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2017 SOUTHEASTERN NATURALIST 16(3):426–442
Distribution, Age Structure, and Growth of Bigheaded
Carps in the Lower Tennessee and Cumberland Rivers
Josey L. Ridgway1,2,* and Phillip W. Bettoli1
Abstract - Invasive Asian carps Hypophthalmichthys nobilis (Bighead Carp) and H. molitrix
(Silver Carp), collectively referred to as bigheaded carps, were introduced to the
US in the 1970s to control noxious algae blooms in aquaculture ponds. Fish subsequently
escaped, and by the 1980s bigheaded carps were widespread and established in the upper
Mississippi River, lower Missouri River, and the Ohio River and some of its tributaries.
We sampled bigheaded carps in the lowermost reservoirs on the Tennessee River (Kentucky
Lake) and Cumberland River (Lake Barkley) in 2015 and 2016 using multiple gears,
including gill nets, hoop nets, electrofishing, and cast nets, to describe their distribution
and estimate several population attributes. Additional electrofishing samples on the Duck
River, a system renowned for its diverse ichthyofauna and mussel communities, revealed
that Silver Carp range extends 220 river kilometers (rkm) upstream below the Columbia
Dam. We collected a total of 737 Silver Carp and 10 Bighead Carp through the course of
this study. The maximum total lengths and ages were 1385 mm and 22 years for Bighead
Carp and 1005 mm and 13 years for Silver Carp. The Silver Carp populations in both
reservoirs had the same pattern of strong year classes (2010, 2011, 2012, and 2015) and
similar growth rates, which were faster than what has been reported for other populations
around the globe. Some young-of-year Silver Carp were collected 180 and 110 rkm upstream
in Kentucky Lake and Lake Barkley, respectively, and they may represent the first
evidence of natural reproduction in those reservoirs or their tributaries.
Introduction
Hypophthalmichthys nobilis Richardson (Bighead Carp) and H. molitrix Valenciennes
(Silver Carp), hereafter collectively referred to as bigheaded carps, are
native to large rivers of eastern Asia and have been introduced to every continent in
the world except Antarctica (Kolar et al. 2007). Bigheaded carps are popular table
fare in overseas countries and have been used for their perceived ability to control
zooplankton and phytoplankton production in polyculture ponds (Kolar et al. 2007).
Bigheaded carps were first introduced to the US in Arkansas in the early 1970s for
aquaculture purposes (Freeze and Henderson 1982). The Arkansas Game and Fish
Commission subsequently propagated and stocked bigheaded carps to assess their
utility as a biological control of excessive plankton and nutrients in wastewater lagoons
(Henderson 1983). Soon thereafter, natural resource agencies and researchers
from several other states began importing and stocking bigheaded carps to initiate
1Department of Biology, Tennessee Technological University, Cookeville, TN 38505. 2Current
address - US Fish and Wildlife Service, Columbia Fish and Wildlife Conservation
Office, 101 Park DeVille Drive, Suite A, Columbia, MO 65203. *Corresponding author -
joseyridgway@gmail.com.
Manuscript Editor: Nathan Franssen
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similar studies with little regard for their potential establishment and impact on
native communities and waterways (Kolar et al. 2007).
Silver Carp were reported from open waterways as early as 1975 in Arkansas,
and a single Bighead Carp was captured from the Ohio River below Smithland
Dam, KY, in 1981 (Freeze and Henderson 1982, Kelly et al. 2011). Reports of natural
reproduction soon followed. Burr et al. (1996) captured young-of-year (YOY)
Silver Carp near Horseshoe Lake, IL (an oxbow lake on the Mississippi River),
and Pflieger (1997) collected young Bighead Carp from Missouri waters in 1989.
Subsequently, these 2 species continued to reproduce in the wild and are now established
in much of the Mississippi, Missouri, and Ohio river basins (Kolar et al.
2007). To date, Silver Carp have been reported in at least 16 states and Puerto Rico,
and Bighead Carp have been found in 23 states and Lake Erie, ON, Canada (Kolar
et al. 2007, USGS 2016).
Bigheaded carps are successful invaders because they tolerate a wide range of
climates, are highly fecund and protracted spawners, grow quickly, and can quickly
overpopulate new waters (Kolar et al. 2007). Bigheaded carps in Asia have a wide
distribution (21.0°N to 43.5°N latitude) in areas with mean annual air temperatures
that vary from -4 °C to 24 °C (Kolar et al. 2007). Although bigheaded carps can naturally
occur in a wide range of habitats including large rivers, reservoirs, lakes, and
ponds, they likely cannot reproduce without access to suitable riverine conditions
because fertilized eggs are semi-buoyant and depend on sufficient shear velocity to
keep them from settling to the bottom and suffocating (Jennings 1988, Laird and Page
1996, Verigin et al. 1978). However, recent research suggests that a reach as short as
25 river kilometers (rkm) could allow bigheaded carp eggs to hatch given sufficient
flows and optimal water temperatures (Murphy and Jackson 2013).
Bigheaded carps are exhibiting rapid population growth in some US watersheds,
and predators are not impeding the invasion because both carp species quickly outgrow
native piscivore gape limitations (Kolar et al. 2007, Schrank and Guy 2002).
For instance, Bighead Carp increased exponentially in Navigation pool 26 of the
Mississippi River near St. Louis, MO, from 1992 to 2001 (Chick and Pegg 2001).
Likewise, Silver Carp increased exponentially in the La Grange Reach of the Illinois
River from 1990 to 2008 (Irons et al. 2011, Sass et al. 2010) and accounted for
nearly a quarter of the total fish biomass in the Illinois River in 2007 (McClelland
and Sass 2008). It is difficult to estimate the extent to which these invasive species
have impacted ecosystem structure and function because relatively little is known
about the ecology of native fish and plankton communities in large river systems
(Dettmers et al. 2001). Nevertheless, there is growing evidence that bigheaded
carps have the ability to influence water quality, alter plankton communities,
compete with native planktivores, displace native fish from optimal habitats, and
transmit diseases (Kolar et al. 2007).
Bigheaded carps can induce a trophic cascade that shifts zooplankton communities
towards smaller individuals (Kolar et al. 2007). Such a trophic cascade
could negatively affect native planktivorous fishes that prey on large zooplankton.
Sampson et al. (2009) concluded that Bighead Carp diets overlapped with
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those of Dorosoma cepedianum Lesueur (Gizzard Shad) and Ictiobus cyprinellus
Valenciennes (Bigmouth Buffalo), and the condition of those 2 native species
declined in the Mississippi River and Illinois River after bigheaded carps became
established (Irons and Sass 2007). Although Sampson et al. (2009) did not observe
substantial overlap in the diets of Bighead Carp and adult Polyodon spathula Walbaum
(Paddlefish), age-0 Paddlefish grew slower when age-0 Bighead Carp were
present (Schrank et al. 2003). Virtually all fishes during their larval stage feed on
similar food resources as bigheaded carps (Chick and Pegg 2001). Therefore, bigheaded
carps could have negative consequences for entire fish co mmunities.
Bigheaded carp are highly mobile in open systems due to their migratory nature;
thus, coordination between states and in some cases national boundaries is necessary
for effective management of these species (Conover et al. 2007). In 2015, the
US Fish and Wildlife Service hosted an inter-agency meeting for states within the
Ohio River basin (Illinois, Indiana, Kentucky, New York, Pennsylvania, Tennessee,
and West Virginia) called the Ohio River Asian Carp Management Meeting. The
purposes of the meeting were, in part, to foster inter-agency collaboration for planning
and reporting, funding strategies, and implementation of management plans.
However, a fundamental understanding of regional population characteristics is
critical to the strategy of such plans (Conover et al. 2007).
By the early 2000s, the leading edge of the bigheaded carp invasion in the
southeast US was in the Tennessee River and Cumberland River drainages (Kolar
et al. 2007). Recent reports by anglers and biologists revealed that bigheaded carps
are advancing in Tennessee waters. However, bigheaded carp populations had not
been studied or systematically sampled throughout Kentucky Lake and Lake Barkley,
the lowest reservoirs on each river system. The objectives of this study were
to (1) document their distribution in the lower Tennessee and Cumberland rivers,
(2) describe the age- and size-class structures of those populations, and (3) estimate
their growth rates.
Study Areas
Kentucky Lake is a mainstem reservoir of the Tennessee River managed by the
Tennessee Valley Authority (Fig. 1). The reservoir filled after the construction in
1944 of Kentucky Dam 35 rkm from the confluence of the Tennessee River and
Ohio River. Kentucky Dam was constructed for power generation, navigation, flood
control, and recreational purposes. Barges and boats pass through the Kentucky
Dam lock on a daily basis. Kentucky Lake flows northerly and spans 298 rkm from
Pickwick Dam, near the Mississippi border, to the western tip of Kentucky. Water
levels vary 1.5 m between winter and summer pools, and the reservoir has 3322 km
of shoreline that encompass ~64,800 ha (Lake Productions 2017). Kentucky Lake is
a run-of-the-river reservoir having a lacustrine downstream and riverine upstream.
The lacustrine portion of Kentucky Lake has many embayments and backwater
areas and is characterized as eutrophic (KDFWR 2016).
The Duck River, Kentucky Lake’s largest tributary, flows freely for 220 rkm
from Columbia Dam in Columbia, TN, to Kentucky Lake. The Duck River has the
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longest unimpounded reach of any river in Tennessee and is world–renowned for
its diverse mussel and fish communities.
Lake Barkley is a mainstem reservoir of the Cumberland River managed by the
Nashville District of the Army Corps of Engineers. The reservoir was formed by
the construction of Barkley Dam in 1966 at rkm 49 (measured from the confluence
of the Cumberland River and Ohio River). Lake Barkley was constructed primarily
for power generation, navigation, flood control, and recreational purposes. Lake
Barkley extends 119 rkm downstream from Cheatham Dam in Tennessee and flows
northwesterly to the western tip of Kentucky. Similar to Kentucky Lake, Lake Barkley
is a run-of-the-river reservoir with a eutrophic lacustrine downstream portion
(KDFWR 2013a). Lake Barkley water levels vary 1.5 m between winter and summer
pools, and the reservoir has >1600 km of shoreline and ~21,000 ha of surface
area (Lake Productions 2017).
Methods
Both Kentucky Lake and Lake Barkley were sampled systematically throughout
their lengths using standardized protocols we developed to assess relative
abundances. Bigheaded carp abundances (as indexed by catch-per-unit-effort
[CPUE] metrics) in Kentucky Lake and Lake Barkley were similar (Ridgway
Figure 1. Locations (black dots) where 10 Hypophthalmichthys nobilis (Bighead Carp) were
collected, 2015–2016.
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2016) and were combined to report total CPUE. Because bigheaded carps are
often difficult to capture, even in areas with high densities (Hayer et al. 2014,
Stancill 2003, Wanner and Klumb 2009, Williamson and Garvey 2005), we deployed
additional gill nets in the reservoirs in response to commercial fisher
sightings and included these data in population structure and growth analyses.
Additionally, we collected electrofishing samples below Barkley Dam (10 min)
and Kentucky Dam (40 min), in the headwaters of each study reservoir (i.e., below
Cheatham Dam [127 min] and Pickwick Dam [60 min]), and on the Duck
River below Columbia Dam [230 min] to document bigheaded carp range in the
lower Tennessee and Cumberland rivers.
We deployed 1.2-m diameter hoop nets with 3.8-cm mesh (2, tied in tandem)
for 3 days in a variety of habitats including backwaters, channel borders, and swift
tailwaters. Hoop nets were deployed in both reservoirs in spring and summer of
2015, and the total number of tandem hoop nets fished was 48. We calculated tandem
hoop net CPUE as mean number of fish collected in 3-day soaks. Experimental
monofilament gill nets were 3.7 m high hobbled down to 2.4 m and fished on the
substrate bottom in winter 2015, fall 2016, and winter 2016. Gangs of nets with 6
mesh sizes ranging from 76 mm to 140 mm square measure (each net consisted of
2 mesh sizes in 30.5-m panels) were fished overnight in shallow backwater areas
with low water velocity. We fished a total of 48 gill net gangs and quantified CPUE
as mean catch in a gang of nets per night. Boat-mounted electrofishing samples
(n = 108) were collected in summer and fall 2015. We quantified CPUE as the mean
number of fish captured per 10 minutes of pedal time. Data on adult and YOY Silver
Carp were not combined in CPUE values to avoid inflating those estimates. We held
the frequency constant at 80 pulses per second and adjusted voltage and amperage
as needed to achieve a 3000-W power output (Stuck et al. 2015). Transects included
a variety of habitat types (i.e., backwaters, channel borders, and swift tailwaters)
typically 2 m deep. Cast nets were 2.7 m in diameter with 1-cm mesh to capture
YOY bigheaded carps in the summer of 2015. We threw cast nets in backwater
habitats without visually targeting any fish; the total number of throws was 480.
We quantified CPUE as mean number of fish captured per throw of the cast net.
Complete details on the sampling design and gears we used to sample bigheaded
carps are provided in Ridgway (2016).
We weighed, measured for total length (TL), sexed via internal examination,
and removed the lapilli otoliths from all bigheaded carps caught. Age determination
in these 2 species is challenging (Kolar et al. 2007, Schrank and Guy 2002),
and various hard structures, such as fin rays, scales, otoliths, and vertebrae, have
been used in previous studies (Hayer et al. 2014; Kamilov 1985, cited in Kolar et al.
2007; Kamilov 2014; Schrank and Guy 2002). At present, there is no consensus for
which bony structure should be used for age estimation. Using methods described
by Schneidervin and Hubert (1986), Hayer et al. (2014) identified growth annuli on
asteriscus otoliths, the largest of the 3 otolith pairs in cyprinids. However, Seibert
and Phelps (2013) recommended using lapilli otoliths because in their opinion they
provided more reliable ages, especially for older Silver Carp. Therefore, the present
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study followed methods similar to Seibert and Phelps (2013) for age determination.
Otoliths were held with forceps, sanded, and burnt golden brown on the reading surface
using a hotplate. Under a dissecting microscope, annuli were illuminated using a
fiber optic filament connected to a light source. The number of annuli in sectioned lapilli
otoliths was counted independently twice; when readings disagreed, the otoliths
were read a third time before a final age was assigned. If a fish was captured in the fall
(October–November 2015) towards the end of the growing season, an annulus was
added to the count. Those fish, and all fish captured between January and June, were
included in growth analyses. Fish collected in the summer (July and August), in the
middle of the growing season, were not included in the growth analyses.
We estimated Silver Carp growth in the reservoirs using the von Bertalanffy
growth model:
Lt = L∞ (1 - e K [t – to]),
where Lt is length at time t, L∞ is the average maximum attainable size, K is the
Brody growth coefficient, and to is the age at which fish length is theoretically
0 (von Bertalanffy 1938). The growth model was fitted using nonlinear least
squares in FAMS Version 1.64 modeling package (Slipke and Maceina 2014).
Because YOY fish had not yet completed a full year’s growing cycle, they were
not included in von Bertalanffy growth modeling. However, Tennessee Wildlife
Resource Agency (TWRA) biologists collected age-1 Silver Carp from Kentucky
Lake near Big Sandy embayment in May 2016 (at the start of the presumed growing
season), and we included length data from those age-1 fish in the von Bertalanffy
growth models.
Results
Only 10 Bighead Carp were collected throughout this study, and none were collected
in Lake Barkley. Five were collected from the reservoirs using standardized
sampling. Gill nets captured 3 Bighead Carp (CPUE = 0.06/net-night), and hoop
nets caught 2 (0.04/three-day soak). Bighead Carp in the Tennessee River were captured
below Kentucky Dam (rkm 36) up to the mouth of Big Sandy Embayment in
Kentucky Lake (rkm 108), and a single Bighead Carp was collected below Barkley
Dam at Cumberland River km 49 (Fig. 1).
A total of 737 Silver Carp (510 Adults and 227 YOY) were captured throughout
this study. The range of Silver Carp extended throughout each reservoir and also into
the Duck River below Columbia Dam at rkm 220 (Fig. 2). Of those fish, 499 were collected
from the reservoirs using standardized sampling protocols. Gill nets captured
240 Silver Carp (CPUE = 5/net-night), electrofishing collected 242 (CPUE = 0.12
adult and 1.96 YOY per 10-minute runs), hoop nets caught 2 (CPUE = 0.04/three-day
soak), and cast net samples totaled 15 YOY Silver Carp (CPUE = 0.03/throw of the
cast net). Young-of-year Silver Carp that we (and other researchers) collected in 2015
were to our knowledge the first juveniles collected in Kentucky Lake and Lake Barkley.
The furthest upstream they were collected in both reservoirs was at rkm 219 in
Kentucky Lake and rkm 166 in Lake Barkley (Fig. 3).
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Bighead Carp in Kentucky Lake varied from 1010 to 1385 mm TL (mean = 1211
mm, SE = 34, n = 8); the largest weighed 35.05 kg. Silver Carp in both reservoirs
varied from 96 to 1005 mm TL (mean = 655 mm, SE = 10, n = 609); the largest
weighed 14.03 kg (Fig. 4). Silver Carp collected below Columbia Dam on the Duck
River varied from 833 to 947 mm (mean TL = 892 mm, SE = 12, n = 10), and the
largest weighed 11.17 kg.
We removed otoliths from 380 of 381 adult Silver Carp collected from the reservoirs,
and age estimates ranged from 3 to 13 years (Table 1). Percent agreement
between 2 blind readings of otoliths from 307 young (age 5 or younger) Silver Carp
was 79%; percent agreement decreased to 63% for 63 older (ages 6–10) fish and
40% for the 10 oldest (age 11 or older) fish. Over all Silver Carp paired readings,
70% agreed, 22% differed by 1 year, and 8% differed by 2 years. The 8 Bighead
Carp varied in age from 8 to 22 years, and percent agreement between 2 blind readings
was 63%; all paired readings differed by 2 years or less. We were able to assign
ages to all Silver Carp and Bighead Carp when there was a discrepancy between the
first 2 readings. Strong year classes of Silver Carp were present in both Kentucky
Lake and Lake Barkley in 2010, 2011, 2012, and 2015 (Fig. 5).
Visual inspection of Silver Carp growth models revealed that they were nearly
identical in Kentucky Lake and Lake Barkley (Ridgway 2016). Therefore, we
Figure 2. Locations where 737 Hypophthalmichthys molitrix (Silver Carp) were collected,
2015–2016. Three Silver Carp were observed but not captured in the Pickwick Dam tailwater.
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Figure 3. A subset of Hypophthalmichthys molitrix (Silver Carp) locations where 227 YOY
Silver Carp were collected, 2015–2016. Young-of-year Silver Carp were observed but not
captured below Barkley Dam.
Table 1. Mean total lengths (TL, mm) and weights (g) by age for male and female Hypophthalmichthys
molitrix (Silver Carp) collected in Kentucky Lake and Lake Barkley, 2015–2016. Age-1 Silver Carp
(n = 20) were collected by Tennessee Wildlife Resource Agency biologists from Kentucky Lake near
Big Sandy embayment, May 2016. Standard errors are in parentheses.
Male Female Both Sexes
Age n TL Weight n TL Weight TL Weight
1 - - - - - - 236 (3.89) 119 (5.93)
3 12 782 5600 5 807 5758 790 (10.65) 5647 (297.60)
4 52 835 6481 58 863 7548 850 (4.00) 7044 (152.67)
5 97 844 6765 83 886 8208 864 (3.73) 7430 (121.53)
6 30 864 7627 19 907 9222 881 (8.71) 8245 (266.08)
7 3 872 8584 3 921 9413 896 (30.07) 8999 (921.49)
8 3 854 7025 2 918 9360 879 (20.27) 7959 (679.09)
10 1 884 7970 2 872 9128 876 (65.36) 8742 (2856.64)
11 5 861 7444 3 881 7932 869 (12.86) 7627 (309.53)
12 - - - 1 904 8555 904 8555
13 - - - 1 882 8400 882 8400
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pooled the mean length-at-age data, and the final von Bertalanf fy model was:
TLt = 881 [1- e-0.881 (t – 0.653)],
where TLt is the predicted size for a given age of interest (t; r2 = 0.997). The model
predicted that Silver Carp reached a mean total length of 770 mm in only 3 years.
Figure 4. Total length-frequency distributions for Hypophthalmichthys molitrix (Silver
Carp) collected using all gears in Kentucky Lake and Lake Barkley in 2015–2016.
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Discussion
In the present study, Bighead Carp were collected from Kentucky Lake and below
Kentucky Dam and Barkley Dam. Although we did not collect any Bighead Carp in
Figure 5. Year classes of Hypophthalmichthys nobilis (Bighead Carp) and H. molitrix (Silver
Carp) collected using all gears in Kentucky Lake and Lake Barkley, 2015–2016.
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Lake Barkley, they have been reported there since 2002 (Kolar et al. 2007). In 2010,
a Bighead Carp was collected 1 reservoir above Lake Barkley in Cheatham Lake at
Cumberland River km 239 (USGS 2016). Bighead Carp were collected in the Tennessee
River upstream of Kentucky Lake in Lake Guntersville and Nickajack Lake
as far back as 1999 (USGS 2016). In 2010, a single Bighead Carp was collected from
Lake Chickamauga below Watts Bar Lake at rkm 853, the furthest known leading
edge of the Bighead Carp invasion on the Tennessee River (USGS 2016).
Prior to this study, the leading edges of Silver Carp invasion in Tennessee waters
were the tailwater below Cheatham Dam in Lake Barkley (Clark et al. 2013) and at
rkm 417 below Wilson Dam in the headwaters of Pickwick Lake (USGS 2016). We
documented that Silver Carp are distributed throughout Kentucky Lake and Lake
Barkley, and for the first time, Silver Carp were collected below Columbia Dam at
Duck River km 220. An old fish ladder on Columbia Dam improves continuity of
native communities, but could also facilitate the upstream progression of the bigheaded
carp invasion.
There is a paucity of information regarding the longevity of bigheaded carps
(Jennings 1988, Kolar et al. 2007). In their native habitats, Bighead Carp were
estimated to reach 16 years, and Silver Carp were estimated to reach 15 years in
China and 20 years in Russia (Kolar et al. 2007). In 2013, a group of federal and
state agencies collectively known as the Ohio River Fisheries Management Team
captured and aged bigheaded carps from the Ohio River. Using otoliths as the aging
structure, a single Bighead Carp was reported to be age 18 (KDFWR 2013b). In the
present study, the oldest Bighead Carp was age 22, which to our knowledge is the
oldest ever reported. Kolar et al. (2007) speculated that bigheaded carp longevity
may be similar to that of Ctenopharyngodon idella Valenciennes (Grass Carp), a
closely related species, which are documented to reach age 32.
The pooled growth rate of Silver Carp was higher in Kentucky Lake and Lake
Barkley than in North Dakota tributaries of the Missouri River (Hayer et al.
2014), the middle Mississippi River (Williamson and Garvey 2005), and the Illinois
and Wabash rivers (Stuck et al. 2015) (Fig. 6). In addition, the growth rate
we observed was higher than Silver Carp in India’s Gobindsager Reservoir (Tandon
et al. 1993, cited in Williamson and Garvey 2005) and Russia’s Amur River
(Nikolskii 1961, cited in Williamson and Garvey 2005). Reaffirming our age
estimates, other researchers using a different aging structure (pectoral fin rays)
reported similarly fast growth of Silver Carp in Kentucky Lake (>800 mm TL at
age 3) and similar strong year classes (2010, 2011, and 2012; A. DeRose, Murray
State University, Murray, KY, unpubl. data).
Although their origin is unknown, we collected YOY Silver Carp from the upper
reaches of Kentucky Lake and Lake Barkley. Those YOY Silver Carp either
hatched in Kentucky Lake and/or Lake Barkley, or they hatched and swam 166–219
rkm from the Ohio River to where they were captured in each reservoir. Spawning
and recruitment in large rivers has been related to increases in both flow and stage
(Schofield et al. 2005, Schrank et al. 2001, Verigin et al. 1978) which are believed
to keep fertilized, semibuoyant eggs suspended before hatching (Jennings 1988,
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Laird and Page 1996, Verigin et al. 1978). Although poorly understood, successful
reproduction has been documented for introduced and native bigheaded carp
populations in reservoir systems of Eurasia and Asia (Kolar et al. 2007). Given optimal
temperature and sufficient flows, Murphy and Jackson (2013) predicted that
bigheaded carps can spawn and successfully reproduce in just 25 rkm, much less
than the previously reported 80–100 rkm (Krykhtin and Gorbach 1981, Nico et al.
2005). Both Kentucky Lake and Lake Barkley are run-of-the-river reservoirs having
swift tailwaters, low retention rates, and expansive backwater embayments which
could serve as nursery grounds. In addition, the lower Duck River has sufficient linear
habitat (223 rkm) and a large backwater delta at the Kentucky Lake confluence.
However, bigheaded carp egg-transport suitability (Murphy and Jackson 2013) is
beyond the scope of this study and has yet to be modeled for these reservoir systems.
We are inclined to believe that they hatched in the reservoir(s) because Silver
Carp larvae utilize backwaters or other flooded areas as nursery grounds (Nikolsky
1963, cited in Kolar et al. 2007). Migrating upstream through a navigation lock at an
early life-history stage would be a heretofore unobserved phenomenon, though not
impossible. Barges have the ability to entrain, retain, and even transport small fish
Figure 6. Von Bertalanffy growth curves for Hypophthalmichthys molitrix (Silver Carp) in
Kentucky Lake and Lake Barkley (this study—observed mean lengths-at-age are shown);
Middle Mississippi River (Williamson and Garvey 2005); Wabash River, IL, and the Illinois
River (Stuck et al. 2015); Missouri River tributaries in South Dakota (Hayer et al. 2014);
India (Tandon et al. 1993 cited in Williamson and Garvey 2005); and the Amur River, Russia
(Nikolskii 1961 cited in Williamson and Garvey 2005).
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(Notemigonus crysoleucas Mitchill [Golden Shiner] 63–122 mm TL were used as a
surrogate for YOY Silver Carp) through locks (Davis et al. 2016). However, transport
distances of up to only 15.5 rkm were examined in that study, and it is difficult
to consider a barge transferring such a substantial number of YOY Silver Carp 200+
rkm unless YOY of incredible abundance were present below the reservoirs. Whether
spawned in the Ohio River or in Tennessee waters, Silver Carp are present in Kentucky
Lake and Lake Barkley across a wide range of sizes and ages.
Eight year classes of Silver Carp were observed and a boom–bust pattern of strong
and weak (or absent) year classes was apparent, which is observed in many fish species.
Not knowing the natal rivers of the adult Asian carp we collected prevented us
from attempting to relate fluctuations in adult year-class strength to environmental
variables per the methods of Maceina and Bettoli (1998). Larval fish should be
sampled spatiotemporally to determine where and when reproduction is occurring.
If successful reproduction varies with environmental variables such as discharge,
then recruitment could be predicted and potentially controlled (DeGrandchamp et al.
2007, Hintz et al. 2017), perhaps by manipulating dam outflow above the reservoirs
(Lohmeyer et al. 2009) or arranging for aggressive harvesting programs.
All of the Bighead Carp we captured were large (≥1009 mm TL) and old (age
8 or older). The absence of young fish and their low densities indicate that recruitment
in Kentucky Lake and Lake Barkley by Bighead Carp is negligible. Similarly,
researchers documenting the leading edges of bigheaded carp invasions in the Ohio
River and tributaries of the Missouri River collected fewer Bighead Carp relative
to Silver Carp (KDFWR 2013b: Silver Carp = 88, Bighead Carp = 34; Hayer et
al. 2014: Silver Carp = 469, Bighead Carp = 8). Long-term monitoring of Bighead
Carp in the Illinois River revealed that year-class strength is highly variable, but
one strong year class can quickly rebuild the population (Irons et al. 2011).
Gill-net catch data suggest Silver Carp are already a large component of the fish
assemblages (Ridgway 2016). Determining the extent to which Silver Carp are increasing
in Kentucky Lake and Lake Barkley requires multiple years of catch data,
and this study could not address that concern. However, studies documenting the
early colonization of bigheaded carps elsewhere reported exponential increases in
Silver Carp (Chick and Pegg 2001, Hayer et al. 2014, Irons et al. 2011, Sass et al.
2010), and Bighead Carp (Irons et al. 2011).
Silver Carp growth rate in Kentucky Lake and Lake Barkley is remarkably fast
relative to other populations, and fish are reaching harvestable size at a young age.
Commercial fishers and markets prefer larger fish because payment depends on
weight, not individual fish. In addition, targeting larger fish greatly reduces handling
and processing effort, bycatch, and bycatch mortality when using gill nets
with larger mesh (Ridgway 2016). Between 2011 and 2015, commercial fishers
removed more than 750,000 kg of bigheaded carp from the reservoirs (N. Jackson,
Kentucky Department of Fish and Wildlife Resources, Murray, KY, unpubl. data).
In 2013, the Kentucky Department of Fish and Wildlife Resources (KDFWR) initiated
a bigheaded carp harvest program, and in 2015 began subsidizing the price of
bigheaded carp to motivate commercial fishers to invest more effort. As a result,
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J.L. Ridgway and P.W. Bettoli
2017 Vol. 16, No. 3
more than 360,000 kg were removed in 2015 alone. Commercial fishing tournaments
hosted by KDFWR have also proved useful, removing more than 37,000 kg in just
2 days in March 2013 (KDFWR 2017). However, periodic standardized sampling is
needed to determine whether removal efforts have reduced carp densities (Bouska
et al. 2014).
Similar to other researchers, we found bigheaded carp to be difficult to capture
using traditional techniques, which translated into low CPUE values during this
study. However, 2 electrified trawling techniques (paupier butterfly frame trawl
and dozer trawl) were recently developed by the Fish and Wildlife Conservation
Office in Columbia, MO, to increase sampling efficiency (Doyle et al. 2015). The
week of 7 November 2016, the paupier trawl sampled Big Bear Embayment of
Kentucky Lake (near Tennessee rkm 11) and collected 1378 Silver Carp in 5 hours
of nighttime electrotrawling pedal time (J. Hammen, US Fish and Wildlife Service,
Columbia, MO, unpubl. data). Continued paupier monitoring in the Kentucky Lake
and Lake Barkley systems is scheduled in 2017.
The findings reported herein constitute the first study of bigheaded carps
throughout Kentucky Lake and Lake Barkley systems, and there is more work to be
done. Continued public education and collaboration with states within the Tennessee
River and Cumberland River basins will be important to effectively control and
manage bigheaded carps. Future monitoring and research in these systems should
aim to deter further migrations upstream, limit natural reproduction, identify potential
ecologic and economic impacts, assess new control techniques, and evaluate
management success.
Acknowledgments
Tennessee Wildlife Resources Agency provided the primary funding for this research. We
received other funding and support from the Center for the Management, Utilization, and Protection
of Water Resources at Tennessee Technological University, and the USGS Tennessee
Cooperative Fishery Research Unit at Tennessee Technological University. Dennis and Ben
Duncan are thanked for their insights as commercial fishers on Kentucky Lake and Lake Barkley.
We thank Tim Broadbent and his staff (TWRA, Region I), Robin Calfee (USGS), Duane
Chapman (USGS), Allison DeRose (Murray State University), Emily Pherigo (USFWS),
Neal Jackson (KDFWR), and David Roddy (TWRA, Fish Division) for providing assistance
and answering queries posed during this research. This manuscript benefitted from the constructive
comments provided by Josh Perkin, Michael Allen, Mark Rogers, and Robin Calfee.
The findings and conclusions in this article are those of the authors and do not necessarily represent
the views of the US Fish and Wildlife Service.
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