0 Scaphirhynchus spp. sturgeon in the middle Mississippi River. We trawled island areas every 7–10 d from April to August 2004–2007. We captured 1,256 individuals ranging from 10 to 193 mm total length over the 4-year study. Mean age-0 sturgeon growth rates ranged from 1.42 to 1.50 mm/d over the 4 years; however, growth rates did not differ among years. Individuals hatched over a 25–65-d period, and peak hatch dates were between 10 and 20 May in all years. Moreover, hatch timing possibly coincided with optimum spawning temperatures of 17–20°...

Habitat Selection and Movement of Naturally Occurring Pallid Sturgeon in the Mississippi River

Abstract The pallid sturgeon Scaphirhynchus albus is a U.S. federally endangered species that occurs in the 320-km middle Mississippi River (MMR). Historic in-channel island habitat has vanished, and the extent of the populations range within the MMR is unknown. We surgically implanted ultrasonic transmitters in 88 adult pallid sturgeon (>600 mm fork length; mean = 791 mm) during 2002–2005 and used boat-mounted hydrophones to quantify their seasonal use of major MMR habitat features (wing dikes, side channels, island side channel tips, tributaries, and main channel; total of 8,629 river kilometers monitored). Distance from habitat features (gravel bars, wing dikes, and island side channels) was quantified during spring, a period considered to be critical for many fish species. We quantified maximum seasonal movement of each fish in the entire MMR with stationary data-logging hydrophones during 2004–2006. Combining data across years and seasons, we found that pallid sturgeon selected the tips of wing dike...

Habitat for Age-0 Shovelnose Sturgeon and Pallid Sturgeon in a Large River: Interactions among Abiotic Factors, Food, and Energy Intake

Abstract The main stems of large rivers throughout the world have been highly modified with little consideration for effects on fishes that rely on these areas to complete their life histories. Particularly important is the ability of riverine habitats to provide foraging opportunities for young fish. We explored how temperature, flow, and food availability influenced diet content, prey selection (Strausss linear selectivity index), and energy condition of age-0 shovelnose sturgeon Scaphirhynchus platorynchus and pallid sturgeon S. albus in major habitat areas (e.g., islands, channel borders, wing dikes, and side channels) of the middle Mississippi River during spring (March–May) and summer (June–August) 2008. Standardized diet mass (dry mass standardized for fish body mass) of the age-0 sturgeon peaked at about 19°C and at a flow velocity of 0.5 m/s. Although potential prey taxa were diverse, the diets for age-0 sturgeon of all sizes were dominated by mayflies (Ephemeroptera) and midge larvae (Chironomi...

Seasonal Comparison of Catch Rates and Size Structure Using Three Gear Types to Sample Sturgeon in the Middle Mississippi River

Abstract We evaluated the efficacy of three gears commonly used to sample shovelnose sturgeon Scaphirhynchus platorynchus, pallid sturgeon S. albus, and lake sturgeon Acipenser fulvescens in large rivers. We stratified habitats and randomly sampled sites with trawls, gill nets, and trotlines in the middle Mississippi River from June 2003 through May 2005 (N = 3,476 samples). A total of 3,523 shovelnose sturgeon, 31 pallid sturgeon, and 13 lake sturgeon were captured. When sample sizes were adequate (based on power analysis), we used a mixed analysis of variance (ANOVA) procedure to determine the relative impact of season and gear type on sturgeon catch per unit effort (CPUE). Kolmogorov–Smirnov tests were used to determine whether the length frequency distributions for each species differed among gears. Overall, the power analyses indicated that there were adequate sample sizes for comparing standardized CPUE for shovelnose sturgeon among seasons; however, too few pallid or lake sturgeon were collected to...