Population Ecology of Introduced Flathead Catfish

Invasive aquatic species are becoming increasingly problematic for aquatic ecologists and resource managers, as the ecological and economic impacts of introductions become better known. The flathead catfish Pylodictis olivaris is a large piscivorous fish native to most of the interior basin of the United States. Via legal and illegal introductions, they have been introduced into at least 13 U.S. states and one Canadian province primarily along the Atlantic slope. I used a variety of capture-recapture models to estimate flathead catfish population parameters in three North Carolina coastal plain rivers (Contentnea Creek, Northeast Cape Fear River, and Lumber River). My estimates using a Jolly-Seber model were hindered by low capture probabilities and high temporary emigration. Reasonable estimates were possible using a robust-design framework to estimate population size and temporary emigration with supplemental information from a radio-telemetry study to examine model assumptions. Population size estimates using a robust design model including temporary emigration ranged from 4 to 31 fish/km (>125-mm total length, TL) of sampling reach. Additional analyses showed high rates of temporary emigration (>90%), independently supported by radio-telemetry results. I also examined flathead catfish diet in these rivers and found that flathead catfish fed on a wide variety of freshwater fish and invertebrates, anadromous fish, and occasionally estuarine fish and invertebrates. Fish or crayfish comprised more than 50% of the stomach contents by percent occurrence, percent-by-number, and percent-by-weight in all rivers and years. A significant difference in the diet composition percent-by-number was found between Contentnea Creek and the Northeast Cape Fear River. Significant differences were not detected between years within Contentnea Creek but were found within the Northeast Cape Fear River. Feeding intensity (as measured by stomach fullness) was highest in the Northeast Cape Fear River associated with a lower mean size of feeding flathead catfish in this river than of those in Contentnea Creek or the Lumber River. A significant correlation between diet item length and flathead catfish total length was found for Contentnea Creek in 2001. This relationship was not significant in the Northeast Cape Fear River in either year. Based on the diet composition data collected in this study and those published on native and introduced flathead catfish populations, I am not able to support or refute the hypothesis that flathead catfish are preferentially feeding on specific species or families. However, the flathead catfish populations examined here are well established, and the greatest impact from selective predation may have occurred immediately following introduction. Based on my findings, flathead catfish could restructure or suppress native fish communities in coastal rivers through direct predation because of their primarily piscivorous food habits. To evaluate the potential ecosystem impact of this invasive species on the native fish community, I developed an ecosystem simulation model (including flathead catfish) based on empirical data collected for a North Carolina coastal river. Model results suggest that flathead catfish suppress native fish community biomass by 5-50% through both predatory and competitive interactions. However, these reductions could be mitigated through sustained exploitation of flathead catfish by recreational or commercial fishers at levels equivalent to those for native flathead catfish populations (6-25% annual exploitation). These findings demonstrate the potential for using directed harvest of an invasive species to mitigate the negative impacts to native species.

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