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2007 SOUTHEASTERN NATURALIST 6(2):321–332
Factors Influencing Paddlefish Spawning in the
Daniel M. O’Keefe1,*, Johanna C. O’Keefe1, and Donald C. Jackson1
Abstract - The early life-history requirements of Polyodon spathula (paddlefish) are
not well understood, in part due to the difficulty of sampling early life stages. Passive
sampling with benthic, mat-style devices effectively collected paddlefish eggs in the
Tombigbee watershed (Mobile River basin) during spring 2005, facilitating identification
and characterization of egg-incubation microhabitats. Eggs were collected
over gravel, sand-impacted gravel, and bedrock substrates at corrected depths ranging
from 1.2 to 7.7 m. Sampling occurred continuously (489 sampler-days) in the
lotic bendway of the Tennessee-Tombigbee Waterway from late February through
April, when water temperatures ranged from 11.5 to 20.8 °C. Of 106 paddlefish eggs
collected from this unique macrohabitat, 95% were taken on either April 6 or April
16. Nine paddlefish eggs were collected in a tributary (Noxubee River) on April 13
after four sampler-days of effort. Water temperatures associated with peak spawning
activity ranged from 16.9 to 19.4 °C, slightly higher than temperatures recorded for
Mississippi River basin populations. A substantial (> 2.74-m) rise in water level
triggered spawning activity, similar to that observed in other systems. Benthic mats
proved useful for delineating paddlefish egg-incubation habitat in areas not subject
to shifting substrate, and could be used in the future to address hypotheses regarding
micro- and macrohabitat suitability.
Many details regarding early life-history and spawning requirements of
Polyodon spathula Walbaum (paddlefish) have historically eluded researchers.
Although efforts to collect early life stages of paddlefish date back to the
early 1900s (Stockard 1907), spawning was not documented until 1960
(Purkett 1961). Since that time, no single method has been used consistently
to sample eggs. Although paddlefish are present throughout large river
systems of the Mississippi and several other Gulf of Mexico drainages,
collection of eggs and subsequent description of spawning or egg-incubation
habitat have occurred only in the Osage River, MO (Purkett 1961), Old
Hickory Reservoir, TN (Pasch et al. 1980, Wallus 1986), and the lower
Yellowstone River, MT and ND (Firehammer et al. 2006).
Published studies have reported collection of eggs from dry gravel bars
after a sudden drop in river level (Purkett 1961) and by sampling with
epibenthic sleds (Pasch et al. 1980, Wallus 1986), ichthyoplankton drift nets
(Pasch et al. 1980, Wallus 1986), and dredges (Purkett 1961). Collection of
larvae (Alexander and McDonough 1983, Lein and DeVries 1998) or youngof-
year (Houser and Bross 1959, Jennings and Wilson 1993, Reed et al.
1Department of Wildlife and Fisheries, Mississippi State University, Mississippi
State, MS 39762. *Corresponding author - firstname.lastname@example.org.
322 Southeastern Naturalist Vol. 6, No. 2
1992, Ruelle and Hudson 1977) is commonly used to provide evidence of
successful spawning. These methods do not provide information regarding
the conditions necessary for successful egg deposition and development, and
only allow coarse-scale delineation of spawning locations (i.e., areas upstream
Passive sampling with benthic, mat-style devices has been used to collect
the adhesive eggs of sturgeons (family: Acipenseridae), facilitating description
of egg incubation habitat and examination of spatial and temporal trends
in spawning activity (Marchant and Shutters 1996, McCabe and Beckman
1990). Suspended, tube-style sampling devices have recently been used to
passively sample paddlefish eggs (Firehammer et al. 2006). Although suspended
devices have the advantage of sampling effectively when shifting
sand substrate renders benthic devices ineffective (Firehammer et al. 2006),
their suspension in the water column does not mimic the position of natural
substrates. The use of suspended sampling devices therefore results in capture
of eggs between the location of spawning and the location of egg
incubation; spawning and egg-incubation locations are not always identical
due to the drifting of eggs before their adhesion to substrates (Purkett 1961).
Determination of egg-incubation locations and monitoring of these locations
over time could provide detailed site-specific information regarding
environmental cues necessary to induce spawning. This is especially important
in regulated rivers, where paddlefish spawning success may hinge on
human decisions regarding dam operation and flow regime (Alexander and
McDonough 1983). Several reviews of paddlefish biology and management
indicated a need for basic knowledge regarding availability of spawning
areas and factors that influence reproductive success (Birstein et al. 1997,
Carlson and Bonislawsky 1981, Graham 1997, Jennings and Zigler 2000,
Russell 1986). Toward this end, the purpose of this study was to: evaluate
the effectiveness of passive sampling with benthic, mat-style devices; determine
the location and characteristics of microhabitats used for paddlefish
egg incubation; and draw inferences regarding environmental factors that
influence spawning in the Tennessee-Tombigbee Waterway.
The Tennessee-Tombigbee Waterway is a comprehensively modified
ecosystem consisting of tailraces, dredged navigation channel segments,
bendways (portions of the former Tombigbee River), and impoundments
(Jackson 1995). Gill netting for paddlefish in 4 tailraces, 15 bendways, 3
impoundments, and 10 tributaries revealed large concentrations of paddlefish
in 2 bendways (O’Keefe 2006). Sampling for paddlefish eggs was
limited to one of these: the bendway between Howell Heflin Dam and
Howell Heflin Lock near Gainesville, AL (Fig. 1). This area is not subject to
channel dredging, snagging, or commercial navigation. Other sections of the
former Tombigbee River in Alabama are part of the navigation channel,
impounded, or subject to flow diversion from the creation of cutoff canals,
2007 D.M. O’Keefe, J.C. O’Keefe, and D.C. Jackson 323
which carry the majority of the waterway’s discharge and commercial barge
traffic. Most bendways are impacted by siltation or formation of sand plugs
at their upstream ends, which further reduce flow (Pennington et al. 1981).
The bendway between Howell Heflin Dam and Howell Heflin Lock is not
subject to excessive sedimentation due to spatial decoupling of the dam and
its navigation lock. This “lotic bendway” is the only portion of the original
river channel that retains the majority of the waterway’s discharge but is not
subjected to commercial navigation and channel maintenance activities. Due
to its relatively undisturbed condition, apparent availability of spawning
habitat, and high adult paddlefish density, the lotic bendway was chosen as a
macrohabitat in which an egg-sampling technique could be evaluated, microhabitats
used for egg incubation could be located and characterized, and
environmental cues related to spawning behavior could be studied.
Potential paddlefish spawning locations were identified prior to the current
study. Purkett (1961) described paddlefish spawning habitat consisting
of shallow gravel bars that were not submerged during normal flow conditions.
Such an exposed gravel bar was located in the lotic bendway at the
mouth of the Noxubee River during concurrent population-dynamics and
radio-telemetry studies (Fig. 1). This is a unique habitat in the mainstem of
Figure 1. Location of egg samplers used to collect paddlefish eggs at shallow (< 3 m)
and deep ( 3 m) locations in the lotic bendway of the Tennessee-Tombigbee
Waterway, AL, below Howell Heflin Dam during spring 2005. One egg sampler was
deployed in the Noxubee River, 7.6 km upstream from its confluence with the
324 Southeastern Naturalist Vol. 6, No. 2
the Tennessee-Tombigbee Waterway; all other exposed gravel bars have
apparently been eliminated through dredging, sedimentation, and impoundment.
In the Noxubee River, a fourth-order tributary that has not been
heavily impacted by channelization or damming, an exposed gravel bar was
located 7.6-km upstream from its confluence. Most egg sampling occurred
in the lotic bendway of the Tennessee-Tombigbee Waterway, but limited
sampling for eggs also occurred at the shallow gravel bar in the Noxubee
River. Within the lotic bendway, egg sampling was conducted in shallow
gravel microhabitats where spawning activity was expected to be heaviest,
and in microhabitats that appeared less suitable for egg incubation (i.e.,
deeper locations and areas with bedrock substrate).
Median discharge above Howell Heflin Dam in Gainesville, AL, was
141 m3 sec-1 over the period March 17, 1978 through March 16, 2004
(station number 02447025; USGS 2005). Median discharge of the Noxubee
River at Geiger, AL, was 10 m3 sec-1 over the period August 1, 1944
through July 31, 2004 (station number 02447025; USGS 2005). Discharge
data were not available for the lotic bendway of the Tennessee-Tombigbee
Waterway below Howell Heflin Dam, where median gage height was 23.10
m (range 22.36 to 34.47 m) over the period of available data from August
19, 1999 through August 18, 2004 (station number 02447026; USGS
2005). Due to high variability of gage height, depths in the lotic bendway
were highly variable. Subsequently, depths reported at sampling locations
were corrected for high and variable water levels by subtracting the difference
between actual gage height and median gage height from the actual
depth measured in the field. Surface-water temperature in the lotic
bendway ranged from 6 to 30 °C, Secchi depth ranged from 9 to 76 cm, and
specific conductance at 25 °C ranged from 70 to 232 S/cm (D.M.
O'Keefe, unpubl. data).
Benthic egg-sampler design was based on mat-style sampling devices
used to collect eggs of Acipenser spp. (Marchant and Shutters 1996,
McCabe and Beckman 1990). Egg samplers were constructed using artificial
substrates of latex-coated hog hair filter material 51 cm wide by 2.54
cm deep. The material was wrapped smooth-side down around a 51-cm
square angle-iron frame such that both sides of the frame were covered
with the filter material. The filter material was attached to the frame
using nuts and bolts. Washers were used to prevent material from slipping
off bolts because holes in the material wore wider with use. Frames were
fastened to anchors with ropes and swivels to prevent twist in the current.
Anchors consisted of scrap iron pieces from 9 to 23 kg. Float lines of 15
to 30-m length and 1-cm diameter were attached to anchors with a swivel.
Most float lines were attached to bullet-shaped floats 35 cm long and 13.5
2007 D.M. O’Keefe, J.C. O’Keefe, and D.C. Jackson 325
cm in diameter, but others were tied to riparian vegetation including
overhanging limbs and exposed roots.
The egg samplers were deployed on shallow gravel bars (< 3 m deep at
median flow) where spawning was expected to occur, at deeper locations
where substrate consisted of gravel or bedrock, and at shallow (< 3 m deep at
median flow) tailrace locations with bedrock substrate (Fig. 1). Sampling
began on February 28, 2005, and concluded on April 25, 2005. Two to
eleven egg samplers (mean of seven) were effectively sampling (i.e., deployed
and retrieved within 14 days) in the lotic bendway at any given time
during this period. One egg sampler was deployed in the Noxubee River
from April 9 to April 13, 2005. Individual egg samplers were deployed at 4-
to 8-day intervals when possible, but high water prevented retrieval of egg
samplers between April 6, 2005 and April 16, 2005.
Upon retrieval, egg samplers were examined for the presence of large
(approximately 3–4 mm diameter), grey, adhesive eggs. Size and coloration
were sufficient to determine that eggs of such description were from
acipenseriform species (Firehammer et al. 2006). Eggs of this description
were removed by cutting the strands of hair to which they were attached.
Periphyton, detritus, and fine substrates accumulated on artificial substrates,
which were rinsed thoroughly before redeployment. Eggs were preserved in
80% ethanol or transported to aerated tanks for hatching. Hatched larvae
were preserved in 80% ethanol upon death. On each date that eggs were
collected, a portion of eggs were preserved as vouchers, and others
were hatched to verify that eggs were paddlefish, and not Scaphirhynchus
suttkusi Williams and Clemmer (Alabama sturgeon). The Alabama sturgeon
is the only other acipenseriform species recorded in the Tombigbee drainage
(Mettee et al. 1996, Ross 2001), and is so rare that capture of its eggs was
unlikely. Paddlefish egg catch per sampler-day was calculated as the sum of
eggs collected over the total number of sampler-days.
Water temperature was measured at 0.5 m below the surface when
samplers were retrieved using a Model 85 Yellow Springs Instrument (YSI,
Inc., Yellow Springs, OH). Water velocity was measured at 0.5 m below the
surface using a Flo-Mate (Marsh-McBirney, Inc., Frederick, MD) at successful
sample sites (i.e., those at which at least one paddlefish egg was
captured) on April 6 and April 13, 2005.
Paddlefish eggs were collected from egg samplers in the lotic
bendway on three dates (March 30, April 6, April 16) and in the Noxubee
River on the one date it was sampled (April 13) during 2005 (Fig. 2). Of
106 paddlefish eggs collected from the lotic bendway, 95% were taken on
either April 6 or April 16. Paddlefish egg catch per sampler-day was 0.23
for the entire study period. Catch per sampler-day was 0.61 for March 30
326 Southeastern Naturalist Vol. 6, No. 2
through April 16 and zero for the remainder of the study. Correct identification
of eggs was verified by hatching a subsample (29%) of the eggs
collected. Thirty-three paddlefish larvae were hatched from eggs collected
on March 30, April 6, April 13, and April 16. Twelve larvae lived
at least two weeks, developed rostrums, and successfully switched from
endogenous to exogenous food sources.
Paddlefish eggs were collected under a wide variety of depth, substrate,
and velocity conditions. Successful artificial substrates were set at depths of
1.2 to 7.7 m at median gage height, representing the full range of depths
sampled. Eggs were collected from samplers placed over gravel at shallow
(< 3 m) and deep ( 3 m) locations, and over bedrock at deep locations. Egg
samplers set in the tailrace of Heflin Dam over bedrock in shallow water did
not collect eggs. High flow deposited coarse sand over some submerged
gravel bars during the study. One egg collected from the Noxubee River
hatched successfully despite adhesion to sand grains, which covered it
completely. On April 6, water velocity ranged from 0.39 to 1.06 m/s at
successful sites in the lotic bendway. On April 13, water velocity at the
successful site in the Noxubee River was 0.99 m/s and temperature was
19.4 °C. Water temperature in the lotic bendway was 18.0 °C on April 6,
16.9 °C on April 9, and 19.4 °C on April 13 and 16.
High water level (which peaked at 8 m above median gage height)
between April 6 and April 16 submerged floats and terrestrial tie-off points
for the egg samplers, making it impossible to examine them during this
Figure 2. Gage height and water temperature in the Tennessee-Tombigbee Waterway,
AL, below Howell Heflin Dam during spring 2005, with number of successful
egg samplers (numerator) and number of functioning egg samplers (denominator)
shown above the abscissa. Collection of one or more paddlefish eggs on an egg
sampler was considered a success.
2007 D.M. O’Keefe, J.C. O’Keefe, and D.C. Jackson 327
period. Five of the egg samplers were retrieved on April 16, two of which
contained no eggs and were deeply buried in gravel and sand. These were not
considered as functioning effectively between April 6 and April 16 and
subsequently were not included when determining the percentage of successful
sampling mats. Two others were partially covered with sand or
gravel and contained 1 or 2 eggs. These were considered as functioning
effectively, as were other partially-covered egg samplers examined on other
dates. The only entirely unimpacted egg sampler retrieved on April 16
contained 19 paddlefish eggs. All substrates not affixed to terrestrial vegetation
shifted position during high water, washing into locations 1.4 to 6.6 m
deeper than original locations.
Passive sampling with benthic, mat-style devices is an effective method
for locating and describing egg-incubation habitat of paddlefish. This
method allows researchers to sample continuously for extended periods of
time and is effective in locations with abundant debris, deep water, and high
velocity. Although mats affixed to floats may experience some downstream
drift during high water, attachment to riparian vegetation eliminates this
problem. Benthic mats are very effective relative to other methods used for
collecting paddlefish eggs. Two of seven methods used to sample paddlefish
eggs and larvae over a 9-year period were successful at capturing eggs in the
Cumberland River (Wallus 1986). Over that period of time, 41 eggs were
collected using an epibenthic sled and an obliquely towed 0.5-m square
ichthyoplankton net (Wallus 1986). During a single season in the present
study, 493 mat-days resulted in the capture of 115 eggs in the lotic bendway
and the Noxubee River.
Firehammer et al. (2006) reported that mat-style egg samplers similar to
those used in our study and previous studies on Acipenser spp. (Marchant
and Shutters 1996, McCabe and Beckman 1990) did not effectively sample
paddlefish eggs in the lower Yellowstone River in Montana and North
Dakota, where shifting sand substrate buried egg samplers. Subsequently,
tube-style egg samplers designed to collect eggs drifting in the water column
were used effectively in the lower Yellowstone River (Firehammer et al.
2006). Mat-style egg samplers successfully collected paddlefish eggs over
relatively stable gravel and bedrock substrates in the Tennessee-Tombigbee
Waterway, but tube-style egg samplers might have reduced problems associated
with sand deposition on samplers following peaks in discharge. The
tube-style sampling devices used by Firehammer et al. (2006) captured 130
acipenseriform eggs (89 genetically confirmed as paddlefish) over two years
with 926 collector days of effort in the lower Yellowstone River. Our higher
catch-per-effort using mat-style devices suggests that mats may be more
effective than tubes where substrate permits their use, but catch rates are not
328 Southeastern Naturalist Vol. 6, No. 2
directly comparable between studies and are dependant on the density of
spawning paddlefish in sampled habitats.
Paddlefish eggs were collected in microhabitats similar to the exposed
gravel bars described by Purkett (1961), and also were collected in large
numbers from deep, high velocity areas with gravel or bedrock substrate.
Purkett (1961) noted that eggs collected from deep areas downstream from
gravel bars were covered with debris or adhered to waterlogged wood,
implying that they came to rest in a depositional zone. In the lotic
bendway, deep bedrock runs were not sites of sediment deposition, but it is
unlikely that eggs would adhere to the clay-rich marl substrate. Artificial
substrates set over deep bedrock were relatively free of leaf litter, fine
substrate, and periphyton growth that accumulated at shallow sites, suggesting
that eggs would experience suitable incubation conditions if they
could adhere to a suitable surface. The artificial substrates provided such a
surface, as might large woody debris (present at the sample site in the form
of sunken, waterlogged timber, even in areas of swift current). Tailrace
spawning in other systems has been inferred by collection of larval paddlefish
found downstream from dams (Hoxmeier and DeVries 1997, Pasch et
al. 1980, Wallus 1986). We collected eggs 2.8 km to 4.5 km downstream
from Howell Heflin Dam, but not from artificial substrates set within
400 m of the dam. This suggested that eggs did not incubate in the tailrace
of Howell Heflin Dam.
Most of the eggs were collected in the Tombigbee watershed at water
temperatures between 16.9 and 19.4 °C when water level was at least 2.74 m
above gage height at median flow. This corresponds to the 2.74-m rise
shown to trigger paddlefish spawning in the Osage River, MO (Purkett
1961). Although paddlefish in the lotic bendway required the same increase
in discharge needed to induce spawning in other systems, spawning occurred
in warmer water than has been observed for Mississippi River basin populations
such as those of the Osage River, MO (15 to 16 °C; Purkett 1961), and
Cumberland and Tennessee rivers, TN (12 to 15 °C; Wallus 1986). A study
of paddlefish in the Mobile River basin found that spawning occurred in
tributaries of the Alabama River, AL, when water temperature ranged from
12 to 17 °C (Lein and DeVries 1998), which is consistent with results from
Mississippi basin populations. However, Hoxmeier and DeVries (1997)
found that paddlefish adults did not vacate a suspected spawning area below
Claiborne Dam on the Alabama River, AL, until water temperature exceeded
24 °C, and suggested that paddlefish of the Mobile River basin may, in some
instances, spawn at warmer temperatures than recorded in the Mississippi
River basin. Our results in the Tombigbee watershed, which is part of the
Mobile River basin, add some credence to this hypothesis. Genetic differences
among paddlefish in the Mobile and Mississippi river basins (Carlson
1982, Epifanio et al. 1996) may explain slight differences in spawning cues.
However, construction of the Tennessee-Tombigbee Waterway created a
2007 D.M. O’Keefe, J.C. O’Keefe, and D.C. Jackson 329
freshwater connection between these two historically isolated stocks, and
interbreeding has therefore been possible since 1985.
Current velocity at paddlefish spawning locations has not been reported
by other authors who have collected eggs (Firehammer et al. 2006, Pasch et
al. 1980, Purkett 1961, Wallus 1986). Spawning and egg-incubation habitatsuitability
curves developed by Crance (1987) indicated a suitability index
of 0.4 or higher for mean water-column velocities of 0.4 to 1.6 m/s. In the
lotic bendway, subsurface water velocities at egg-incubation locations were
at the lower to middle portion of this range. Sub-surface velocity is greater
than mean velocity in deep rivers (Allan 1995), suggesting that mean velocities
at some egg-incubation locations in the Tombigbee watershed were
slightly lower than expected by Crance (1987). However, velocity measurements
were taken during our study only on selected dates, and do not
represent the full range of current conditions that incubating eggs experienced.
Velocity measurements were taken in the lotic bendway on April 6,
when gage height was at its lowest for the period of March 31 to April 17.
This fact suggests that recorded sub-surface velocities represented the minimum
values encountered by the majority of eggs incubating at sampled
locations in the lotic bendway.
Egg collection facilitated description of date, discharge, substrate, depth,
water temperature, and current velocity conditions associated with spawning
and subsequent egg incubation. In highly regulated rivers such as the Tennessee-
Tombigbee Waterway, determining environmental spawning cues is
especially important because timing and duration of high discharge (which
are controlled, to some extent, by human activity) must correspond with the
appropriate date and water temperature for successful paddlefish spawning
Collection of paddlefish eggs in the lotic bendway and an unregulated
tributary (Noxubee River) suggests that macrohabitats not subjected to
channelization, navigation traffic, or impoundment are used as egg incubation
habitat. The suitability of these relatively undisturbed environments as
compared with more degraded environments was not directly addressed by
this study. Although shallow (< 3 m deep) gravel microhabitat is rare in
altered mainstem Tennessee-Tombigbee Waterway macrohabitats, deeper
gravel and bedrock areas are common in certain areas of the navigation
channel (D.M. O'Keefe, unpubl. data). Based on egg-sampler success at
deep gravel and bedrock locations in the lotic bendway, these areas could
provide suitable egg-incubation habitat, assuming that adult paddlefish
would elect to spawn in the absence of proximal shallow gravel habitat.
Radio-telemetry data from paddlefish tagged in the lotic bendway indicate
that paddlefish show strong preference for the lotic bendway macrohabitat,
although some individuals moved into the Noxubee River or navigation
channel when conditions were ideal for spawning (O’Keefe 2006).
Regardless of the degree to which paddlefish preferenially spawn in
areas relatively free from disturbance, these areas should be protected
330 Southeastern Naturalist Vol. 6, No. 2
from excessive human disturbance because of their demonstrated role in
the life cycle of paddlefish. Graham (1997) suggested that all potential
and known paddlefish spawning areas should be protected from detrimental
human disturbance because of widespread degradation of historically
suitable spawning habitats. The limited distribution of adult paddlefish in
the Tennessee-Tombigbee Waterway and their restricted movements
(O’Keefe 2006) further suggest that the lotic bendway and Noxubee
River are critical to their continued survival in the Tombigbee watershed
and worthy of protection.
Documentation of paddlefish spawning and egg-incubation locations is
uncommon, but necessary for protection of critical habitats. We found that
passive sampling with benthic mats effectively collected paddlefish eggs in
areas not subjected to shifting substrates. The use of benthic mats allowed us
to document and characterize paddlefish egg-incubation habitats, and examine
patterns of paddlefish spawning activity in relation to abiotic factors.
Beyond the scope of our study, our methods could be used to investigate the
relative suitability of multiple macrohabitats (e.g., channelized vs.
unchannelized stream reaches) in addition to facilitating more detailed description
of egg-incubation microhabitat characteristics.
Field work for this project was completed with the assistance of M. Kashiwagi
and A. Pollack. Phillip Bettoli and two anonymous reviewers improved this paper
with their helpful comments. Funding was provided through the Mississippi Department
of Wildlife, Fisheries, and Parks under Federal Aid Project T-1. This paper is
approved as manuscript number WF-230 of the Forest and Wildlife Research Center,
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