Trophic and Population Ecology of Introduced Flathead
Catfish Pylodictis olivaris in the Tar River, North Carolina
Daniel J. Walker, Jordan Holcomb, Robert Nichols, and Michael M. Gangloff
Southeastern Naturalist, Volume 14, Issue 1 (2015): 9–21
Full-text pdf (Accessible only to subscribers.To subscribe click here.)
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D.J. Walker, J. Holcomb, R.Nichols, and M.M. Gangloff
22001155 SOUTHEASTERN NATURALIST Vol1.4 1(41,) :N9–o2. 11
Trophic and Population Ecology of Introduced Flathead
Catfish Pylodictis olivaris in the Tar River, North Carolina
Daniel J. Walker1,2,*, Jordan Holcomb1,3, Robert Nichols4, and Michael M. Gangloff1
Abstract - Pylodictis olivaris (Flathead Catfish) are large piscivores native to western Gulf
of Mexico drainages that have been widely introduced into Atlantic Slope drainages with
largely unknown consequences for native lotic faunas. From 2009–2011, we assessed the
diet, demography, growth, and spatial distribution of Flathead Catfish in the lower Tar River
in east-central North Carolina. We documented current presence of Flathead Catfish using
electrofishing at 27 sites in the Tar River and its tributaries Fishing and Sandy creeks and examined
diet and growth rates in the lower Tar River population. Stomach contents revealed
that Tar River Flathead Catfish are primarily piscivorous but also consumed a diverse range
of prey items. Canonical correspondence analysis found that Flathead Catfish ≥500 mm TL
appeared to consume centrarchids at greater rates than smaller Flathead Catfish, suggesting
a shift to larger prey in larger, older fish. Body-condition analysis found that condition did
not change with body size, suggesting that the lower Tar River population has likely not
yet over-exploited its resource base. Upstream distribution of Flathead Catfish in the upper
Tar River and Fishing Creek, two important refuges for numerous imperiled lotic taxa in
this fragmented drainage, appears restricted by two small dams. Our data suggest a need for
continued monitoring for natural and human-mediated Flathead Catfish range expansions
into sensitive reaches as well as empirical study of possible species-, assemblage-, and
ecosystem-level effects of this apex predator on imperiled freshwater biota in the Tar River.
Moreover, tabling the removal of some small dams in the Tar Drainage may be a prudent
action capable of protecting sensitive taxa, at least in the sh ort-term.
Introduction
Pylodictis olivaris (Rafinesque) (Flathead Catfish) are native to Gulf of Mexico
drainages extending from the Mobile Drainage west to the Rio Grande Drainage.
Flathead Catfish are omnivorous with a strongly piscivorous diet (Minckley and
Deacon 1959, Pine et al. 2005). This large-bodied (>1.5 m TL) catfish has been
widely introduced in the middle and southeastern Atlantic slope drainages (Jackson
1999). Flathead Catfish now occur from the St. Johns River system in Florida north
to the Delaware River in New Jersey and Pennsylvania (Brown et al. 2005, Fuller
and Neilson 2013).
Flathead Catfish introductions coincided with severe and sometimes precipitous
native fish declines in several drainages. Thomas (1995) documented a swift
1Appalachian State University, Biology Department, 572 Rivers Street, Boone, NC 28608-
2028. 2University of Tennessee, Forestry Wildlife and Fisheries Department, 274 Ellington
Plant Sciences Building, 2431 Joe Johnson Drive, Knoxville, TN 37996-4800. 3Florida Fish
and Wildlife Conservation Commission, 7386 NW 71st St. Gainesville, FL, 32653. 4North
Carolina Wildlife Resources Commission, Raleigh, NC 27606. *Corresponding author -
dwalke44@vols.utk.edu.
Manuscript Editor: Nathan Franssen
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decline in Lepomis auritus (L.) (Redbreast Sunfish) and Ameiurus spp. (Bullhead
Catfish) catch rates in both creel and electrofishing surveys with a simultaneous
sharp increase in catch rates of introduced Flathead Catfish in the Altamaha River,
GA. Flathead Catfish can affect native fish assemblages through predation and
indirectly by competition for other resources (e.g., food, spawning habitat, cover;
Baumann and Kwak 2011). These findings raised concerns among fisheries managers
and researchers over the impact of introduced Flathead Catfish populations on
native fish assemblages (Bringolf et al. 2005).
The first documented introduction of Flathead Catfish on the Atlantic slope of
North Carolina occurred in 1966, when the North Carolina Wildlife Resources
Commission (NCWRC) released 11 mature individuals into the Cape Fear River,
subsequently establishing a viable population of Flathead Catfish (Guier et al.
1984). In North Carolina, introduced populations of Flathead Catfish are established
in Coastal Plain and Piedmont reaches of the Neuse, Cape Fear, Tar, Pee Dee,
and Catawba rivers (Fuller and Neilson 2013, Kwak et al. 2006). Recent research
on the Cape Fear population of Flathead Catfish assessed diet, prey selectivity,
feeding chronology, and hypothesized possible impacts of Flathead Catfishes on the
native fish community (Baumann and Kwak 2011, Pine et al. 2005). Those studies
determined that the Flathead Catfishes of the Cape Fear River were opportunistic
generalists, and that an ontogenetic dietary shift occurred at 300 mm TL, whereby
larger individuals fed significantly more on larger prey, especially clupeids and
centrarchids. Additionally, Flathead Catfish did not prey on imperiled species found
in the Cape Fear River (Baumann and Kwak 2011). Although these studies did not
show a correlation between increasing Flathead Catfish numbers and declining
native fish populations, stomach-content analysis and diet-selectivity calculations
suggested potential impacts on native fishes, especially sunfishe s.
The Tar River Drainage is the northernmost watershed occurring entirely within
North Carolina (Fig. 1; Philips 1989). The Tar River drainage supports populations
of rare and imperiled aquatic species recognized at both the state and federal level
including Noturus furiosus (Jordan and Meek) (Carolina Madtom), Ambloplites
cavifrons (Cope) (Roanoke Bass), Elliptio steinstansana (Johnson and Clarke) (Tar
Spinymussel), Alasmidonta heterodon (Lea) (Dwarf Wedgemussel), and Orconectes
carolinensis (Cooper and Cooper) (North Carolina Spiny Crayfish) , as well
as numerous endemic species (Clamp 1999, USFWS 2012). Additionally, the Tar
River supports a population of anadromous Alosa sapidissima (Wilson) (American
Shad) and other important food and game fishes that may be negatively affected by
Flathead Catfish (Layher and Boles 1980, Menhinick 1991) .
Specific dates of introduction of Flathead Catfish into the Tar River are not
clear, although it was known to be introduced in the Roanoke-Chowan drainage by
the 1980s (Hocutt and Wiley 1986). The establishment of Flathead Catfish in the
Tar River is suspected to have occurred in the 1990s (Homan 2010). However, no
prior studies have assessed ecological effects of Flathead Catfish in the Tar River
drainage. The North Carolina Wildlife Resources Commission began conducting
catfish-specific surveys of the lower Tar River in 2006, and documented increasing
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Flathead Catfish catch rates and simultaneous decreases of Ameiurus catus (L.)
(White Catfish) catch rates (NCWRC 2009). Between 2006 and 2008, the relative
abundance of Flathead Catfish increased from 14% to 56% of all catfish captured
in the Tar River, whereas White Catfish abundance dropped from 53% to 32% of
the catfish sampled in the Tar River during the same time period. Additionally, the
largest Flathead Catfish reported by the NCWRC from the Tar River weighed ~23
kg at age 9 y.
The objectives of our study were to (1) document locations where Flathead
Catfish were collected in the Tar River Drainage, (2) investigate age and growth
relationships, and (3) determine diet and describe feeding habits to assess potential
Flathead Catfish impacts on Tar River ecosystems. These data will establish baseline
presence and population growth-rate parameters as well as provide a means
for assessing the condition factor and nutritive status of Flathead Catfish in the Tar
River. Furthermore, quantifying the condition factor of Flathead Catfish will allow
us to make inferences about the effects of potential resource scarcity on these fish.
Materials and Methods
Presence surveys
The occurrence of Flathead Catfishes in the upper Tar River was documented as
part of an ongoing study of the impacts of small dams on North Carolina fish assemblages
(Fig. 1). We conducted surveys in 27 wadeable reaches of the Tar River,
Sandy Creek, and Fishing Creek using a Smith-Root 12B backpack electroshocker
(Smith-Root Inc., Vancouver, WA). In sections of reaches too deep to use the
backpack electroshocker, we surveyed with 3.7-m-wide by 1.8-m-deep seines with
4.8-mm2 mesh. Each reach was approximately 150 m in length. Three replicates
of four mesohabitats (run, riffle, pool, and bank) were sampled for approximately
100 s each (at least 1200 s/reach). If a new species was detected on the last replicate
Figure 1. Map of Tar River watershed study sites in North Carolina showing the locations of
dietary study site (square), fish community survey site locations (open circles), sites where
Flathead Catfish were detected (filled circles), and dams (crosse s).
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of a given mesohabitat, that mesohabitat was sampled for an additional 50 s or until
no new species were detected. In addition to electro-fishing, one sampling period
(3 nights) was conducted at the NC Highway 42 crossing of the Tar River using
conventional (hook-and-line) and set-line angling.
Stomach-content analysis
We collected Flathead Catfish from the lower Tar River in Greenville, Pitt County,
NC, between August and October 2010 (Fig. 1) using a Smith-Root 7.5 GPP
boat-mounted electrofisher emitting low-frequency, pulsed DC (15 pulse, 240–340
volts, 1–1.5 amps). We captured 20 individuals during two morning sampling efforts
specifically targeting Flathead Catfish that covered approximately 2 river
km, whereas the remainder of specimens were collected opportunistically during
NCWRC sampling efforts in autumn 2010 (ntotal = 71). All fish sampled were euthanized
with a lethal (>500 ppm) dosage of tricainemesylate (MS-222). We stored
all fish collected after 25 August 2010 in freezers prior to processing. We assigned
fish-identification numbers and recorded their mass (g) and total length (TL in mm).
Body condition, a relative measure of the physical fitness of members of a population,
was calculated by log-transforming and plotting weight-at-length data using
Studentized residual scores as an index of body condition (Jako b et al. 1996).
Methods for stomach-content analysis were adapted from Hyslop (1980). In the
field, we made a ventral incision in each fish from the vent to the breast and attached
cable ties around the base of the esophagus and beginning of the intestinal tract to
contain the contents of the stomach, which was then excised from the rest of the
digestive tract. Stomachs were then placed in labeled Whirl-Paks (Nasco Inc., Fort
Atkinson, WI) and preserved in 10% formalin for transport back to the lab. We
injected the largest stomachs with formalin to ensure preservation.
In the lab, stomachs were soaked in water to remove excess formalin, and then
patted dry before being weighed whole. We opened stomachs with an incision along
the longest axis of the organ and collected and weighed separately any contents.
Stomach fullness (Ft), an indicator of feeding intensity, was calculated as a percentage
of total mass using the equation put forth by Hyslop (1980) :
Ft = [(Wt. (wet) of stomach contents)/(Wt. (wet) of fish)] * 100
We then identified stomach contents to the lowest feasible taxon. We were unable
to conclusively identify much of the gut-content material because of delays
between collection and processing as well as damage to the contents due to the
strong pharyngeal jaws of Flathead Catfish. Bones, scales, and exoskeletons were
grouped together as unidentifiable fish or invertebrate parts, respectively. Plant and
inorganic materials were classified as detritus.
Otolith analysis
Otolith analysis has been verified as an accurate means of estimating the age
of Flathead Catfish (Nash and Irwin 1999). We removed lapilli otoliths in the field
via a dorsal incision through the supraoccipital bone (Long and Stewart 2010).
When possible, both lapilli otoliths were removed so that we could read them
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2015 Vol. 14, No. 1
independently to increase the accuracy of age estimations. We then packaged otoliths
in labeled vials for transport back to the lab. In the lab, otoliths were prepared
for reading following Buckmeier et al. (2002). We processed otoliths individually
and independently of fish-size data. First, otoliths were soaked in 95% ethanol to
withdraw moisture and aid in removal of any remaining tissue. We then placed dried
otoliths directly on a hotplate at 300 °C until they changed to a golden brown color
(typically ≤5 min). Preparing otoliths in this manner increased annuli visibility and
contrast to the bone matrix. Annuli are formed each spring, so the number of annuli
corresponds to the age of the fish in years (Buckmeier et al. 2002). We mounted
otoliths to individual microscope slides using Crystalbond 509 thermoplastic adhesive
(Crystalbond; Buehler, Lake Bluff, IL) perpendicular to the slide, and with
the posterior end of the otolith in contact with the glass. After the adhesive had set,
otoliths were sectioned by grinding the anterior end down with a table-mounted
power sander, and then wet-polished by hand under a dissecting microscope with
600-grit sandpaper until the annuli could be seen radiating from the nucleus of the
otolith. Two independent readers counted annuli outward from the nucleus. Any
differences in observed age were resolved by re-examining the otol iths together.
Statistical analysis
Lengths of the Flathead Catfish (mm TL) were plotted against their masses (g)
after we linearized the data using a LOG10 transformation. We fit a linear regression
to the data and used Studentized residual scores as an index of body condition (Jakob
et al. 1996, Sutton et al. 2000). The residual scores provide a relative measure of body
condition for each individual, and standardizing the residuals by dividing each by its
error allows for direct comparison of body condition across size classes. To identify
outliers, we established a threshold of ±2.5 Studentized residual score. We assessed
growth rates by plotting the length of each fish against its age determined through
otolith analysis and then produced a von Bertalanffy growth curve using Fisheries
Analysis and Management Software (FAMS v. 1.0; American Fisheries Society,
Bethesda, MD). To assess ontogenetic diet shifts, we created a matrix comprised of
the presence–absence data of the diet contents and stomach fullness of the Flathead
Catfish collected with stomach contents (n = 36). Canonical correspondence analysis
(CCA; PC-ORD v. 4, MjM Software, Gleneden Beach, OR) was conducted to identify
relationships among total length, stomach fullness, and the presence of stomach
contents identified from Flathead Catfish. Stomach fullness characterized the horizontal
axis, and total length characterized the vertical axis. We further investigated
ontogenetic diet shift by comparing total length of Flathead Catfish individuals to
their stomach fullness. These analyses were used to establish a length threshold delineating
a potential ontogenetic diet shift.
Results
Presence surveys
Flathead Catfish were found at 4 of 27 sites in the upper and mid-reaches of the
mainstem Tar River and its tributaries (Fig. 1). We found Flathead Catfish in the Tar
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River several km upstream of the partially breached Webb’s Mill Dam near Spring
Hope, NC. These records are the first Flathead Catfish detected upstream of that
dam. Flathead Catfish were not detected in the Tar River upstream of Louisburg,
where a small dam may be serving as a barrier, or in Fishing Creek upstream of
Bellamy’s Mill Dam (Fig. 1).
Growth analysis
Linear regression of LOG10(mass) by LOG10(TL) revealed a strongly linear
relationship between mass and length (R2 = 0.99, df = 1, F = 11945.1, P < 0.001;
Fig. 2). The Studentized residual scores calculated from this regression (mean=
0.004, SD = 1.0171) indicated only 2 fish with outlying body condition. Both fish
had greater than expected body condition (Studentized residual= 2.759 and 3.001,
respectively).
Flathead Catfish ages ranged from less than 1 to 8 years old. The mean age was 3.15 y,
the approximate age of sexual maturity in Flathead Catfish (Minckley and Deacon
1959). The mean TL (mm) calculated for each age class (n = 54; Fig. 3) was entered
into the FAMS von Bertalanffy growth-function solver. An optimal solution was
found after 20 iterations: R2 = 0.91, P = 0.0003, Linf = 3561.95, K = 0, t0 = -1.124.
Stomach-content analysis
The majority of stomach contents analyzed were categorized as fish or fish remnants,
and these items comprised >60% of stomach contents (Table 1). The CCA
results demonstrated a trend toward greater piscivory in larger fish, as the smaller
Flathead Catfish (less than 500 mm TL) were associated with Cyprinid and unidentifiable
invertebrate prey items (Fig. 4A). Flathead Catfish ≥500 mm TL exhibited a shift
towards fuller stomachs and were associated with centrarchid and ictalurid prey
types (Fig. 4B). Centrarchids were the most frequently identified fish, which adult
(≥500 mm TL) Flathead Catfish appeared to consume at an elevated rate compared
Figure 2. Relationship
between LOG10-
transformed Flathead
Catfish length and
weight in the Tar River.
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to sub-adults. Additionally, adult Flathead Catfish consumed more ictalurid catfish
than sub-adults. Other items detected in Flathead Catfish stomachs include crayfishes
(Cambaridae), bivalves (Corbicula fluminea (O.F. Müller) [Asian Clam]),
and both organic and inorganic detritus (i.e., bark, leaves, and gravel). Stomach
fullness ranged from 0 to 6.2% of body mass. Mean stomach fullness (for fishes
with prey in their stomachs) was 0.9%.
Discussion
Flathead Catfish populations are robust in the lower Tar River downstream of
Webb’s Mill Dam near Spring Hope, NC. Multiple size classes (including 2 individuals
weighing 15.7 and 16.5 kg, respectively) were found approximately 600 m
Figure 3. Mean length-atage
for Flathead Catfish in
the lower Tar River, NC.
Error bars display ± 1 SE.
Solved Von Bertalanffy
growth function: Lt =
3561.952(1-e-0.036(t- (-1.124)).
Figure 4 (following page). (A). Canonical correspondence analysis of total length, stomach
fullness (Ft), and stomach contents. Centrarchid and ictalurid prey items correspond with
greater stomach fullness, and a shift from Cyprinid and invertebrate prey items to larger
prey types appears to occur at 500 mm TL (median TL = 476 mm). (B). Stomach fullness
of Flathead Catfish at total length.
Table 1. The percent occurrence of prey items identified from the stomach s of Flathead Catfish.
% occurrence
Prey Catfish less than 500 mm TL Catfisth ≥500 mm TL Total
Centrarchidae 4.76 19.36 13.46
Ictaluridae 0.00 9.68 5.77
Cyprinidae 9.52 0.00 3.85
Cambaridae 4.76 6.45 5.77
Corbiculidae 4.76 3.23 3.85
Identified fish 14.29 16.13 15.39
Unidentified fish 42.86 38.71 40.39
Unidentified invertebrate 14.29 3.23 7.69
Detritus 19.05 19.36 19.23
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downstream of the Spring Hope dam. It is likely Flathead Catfish occur throughout
the Lower Tar River and its major tributaries from Webb’s Mill Dam downstream
to the Albemarle Sound Estuary. Because of the large unsampled reach between
our study sites upstream of Webb’s Mill Dam, it is difficult to speculate how far
upstream Flathead Catfish have extended their range in the Tar River. It is likely
that a lowhead dam just upstream of Louisburg, NC, is the upstream-most barrier to
Flathead Catfish range expansion in the Tar River. However, we have not sampled
the Tar River near the Louisburg Dam, so the upstream distribution of Flathead
Catfish in the Tar remains unresolved.
Surveys also failed to detect Flathead Catfish at 7 sites upstream and 2 sites
downstream of Bellamy’s Mill Dam in Fishing Creek. Bellamy’s Mill Dam, completed
ca. 1859, is the first major barrier in the Fishing Creek sub-drainage. Because
the confluence of Fishing Creek and the Tar River is downstream of Rocky Mount
Mill Pond (the first major barrier to possible Flathead Catfish upstream distribution
on the Tar River) and Flathead Catfish were detected well downstream of this
structure, it seems likely that they also occur in lower Fishing Creek. However, we
only sampled 2 sites downstream from Bellamy’s Mill in Fishing Creek. We did not
detect Flathead Catfish at 3 sites in Sandy Creek including 2 sites downstream from
Laurel Mill Dam, the first anthropogenic barrier in the Swift Creek sub-drainage.
The prevalence of beaver dams in Sandy and Swift creeks, combined with a lack of
habitat and forage, may limit Flathead Catfish distribution in this lower Tar River
sub-drainage.
The results of the diet and population analyses are consistent with those of
other studies of invasive Flathead Catfish populations in Atlantic Slope drainages.
Flathead Catfish in the Tar River are strongly piscivorous but also consume a diverse
range of prey items, as was found in previous studies of other populations
(Baumann and Kwak 2011, Minckley and Deacon 1959, Pine et al. 2005). The
trend in our analysis of the stomach contents indicates that large, older fish were
more likely to have consumed centrarchids. Larger fish also consumed ictalurids
(most probably Ictalurus punctatus (Rafinesque) [Channel Catfish] and Ameirus
spp. [bullhead catfish]) at elevated rates, again consistent with an ontogenetic
diet shift found in previous studies of other populations, though the results of our
CCA suggest that the ontogenetic shift occurs around 500 mm TL in the Tar River
population and not 300 mm TL as reported from the Cape Fear River population
of Flathead Catfish (Baumann and Kwak 2011). This finding may have important
trophic and economic consequences for Tar River ecosystems and fisheries
because both centrarchids and ictalurids are important mesopredators and game
fishes as well as endangered freshwater-mussel hosts in most Atlantic Slope lotic
ecosystems (Michaletz and Dillard 1999). Cape Fear River Flathead Catfish were
strongly piscivorous, but prey selection was generally random and did not target
naive (i.e., not co-evolved) fishes (Pine et al. 2005). Although few studies have
shown a demonstrable downward trend in mesopredatory and game fishes in systems
recently invaded by Flathead Catfish, Thomas (1995) documented population
shifts within Altamaha River, GA, native mesopredatory guilds coincident with
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the establishment of Flathead Catfish. A similar decline in native mesopredator
catch was also documented in earlier catfish surveys of the Tar River (NCWRC
2009). Dietary trends may reflect ontogenetic diet shifts linked to a gape threshold,
above which it is easier for adult Flathead Catfish to consume larger and betterdefended
prey. This prey base of larger fish, in turn, may eventually allow more
rapid increases in weight with age and lead to growth estimates similar to other
studies conducted on older Flathead Catfish populations (Jackson 1999, Minckley
and Deacon 1959). Alternately, the stomach-fullness data could be driven by gastric
evacuation rates or feeding chronology of the Flathead Catfish i n the Tar River.
Our data also indicate Tar River Flathead Catfish were younger (max age 8 y)
on average compared to other North Carolina Atlantic Slope populations (e.g.,
maximum age in Neuse River = 14 y, in Cape Fear River = 17 y, and in Lumber
River = 12 y; Kwak et al. 2006) as well as other southeastern coastal plain rivers
(e.g., maximum age in Satilla River = 10 y and in Ocmulgee River = 16 y;
Sakaris et al. 2006), suggesting that this is a relatively young population. However,
the NCWRC (2009) study of Flathead Catfish in the Lower Tar River from
2006–2008 documented a 9-year-old Flathead Catfish. While that fish was older
than the fish encountered in this study, it is still younger than the published maximum
ages of Flathead Catfish in other drainages. Both studies of ages of Flathead
Catfish in the Tar River relied on boat electrofishing, so the discrepancies in
maximum age encountered may be attributable to differences in sampling effort.
In this study, 51 of the 71 Flathead Catfish were collected as by-catch during fall
sampling for other species, whereas the 9-year-old Flathead Catfish was collected
during a multi-year catfish-specific sampling effort. During the two morning
sampling efforts of boat electrofishing specifically targeting Flathead Catfish
conducted during the collection period of this study, we made anecdotal observations
of the effectiveness of electrofishing on Flathead Catfish. It appeared that
the larger, and presumably older, fish were less responsive to the electricity and
may be underrepresented in electrofishing surveys.
Sakaris et al. (2006) found that introduced populations of Flathead Catfish
in Georgia grew at substantially increased rates relative to native populations.
If the Tar River Flathead Catfish populations are relatively young, as both the
NCWRC age report and our age data suggest, the effects they exert on the native
fish communities (and on the Tar River ecosystem in general) could increase
over time if the population grows in age, size classes, and number of fish. As
more Flathead Catfish in the Tar River population mature and surpass our observed
500-mm-TL ontogenetic length threshold, reductions in the prey base of
large fish (Ictaluridae and Centrarchidae) may become more apparent. Concurrently,
increases in the number of juvenile Flathead Catfish (less than 500 mm) may
significantly impair benthic communities and lower trophic levels (benthic macroinverterbrates,
Cambaridae).
Our residual score data did not identify any fish with significantly lowered
body condition. Across life-history stages, Flathead Catfish in the lower Tar River
generally exhibit average or improved body condition. This finding may indicate
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that both juvenile and adult Flathead Catfish in the lower Tar River are not currently
over-exploiting their prey base, or that current prey-consumption rates of
Flathead Catfish have not depleted the prey base. The lower Tar River in the study
reach had abundant habitat for all flathead life-history stages (i.e., vegetated
shallows for juveniles and sub-adults, and structure near deeper water for larger
adults; Minckley and Deacon 1959). If over-exploitation were occurring, we
would expect to see decreased condition-index scores at some life-history stage.
Moreover, Flathead Catfish growth rates in the lower Tar River population are
somewhat higher than those reported from other Atlantic Slope drainages (Kwak
et al. 2006, Sakaris et al. 2006).
Our study is the first to examine life-history attributes of Flathead Catfishes in
the Tar River. We found that Tar River Flathead Catfish consume a range of prey
items including fishes, mollusks, and crayfish, consistent with other studies of the
diet of invasive Flathead Catfish populations. Because there were few changes
in condition with age, we believe that the Tar River Flathead Catfish population
has not exceeded its carrying capacity, and given the relatively young ages found
among sampled individuals, this population appears to be robust. This finding is
alarming because Flathead Catfish have already been correlated with a decrease
in the native ictalurid catch rate in the Tar River, and appear to have negatively
affected centrarchid and ictalurid populations in other systems. Further, imperiled
centrarchids and ictalurids in the Tar River basin (Roanoke Bass and Carolina Madtom)
may be at increased risk of local extirpations due to Flathead Catfish. Given
their broadly generalist feeding tendencies across their life history, Flathead Catfish
are not only a potential threat to sensitive lotic taxa, but aquatic communities as a
whole in the Tar River and its tributaries.
In their analysis of 762 animal extinctions, Clavero and Garcia-Berthou (2005)
found that invasive species were the second-leading cause of extinction among
North American fishes. The importance of understanding the trophic and management
implications of introduced Flathead Catfish in the Tar River drainage and
along the Atlantic Slope cannot be overstated. These data are critical to managers
tasked with creating effective policies to protect native species and ecosystems
from large, invasive game fishes. The popularity of Flathead Catfish as a food fish
further complicates the issue. In public meetings, citizens have demanded that the
state of North Carolina introduce Flathead Catfish to other drainages, threatening to
do so themselves if the state agencies refuse (B. Tracy, NC Department of Environment
and Natural Resources Division of Water Quality, Raleigh, NC, pers. comm.).
The tradeoff between managing endemic species and sport fish highlights one of
the key difficulties facing aquatic resource conservation in the Tar River and across
the globe. Although the majority of scientific evidence suggests that introduced
Flathead Catfish may have widespread and potentially dramatic effects on naïve
ecosystems, unauthorized introductions by sportsmen appear likely to continue to
expand this species’ range. Targeted education and substantial penalties need to be
included in management strategies designed to conserve the Tar River Drainage
ecosystem and its imperiled species.
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Acknowledgments
We would like to acknowledge the invaluable technical and logistical assistance from
the North Carolina Wildlife Resources Commission. Financial support for this research
came from a grant to M.M. Gangloff from the North Carolina Department of Environment
and Natural Resources Albemarle-Pamlico National Estuarine Program and a grant to D.J.
Walker from the North Carolina Association of Environmental Professionals. Members of
the Aquatic Conservation Research Lab at Appalachian State University assisted with fieldwork
and provided helpful comments on earlier drafts of this ma nuscript.
Literature Cited
Baumann, J.R., and T.J. Kwak. 2011. Trophic relations of introduced Flathead Catfish in an
Atlantic River. Transactions of the American Fisheries Society 140:1120–1134.
Bringolf, R.B., T.J. Kwak, W.G. Cope, and M.S. Larimore. 2005. Salinity tolerance of Flathead
Catfish: Implications for dispersal of introduced populations. Transactions of the
American Fisheries Society 134:927–936.
Brown, J.J., J. Perillo, T.J. Kwak, and R.J. Horwitz. 2005. Implications of Pylodictis olivaris
(Flathead Catfish) introductions in the Delaware and Susquehanna drainages. Northeastern
Naturalist 12:473–484.
Buckmeier, D.L., E.R. Erwin, R.K. Betsill, and J.A. Prentice. 2002. Validity of otoliths
and pectoral spines for estimating ages of Channel Catfish. North American Journal of
Fisheries Management 22:934–942.
Clamp, J.C. (Compiler). 1999. A report on the conservation status of North Carolina’s freshwater
and terrestrial crustacean fauna. North Carolina Wildlife Resources Commission,
Raleigh, NC. 100 pp.
Clavero, M., and E. Garcia-Berthou. 2005. Invasive species are a leading cause of animal
extinctions. Trends in Ecology and Evolution 20:110.
Fuller, P., and N. Neilson. 2013. Pylodictis olivaris. USGS Nonindigenous Aquatic
Species Database. Available online at http://nas.er.usgs.gov/queries/FactSheet.
aspx?speciesID=750. Accessed 19 April 2013.
Guier, C.R., L.E. Nichols, and R.T. Rachels. 1984. Biological investigation of Flathead Catfish
in the Cape Fear River. Proceedings of the Annual Conference of the Southeastern
Association of Fish and Wildlife Agencies 35:607–621.
Hocutt, C.H., and E.O. Wiley. 1986. The Zoogeography of North American Freshwater
Fishes. Wiley, Hoboken, NJ. 866 pp.
Homan, J. 2010. Population characteristics of Flathead Catfish and White Catfish in the Tar
River. Catfish Management Technical Committee 2010 State Project Reports. American
Fisheries Society Southern Division.
Hyslop, E.J. 1980. Stomach contents analysis: A review of methods and their application.
Journal of Fish Biology 17:411–429.
Jackson, D.C. 1999. Flathead Catfish: Biology, fisheries, and management. Pp. 23–35, In
E.R. Irwin, W.A. Hubert, C.F. Rabeni, H.L. Schramm, Jr., and T. Coon (Eds.). Catfish
2000. American Fisheries Society, Symposium 24, Bethesda MD. 532 pp.
Jakob, E.M., S.D. Marshal, and G.W. Uetz. 1996. Estimating fitness: A comparison of bodycondition
indices. Oikos 77:61–67.
Kwak, T.K., D.S. Waters, and W.E. Pine III. 2006. Age, growth, and mortality of introduced
Flathead Catfish in Atlantic rivers and a review of other populations. North American
Journal of Fisheries Management 26:73–87.
Southeastern Naturalist
21
D.J. Walker, J. Holcomb, R.Nichols, and M.M. Gangloff
2015 Vol. 14, No. 1
Layher, W.G. and R.J. Boles. 1980. Food habits of the Flathead Catfish, Pylodictis olivaris
(Rafinesque), in relation to length and season in a large Kansas reservoir. Transactions
of the Kansas Academy of Science 83:200–214.
Michaletz, P.H., and J.G. Dillard. 1999. A survey of catfish management in the United
States and Canada. Fisheries 24:6–11.
Minckley, W.L., and J.E. Deacon. 1959. Biology of the Flathead Catfish in Kansas. Transactions
of the American Fisheries Society 88:344–355.
Nash, M.K. and E.R. Irwin. 1999. Use of otoliths versus pectoral spines for aging adult
Flathead Catfish. Pp. 309–316, In E.R. Irwin, W.A. Hubert, C.F. Rabeni, H.L. Schramm,
Jr., and T. Coon (Eds.). Catfish 2000. American Fisheries Society, Symposium 24,
Bethesda MD. 532 pp.
North Carolina Wildlife Resources Commission (NCWRC). 2009. Tar River is home to
trophy Flathead Catfish. Fisheries Research Summary.
Philips, J.D. 1989. Nonpoint-source pollution control effectiveness of riparian forests along
a coastal plain river. Journal of Hydrology 110:221–237.
Pine III, W.E., T.J. Kwak, D.S. Waters, and J.A. Rice. 2005. Diet selectivity of introduced
Flathead Catfish in coastal rivers. Transactions of the American Fisheries Society
134:901–909.
Sakaris, P.C., E.R. Irwin, J.C. Jolley, and D. Harrison. 2006. Comparison of native and in -
troduced Flathead Catfish populations in Alabama and Georgia: Growth, mortality, and
management. North American Journal of Fisheries Management 26:867–874.
Sutton, S.G., T.P. Bult, and R.L. Haedrich. 2000. Relationships among fat weight, body
weight, water weight, and condition factors in wild Atlantic Salmon parr. Transactions
of the American Fisheries Society 129:527–538.
Thomas, M.E. 1995. Monitoring the effect of introduced Flathead Catfish on sport-fish
populations in the Altamaha River, Georgia. Proceedings of the Annual Conference of
the Southeastern Association of Fish and Wildlife Agencies 47:531–538