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2006 SOUTHEASTERN NATURALIST 5(4):693–710
Distribution and Abundance of American Eels in the
White Oak River Estuary, North Carolina
Joseph E. Hightower1,* and Cynthia Nesnow2,3
Abstract - Apparent widespread declines in abundance of Anguilla rostrata
(American eel) have reinforced the need for information regarding its life history
and status. We used commercial eel pots and crab (peeler) pots to examine the
distribution, condition, and abundance of American eels within the White Oak
River estuary, NC, during summers of 2002–2003. Catch of American eels per
overnight set was 0.35 (SE = 0.045) in 2002 and 0.49 (SE = 0.044) in 2003. There
was not a significant linear relationship between catch per set and depth in 2002 (P
= 0.31, depth range 0.9–3.4 m) or 2003 (P = 0.18, depth range 0.6–3.4 m). American
eels from the White Oak River were in good condition, based on the slope of a
length-weight relationship (3.41) compared to the median slope (3.15) from other
systems. Estimates of population density from grid sampling in 2003 (300 mm and
larger: 4.0–13.8 per ha) were similar to estimates for the Hudson River estuary, but
substantially less than estimates from other (smaller) systems including tidal creeks
within estuaries. Density estimates from coastal waters can be used with harvest
records to examine whether overfishing has contributed to the recent apparent
declines in American eel abundance.
Anguilla rostrata LeSueur (American eel) is an ecological generalist that
is widely distributed along the eastern coast of North America (Haro et al.
2000, Helfman et al. 1984). It is a catadromous species that matures in
freshwater, spawns in the Sargasso Sea, and dies after spawning (Haro et al.
2000, McCleave et al. 1987, Tesch 1977). The leptocephalus larvae migrate
northward and eastward through passive drift for about 1 year before metamorphosing
into glass eels (Helfman et al. 1987, McCleave et al. 1987). As
glass eels enter coastal rivers and estuaries, they transform into elvers
(Helfman et al. 1987). Once in a river or creek, they move into the yellow
phase of their life history (sexually immature juveniles of intermediate size),
which is the primary feeding and growth stage (Jessop 2000, Oliveira 1997).
Sexual maturity typically occurs at a length and age of 300–400 mm and 5–
13 years for males and 500–700 mm and 9–17 years for females (Helfman et
al. 1987, Oliveira 1999, Oliveira and McCleave 2000). At the final metamorphosis,
American eels enter the silver phase and begin their migration to the
1US Geological Survey, North Carolina Cooperative Fisheries and Wildlife Research
Unit, Department of Zoology, North Carolina State University, Raleigh, NC
27695-7617. 2North Carolina Cooperative Fish and Wildlife Research Unit, Department
of Zoology, North Carolina State University, Raleigh, NC 27695-7617. 3Current
address - USDA, APHIS, VS Eastern Regional Office, Raleigh, NC 27606.
*Corresponding author - firstname.lastname@example.org.
694 Southeastern Naturalist Vol. 5, No. 4
Sargasso Sea. American eels are assumed to be a panmictic (randomly
breeding) population due to their catadromous life-history strategy (Haro et
al. 2000). This assumption is supported by recent genetic analyses (Wirth
and Bernatchez 2003).
The development in the 1990s of substantial commercial fisheries for
elvers for overseas aquaculture, along with various indicators of declining
abundance, have led to concerns about the status and management of American
eel populations (Dixon 2003, Haro et al. 2000, Jessop 2000). Population
indices from the eastern United States and Canada suggest a widespread
decline in abundance, potentially due to habitat loss, overfishing, parasitism,
pollution, or a change in oceanic conditions (Haro et al. 2000). Habitat loss
could be substantial; Busch et al. (1998) estimated that about 84% of the
river and stream habitat once available to migratory fishes is now located
Population size or density of American eels has been estimated in many
systems, ranging from tidal creeks and estuaries to inland first-order
streams. Most of these estimates are from wadeable sites that are often
closed off using block nets. Only a few studies have been conducted within
estuaries (e.g., Bozeman et al. 1985, Ford and Mercer 1986, Morrison and
Secor 2004), even though most harvest of yellow eels occurs in coastal
waters (ASMFC 2000). The objectives of this study were to examine the
distribution, condition, and abundance of American eels within the White
Oak River estuary, NC. Information about American eel abundance and life
history in a coastal estuary should aid in evaluating the impact of fishing
versus other potential causes for population declines.
Sampling was conducted during May–June 2002 and July–August 2003
at estuarine sites within the White Oak River (Fig. 1), a brackish-freshwater
river that lies entirely in the southern outer coastal plain of North Carolina
(NCDENR 2005). Land to the west of the river is in private ownership,
whereas much of the watershed to the east is contained within the Croatan
National Forest. Most of the surrounding land area is forested, except for
substantial agricultural land use on the western side of the basin (NCDENR
2001). This section of the river is bordered by saltwater marshes and has
generally good water quality, although dissolved oxygen, pH, and fecal
coliform standards were exceeded in more than 20% of observations
(NCDENR 2005). Excess nutrient inputs to the mainstem river (especially
runoff from subdivisions and agricultural operations) are of concern
American eels were captured primarily using commercial eel pots made
of 1.3-cm wire mesh and measuring 62 cm in length with a 24-cm diameter
circular entry ring. Typically, pots were set in late afternoon with fresh
Callinectes sapidus Rathbun (blue crab) as bait and checked the following
morning. Location (UTM coordinates) and water depth were recorded at
2006 J.E. Hightower and C. Nesnow 695
each site a pot was fished. In 2002, American eels were also obtained from a
commercial crabber (R. Howell, Swansboro, NC), who fished about 200
peeler crab pots daily in this section of the estuary. Peeler pots, which
capture female blue crabs that are about to molt, have 2.54-cm mesh and are
not fitted with escape rings because there is no minimum-size regulation for
In 2002, there were 283 overnight sets of commercial eel pots on 12
occasions between May 16 and June 6. Pots were fished along the shoreline
(Fig. 1) at depths ranging from 0.9 to 3.4 m (mean depth = 1.6 m). The study
area for the capture-recapture experiment was assumed to be a 1.29-km2 area
downstream of Stella, NC, and demarcated by the NC Division of Marine
Fisheries upper boundary for commercial shrimping to the south and a
peninsula of land to the north (Fig. 1). American eels captured in peeler pots
were not included in the capture-recapture analyses because the specific
location of each capture was not recorded.
Figure 1. Location of White Oak River on North Carolina coastline. Inset A shows
2002 capture-recapture study area (denoted by linesacross the river channel) and eel
pot sampling sites where catch of American eels was zero (open circles) or greater
than zero (closed circles). Inset B shows grid design for 2003 capture-recapture
studies. American eels were captured at every grid location except the open circle in
grid #1. Numbers denote the order in which grids were sampled (#1: 15–28 July, #2:
29 July–8 August, #3: 10–19 August).
696 Southeastern Naturalist Vol. 5, No. 4
In 2003, 572 sets were made on 29 occasions between July 15 and
August 19 using commercial eel pots fished at depths ranging from 0.6 to
3.4 m (mean depth = 1.1 m). Most sets (434 of 572) were overnight, but
113 were 2-day sets and 25 were 3-day sets. A grid design (Morrison and
Secor 2004) was adopted for 2003 in order to characterize movement
patterns more rigorously. We established grids with four columns, five
rows, and a spacing of 100 m between pots (Fig. 1). The grid spacing was
thought to be important because Morrison and Secor (2004) reported that
American eels in the Hudson River were attracted to a bait plume and
immigrated into their grids of baited eel pots. They attempted to define a
grid spacing so that pots would be sampling the entire grid but would not
have overlapping areas of attraction. They concluded that a spacing of 200
meters would have low interference between pots, but would still insure
complete coverage of the grid. The average distance between our capture
and recapture sites in 2002 was 185 m (see Results). Using a grid spacing
of 100 m in 2003 may have resulted in some interference between pots, but
should provide more detailed information about movements. We used three
grids, with the order of grids designed to determine whether American eels
marked upstream or downstream of the current grid site would migrate into
that grid. Capture-recapture estimates of population size and density were
made separately for each grid, so that an American eel was considered
unmarked until captured within that grid. We converted each set of capture
histories into a summary table referred to as a full-m array (Burnham et al.
1987), and used that summary table to calculate mean recapture rate for
each number of days at large.
Captured American eels were measured (total length) to the nearest mm
and weighed to the nearest 25 g. Following the technique described by
Sorensen et al. (1983), each unmarked American eel received an individual,
three-digit freeze brand on the left midline using liquid nitrogen and a small
slotted cattle brander (Everhot Manufacturing Company, Maywood, IL).
After processing and recovery, American eels were released in the same area
Statistical analyses were done using SAS JMP© software and a significance
level of = 0.05. Correlation analysis was used to determine if catchper-
unit-effort (CPUE) was affected by depth at the sampling location, and
whether distance moved between capture and recapture sites was affected by
time at large, length at tagging, or weight at tagging. A one-way analysis of
variance (ANOVA) was used to test whether CPUE differed for 1-, 2-, and 3-
day soak times. Information about the factors affecting catch rates and
movements will aid in developing an appropriate capture-recapture design
for American eels. Length and weight data from initial captures were used to
construct a log10-scale length-weight relationship as a measure of summer
condition. An analysis of covariance was used to test for differences between
years in the length-weight relationship. A t-test was used to compare
2006 J.E. Hightower and C. Nesnow 697
mean lengths of American eels captured in commercial eel pots and crab
peeler pots. Summer movement patterns for recaptured American eels were
examined using ArcView GIS software with the Animal Movements extension
(Hooge and Eichenlaub 1997).
Estimates of population size were made using MARK capture-recapture
software (available at http://www.cnr.colostate.edu/~gwhite/mark/
mark.htm; Cooch and White 2003). The key question to address was whether
a closed or open population model was more appropriate (Otis et al. 1978).
Morrison and Secor (2004) detected immigration of American eels into grids
of baited eel pots fished in the Hudson River estuary, and used open population
models to estimate abundance. These open models are more correct in
biological terms, but are also more demanding because of the increased
number of parameters to estimate. We used closed models because our
capture histories were relatively sparse and because our data indicated that
most movement in the White Oak River estuary was over short distances
(see Results and Discussion sections). If substantial immigration occurred,
our estimates of population density would be too high. Also, because of the
sparse capture histories, we considered only simple models with capture
probabilities equal for initial-capture and recapture events.
Alternative models were compared using Akaike’s Information Criterion
(AIC), as corrected for small sample size (AICc; Burnham and Anderson
1998). The AICc criterion is useful for selecting the model that best explains
the variation in the data with the fewest parameters (Cooch and White 2003).
AICc values are on a relative scale, so alternative models can be compared
using differences in AICc values (AICc). Models having a AICc within
1–2 of the best model have substantial support, whereas those with a AICc
value of 3–7 have considerably less support (Burnham and Anderson 1998).
Models with a AICc > 10 have essentially no support and fail to explain
some substantial structure in the data.
Excluding within-year recaptures, 488 American eels were captured
during the two years (N2002 = 270, N2003 = 218). We did not observe in 2003
any American eels that had been freeze-branded in 2002. If the marks were
not retained between years, some of the 218 individuals from 2003 may have
been previously captured. Total length ranged from 224 to 709 mm (N =
487) and weight ranged from 25 to 800 g (N = 297). Based on length and
weight data for 296 individuals, there was a significant overall lengthweight
relationship (r2 = 0.85, P < 0.0001). The slope was not different
between years, although intercepts differed significantly:
2002: log10 weight = -6.616 + 3.408 log10 total length
2003: log10 weight = -6.683 + 3.408 log10 total length
The higher intercept that we observed for 2002 may have been due to the
earlier sampling (mid-May to early June) relative to 2003.
698 Southeastern Naturalist Vol. 5, No. 4
American eels were caught in 66 of 283 overnight sets (23%) of commercial
eel pots in 2002, resulting in a total catch of 100 and an average CPUE
of 0.35 American eels per set (SE = 0.045). Of the 100 captures, 19 were
recaptures of previously marked individuals. There was not a significant
linear relationship between catch per set and depth (P = 0.31). The distance
moved between capture and recapture sites in 2002 ranged from 52 to 1204
m and averaged 185 m (SE = 58.0 m). There was not a significant correlation
between distance moved and time at large (P = 0.22), length at tagging (P =
0.58), or weight at tagging (P = 0.48). All but one recapture occurred on the
same shoreline as the initial capture (Fig. 2). Days at large for recaptured
individuals ranged from 1 to 15.
There were 221 American eels caught in commercial peeler pots fished
from April to June 2002. American eels captured in the larger-mesh peeler
pots were substantially larger than those captured in the eel pots (mean = 422
mm for eel pots, mean = 544 mm for peeler pots; Fig. 3).
In 2003, American eels were caught in 211 of 572 sets (37%), resulting in a
total catch of 317. Of the 217 different individuals captured, 142 were caught
once, 56 were caught twice, 15 were caught three times, and 4 were caught four
times. The size distribution was similar to that observed in eel-pot sampling
during 2002 (Fig. 3). When grouped into daily intervals, there was a significant
difference in CPUE for 1-, 2-, and 3-day sets. CPUE was 0.49 American eels/
set (SE = 0.044) for 434 one-day (overnight) sets, 0.79 (SE = 0.087) for 113
two-day sets, and 0.56 (SE = 0.184) for 25 three-day sets. There was not a
significant linear relationship between catch per set and depth (P = 0.18).
Estimates of population size for 2002 were similar for a model allowing
time-specific capture probabilities (N = 204.6, 95% CI = 149–307) and a
reduced model with capture probability constant over time (N = 209.8, 95%
CI = 152–316). These estimates would apply to the size range vulnerable to
capture by eel pots, or about 300 mm and larger. Capture probabilities for
the full model were low and variable (Fig. 4). A reduced model with constant
capture probability had no support based on a AICc value > 10. Based on
the full model, the estimated density of American eels ( 300 mm) within the
study area was 2.0 per ha, assuming that all depths were effectively sampled
and used equally.
In 2003, there were 11 sampling occasions (days) for grid #1 with 48
captures of 32 different individuals. For grid #2, there were 10 occasions and
148 captures of 101 different individuals. For grid #3, there were 8 occasions
and 119 captures of 89 different individuals. Morrison and Secor (2004)
reported temporary trap-shy behavior for American eels at large 1 d after
tagging; however, we did not find any indication of a lower recapture rate for
newly tagged individuals. Based on averages over the three grids, mean
recapture rates were 0.11, 0.13, 0.09, 0.02, 0.02, 0.04, and 0.02 for tagged
American eels at large for 1–7 days, respectively.
Most recaptures were of American eels marked within the same grid
(Fig. 2). None of the 75 American eels recaptured in grid #2 had been
2006 J.E. Hightower and C. Nesnow 699
marked in grid #1 (a distance of about 2.5 km). Of the 35 recaptures in grid
#3, two had been marked in grid #2 (a distance of about 1.7 km) and 3 had
been marked in grid #1 (a distance of about 0.8 km). For the capturerecapture
estimate in grid #3, the animals first marked in another grid were
treated as unmarked at the first capture in grid #3.
For grid #1, the best model used a constant capture probability. Estimated
population size within the grid was 50.0 (95% CI = 39–76), with an
Figure 2. Movement
eels in the
White Oak River,
(panel A) and
J u l y – A u g u s t
2003 (panel B).
Open circles in
panel A denote
all 2002 sampling
numbers in panel
B denote the order
700 Southeastern Naturalist Vol. 5, No. 4
estimated capture probability of 0.08 (95% CI = 0.06–0.13). These estimates
would apply to American eels vulnerable to capture by eel pots, or about 300
mm and larger. The model with time-specific capture probabilities was
lower ranked but still feasible (AICc = 1.89). It resulted in a very similar
estimate of population size (N = 48.6, 95% CI = 39–74) and capture probabilities
ranging from 0.02 to 0.21 (Fig. 5). Based on the highest-ranked
model, the estimated density of American eels (300 mm and larger) within
grid #1 was 4.0 per ha.
For grid #2, the best model allowed capture probabilities to vary over
time. For that model, estimated population size within the grid was 161.8
Figure 3. Frequency distributions for total length of American eels captured in the
White Oak River, NC, using standard eel pots and peeler crab pots during 2002 and
eel pots during 2003.
Figure 4. Estimated
American eel capture
probabilities and 95%
based on eel pot sampling
within the White
Oak River, NC, in
2006 J.E. Hightower and C. Nesnow 701
Figure 5. Estimated
A m e r i c a n
eels, based on
eel pot sampling
the White Oak
River, NC, in
shown for grid
#1 for comparison,
model with a
to slightly better
fit the data.
(95% CI = 137–203) and density (300 mm and larger) was 12.7 per ha.
Capture probabilities were similar in magnitude to estimates for grid #1
and ranged from 0.05 to 0.19 (Fig. 5). The model with a constant capture
probability was not considered feasible based on a AICc value greater
702 Southeastern Naturalist Vol. 5, No. 4
For grid #3, the best model allowed capture probabilities to vary over
time. For that model, estimated population size within the grid was 170.6
(95% CI = 136–232) and density (300 mm and larger) was 13.8 per ha.
Capture probabilities were similar in magnitude to estimates for grids #1
and #2 and ranged from 0.02 to 0.16 (Fig. 5). The model with a constant
capture probability was not considered feasible based on a AICc value
greater than 10.
American eels were widely distributed in the White Oak River estuary,
based on eel pot and peeler pot captures. For example, we captured American
eels at 59 of 60 locations within grids sampled during 2003. We did not
detect any relationship between catch rate and water depth, although most
sites sampled were shallow (< 3 m). Another indication of a widespread
distribution is that American eels tend to occur in peeler pots set at all depths
(R. Howell, pers. comm.). Morrison and Secor (2004) sampled American
eels at depths of 2–10 m within the Hudson River estuary. Within Chesapeake
Bay, American eels occurred in trawl samples at depths ranging from
1 to 33 m, although there was some indication of a preference for depths of
4–10 m (Geer 2003).
American eels captured in this study ranged from 224 to 709 mm total
length. This size range is similar to that observed in other studies where 1.3-
cm wire mesh eel pots were used (Helfman et al. 1984, Morrison and Secor
2003). Helfman et al. (1984) compared eel-pot and rotenone samples from
the Altamaha River, GA. They found size distributions to be similar above
250 mm and concluded that eel pot sampling was representative of the
underlying population over the range 200–800 mm. However, some results
suggest that selectivity of eel pots may increase up to about 400 mm. Our
size distributions from eel-pot sampling were more or less flat from about
300 to 500 mm, although the distributions were irregular due to small sample
sizes. Modes of length distributions for Hudson River eel-pot sampling
ranged from 380 to 460 mm (Morrison and Secor 2003). In comparison to
these results, the mean length of American eels captured by trawling (with a
6.35-mm liner) in Chesapeake Bay from 1979 to 1999 was 268 mm (Geer
2003). Size selectivity in eel-pot sampling could be due to escape through
the trap mesh sizes or to behavioral interactions.
American eels larger than about 500–550 mm were uncommon in our
eel-pot and peeler-pot samples. A decrease in abundance at 500 to 550 mm
has been observed in several other systems, and has been attributed to the
outmigration of female silver eels (Morrison and Secor 2003, Oliveira 1999,
Oliveira and McCleave 2000). We did not determine the sex of our captured
American eels, although results of other studies (Barber 2004; Hansen and
Eversole 1984; Krueger and Oliveira 1997; Morrison and Secor 2003;
Oliveira 1997,1999) have shown that American eels with a total length
greater than 400 mm are almost all females.
2006 J.E. Hightower and C. Nesnow 703
American eels in the White Oak River estuary appeared to be in good
condition based on the observed length-weight relationship. The estimated
slope (3.41) was higher than the median slope (3.15) from other coastal or
inland locations (Table 1). A value of 3 for the length-weight exponent
indicates isometric growth (a consistent length-weight relationship),
whereas a value greater than 3 indicates allometric growth with an increase
in robustness over time. We did not account for gut fullness, which may have
been artificially increased due to feeding within the trap. Weight measurements
could be biased by about 2–6% due to gut fullness (Helfman et al.
1984, Morrison and Secor 2003).
The movement that we observed for marked individuals was primarily
over short distances. Both in the along-shore sampling of 2002 and the grid
sampling of 2003, most recaptures occurred at sites close to the original
tagging location. Limited movement of American eels has been observed in
several estuarine sites along the east coast of the United States (Bozeman et
al. 1985, Ford and Mercer 1986, Helfman et al. 1983, Morrison and Secor
2003). The level of short-term movement in the White Oak River estuary is
probably related to the distribution of prey. Observations from peeler pot
Table 1. Estimated slope of length-weight relationship for American eels collected at sites from
Newfoundland to Georgia.
Study Location Slope
Gray and Andrews (1971) Indian Pond, NF, Canada 3.44
Topsail Barachois, NF, Canada 3.44
Burnt Berry Brook, NF, Canada 3.27
Salmon River, NF, Canada 3.08
Jessop (1987) Medway, NS, Canada 2.94
LaHave, NS, Canada 3.06
Eel Brook Rivers, NS, Canada 3.17
Oliveira and McCleave (2000) Sheepscot River, ME 3.15
Medomak River, ME 3.07
East Machias River, ME 3.07
Pleasant River, ME 3.07
Morrison and Secor (2003) Hudson River, New York 3.20
Moser et al. (2000) Roanoke River - inland, NC 3.56
Currituck Sound - northern, NC 3.48
Currituck Sound - southern, NC 2.98
Pasquotank River mouth, NC 3.26
Yeopim River, NC 3.34
Roanoke Sound, NC 3.04
Chocowinity Bay, NC 3.16
Neuse River, NC 3.04
White Oak River, NC 3.20
Beaufort area, NC 3.14
Cape Fear River - inland, NC 2.94
This study White Oak River estuary 3.41
Harrell and Loycano (1982) Cooper River, SC 3.34
Michener and Eversole (1983) Charleston Harbor, SC 3.01
Hansen and Eversole (1984) Cooper River, SC 3.07
Helfman et al. (1984) Altamaha River, GA 3.25
704 Southeastern Naturalist Vol. 5, No. 4
fishing indicate that American eels shift their distribution within the estuary
based on changes in salinity and prey species such as blue crabs (R. Howell,
pers. comm.). Bozeman et al. (1985) suggested that movement of American
eels may be reduced in estuaries compared to temperate freshwater lakes
because of the high productivity of estuaries.
Our estimates of population density from grid sampling in 2003 ranged
from 4.0–13.8 per ha. The estimates would apply to American eels roughly
300 mm and larger, based on observed size distributions. We obtained a
lower density estimate (2.0 per ha) in 2002, but there was greater uncertainty
about the area sampled. The grid sampling in 2003 provides a clearer
basis for the default area sampled. Our density estimates from grid sampling
are similar to those obtained from the Hudson River estuary (1–30
per ha; Morrison and Secor 2004). Both studies are based on eel-pot sampling
of estuarine shoal waters with bottom sediments of fine clay and silt.
Both sets of estimates are substantially lower than estimates from other
(smaller) streams and rivers, including some estuaries. Oliveira and
McCleave (2000) used three-pass electrofishing within blocked sections of
four wadeable rivers in Maine, and obtained estimates ranging from 840 to
2180 per ha. Their estimates included all individuals greater than 100 mm
total length. Oliveira (1997) used five-pass electrofishing within blocked
sections of a wadeable Rhode Island river and obtained estimates ranging
from 450 to 3230 per ha. His estimates included all individuals greater than
160 mm total length. Ford and Mercer (1986) used capture-recapture methods
and estimated that the density of American eels > 150 mm was 875 per
ha in several Massachusetts tidal creeks. The average density of American
eels in wadeable Maryland tributaries to Chesapeake Bay, based on twopass
electrofishing of blocked stream segments, was 430 per ha (Wiley et
al. 2004). Average densities for medium (250–374 mm) and large (> 374
mm) American eels in wadeable coastal plain streams in Virginia were 370
and 52 per ha, respectively (Smogor et al. 1995). Sampling was done using
two passes with an electric seine in blocked sections of streams. The
estimated density of American eels 200 mm in a Georgia tidal creek,
based on capture-recapture methods, was 182 per ha (Bozeman et al.
1985). As noted by Morrison and Secor (2004) for the Hudson River, any
adjustments for the size range of American eels sampled would not account
for the substantial difference between the White Oak estimates and those of
The difference in density between the White Oak River estuary and
other systems (except for the Hudson River estuary) may be even greater
than estimated here, if immigration into our study areas was a significant
bias. In 2003, 5 of 35 recaptures at grid #3 were marked at the other two
grids, at distances of 0.8 and 1.7 km. This movement between grids could
be strictly random and could have occurred prior to the start of grid sampling,
but likely indicates that the baited pots are sampling a larger area
2006 J.E. Hightower and C. Nesnow 705
than defined by the grid. Immigration would cause the closed-model population
estimates to be too high because the area sampled would be greater
than the assumed size. The movement between grid sites that we detected
in 2003 suggests a limited response to the presence of baited eel pots. Also,
shorelines provided boundaries that may have limited migration somewhat,
particularly for grids 1 and 2. On the other hand, the grids were separated
by 0.8–2.5 km, so a higher rate of immigration could have occurred for
unmarked animals closer to each grid. Morrison and Secor (2004) observed
substantial immigration into grids of baited pots in the Hudson River
estuary, and they used open population models to allow for immigration
into their grids. We did not consider open models to be a feasible alternative
in our study because of our relatively low capture probabilities. We
also did not account for the trap-shy behavior reported by Morrison and
Secor (2004). Our recapture rates were similar for American eels at large
for 1–3 days, whereas Morrison and Secor (2004) found a lower rate of
recapture one day after tagging. Possible explanations for the trap-shy
behavior that they observed were a short-term effect of the anesthetic,
reduced interest in feeding after feeding in the trap, and overlooking the
brands (Morrison and Secor 2004). Bozeman et al. (1985) reported a significantly
longer interval from tagging to recapture for American eels with
full (mean 2.83 d) versus empty (mean 2.02 d) guts.
It does not appear that harvest would account for the relatively low
density estimates in the White Oak and Hudson rivers, compared to other
systems. Commercial fishing for American eels is limited in the White Oak
River (R. Howell, pers. comm.), and there is only a small bait fishery in the
Hudson River (Morrison and Secor 2003). It may be that estuarine shoal
areas with soft sediments support lower densities of American eels than
other habitat types, perhaps due to the low habitat complexity. Geer (2003)
reported that trawl catches of American eels in Chesapeake Bay tended to
occur in association with detritus, hydroid, and shell habitats. Also, catches
were considerably higher within tributaries near or above the freshwater
interface compared to the mainstem Chesapeake Bay. Wiley et al. (2004)
reported that sites in Maryland streams with only one or two velocity-depth
regimes (e.g., only deep-slow, shallow-slow) had significantly lower densities
than sites with more velocity-depth regimes. It would seem reasonable
that the amount of cover would be an important variable, although
Wiley et al. (2004) did not detect a relationship between American eel
abundance and the amount of instream habitat. Smogor et al. (1995) detected
a positive relationship between American eel density and cover for
small (61–249 mm) and medium-sized (250–374 mm) but not large (> 374
mm) American eels.
For future capture-recapture studies, a key step would be to minimize the
biases associated with use of baited pots, such as immigration into the study
area (bait attraction) and trap-shy or trap-happy behavior. Carton and
706 Southeastern Naturalist Vol. 5, No. 4
Montgomery (2003) found that Anguilla dieffenbachii Steindachner (longfin
eel)and Anguilla australis Richardson (shortfin eel) were attracted to the
scent of bait, and that initial detection of a food odor resulted in direct
upstream movement toward the odor source. The eels in their study appeared
to detect and track the margins of the odor plume. Capture-recapture studies
could be done using unbaited fyke nets, which are inexpensive and easy to
use (Jellyman and Graynoth 2005). Unbaited fyke nets were found to be
relatively ineffective for trapping longfin and shortfin eels (Jellyman and
Graynoth 2005), but have proven effective in American eel research in the
Maritime Provinces (Cairns et al. 2004; D. Cairns, Fisheries and Oceans
Canada, Charlottetown, PEI, Canada, pers. comm.).
Another approach for reducing these biases would be to use different
methods for marking and recapture sampling (Morrison and Secor 2004,
Ricker 1975). For example, baited pots or traps have been used in most
estuarine studies targeting American eels (e.g., Bozeman et al. 1985, Ford
and Mercer 1986, Morrison and Secor 2003), but a bottom trawl using a
6.35-mm liner regularly captured American eels in the Chesapeake Bay
(Geer 2003). Another approach for reducing biases associated with baited
pots would be to use permanent marks (e.g., passive integrated transponder
or coded wire tags) so that sampling could be conducted over short periods at
longer intervals (e.g., one overnight set per week). This would eliminate bias
due to a short-term trap-shy behavior as well as gradual immigration into a
study grid due to the bait plume.
One of the key gaps in information listed in the Fishery Management Plan
for American eel (ASMFC 2000) is regarding the impacts of fishing. Traditional
stock assessment methods such as catch-at-age analysis (Hilborn and
Walters 1992) are not feasible because age composition is not routinely
determined for American eel catches. Spatially-based management using
capture-recapture methods may be a more practical alternative. For example,
Morrison and Secor (2004) used density estimates from grid sampling to
estimate overall abundance of American eels in the Hudson River estuary (for
areas 2–10 m deep). Capture-recapture estimates provide a much stronger
basis for management than relative abundance indices, because population
density and harvest within a particular area can be used to estimate the harvest
rate. Management can then be based on the difference between the current
harvest rate and a target rate, based on American eel life-history characteristics.
A similar approach has been proposed for management of the A. anguilla
Linnaeus (European eel), based on an estimated carrying capacity of 10 kg per
ha in freshwater habitats (ICES 2001). The management goal in that approach
would be to maintain spawning escapement of silver eels above some percentage
(e.g., 30%) of carrying capacity (ICES 2001). It is interesting to note that
the density estimates for the White Oak River estuary (4–14 per ha) would
correspond to about 1–3 kg per ha, based on the average weight of American
eels captured during 2002–2003.
2006 J.E. Hightower and C. Nesnow 707
The first step in carrying out this spatially-based management approach
would be to develop geographic information systems (GIS) layers
for annual harvest by habitat type or management unit. Capture-recapture
studies within these habitat or management units would then provide the
quantitative information needed for effective management of this important
We thank the US Forest Service who provided funding for this research. We also
appreciate the contributions of field technicians P. Hubert, N. Jeffers, S. Minis, and
A. Price. Helpful comments on an earlier version of this manuscript were provided by
D. Fox and W. Starnes. Advice and assistance was also provided by J. Armstrong, J.
Crutchfield, R. Graham, D. Hewitt, B. Jessop, C. Oakley, W. Patrick, W. Pine, K.
Pollock, A. Read, P. Read, D. Secor, C. Vida, M. Whitacre, and W. Wise. The
College of Agriculture and Life Sciences, NC State University, assisted with travel
expenses for the field studies. We greatly appreciate the assistance of R. and G.
Howell of Swansboro, who provided logistical support, eels captured during crabbing
operations, and valuable background information about American eels and
commercial fishing operations.
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