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 and is a useful method for sampling the benthic fish community in moderate-to large-size river systems.
We investigated variation of incidentally captured turtle mortality in response to environmental factors and passive fishing techniques. We used Long Term Resource Monitoring Program (LTRMP) data collected from 1996 to 2001 in the unimpounded upper Mississippi River (UMR) adjacent to Missouri and Illinois, USA. We used a principle components analysis (PCA) and a stepwise discriminant function analysis to identify factors correlated with mortality of captured turtles. Furthermore, we were interested in what percentage of turtles died from passive fishing techniques and what technique(s) caused the most turtle mortality. The main factors influencing captured turtle mortality were water temperature and depth at net deployment. Fyke nets captured the most turtles and caused the most turtle mortality. Almost 90% of mortalities occurred in offshore aquatic areas (i.e., side channel or tributary). Our results provide information on causes of turtle mortality (as bycatch) in a riverine system and implications for river turtle conservation by suggesting management strategies to reduce turtle bycatch and decrease mortality of captured turtles.
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 information on community structure. We found species richness was greater at wing dikes for both adult and age‐0 fishes when compared with main channel borders. Stepwise multiple regression revealed significant relationships between adult species richness and passive gear deployment (e.g., hoop nets and mini‐fyke nets), physical habitat type, and river elevation, as well as interactions between physical habitat and passive gears, and physical habitat and transparency (i.e., Secchi depth). This model explained 56% of the variance in adult species richness. Approximately 15% of the variation in age‐0 species richness was explained by the sample period, sample date, transparency, physical habitat, and depth of gear deployment. Long‐term impacts of river modifications on fishes have not been well documented in many large river systems and warrant further study. The findings from this study provide baseline ecological information on fish assemblages using main channel borders in the unimpounded upper Mississippi River, information that will aid managers making channel maintenance decisions in large river systems.
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 make statistical comparisons. Shovelnose sturgeon catch rates varied among gears and seasons; season and gear type interacted in the ANOVA model. Among gear types, 5.08‐cm gill nets produced the highest standardized CPUE, and catch rates were highest during spring. The length frequency distributions of shovelnose sturgeon depended on gear type. Shovelnose sturgeon can be sampled in large rivers using stratified random sampling with this combination of gear. However, pallid sturgeon and lake sturgeon may be too rare to sample with this approach.
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