Browsing by Author "Emmel, Jason A."

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    Feeding Ecology and Distribution of an Invasive Apex Predator: Flathead Catfish in Subestuaries of the Chesapeake Bay, Virginia
    Schmitt, Joseph D.; Emmel, Jason A.; Bunch, Aaron J.; Hilling, Corbin D.; Orth, Donald J. (2019-04)
    Native to the central United States, Flathead Catfish Pylodictus olivaris have invaded Atlantic coast rivers from Florida to Pennsylvania. They are now invasive in several subestuaries of the Chesapeake Bay, yet contemporary accounts of their distribution do not exist. Due to their piscivorous nature, Flathead Catfish could have deleterious impacts on native ichthyofauna, yet their feeding ecology has not been well described in these systems. We used a large-scale, stratified random sampling effort to describe the current distribution and feeding ecology of Flathead Catfish in Virginia tidal rivers. Low-frequency electrofishing was conducted at more than 1,500 sites in the James, Pamunkey, Mattaponi, and Rappahannock rivers in eastern Virginia, resulting in 766 Flathead Catfish being captured in the James, Pamunkey, and Mattaponi rivers. Flathead Catfish are abundant in the tidal James River from Richmond, Virginia, to the confluence of the Chickahominy River. A relatively new but established population was also observed in the Pamunkey River, where the highest observed densities of Flathead Catfish occurred near Williams Landing (37 degrees 36 ' 21.49 '' N, 77 degrees 5 ' 33.42 '' W) in New Kent County, Virginia. Stomachs collected from 731 Flathead Catfish revealed that they are piscivores that feed heavily on Gizzard Shad Dorosoma cepedianum, White Perch Morone americana, and various Alosa species. Analysis of trophic level, diet breadth, and feeding strategy demonstrated that Flathead Catfish are piscine specialists that occupy trophic positions indicative of an apex predator. Our results show that Flathead Catfish could have substantial per capita impacts on at-risk native species including American Shad Alosa sapidissima, Blueback Herring A. aestivalis, and Alewife A. pseudoharengus as they make seasonal migrations in and out of these river systems. Moreover, future range expansion of Flathead Catfish into the Rappahannock River is plausible, as established populations now exist in adjacent tributaries.
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    Growth and Mortality of Invasive Flathead Catfish in the Tidal James River, Virginia
    Hilling, Corbin D.; Bunch, Aaron J.; Emmel, Jason A.; Schmitt, Joseph D.; Orth, Donald J. (2019-12)
    Invasive species are a major threat to biodiversity of native fishes in North America. In Atlantic coastal rivers of the United States, large catfishes introduced from the Gulf of Mexico drainages have become established and contributed to native species declines. Flathead Catfish Pylodictis olivaris were introduced to the Chesapeake Bay drainage in the 1960s and 1970s in the James and Potomac river systems in the eastern United States. Diet studies have found James River Flathead Catfish function as apex predators and are known to consume at-risk Alosa spp. To limit further range expansion and impacts to native species, resource management agencies need information on population characteristics to support population assessments and management plan development. Thus, we examined temporal trends in growth rates and estimated total instantaneous mortality for tidal James River Flathead Catfish collected by Virginia Department of Game and Inland Fisheries from 1997 to 2015. Parameters of the von Bertalanffy growth model with length-at-age observations pooled across sampling years were estimated as L-infinity = 1,059 mm, k= 0.231/y, and t(0) = 0.55 y. Flathead Catfish growth differed among sampling years, especially for the years 2007 and 2014, which had the largest sample sizes. However, there were no obvious temporal trends in growth trajectories. James River Flathead Catfish tend to grow much faster than most populations used in development of the relative growth index, but the species is known to grow faster in its nonnative range. Consequently, scientists and managers should use caution when applying growth indices if native and nonnative populations are not expressly considered in development of the index. We estimated total instantaneous mortality as Z 0.50 and mean natural mortality from six estimators as M 0.30. A lack of older individuals in the population means that mortality rates may be overestimated as a result of gear selectivity or ongoing maturation of the population. These data provide information to support future work examining the species in the James River and development of population models to evaluate management strategies and management plans.