Trophic Relations of Introduced Flathead Catfish in an Atlantic River

The flathead catfish Pylodictis olivaris is a large piscivore that is native to the Mississippi and Rio Grande river drainages but that has been widely introduced across the United States. River ecologists and fisheries managers are concerned about introduced flathead catfish populations because of the negative impacts on native fish communities or imperiled species associated with direct predation and indirect competition from this apex predator. We studied the trophic relations of introduced flathead catfish in an Atlantic river to further understand the effects on native fish communities. Crayfish (Astacidea) occurred most frequently in the flathead catfish diet, while sunfish Lepomis spp. comprised the greatest percentage by weight. Neither of two sympatric imperiled fish species (the federally endangered Cape Fear shiner Notropis mekistocholas and the Carolina redhorse Moxostoma sp., a federal species of concern) was found in any diet sample. An ontogenetic shift in diet was evident when flathead catfish reached about 300 mm, and length significantly explained the variation in the percent composition by weight of sunfish and darters Etheostoma and Percina spp. Flathead catfish showed positive prey selectivity for taxa that occupied similar benthic microhabitat, highlighting the importance of opportunistic feeding and prey encounter rates. Flathead catfish displayed a highly variable diel feeding chronology during July, when they had a mean stomach fullness of 0.32%, but then showed a single midday feeding peak during August (mean fullness = 0.52%). The gastric evacuation rate increased between July (0.40/h) and August (0.59/h), as did daily ration, which more than doubled between the 2 months (3.06% versus 7.37%). Our findings increase the understanding of introduced flathead catfish trophic relations and the degree of vulnerability among prey taxa, which resource managers may consider in fisheries management and conservation of native fish populations and imperiled species. The introduction of nonnative species has become a growing concern for fisheries biologists because second to habitat degradation and loss, introduced species are the greatest threat to native aquatic biodiversity (Wilcove et al. 1998; Lodge et al. 2000; Jelks et al. 2008). A total of 754 fish taxa in the United *Corresponding author: [email protected] 1Present address: North Carolina Wildlife Resources Commission, Division of Inland Fisheries, North Carolina State University Centennial Campus, 1751 Varsity Drive, Raleigh, North Carolina 27606, USA. 2The unit is jointly supported by North Carolina State University, North Carolina Wildlife Resources Commission, U.S. Geological Survey, U.S. Fish and Wildlife Service, and Wildlife Management Institute. Received September 7, 2010; accepted February 26, 2011 States have been introduced or translocated to waters outside their native range with an economic toll of US$5.4 billion annually (Fuller et al. 1999; Pimentel et al. 2005). Humans enhance the viability of fish introductions by altering the physical properties of natural environments and by introducing robust species 1120 D ow nl oa de d by [ N or th C ar ol in a St at e U ni ve rs ity ], [ T om K w ak ] at 1 3: 09 2 2 A ug us t 2 01 1 TROPHIC RELATIONS OF INTRODUCED FLATHEAD CATFISH 1121 to create sport fisheries (Moyle and Light 1996a; Rahel 2000; Pine et al. 2007; Johnson et al. 2008). The introduction of piscivores has unpredictable ecological effects on native species that range from changes in microhabitat use owing to competition for food and space to direct extirpation from vulnerability to a new form of predation (Courtenay 2007; Moyle and Light 1996a, 1996b). Introduced flathead catfish Pylodictis olivaris is considered among the most ecologically harmful introductions in the United States owing to their large size, ability to establish new populations, obligate carnivorous feeding, and associated effects on native species (Fuller et al. 1999; U.S. Fish and Wildlife Service memorandum dated November 3, 1999). In local systems the desire of anglers to pursue an aggressive catfish that has the ability to reach a large size and has good flavor, has resulted in domestic introductions of the flathead catfish outside of its native range (Jenkins and Burkhead 1994; Jackson 1999). Once introduced, populations establish themselves and expand rapidly within a system (Guier et al. 1984; Ashley and Buff 1988; Thomas 1995). Attitudes among biologists, resource managers, and anglers about this introduced species vary because of its value as a sport fish and its ability to alter fish communities and traditional fisheries (Jackson 1999). The piscivorous flathead catfish is native to the southern Great Lakes and the Mississippi, Mobile, and Rio Grande river drainages (Jenkins and Burkhead 1994; Jackson 1999). It has been widely introduced outside its native range, including to the Atlantic Slope from the Flint River of Georgia to the Delaware and Susquehanna rivers in Pennsylvania (Quinn 1989; Fuller et al. 1999; Jackson 1999; Brown et al. 2005). Flathead catfish are the second largest ictalurid in North America; total lengths can exceed 1 m, weights can be over 50 kg, and life spans may reach 28 years (Jackson 1999; Kwak et al. 2006). Flathead catfish are highly mobile in their introduced and native ranges (Kwak et al. 2004; Vokoun and Rabeni 2005; Malindzak 2006); introduced riverine flathead catfish occupy annual linear ranges from 13 to 28 km, which are seasonally greater during spawning (Kwak et al. 2004; Malindzak 2006). Flathead catfish are primarily nocturnal (Minckley and Deacon 1959; Quinn 1989; Malindzak 2006), showing little activity during the day (Malindzak 2006; Vokoun and Rabeni 2006). Anecdotal evidence suggests that feeding is crepuscular (Minckley and Deacon 1959; Quinn 1989), but flathead catfish diel feeding chronology has not been quantitatively described. Ontogenetic shifts in diet occur in most fishes (Gerking 1994) and are influenced by biotic and environmental factors. During early life stages, flathead catfish are invertivores, feeding on aquatic insects and crayfish, but as they mature and grow (over 300 mm) they become obligate carnivores, feeding mainly on live fish (Layher and Boles 1980; Jackson 1999; Herndon and Waters 2002; Pine 2003). Along with fish size and environment, the ontogenetic shift in diet is affected by the abundance of available prey items (Minckley and Deacon 1959; Haas et al. 2001). Flathead catfish interact intensively with other fishes by direct predation and indirect competition for food sources. Flathead catfish are opportunistic feeders, consuming the largest and most abundant prey item at the time of feeding (Minckley and Deacon 1959; Swingle 1967; Turner and Summerfelt 1971). They consume large amounts of prey, and the large gape width of their mouths allows them to consume prey items that are large relative to their size (Turner and Summerfelt 1971; Herndon and Waters 2002). The dominant prey items found in the diet of flathead catfish are fishes of the families Centrarchidae, Clupeidae, and Ictaluridae, as well as crayfish and aquatic insects (Guier et al. 1984; Ashley and Buff 1988; Quinn 1989; Weller and Robbins 2001). However, only one study of diet selectivity has been published for the species (introduced populations in coastal rivers, Pine et al. 2005), and no estimate of gastric evacuation or daily ration appears in the literature for native or introduced populations. Introduced flathead catfish have raised concern among river ecologists and fisheries managers because of the possible negative effects on native fish communities (Guier et al. 1984; Bart et al. 1994; Thomas 1995; Ashley and Rachels 2000; Weller and Robbins 2001). The greatest effect on native fish populations is reported to occur shortly after flathead catfish have been introduced into a system (Ashley and Buff 1988; Thomas 1995; Jackson 1999; Kwak et al. 2004). Food-web-simulation modeling by Pine et al. (2007) projected declines of up to 50% in the biomass of native fish groups after the establishment of introduced flathead catfish. In contrast, several investigators could not detect an adverse effect on native fish populations (Ashley and Buff 1988; Quinn 1989). One major reason for concern is that flathead catfish have been introduced into waters that are inhabited by rare and endangered fishes, and the impact of cooccurrence with these species is unknown. In the Cape Fear River basin of North Carolina, flathead catfish co-occur with two species of concern, the federally endangered Cape Fear shiner Notropis mekistocholas, a shiner with only five known populations remaining (Hewitt et al. 2006, 2009), and the rare and undescribed Carolina redhorse (Moxostoma sp.), a federal species of concern (Starnes et al. 2005). Introduced flathead catfish also may feed upon juvenile Gulf sturgeon Acipenser oxyrinchus desotoi (Fuller et al. 1999), which could result in declines of this federally threatened benthic species. Such an occurrence was recently documented (June 2010) for the closely related Atlantic sturgeon A. o. oxyrinchus in the Satilla River, Georgia (Flowers et al. 2011). Understanding the relationships between flathead catfish and these rare and endangered species is fundamental to documenting the effects on imperiled fish populations. Determining feeding behaviors and how a fish’s diet affects its energy and growth is essential to understanding its ecological role (Bowen 1996). Thus, we initiated research on the trophic relations of introduced flathead catfish to fill the gaps in understanding the dietary requirements of this introduced predator D ow nl oa de d by [ N or th C ar ol in a St at e U ni ve rs ity ], [ T om K w ak ] at 1 3: 09 2 2 A ug us t 2 01 1 1122 BAUMANN AND KWAK and its potential ecological effects on native species. Our research objectives were to (1) quantify the diet of the flathead catfish in terms of prey selection and the relative importance of prey items, as well as to determine ontogenetic shift in diet; (2) quantify feeding selectivity using the frequency of prey items in stomach contents relative to their availability in an inland river system, as related to the potential impact this introduced species has on native fish communities and imperiled species (Chesson 1978; Pine et al. 2005); (3) determine diel periodicity in feeding; and (4) calculate daily ration and gastric evacuation rate to quantify predation as a dynamic function (Bromley 1994).