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2018 SOUTHEASTERN NATURALIST 17(3):438–455
Health and Genetic Structure of the American Eel in Florida
Kimberly I. Bonvechio1,*, Brandon Barthel2, and Jessica Carroll2
Abstract - We collected biological, health, and parasite-infection (e.g., Anguillicoloides
crassus [a swim-bladder nematode]) data for 609 Anguilla rostrata (American Eel), and population-
genetics data from a subset of 299 individuals captured throughout Florida from 2014
through 2016. We did not find evidence of genetic differentiation between groups and concluded
that a single American Eel stock extends through the Atlantic-coast drainages into the
Gulf of Mexico. We found spatial and seasonal differences in swim-bladder condition and in
prevalence and incidence of A. crassus infection in Florida’s American Eels. Average values
for the ratio of swim-bladder to length and health indices were similar between regions,
among seasons, and between uninfected and infected American Eels. Information gathered by
this study will be important in future conservation and management efforts.
Introduction
Anguilla rostrata Lesueur (American Eel) is a facultatively catadromous species
(McCleave and Edeline 2009) with a large geographic distribution in the Western
Atlantic Ocean, Caribbean Sea, and Gulf of Mexico drainages, stretching from
Greenland (Møller et al. 2010) to Venezuela (Benchetrit and McCleave 2016).
Based on a review of available data on larval American Eels, Miller et al. (2015)
suggested that the American Eels spawn in the western Sargasso Sea, between
longitudes 75°W and 60°W, from February through April. After hatching, the
transparent, leaf-like leptocephali are transported by currents to areas throughout
the species’ range in the northwestern Atlantic, Caribbean Sea, and Gulf of Mexico.
Before they enter coastal estuaries, American Eels metamorphose into the glass-eel
life stage, in which they transform to an eel-like morphology but still lack pigment.
As they gain pigment and increase in size, the eels transform into elvers and immature
yellow eels, which can remain in marine or estuarine waters or travel upstream
into lakes and rivers (McCleave and Edeline 2009). It can take as long as 30 years
for individuals to mature into silver eels, at which point they migrate back to the
Sargasso Sea to spawn (Helfman et al. 1987, McCleave and Edeline 2009).
Based on a restriction-fragment–length polymorphism analysis of the mitochondrial
DNA (mtDNA) genome, Avise et al. (1986) reported that there was no genetic
divergence between American Eels collected from a number of sites from Maine
to Louisiana, including the Carabelle River from the Gulf coast of Florida. Côté
et al. (2013) conducted a more expansive study, using 18 microsatellite markers
to genotype American Eels from Canada to northeast Florida. Results from both
1Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute,
Eustis Fisheries Lab, Eustis, FL 32726. 2Florida Fish and Wildlife Conservation Commission,
Fish and Wildlife Research Institute, St. Petersburg, FL 33701. *Corresponding
author - Kim.Bonvechio@MyFWC.com.
Manuscript Editor: Benjamin Keck
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studies were consistent with a panmixia hypothesis, which posited that a single
breeding-stock spawned in the Sargasso Sea and the resulting larvae dispersed into
drainages across its range. The more robust study by Côté et al. (2013), however,
did not include tissue samples collected south of St. Augustine, FL, and excluded
American Eels from Gulf of Mexico and Caribbean Sea drainages. While the
mtDNA analysis conducted by Avise et al. (1986) would have been expected to
identify multiple breeding-populations that had been isolated from one another for
an extended period, it might not have had the resolution needed to detect finer-scale
differentiation. Studies using microsatellite DNA markers are more likely to detect
fine-scale genetic differences because they typically include multiple, independent
nuclear loci that are inherited in a Mendelian fashion, while mtDNA surveys are
restricted to a single, maternally transmitted DNA molecule. It is possible to imagine
a scenario in which offspring of American Eels that spawned in one portion of
the Sargasso Sea are more likely to settle in the Gulf of Mexico than in other areas.
This situation could lead to fine-scale genetic differences that could be detected at
a minority of the nuclear genetic loci included in a microsatellite-markers study,
while there would continue to be sufficient gene flow to prevent mtDNA divergence
between regions. Further, Miller et al. (2015) analyzed nearly 10,000 records of
American Eel larvae collected from locations throughout the Atlantic basin from
1863 to 2007 and found that a small number (~0.2 %) had been collected in the Gulf
of Mexico and western Caribbean Sea, which suggested that spawning might have
occurred outside the Sargasso Sea. In Florida, American Eels inhabit drainages on
both the Atlantic and Gulf of Mexico coasts and have been assumed to be part of the
same breeding stock. But whether spatial structure exists between American Eels
along both coasts of the state has not been evaluated using methods that are best
suited to identify such a structure.
The American Eel faces a myriad of threats, which include changing oceanic
currents, habitat degradation, disease, and overfishing (ASMFC 2012). In 2006,
approximately 24% of American Eels subsampled from the commercial fishery in
the St. Johns River, FL, were found to be infected with the nonnative swim-bladder
nematode Anguillicoloides crassus (Kuwahara, Niimi, & Hagaki; Florida Fish and
Wildlife Conservation Commission [FWC], Eustis, FL, unpubl. data). Although
not considered harmful to its native host, Anguilla japonica Temminck & Schlegel
(Japanese Eel), this parasite is considered pathogenic in other anguillids (Kennedy
2007). It was first observed in an American Eel at an aquaculture facility in Texas
and subsequently, in 1995, in a wild individual in South Carolina (Fries et al. 1996,
Johnson et al. 1995). The nematode’s range extends north to Canada, where it was
first observed in wild American Eels in 2007 (Rockwell 2009). An American Eel
becomes infected by eating an intermediate or paratenic host of the larval parasite.
The larval parasite travels to the swim bladder, where it completes its life cycle (De
Charleroy et al. 1990), causing hemorrhaging, thickening of the swim-bladder wall,
and increased stress to the American Eel (Kennedy 2007, van Banning and Haenen
1990). Laboratory studies have shown that the life cycle is completed in about 3
months at 20–22 °C (Moravec et al. 1994), but development time can vary based
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on salinity (Kennedy and Fitch 1990, Kirk et al. 2000), temperature (Knopf et al.
1998), and availability of suitable hosts (Kirk et al. 2000). Nematode infections
may affect the function of the swim bladder and impact the eel’s ability to withstand
environmental stressors, such as low-oxygen situations, or to migrate to the
Sargasso Sea to spawn (Lefebvre et al. 2007).
Following both its 2012 American Eel Benchmark Stock Assessment and its 2017
Assessment Update, the Atlantic States Marine Fisheries Commission declared that
the American Eel stock was in a state of decline (ASMFC 2012, 2017). The American
Eel is managed as a single stock; thus, it is important to have information about
the species throughout its range to develop effective conservation and management
strategies. Unfortunately, little is known about the American Eel in Florida, particularly
those in river systems that drain to the Gulf of Mexico. Collecting information
on the health, growth, and genetic structure of American Eels in Florida’s freshwater
systems would provide much-needed insight into how the species persists within
its southern range. We developed a study that was intended to: (1) assess the overall
health of the American Eel in different systems throughout Florida, with focus on the
presence and severity of A. crassus infestation; (2) collect data on basic biological
parameters, including age and growth of American Eels inhabiting Florida Waters;
and (3) assess the population genetics of American Eels inhabiting Florida waters,
with emphasis on comparing individuals between Gulf and Atlantic drainages. This
study was expected to provide managers with critical information on the biology, life
history, and status of the American Eel in Florida.
Methods
American Eels were collected from 72 bodies of water from February 2014
through May 2016. Most specimens were collected during electrofishing surveys
for FWC’s Freshwater Long-term Monitoring Program, but other American Eels
were collected incidentally during sampling for other projects, and 1 was caught
by an angler. Biologists were instructed to freeze any American Eels that they
encountered until the specimen(s) could be processed in the laboratory. For each
eel, we recorded total length (nearest mm), swim-bladder length (nearest mm),
total-body weight (nearest g), liver weight (nearest mg), and spleen weight (nearest
mg). From these measurements, we calculated each individual’s length-ratio index
(LRI; swim-bladder length/eel length; Palstra et al. 2007), hepato-somatic index
(HSI; liver weight/eel weight), and spleen-somatic index (SSI; spleen weight/eel
weight). We also assessed the condition and infection of the swim bladders using
swim-bladder degenerative index (SDI) scoring for opacity, thickness, and presence
of pigmentation and exudate (Lefebvre et al. 2002).
We used sagittal otoliths to determine eel age. We removed cleaned, and dried
both sagittal otoliths from each eel and stored them in vials. We processed the left
otolith for age determination unless it was broken through the core or missing, in
which case, the right was processed. We marked the core with a pencil, embedded
the otolith in a 2-part epoxy, and mounted it on card stock with hot glue. We
sectioned otoliths with a Buehler Isomet low-speed saw (Buehler, Lake Bluff, IL)
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equipped with 4 equally spaced diamond wafering-blades. One transverse cut with
this multiblade technique yields three ~400-μm–thick sections that encompass
both the core and the entire region surrounding the core (VanderKooy 2009). After
processing, we mounted sections on glass slides with Flo-Texx® mounting medium.
We employed at least 2 blind reads to examine sectioned otoliths on a stereomicroscope
using transmitted light. These reads were conducted either by a single reader
examining the otolith on separate occasions, or by 2 readers each examining the
otolith once. We conducted a third read to resolve the discrepancy if age estimates
differed between reads.
For determinations of overall eel health and index of swim-bladder condition,
we separated American Eels into 5 regions: St. Johns River, Other Northeast,
West, Panhandle, and South (Fig. 1). The St. Johns River region included any
American Eels collected from the main river and its tributaries and was included
as a distinct region, separate from other systems in northeast Florida, because
the commercial American Eel fishery in Florida currently only operates in the
St. Johns River system. Unlike the first 2 regions where American Eels ingress
from the Atlantic Ocean, the Panhandle and West regions refer to American Eels
collected from Gulf of Mexico drainages. The West region extends from the
Steinhatchee River south to Charlotte Harbor; the Panhandle extends west of
the Steinhatchee River to the Perdido River (Florida/Alabama border). American
Eels may enter south Florida systems, such as Lake Okeechobee and the
Figure 1. Map depicting American Eel collection locations and regions.
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Kissimmee River via rivers and canals that connect with either the Atlantic or the
Gulf; thus, we considered this area separately.
We calculated the average LRI, HSI, SSI, and number of swim-bladder
nematodes and determined prevalence (% of eels with active infections) for all
specimens by region and, when possible, by season (i.e., winter = January–March,
spring = April–June, summer = July–September, fall = October–December) within
regions. American Eel samples were collected incidentally from non-target surveys
in a nonsystematic way, so many regions and seasons had low sample sizes that
precluded formal statistical analysis.
We genotyped a subset of the collected American Eels. For each specimen,
we removed and placed into a vial of 95% ethanol a small piece of fin tissue. We
extracted DNA using PUREGENE DNA purification kits (Gentra Systems, Minneapolis,
MN) in accordance with the manufacturer’s directions. We adjusted
recovered DNA to a concentration of 50 ng/μl with low tris-EDTA buffer and ran
6 polymerase chain reactions (PCR) to amplify 17 of the microsatellite markers
Côté et al. (2013) used to study the American Eel. The PCR profile included initial
denaturation at 95 °C for 5 min followed by 30 cycles of 30 s of denaturation at 94
°C, 90 s of annealing at 57 °C, and an extension of 60 s at 72 °C. There was a final
extension of 30 min at 72 °C. The forward primers had 5' labels of FAM, HEX, or
NED, and we used a Genescan 500 ROX-size standard run on an ABI 3130 genetic
analyzer (Applied Biosystems, Foster City, CA) to visualize allele sizes. We conducted
allele scoring with GENEMAPPER version 3.7 (Applied Biosystems).
We excluded specimen genotypes from data analyses if they displayed evidence
of DNA contamination (i.e., when we observed >2 alleles at a locus) or were genotyped
at fewer than 13 loci. We calculated observed and expected heterozygosity
(HO and HE, respectively) using the program ARLEQUIN (Excoffier et al. 2005)
and estimated the inbreeding coefficient (FIS) with the program GENETIX (Belkhir
et al. 1996–2004). We tested whether each locus conformed to Hardy–Weinberg expectations
using the exact-test approach in ARLEQUIN. We determined statistical
significance (α < 0.05) following sequential Bonferroni correction for all analyses
involving multiple comparisons (Rice 1989).
The primary objective of the genetic component of the study was to determine
whether American Eels captured in Atlantic coast drainages were genetically different
from those captured in streams that drain into the Gulf of Mexico. We pooled
specimens from the 2 sets of drainages into different groups (Atlantic vs. Gulf
regional groups of American Eels). A third regional group was composed of specimens
captured in Lake Okeechobee and south Florida canals because these systems
are hydrologically connected to both the Atlantic and Gulf coasts. In addition to the
comparison of the pooled specimens, we evaluated whether there was evidence of
differentiation between American Eels captured in the St. Johns River (an Atlantic
drainage) versus the Apalachicola River (a Gulf drainage) because these systems
had much larger sample sizes than any of the other drainages. Although we anticipated
that this analysis would produce similar results to comparisons involving
the Atlantic and Gulf coast pooled groups, it was possible that pooling American
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Eels from different drainages into groups may have obscured differences that
could be identified only when specimens from the 2 river systems were considered
separately. We employed the program GENEPOP (Raymond and Rousset 1995) to
test for genetic differentiation (i.e., differences in allele frequency) between the
groups and river systems at each individual locus. We evaluated multilocus genetic
differentiation by estimating pairwise FST values between the groups and river systems
using ARLEQUIN.
We also estimated the effective population size, or number of American Eels contributing
offspring to the next generation, assuming all the American Eels belong to a
single population. We made the estimate using the linkage-disequilibrium estimator
from the program NeESTIMATOR (Do et al. 2014); the critical value was set to 0.02,
and we employed the jackknife method to calculate 95% confidence intervals.
Results
A total of 609 American Eels was collected from 72 systems throughout Florida
(Fig. 1); the majority (78%) were collected from the St. Johns River (SJR) and
Panhandle regions (Table 1). Sample size varied by season, with summer samples
generally the most poorly represented and even lacking for some regions (Table 1).
Broad variation in size (136–804 mm total length) and age (0–12 y) of American
Eels were represented in the collection, encompassing the elver (immature) stage
to the silvering (mature) stage (Figs. 2, 3; Table 1). The 5 silvering females varied
in size from 606 mm to 800 mm TL and in age from 4–7 y. Two of these eels were
collected in the South and West regions in December, and the remaining individuals
were collected from the Panhandle region in October or November. Of the 589 aged
individuals, the majority (69%) were aged 3–5 y (Fig. 3), and there was considerable
variation in the size at age for most age classes (Fig. 4).
The parasitic nematode A. crassus was present in as many as 78% of the
American Eels in a single region’s sample and was most common in northeastern
Florida (Table 2). The Panhandle American Eels had a low (13%) prevalence of
infection, and all but 1 American Eel in the West and South regions were uninfected
Table 1. Sample characteristics for American Eels collected from freshwater systems in 5 regions of
Florida from 2014 through 2016. Characteristics include size variation, age variation, seasons when
collections occurred (ALL = all seasons, WIN = January–March, SPR = April–June, FALL = October–
December), and n = sample size.
Continental
Total length age
min–max min–max
Region (mm) (y) Seasons collected Total n Genetics n
St. Johns River 147–669 1–9 ALL 270 110
Other Northeast 299–577 4–7 WIN, SPR, FALL 18 0
Panhandle 136–804 0–12 ALL 206 133
West 266–800 1–8 WIN, SPR, FALL 50 31
South 158–758 0–11 ALL 65 25
Overall 136–804 0–12 ALL 609 299
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(Table 2). Average intensity (number of parasites/eel) followed the same pattern
as prevalence across regions (Table 2). In infested American Eels, the number of
nematodes ranged from 1 to 52, with average intensity ± 1 SD higher for SJR (4.34
± 6.32 nematodes/eel) and Other Northeast (6.14 ± 7.67 nematodes/eel) than the
Panhandle (2.27 ± 1.91 nematodes/eel). In addition to regional differences, parasite
intensity and prevalence may also vary seasonally. In particular, we detected a
measurable drop in both parasite prevalence and intensity during the summer, but
low sample size (n = 0–6) in the SJR region precluded formal analysis (Fig. 5).
Figure 2. Length–frequency distribution of American Eels collected from Florida’s freshwaters
during 2014–2016.
Figure 3. Age distribution of American Eel collected from Florida’s freshwaters during
2014–2016. The continental age of the fish is represented here (ICES 2009).
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Table 2. Summary statistics for prevalence (% of fish with active infection) and intensity (number
of parasites per fish) of A. crassus infection and swim-bladder condition (length ratio index [LRI]
and swim-bladder degenerative index [SDI]) and fish health (hepato-somatic index; HSI and spleen
somatic index; SSI) indices for American Eel collected from freshwater habitats within 5 different
geographic regions of Florida. The averages (standard deviation) are presented for each index and
intensity, % of fish for prevalence, and number of fish collected (n) for each region and for all fish
combined.
Intensity Prevalence
Region LRI SDI HSI SSI (#/fish) (%) n
Other Northeast 0.17 (0.05) 1.39 (0.92) 0.93 (0.16) 0.13 (0.06) 4.78 (7.21) 78 18
Panhandle 0.17 (0.04) 0.32 (0.66) 0.83 (0.41) 0.11 (0.07) 0.29 (1.01) 13 206
South 0.19 (0.03) 0.02 (0.12) 0.91 (0.19) 0.09 (0.04) 0.05 (0.37) 2 65
St. Johns River 0.18 (0.05) 1.31 (1.15) 1.04 (0.25) 0.17 (0.09) 2.21 (5.00) 51 270
West 0.16 (0.04) 0.04 (0.20) 0.94 (0.26) 0.09 (0.04) 0.00 (0.00) 0 50
All fish combined 0.18 (0.04) 0.73 (1.03) 0.94 (0.32) 0.13 (0.08) 1.22 (3.77) 29 609
Figure 4. Total length (mm) and continental age of American Eel specimens collected from
4 regions of Florida during 2014–2016. Data points above the dashed line at 400 mm are assumed
to represent females, based on Harrell and Loyacano (1980). With an arbitrary birth
date of 1 January, we adjusted age for time of year fish were collected (i.e., age + [days past
1 Jan/364]).
Figure 5 (following page). Average swim-bladder condition (LRI, SDI) and overall health
(HSI, SSI) indices for American Eels collected from 4 regions of Florida from 2014
through 2016. Values are calculated and presented by (winter = January–March, spring =
April–June, summer = July–September, and fall = October–December). Sample sizes (n),
prevalence (% of fish with active infection), and intensity (number of parasites per fish) of
A. crassus are also presented.
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Figure 5. [Caption on preceding page.]
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We calculated 2 swim-bladder–condition indices (SDI, LRI) and 2 health indices
(HSI, SSI) to assess the overall American Eel health. Aside from the SDI, average
index-values were similar across regions and between infected and uninfected
American Eels within regions (Tables 2–4). Presumably due to damage to the swim
bladder from present and past A. crassus infections, the average SDI was higher
for areas where the nematode is known to exist (Table 2). The SDI has a maximum
value of 6; the largest value observed for American Eels in this study was 5 (Fig. 6).
Even when we did not observe nematodes, the SDI value was as high as 4, and in
the South and West regions, all SDI scores were 0, with the exception of 1 infected
individual that had a score of 1 (Fig. 6). In addition to spatial differences, we also
observed seasonal effects in the average SDI for the Panhandle and SJR regions,
Table 4. Summary statistics, by season, for overall and swim-bladder health indices for Panhandle
American Eels with (present) and without (absent) active A. crassus infections. We include the mean
± 1 SD and sample size (n) for length-ratio index (LRI), swim-bladder degenerative index (SDI),
hepato-somatic index (HSI), and spleen somatic index (SSI). Eels were collected from March 2014
through May 2016 and are grouped by season (winter = January–March, spring = April–June, summer
= July–September, and fall = October–December). No infected eels were collected in summer.
Infection
Season Status n LRI SDI HSI SSI
Winter Absent 41 0.16 ± 0.04 0.32 ± 0.61 0.95 ± 0.77 0.10 ± 0.08
Present 9 0.17 ± 0.04 1.78 ± 0.44 0.82 ± 0.42 0.10 ± 0.05
Spring Absent 14 0.18 ± 0.04 0.43 ± 0.65 1.07 ± 0.24 0.10 ± 0.03
Present 3 0.18 ± 0.02 1.67 ± 1.15 1.16 ± 0.46 0.08 ± 0.03
Summer Absent 48 0.17 ± 0.04 0.02 ± 0.14 0.62 ± 0.14 0.11 ± 0.09
Present 0 --- --- --- ---
Fall Absent 76 0.17 ± 0.05 0.07 ± 0.25 0.83 ± 0.21 0.13 ± 0.05
Present 14 0.21 ± 0.04 1.43 ± 0.76 0.86 ± 0.20 0.12 ± 0.05
Table 3. Summary statistics, by season, for overall and swim-bladder health indices for St. Johns River
American Eels with (present) and without (absent) active A. crassus infections. The means ± 1 SD and
sample sizes (n) are presented for length-ratio index (LRI), swim-bladder degenerative index (SDI),
hepato-somatic index (HSI), and spleen somatic index (SSI). Eels were collected from February 2014
through May 2016 and are grouped by season (winter = January–March, spring = April–June, summer
= July–September, and fall = October–December).
Infection
Season Status n LRI SDI HSI SSI
Winter Absent 59 0.16 ± 0.05 0.81 ± 1.21 1.08 ± 0.28 0.17 ± 0.06
Present 70 0.19 ± 0.04 2.13 ± 0.88 1.12 ± 0.25 0.20 ± 0.12
Spring Absent 42 0.16 ± 0.05 0.38 ± 0.62 0.91 ± 0.19 0.14 ± 0.06
Present 33 0.17 ± 0.04 1.94 ± 0.70 0.99 ± 0.23 0.17 ± 0.08
Summer Absent 4 0.19 ± 0.03 0.50 ± 0.58 1.05 ± 0.36 0.15 ± 0.08
Present 2 0.21 ± 0.01 0.50 ± 0.71 1.19 ± 0.34 0.11 ± 0.07
Fall Absent 27 0.19 ± 0.05 0.67 ± 1.14 0.94 ± 0.24 0.14 ± 0.06
Present 32 0.20 ± 0.04 1.66 ± 0.75 1.05 ± 0.23 0.16 ± 0.07
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where the majority (92%) of infected American Eels were collected (Tables 3, 4;
Fig. 5). We noted a reduction in the average SDI values in the summer, as compared
to other seasons of the year (Fig. 5).
We determined genotypes for 299 American Eels collected from 16 watersheds.
The levels of genetic variation were very similar to those reported by
Côté et al. (2013; Table 5). FIS values indicated that most markers had more
Table 5. Genetic-diversity estimates for American Eel specimens (n = 299) sampled from Florida
freshwater systems during the period 2014–2016. The presented metrics include number of alleles
(Na), observed heterozygosity (HO), expected heterozygosity (HE), inbreeding coefficient (FIS), Hardy–
Weinberg equilibrium-test result (P), and the first reference for each locus. An asterisk (*) indicates a
statistically significant difference.
Locus Na HO HE FIS P Reference
ARO-54 21 0.729 0.850 0.144 0.0004* Wirth and Bernatchez 2003
ANG-101 41 0.891 0.957 0.068 0.0000* Wirth and Bernatchez 2003
ANG-114 59 0.916 0.967 0.052 0.2057 Wirth and Bernatchez 2003
AjfaBP 18 0.569 0.600 0.052 0.3882 Als et al. 2011
AJMS-06 31 0.817 0.865 0.057 0.0312 Tseng et al. 2001
AJTR-24 58 0.834 0.955 0.129 0.0567 Ishikawa et al. 2001
AJTR-25 43 0.850 0.874 0.030 0.3565 Ishikawa et al. 2001
AJTR-37 22 0.869 0.886 0.021 0.2932 Ishikawa et al. 2001
AJTR-45 96 0.920 0.976 0.057 0.4023 Ishikawa et al. 2001
AAN-03 10 0.470 0.494 0.054 0.0679 Daemen et al. 1997
AAN-05 9 0.515 0.476 -0.081 0.7491 Daemen et al. 2001
AangCT53 23 0.636 0.635 0.001 0.4041 Wielgoss et al. 2008
AangCT68 25 0.895 0.917 0.023 0.7209 Wielgoss et al. 2008
AangCT76 38 0.887 0.936 0.051 0.2923 Wielgoss et al. 2008
AangCT82 21 0.515 0.539 0.048 0.3051 Wielgoss et al. 2008
AangCT87 59 0.911 0.955 0.045 0.0757 Wielgoss et al. 2008
AangCT89 36 0.926 0.941 0.015 0.0640 Wielgoss et al. 2008
Figure 6. Relationship between the swim-bladder degenerative index (SDI) and number of
nematodes present in the swim bladders of American Eel specimens collected from freshwater
habitats in Florida from 2014 through 2016.
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homozygotes than expected under Hardy–Weinberg equilibrium, but only2 loci
had statistically significant heterozygote deficits after Bonferroni correction
(ARO-54 and ANG-101). Although Côté et al. (2013) reported higher FIS values
for both of these markers than those we report, those authors determined that the
vales were not out of the HWE.
We detected no differentiation between the Atlantic and Gulf Groups or between
the Atlantic and South Florida groups for any of the 17 loci tested. One locus
(AangCT76) was genetically differentiated between the Gulf and South Florida
groups. The South Florida group, however, had a particularly small sample size, so
sampling error may have affected allele frequencies collected for that group. The
pairwise FST estimates were not significant for any of the comparisons (Atlantic versus
Gulf FST = 0.00035, P = 0.1841; Atlantic versus south Florida FST = −0.00123,
P = 0.7510; Gulf versus south Florida FST = −0.00021, P = 0.2305). We found no
loci to be genetically differentiated in the direct comparison between American
Eels from the St. Johns (n = 110) and Apalachicola (n = 107) rivers, and the pairwise
FST estimate was not significant (F ST = 0.00008, P = 0.4078).
The estimated effective-population size for the American Eels sampled in Florida
was 7698 (95% CI, 2698–infinity). This result is similar to the estimate from
Côté et al. (2013) for 2 life stages of American Eels captured from numerous locations
between Florida and Newfoundland (Ne = 10,532, 95% CI = 9312–11,752).
The confidence intervals from Côté et al. (2013) are likely narrower because the
sample size in that study was much larger than in the present study.
Discussion
Little information is available about the American Eel in its southern range,
particularly in regards to comparing those in drainages closest to the putative
spawning location in the Sargasso Sea compared to those having access to the Gulf
of Mexico. Thus, this project attempted to collect baseline data that could inform
management decisions and direct future research efforts. Using non-target surveys,
we gathered information on the basic biology and health, the intensity and prevalence
of the nonnative swim-bladder nematode A. crassus, and the broad-scale
patterns of genetic relationships in American Eels throughout Florida. We could not
determine the sex of specimens that had been frozen due to artifacts in frozen tissue
that make interpretation difficult. However, 69% of the more than 600 American
Eels collected from water bodies during this study were 400 mm TL or larger, suggesting
that most were female (Harrell and Loyacano 1980). This result agrees with
those of other studies showing that the sex ratio of American Eels in upstream waters
tends to be skewed towards slow-growing females (Côte et al. 2015, Davey and
Jellyman 2005). Our female-skewed sample was expected because all but 1 American
Eel was collected by electrofishing in upstream freshwater (less than 1–2 ppt), where
American Eel density can be relatively low (Davey and Jellyman 2005, Krueger
and Oliveira 1999) and to which slow-growing females are known to migrate and
develop (Côte et al. 2015). We collected 5 silvering females, varying from 606 mm
to 800 mm TL and from 4 to 7 y in age. Two of these American Eels were collected
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2018 Vol. 17, No. 3
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in the South and West regions in December, the others from the Panhandle region in
October and November. This finding coincides with observations of silver American
Eel migration in Georgia, which begins in October (Facey and Helfman 1985).
Additional efforts are needed to fully assess growth, sex ratio, and life history of
Florida’s American Eel populations.
We did not find compelling evidence of genetic differentiation between any
geographically defined groups of American Eels evaluated in this study. All pairwise
FST estimates were extremely small and statistically nonsignificant. While we
found that American Eels from the Gulf of Mexico drainages and South Florida
had different allele frequencies at 1 locus; the south Florida group consisted of a
small number of American Eels and so may not have been representative of allelic
distribution of the American Eels in this region. Additional samples should be collected
and analyzed to acertain whether this finding was the result of population
structure rather than small sample size. Our study evaluated much larger samples
from the Gulf of Mexico and Atlantic regions and found that the American Eels
from these 2 regions were part of the same genetic population. Côté et al. (2013)
found that American Eels captured in Atlantic coastal drainages from Newfoundland
to Florida were from a single, panmictic population. Our results suggest that
this population extends beyond the Atlantic coast at least as far as the Gulf Coast
drainages in the Florida panhandle.
Citing the collection of small larval American Eels in the western Caribbean,
Miller et al. (2015) noted the possibility of another spawning location outside the
Sargasso Sea. However, our results indicate that larval American Eels from the Sargasso
Sea are at least capable of reaching the eastern Gulf of Mexico, although the
pathway of that travel remains unknown. Given that American Eels of every age
class, from 0 to 12 y, were collected for this study, recruitment of American Eels is
occurring annually into this region. After growing and developing, these American
Eels must then either migrate back to the Sargasso Sea to spawn, or they represent a
reproductive sink for the stock and do not contribute gametes to the next generation.
Our results do not exclude the possibility that another spawning ground exists nor
that these Sargasso Sea-spawned American Eels, when mature, would migrate back
to an area other than the Sargasso Sea to spawn. Thus, additional samples from Gulf
of Mexico drainages in other states would be required in order to determine whether
the American Eel makes up a single panmictic population throughout its range.
We found spatial differences in the prevalence and incidence of A. crassus infection
in Florida American Eels, with the highest rates detected in Northeast Florida
and the St. Johns River. Furthermore, the evidence of swim-bladder damage, as
indicated by SDI scores up to 4 (out of a possible 6), for American Eels without
active infections suggest that these and perhaps other eels in these areas may have
had previous infections. Machut and Limburg (2008) posited that parasite-infection
rates may be elevated in urbanized areas, but this suggestion fails to explain the
lack of nematodes in other Florida American Eels. For example, the Southeast
Florida region is highly urbanized and channelized, and none of the American Eels
collected in human-made canals in this region were infected with the nematode
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2018 Vol. 17, No. 3
or had nonzero SDI scores. Another possible explanation is that the nematode
was transported by ballast water that contained infected hosts, such as copepods
(De Charleroy et al. 1990). This scenario, too, fails to explain the distribution of
nematode occurrence in Florida because the major Florida ports of Miami, Fort
Lauderdale, and Tampa are all areas where the nematode has not been observed in
wild American Eels. Kennedy (2007) suggested that the primary pathway of the
nematode is movement of infected American Eels between areas for purposes such
as stocking, aquaculture, and bait. To our knowledge, there has been no large-scale
stocking or aquaculture of American Eels in Florida, but the 2 areas where the
nematodes are known to exist in Florida (Panhandle and Northeast Florida regions)
were home to the only historical commercial American Eel fisheries in Florida.
Since 2000, most (≥80%) American Eels have been harvested for human consumption
(as opposed for use as bait), but annual exports vary greatly, ranging from 0
to 100%. In years with high exports, out-of-state dealers picked up and transported
live American Eels from various locations along the Atlantic coast states, including
Florida, and may have accidentally released nematode larvae into state waters
during water exchanges. Other avenues of introduction, including the importation
of infected bait American Eels from other Atlantic coast drainages, may also have
played a role in the establishment of this nonnative parasite in some areas. Additional
efforts should be made to determine the extent of the nematode’s distribution
in Florida waters. If it is limited to certain regions, future efforts to establish bait
markets or aquaculture ventures should consider ways of reducing the possibility
of spreading infection to unaffected populations.
Salinity may play a role in the distribution of the nematode within a region.
Under laboratory conditions, hatching of nematode eggs and survival of larval
stages have been shown to be inhibited, although not completely, in high-salinity
conditions (Kennedy and Fitch 1990). Field observations coincide with these results.
For example, Neto et al. (2010) found that prevalence of the nematode in
Anguilla anguilla L. (European Eel) followed the salinity gradient in the Tagus
Estuary, Portugal, with decreasing prevalence as salinity increased. Denny et al.
(2013) found similar results for American Eel in Bras d’Or Lakes, NS, Canada,
with prevalence of the nematode higher in riverine areas. All but 1 of the American
Eels included in our study were obtained by electrofishing, a gear that is limited to
freshwater environments; therefore, we could not address the role of salinity in the
distribution of the nematode in our samples. Temperature may also play a role in
the A. crassus life cycle and ultimately how the nematode affects American Eels in
Florida. Although we collected only a limited number of specimens in summer, our
data suggest that average SDI, prevalence, and intensity of infection are reduced
during the period from July to September, when waters are generally warmest in
Florida. Thomas and Ollevier (1993) found the incubation time of nematode eggs
decreased as temperature increased, from 5 °C to 30 °C. However, Schippers et al.
(1991) found that 58–87% of nematodes died when water temperatures were raised
to 36.5 °C. Thus, it is possible that high water-temperatures can impact the survival
of A. crassus during the summer months in Florida and reduce their incidence in
American Eels.
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2018 Vol. 17, No. 3
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Alternatively, average SDI, prevalence, and intensity of infection may be reduced
in summer months due to other factors, such as increased mortality of American
Eels. Gollock et al. (2005) determined that high temperature alone would be unlikely
to cause mortality in American Eels, but Lefebvre et al. (2002) attributed a drop in
the swim-bladder index of European Eel to the death of individuals more severely
infected by A. crassus during the warmest months. Lefebvre et al. (2007) showed
that inducing oxygen stress led to increased mortality of American Eels that had
swim-bladder damage. We did not observe differences in health between infected
and uninfected American Eels. Average LRI, SSI, and HSI values were similar between
regions, among seasons within regions, and between uninfected and infected
American Eels in the Panhandle and St. Johns River regions, where most of the infected
American Eels were collected. Furthermore, condition (weight as a function of
length) was nearly identical between infected and uninfected American Eels in both
the Panhandle and St. Johns River regions. This finding coincides with Machut and
Limburg’s (2008) observations that condition of yellow eels was not related to nematode
prevalence or intensity. Still, it remains unclear what effect the combination of
localized stressors, including high temperature, low dissolved oxygen, and environmental
toxins, might have on American Eels in Florida in summer or to their capacity
to return to the Sargasso or another spawning area, should one exist.
Published information on the American Eel in Florida is extremely limited. This
study provides important baseline data about its basic health and biology, the distribution
of A. crassus, and the population genetics of the American Eel in Florida. Given
that A. crassus infections diminish swim-bladder function and may impair the ability
of infected individuals to reach the Sargasso Sea to spawn (Kennedy 2007), additional
efforts must be made to describe the distribution of the nematode in Florida waters
and to limit its spread there and beyond. Furthermore, the genetic information produced
by this study has implications for American Eel conservation and management,
but additional population-genetics work is needed for Gulf of Mexico drainages in
other states. Finally, for the future management and conservation of this species,
it is critical that future efforts focus on describing basic population characteristics
including abundance, sex-specific growth rates, and sex ratio, and life-history characteristics,
such as age at maturation and timing of inland and outgoing migration, for
Florida and other Gulf of Mexico American Eel populations.
Acknowledgments
We thank all of the personnel with the Florida Fish and Wildlife Conservation Commission
who collected, stored, and transported samples for processing. We extend special
appreciation those who helped with laboratory work, including T. Alfermann, M. Bakenhaster,
J. Benton, G. DelPizzo, J. Feltz, N. Feltz, D. Gandy, S. Hamby, J. Hill, J. Holder,
Y. Kiryu, E. Lundy, W. Porak, D. Richard, C. Steward, and A. Strickland. We are also grateful
to N. Balk, M. Cantrell, B. Crowder, A. Strickland, and N. Trippel for their reviews of
previous versions of this manuscript. This project was developed with financial assistance
provided by the Fish and Wildlife Foundation of Florida, Inc., through the Conserve Wildlife
Tag grant program (CWT 1516-06).
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2018 Vol. 17, No. 3
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