Trophic Relations of Introduced Flathead Catfish in a North Carolina Piedmont River
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2007-10-15
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Abstract
The flathead catfish Pylodictis olivaris is a large piscivious ictalurid that is native to the Mississippi and Rio Grande river drainages, but has been widely introduced across the United States. I studied the trophic relations of introduced flathead catfish in the upper Cape Fear River basin, located in the piedmont region of North Carolina. My specific objectives for this study were to (1) quantify the diet of the flathead catfish and determine an ontogentic shift in diet; (2) determine selectivity for different prey fishes based on their occurrence in the flathead catfish diet and abundance in the river system; (3) determine diel chronology in feeding; (4) calculate daily ration and gastric evacuation rate to quantify the rate of food consumption; and (5) conduct field experiments to elucidate the mechanisms of the predator-prey relationship by determining preferences in introduced flathead catfish and channel catfish feeding between prey species, prey location in the water column, and accessibility to cover.
River ecologists and fisheries managers are concerned with introductions of flathead catfish because of negative impacts to native fish communities associated with direct predation and indirect competition from these apex predators. There are also concerns with introductions that result in co-occurrence with imperiled species, and within my study site, introduced flathead catfish occur with the federally endangered Cape Fear shiner Notropis mekistocholas and the Carolina redhorse Moxostoma sp., a federal species of concern.
I sampled a section of the Deep River in North Carolina that was hydrologically divided into unimpounded and impounded reach, to quantify diet and determine diet selectivity. A second study site, located at the confluence of the Deep and Haw rivers where the Cape Fear River is formed, was sampled in conjunction with the first field site to determine diel feeding chronology, daily ration, and gastric evacuation rate. Flathead catfish were collected using non-lethal, low-frequency, pulsed-DC electrofishing, and diets were sampled using non-lethal, pulsed gastric lavage. A randomized prey curve determined that the number of stomachs sampled was sufficient to accurately describe flathead catfish diet.
The prey taxon with the greatest occurrence in the diet was crayfish, while sunfish composed the greatest percent of the diet by weight; neither imperiled fish species was found in any stomach sampled. An ontogenetic shift in diet was evident when flathead catfish reached about 300 mm in total length, and flathead catfish length significantly explained variation in percent-composition-by-weight of crayfish, sunfish, and darters. Flathead catfish showed positive prey selectivity for taxa that occupied similar benthic microhabitat as this predator, highlighting the importance of prey encounter rates to the predatory behavior of flathead catfish. Flathead catfish fed throughout the 24-h period and displayed a highly variable diel feeding chronology for July with a mean stomach fullness of 0.32%, but showed a single mid-day peak in feeding during August (mean fullness = 0.52%). The gastric evacuation rate for flathead catfish increased between July (0.40⁄h) and August (0.59⁄h), as did daily ration, which more than doubled between the two months (3.06% in July, 7.37% in August).
A tethering experimental approach proved effective in determining prey selection dynamics for two contrasting large catfish species in the field, when they were presented choices among prey differing in species, location in the water column, and access to shelter from the predator. The flathead catfish, an obligate carnivore, showed no preference among all three treatment effects, whereas channel catfish, a feeding generalist, showed strong specificity for prey species and location of that prey item in the water column. These research findings under controlled conditions in a field setting offer additional insight into prey selection dynamics of these introduced catfish predators as it occurs in a natural setting that could not be gained by traditional sampling and observational approaches. Understanding the trophic relations of introduced flathead catfish and the degree of vulnerability among prey taxa will allow resource managers to make science-based decisions that may decrease the impacts of introduced flathead catfish on native fish populations and allow enhanced protection for imperiled species.
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introduced flathead catfish, Pylodictis olivaris, trophic relations, diet selectivity, diel feeding chronology, daily ration, evacuation rate, pulsed gastric lavage, tethering experiments, diet, Cape Fear River, North Carolina
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MS
Discipline
Fisheries and Wildlife Sciences
