2008 SOUTHEASTERN NATURALIST 7(3):475–482
Size of Spawning Population, Residence Time, and
Territory Shifts of Individuals in the Spawning
Aggregation of a Riverine Catostomid
Timothy B. Grabowski1,2,* and J. Jeffrey Isely3
Abstract - Little is known about the behavior of individual fish in a spawning
aggregation, specifically how long an individual remains in an aggregation. We
monitored Moxostoma robustum (Cope) (Robust Redhorse) in a Savannah River
spawning aggregation during spring 2004 and 2005 to provide an estimate of the
total number of adults and the number of males comprising the aggregation and
to determine male residence time and movements within a spawning aggregation.
Robust Redhorse were captured using prepostioned grid electrofishers, identified
to sex, weighed, measured, and implanted with a passive integrated transponder.
Spawning aggregation size was estimated using a multiple census mark-andrecapture
procedure. The spawning aggregation seemed to consist of approximately
the same number of individuals (82–85) and males (50–56) during both
years of this study. Individual males were present for a mean of 3.6 ± 0.24 days
(± SE) during the 12-day spawning period. The mean distance between successive
recaptures of individual males was 15.9 ± 1.29 m (± SE). We conclude that males
establish spawning territories on a daily basis and are present within the spawning
aggregation for at least 3–4 days. The relatively short duration of the aggregation
may be the result of an extremely small population of adults. However, the behavior
of individuals has the potential to influence population estimates made while
fish are aggregated for spawning.
Fishes have been hypothesized to form spawning aggregations for a
number of reasons including spawning habitat limitations, fertilization
enhancement, and swamping potential egg predators (Claydon 2005,
Domeier and Colin 1997). Although this behavior is exhibited over a
wide range of taxa, little is known about the length of time individuals
remain a part of a spawning aggregation or how the positions or territories
of individuals change over that period of time. The majority of work
on spawning aggregations has focused on coral reef and estuarine fishes.
Serranids, such as Epinephelus guttatus (Linnaeus) (Red Hind; Colin et
al. 1987, Whiteman et al. 2005) and E. striatus (Bloch) (Nassau Grouper;
Smith 1972); lutjanids such as Lutjanus cyanopterus (Cuvier) (Cubera
1Department of Biological Sciences, Clemson University, Clemson, SC 29634-0326.
2Current address - Institute of Biology, University of Iceland, Sturlugata 7, Is-101
Reykjavik, Iceland. 3US Geological Survey, South Carolina Cooperative Fish and
Wildlife Research Unit, Clemson University, Clemson, SC 29634-0372. *Corresponding
author - firstname.lastname@example.org.
476 Southeastern Naturalist Vol.7, No.3
Snapper; Heyman et al. 2005); and Centropomus undecimalis (Bloch)
(Common Snook; Lowerre-Barbieri et al. 2003) are known to form aggregations
that last weeks. It has been noted that individuals of these fishes
likely enter and leave aggregations (Lowerre-Barbieri et al. 2003, Whiteman
et al. 2005), but it is generally not known how long an individual
remains in an aggregation. It is unknown if riverine species in spawning
aggregation behave in a similar manner.
Moxostoma robustum (Cope) (Robust Redhorse) is an imperiled,
large, riverine catostomid found in three Atlantic slope drainages in North
Carolina, South Carolina, and Georgia (Bryant et al. 1996, Cooke et al.
2005). This species has been reported to form spawning aggregations
in shallow, flowing water over a clean gravel substrate (Bryant et al.
1996, Grabowski and Isely 2007a) similar to other members of its genus
(Jenkins and Burkhead 1994, Page and Johnston 1990). Males establish
territories that are vigorously defended from rivals. However, these fish
generally spawn as a “tremoring trio” consisting of a female flanked by
two males (Jenkins and Burkhead 1994, Page and Johnston 1990). The
formation of at least two Robust Redhorse spawning aggregations during
late spring has been documented in the lower Savannah River in South
Carolina and Georgia (Grabowski and Isely 2006, 2007a). Both aggregations
are associated with relatively small, mid-channel gravel bars.
Individuals make upstream migrations potentially in excess of 100 km to
reach these habitats and seem to have a high degree of fidelity to one of
the gravel bars (Grabowski and Isely 2006). It is not known how long the
same individuals remain in an aggregation or if they maintain the same
position throughout their residence in the aggregation. These are important
considerations for monitoring programs for this species, since sampling
programs are frequently focused on spawning aggregations (Robust
Redhorse Conservation Committee 2002), which could contribute bias in
As part of a larger study examining spawning habitat partitioning among
catostomids in the Savannah River, we marked and recaptured a significant
number of Robust Redhorse (Grabowski and Isely 2005, 2007a). Our objectives
were to provide an estimate of the size of the spawning population
of Robust Redhorse and to document the residence time and movements
of individuals within these spawning aggregations. We further attempted to
correlate residence time to size and interpreted differences in residence time
We captured Robust Redhorse from the lower Savannah River below
New Savannah Bluff Lock and Dam (NSBLD) in Augusta, GA, between
river kilometers (rkm) 300 and 280. Our efforts were restricted to the two
2008 T.B. Grabowski and J.J. Isely 477
mid-channel gravel bars that occur within this reach. These structures are
unique in the lower Savannah River and have been identified as important
spawning habitat for catostomids (Grabowski and Isely 2006, 2007a). The
upstream gravel bar (hereafter referred to as the upper gravel bar) is located
at rkm 299.4, immediately downstream of the tailrace of NSBLD. This
structure encompasses an area of approximately 25,500 m2 and consists of
a thin layer of gravel mixed with sand. The upper gravel bar attracts spawning
aggregations of Carpiodes spp. (carpsuckers), Minytrema melanops
(Rafinesque) (Spotted Sucker), Moxostoma collapsum (Cope) (Notchlip
Redhorse), and a relatively small number of Robust Redhorse (Grabowski
and Isely 2007a). The other gravel bar (hereafter referred to as the lower
gravel bar) is approximately 16 rkm downstream of NSBLD and is only
about 4200 m2 in area. The lower gravel bar is composed of a relatively
thick layer of coarse gravel over sand and appears to be used as spawning
substrate only by Robust Redhorse (Grabowski and Isely 2007a).
We captured Robust Redhorse primarily from the lower gravel bar using
an array of six prepostioned grid electrofishers (PGEs) as described in
Grabowski and Isely (2005). Sampling was conducted every other day from
02–14 May 2004 and daily from 07–18 May 2005. We measured, weighed,
and checked each captured fish for tags. We determined the sex of each
individual based upon development of secondary sexual characteristics
(such as nuptial tubercle development) and expression of gametes in reproductively
active individuals. We retained at least one digital photograph
of each individual to serve as a voucher. A passive integrated transponder
(PIT) tag (Biomark, Inc., Boise, ID) was implanted into the dorsal musculature
immediately posterior to the dorsal fin of untagged individuals prior
to release. Various state and federal agencies and researchers have tagged
Robust Redhorse on the lower Savannah River since 1998, so fish possessing
existing PIT tags were not re-marked, but were included in the study. All
fish were allowed to recover (approximately 20 minutes) prior to release.
Each PGE was operated a maximum of two times a day for a total of 20–30
seconds. The position of the center point of each PGE was recorded with a
WAAS-enabled handheld GPS unit (Garmin International, Olathe, KS) prior
to sampling, and this position served as the location of all Robust Redhorse
captured within each PGE.
Estimates of the total number and number of males in the spawning aggregation
were made for each year using a multiple census mark-recapture
model (Darroch 1958, Seber 1986). GPS waypoints were used to determine
distance between capture and recapture locations. Pearson’s correlation was
used to evaluate the relationship between residence time and total length of
individuals. A significance level of α = 0.05 was used for all tests. Means are
reported ± standard error.
478 Southeastern Naturalist Vol.7, No.3
A total of 169 Robust Redhorse have been captured and tagged (48 prior
to this study, 121 during it) in the lower Savannah River consisting of 109
males (mean = 590.7 ± 3.18 mm TL; range = 450–692 mm TL), 42 females
(mean = 636.2 ± 7.80 mm TL; range = 368–710 mm TL), and 18 individuals
of unknown sex (mean = 583.4 ± 11.98 mm TL; range = 420–621 mm TL).
We recaptured a total of 53 fish in 2004 and 2005. Three of these individuals
were tagged and recaptured on the upper gravel bar, while the remainder was
tagged and recaptured from the lower gravel bar. All fish were recaptured
from the gravel bar where they were originally tagged; we did not encounter
any movement between spawning aggregations. The majority of recaptured
individuals were male (n = 51). We estimated that the spawning aggregation
on the lower bar in 2004 consisted of about 83 individuals (95% C.I. = 66–
100) of which 51 (95% C.I. = 40–62) are male. For 2005, we estimated that
the spawning aggregation on the lower bar consisted of approximately 85
individuals total (95% C.I. = 74–95) of which about 56 (95% C.I. = 51–62)
During 2005, we captured 14 individuals multiple times. These individuals
ranged in size from 530 to 634 mm TL (mean = 593.5 ± 8.47 mm TL;
range = 530–634), and all were male. One individual was recaptured twice,
while the remaining fish were only recaptured once each. The recaptured
males averaged a minimum residence time in the spawning aggregation of
3.6 ± 0.24 days (range = 3–5 days) while the aggregation was present on the
lower gravel bar for 12 days in 2005. There was no significant relationship
between male size and residence time (r = 0.32; P = 0.25). No individuals
were recaptured in the same PGE, but three pairs of individuals were recaptured
together. The estimated mean distance between capture locations was
approximately 15.9 ± 1.29 m (range = 6.7–24.0 m).
Robust Redhorse in the lower Savannah River form large spawning aggregations
relative to the size of the available spawning habitat, leaving the
aggregation vulnerable to decreased reproductive success due to nest-site
superimposition (Grabowski and Isely 2007a) and the dewatering of nest
sites due to fl uctuations in river fl ow (Grabowski and Isely 2007b). Despite
the appearance of localized, high-density aggregations of Robust Redhorse,
it seems that the overall spawning population in the lower Savannah River
is small. It is also possible that the assumptions of no mortality or recruitment
inherent in the multiple census model (Darroch 1958, Seber 1986)
were violated. While it is unlikely that there was an appreciable amount of
mortality and recruitment occurring during the spawning season, the movement
of individuals into and out of the spawning aggregation may have an
effect similar to that of recruitment and mortality on the population estimate
2008 T.B. Grabowski and J.J. Isely 479
(Kendall 1999, Williams et al. 2001) at the small temporal and spatial scales
at which the model was employed. Our estimates may actually be a better
refl ection of the number of individuals present on the gravel bar at any given
time and probably underestimate the number of individuals that comprise the
aggregation over the entire spawning period. Further studies on the behavior
of Robust Redhorse in spawning aggregations are necessary to be able
to select a model to adequately address this turnover and more accurately
estimate population size.
The sensitivity of multiple census models to the turnover of
individuals in the spawning aggregation is potentially illustrated in our
mark-recapture data and the disparity between the estimates of the number
of males and the total number of individuals. Males seem to comprise
more than two-thirds of the actively spawning adults in the spawning aggregation
of a catostomid species at any given time. This finding is most
likely an artifact of sampling due to the mating system of redhorses where
a single female spawns as part of a triad with two males (Jenkins and Burkhead
1994, Page and Johnston 1990) and not an actual deviation from a
1:1 sex ratio in the population. Low catch rates of female catostomids in
spawning aggregations have been noted previously (Grabowski and Isely
2005, Vokoun et al. 2003). Females are likely present in the vicinity of
the aggregation in deep-water holding areas, but spend much less time
within the spawning aggregations since they do not establish or maintain
territories. The lack of female Robust Redhorse recaptures suggests they
are present in the spawning aggregation for much shorter periods of time
and thus are less susceptible to capture. Females of other species such as
Morone saxatilis (Walbaum) (Striped Bass; Carmichael et al. 1998) and
Plectropomus leopardus (Lacepède) (Coral Trout; Zeller 1998) have been
observed to spend less time within spawning aggregations than male conspecifics.
While it is true that fewer females were captured and tagged,
the number recaptured was still disproportionately small. It is also possible
that differential stress of capture and handling would cause female
Robust Redhorse to cease spawning as has been noted in other species
(Pankhurst and Van Der Kraak 1997, Zeller 1998). This possibility should
be investigated further even though no such effects have been noted previously
with radio-tagged Robust Redhorse (T.B. Grabowski, unpubl.
data; D. Coughlan, Duke Energy, Huntersville, NC, pers. comm.) when
handled in reproductive condition.
In contrast, male Robust Redhorse appear to remain within a spawning
aggregation for a considerable portion of the spawning period. We did
not find any evidence of a correlation between male length and residence
time. This finding may be an artifact of a small sample lacking representatives
from the smallest and largest size classes; however, our results are
consistent with other studies examining Coral Trout (Zeller 1998) and Gadus
morhua Linnaeus (Atlantic Cod; Robichaud and Rose 2003). Males
480 Southeastern Naturalist Vol.7, No.3
seem to establish multiple territories during their time within the spawning
aggregation. While it seems likely that the disturbance caused by our
sampling induced captured males to establish new territories, there are
two lines of evidence to suggest that males normally shift the locations
of their territories during their time in a spawning aggregation. First, the
spawning aggregations appeared to disband during the night and reform
each day. Individuals were not observed holding positions on the lower
gravel bar until several hours after sunrise (Grabowski and Isely 2005,
2007a). Additionally, flow fluctuations in the Savannah River would have
forced males to change position with changing water levels (Grabowski
and Isely 2007b). There is some evidence to suggest individual males
may recognize other males and form partnerships to establish and codefend
territories; however, our study lacked sufficient spatial resolution
to definitively say these males were immediately adjacent to one another
and co-defending a territory.
Based on our observations, we conclude that male Robust Redhorse
establish daily spawning territories based on prevailing conditions at the
spawning site and possibly their relative, time-specific, dominance hierarchy.
Females select their mates and likely expel their entire spawn over
a relatively short period (1–2 days). Although both sexes may spawn with
multiple partners, males expend their sperm supply over a longer period of
several days. The relatively short duration of the aggregation may be the
result of an extremely low abundance of adults.
We thank A. Aranguren, P. Ely, L. Hunt, S. Lamprecht, G. Looney, K. Meehan,
M. Noad, N. Ratterman, F. Sessions, J. Shirley, and S. Young for their assistance in
the field. E. Irwin and P. Sakaris provided technical support with grid electrofisher
design and operation. We thank E. Eidson and the Phinizy Swamp Nature Park for
logistical assistance. This manuscript benefitted from the comments and suggestions
of D. Coughlan, C. Gagen, and an anonymous reviewer. Cooperating agencies for
the South Carolina Cooperative Fish and Wildlife Research Unit are the US Geological
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