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2008 SOUTHEASTERN NATURALIST 7(4):679–690
Life-history Plasticity Among Stream Sites in the
Chattahoochee River Drainage, Alabama
Lemuel R. Casten1 and Carol E. Johnston1,*
Abstract - Cyprinella venusta (Blacktail Shiner) were studied at four sites to investigate
life-history parameter variation in Alabama streams. Consistent with life-history
theory, fish from a perceived degraded site were significantly smaller than fish from
sites that had less apparent environmental disturbance. Fish from the degraded site had
the smallest size at maturity and the smallest propagule size. Egg diameters were not
related to standard length of fish among sites. There was no significant difference in
mature egg diameters; however, ripening egg diameters differed among sites. Clutch
sizes adjusted for standard length varied among sites. There was no difference in
gonad mass in females for all spawning months; however, there were differences in gonad
mass in males in July. The gonadosomatic index peaked in July for both males and
females. Reproductive males were still present in September for three sites, but all females
from all sites had become latent by September, indicating that the spawning season
had ended. The results of this study suggest that Cyprinella venusta has the ability
to alter life-history parameters and this may be a factor contributing to its persistence
in habitats where other species are declining.
Phenotypic plasticity, or the ability of an organism to change its phenotype
in response to environmental changes, is usually thought of as an
adaptive strategy for dealing with differing environments (Miner et al.
2005, Sultan 2000). These phenotypic changes may have effects beyond
the individual organism, and may alter ecological processes in the environment
by changing many direct and indirect ecological interactions (Miner
et al. 2005). Morphological plasticity has been well documented in fishes,
and is common in many species of animals with broad geographic ranges
(Robinson and Parsons 2002, Stearns 1992). Local adaptation is a process
that also may produce differences in morphology, but involves genotypic
changes. With some exceptions (e.g., Gasterosteus aculeatus L. [Stickleback];
Kristjánsson et al. 2002), genetic changes involve relatively long time
periods and some degree of isolation.
Life-history parameters, and reproductive traits in particular, are known
to vary across environmental gradients in some fishes (Heins 1991, Mazzoni
and Iglesias-Rios 2002). The documentation of such phenotypic plasticity is
the first step towards understanding the persistence or extirpation of fishes
exposed to various environmental changes.
In the Chattahoochee River drainage, Cyprinella venusta (Girard)
(Blacktail Shiner) persists and is even increasing its distribution (Johnston
and Farmer 2004), whereas a sympatric species, Cyprinella callitaenia
1Fish Biodiversity Lab, Department of Fisheries, Auburn University, Auburn, AL
36849. *Corresponding author - email@example.com.
680 Southeastern Naturalist Vol. 7, No. 4
(Bailey and Gibbs) (Bluestripe Shiner), has been extirpated from much of
its native range (Johnston and Farmer 2004, Shepard et al. 1995). Other species
sympatric with Blacktail Shiner, such as Notropis amplamala (Pera and
Armbruster) (Longnose Shiner), Lythrurus atrapiculus (Snelson) (Blacktip
Shiner), Notropis cummingsae (Myers) (Dusky Shiner), and Notropis hypsilepis
(Suttkus and Raney) (Highscale Shiner), are also decreasing in abundance
and distribution. The proposed reasons for these findings include changes in
stream hydrology, sediment input, and temperature (Johnston and Farmer
2004), although less obvious biotic factors are almost certainly important.
The objective of this study is to investigate life-history variation in
Blacktail Shiner in tributaries to the Chattahoochee River, AL. The persistence
of Blacktail Shiner at our study sites could be due to the ability to alter
life-history parameters in response to disturbance.
Four sites from Chattahoochee River drainage were chosen for this study
(Fig. 1). Sites chosen in Uchee Creek are by County Road 39 (Russell County,
AL, 32°20'42"N, 85°3'16"W), and Highway 169 (Russell County, AL,
32°22'42"N, 85°10'53"W), (referred to as Uchee Creek 39 and Uchee Creek
169 hereafter) and Moffits Mill (Lee County, AL, 32°30'24"N, 85°10'48"W).
An additional site was located in an adjacent stream, Wacoochee Creek (Lee
County, AL, 32°37'21"N, 85°8'0"W). The sites were perceived as ranging
Figure 1. Sampling localities of Cyprinella venusta (Blacktail Shiner) within the
Chattahoochee River drainage in Alabama.
2008 L.R. Casten and C.E. Johnston 681
from relatively pristine with moderate to high species richness (Wacoochee
Creek and Uchee 169), to intermediate disturbance and species richness
(Moffits Mill), to highly degraded with low species richness (Uchee Creek
39) (Tables 1 and 2). The Uchee 39 site would be predicted to have a much
higher species richness based on stream size and historical collections. Potential
predators include bass and pickerel species, which were present in
collections from Moffits Mill and Uchee 169, but absent from Uchee 39 and
Wacoochee. The fish assemblages at all sites included native species, with
no exotics present. All of these sites are thought to be within the dispersal
distance of individual fish, or in close enough proximity for genetic mixing
to occur (Fig. 1; Johnston 2000), and were chosen so as to reduce the possibility
of local adaptation.
Table 1. Characteristics of study sites. Habitat heterogeneity is based on categorical estimate
and coefficient of variation of transect measurements of physical habitat parameters (water
depth, fl ow, stream width, substrate).
Site richness Riparian zone Substrate heterogeneity
Wacoochee 11 Forested Gravel, sand Moderate
Uchee 169 20 Forested Gravel, sand High
Moffits Mill 10 Partly forested Bedrock, gravel Moderate
Uchee 39 7 None Clay, sand, bedrock Low
Table 2. Descriptive statistics for habitat measurements (July 2005) and ANOVA results. Means
with an * differed from others (Duncan post-hoc test).
Variable Site n Mean SD CV
Depth, m Wacoochee 21 0.31 0.15 0.48
Moffits Mill 15 0.34 0.27 0.79
Uchee 169 12 0.67* 0.42 0.62
Uchee 39 14 0.57* 0.33 0.57
F = 5.4, P = 0.002
Flow, m/sec Wacoochee 21 0.25* 0.16 0.64
Moffits Mill 15 0.35 0.31 0.88
Uchee 169 12 0.53** 0.32 0.60
Uchee 39 14 0.40 0.21 0.52
F = 3.4, P = 0.022
Width, m Wacoochee 6 11.0* 1.8 0.16
Moffits Mill 4 23.9** 4.5 0.18
Uchee 169 4 16.0 1.2 0.07
Uchee 39 4 24.0** 0.37 0.01
F = 33.2, P = 0.000
Temperature, °C Wacoochee 6 25.4 0.66 0.03
Moffits Mill 4 25.6 0.22 0.008
Uchee 169 4 25.4 1.0 0.04
Uchee 39 4 27.5* 1.7 0.06
F = 4.1, P = 0.028
Turbidity, NTU Wacoochee 6 10.1 2.3 0.22
Moffits Mill 4 8.4 1.1 0.13
Uchee 169 4 14.1 3.9 0.27
Uchee 39 4 11.8 5.1 0.43
F = 2.1, P = 0.143
682 Southeastern Naturalist Vol. 7, No. 4
Uchee Creek has shown a decline in fish species distribution and abundance
(Johnston and Farmer 2004, Walser and Bart 1999). Reasons for the
decline in species populations may be changes in land use resulting in siltation,
deterioration of water quality, and recent water draw down for mining
and irrigation (Johnston and Farmer 2004, Howard 1997, Walser and Bart
1999). Fish diversity is especially low in mainstem sites, where species richness
may be as low as 2 species (Johnston and Farmer 2004).
Uchee Creek 39 was perceived to be the most degraded of the four sites,
having eroded banks, no riparian zone, clay-sand substrates, and sluggishto-
fast fl ows. There was obvious clear-cutting in the area. Uchee Creek 169
is a typical Coastal Plain stream with sandy gravel and mud substrates and
sluggish-to-fast fl ows. The riparian area is rich in vegetation, and snags are
present in the stream. Though Uchee Creek is heavily infl uenced by agricultural
activity, this site remains relatively undisturbed. Moffits Mill on Little
Uchee Creek lies directly on the Fall Line and is characterized by a large shoal
with bedrock substrate and fast fl ows. The riparian zone is partially intact at
this site. Because of the presence of both disturbed and relatively unchanged
sites within the watershed, the sites in Uchee Creek provide a framework for
comparison of potential life-history changes among sites. We included a site
from an adjacent stream, Wacoochee Creek, for comparison to these Uchee
Creek sites. The site on Wacoochee Creek is completely forested and has relatively
high species richness, and we considered it relatively pristine.
Materials and Methods
Blacktail Shiner were collected monthly from May to November 2005
using a backpack shocker and seine in four streams along the Chattahoochee
River drainage. From May to July, sampling frequency was twice a month
because this is the time when Blacktail Shiner actively spawn. Sampling was
conducted once a month from August to November. Samples from May to
September were used to quantify life-history parameters. May to November
samples were used to determine sex ratios for each site.
Physical habitat measurements were taken for each collection at each
site, and include: water depth (m) and velocity (fl ow, m/sec), substrate type
(bedrock, cobble, gravel, sand, or clay; following a modified Wentworth
scale; Ross et al. 1990), turbidity (NTU), temperature (°C), and stream
width (m). Measurements were taken at approximately equal intervals along
1–3 transects perpendicular to water fl ow. Water velocity was measured
using a Marsh-McBirney fl ow meter, and turbidity was measured using a
LaMotte model 2020 turbidimeter (average of 3 measurements per site).
All habitat types were sampled to allow for the collection of a representative
number of Blacktail Shiners inhabiting various habitats. Fish collected
were anesthetized using tricaine methanesulfonate (MS 222), and fixed
in neutral 10% buffered formalin before processing. Fish were sexed and
measured to the nearest 0.01 mm standard length (SL) with dial calipers.
Ovaries were removed from each specimen to assess gonad stage. Ovarian
condition of females was categorized using the classification of Heins and
2008 L.R. Casten and C.E. Johnston 683
Baker (1993): (1) latent (LA), (2) early maturing (EM), (3) late maturing
(LM), (4) mature (MA), (5) ripening (MR), and (6) ripe (RE). Ovarian stages
that were MA, MR, and RE were considered reproductive and LA, EM, and
LM females were considered non-reproductive (Heins and Baker 1993).
Reproductive condition of males was determined by visual examinations of
testes. Mature males have large and opaque testes. Males that have small and
transparent testes were considered latent.
Seasonal change in gonad mass for both sexes was determined using
the gonadosomatic index (GSI). Gonads and eviscerated specimens were
air dried for 2 hours (Johnston and Knight 1999) and weighed to the nearest
0.0001 grams. GSI was calculated as dry gonad mass/dry somatic mass
(mass of eviscerated specimen) X 100 (Johnston and Knight 1999).
Clutch size and egg size (diameter) for each clutch was determined following
Heins and Baker (1993). Clutch size was quantified by counting the
number of all mature eggs in mature (MA) and ripening (MR) females. Ripe
(RE) females were not used because they may have already spawned before
sampling collection and will not represent complete clutches. Females
showed distinct groups of early maturing (EM), late maturing (LM), mature
(MA), mature ripening (MR), and ripe (RE) eggs. Only MA and MR egg
classes were measured and analyzed in this study. Due to a limited number
of females with clutches having RE eggs, RE egg measurements were not included
in statistical analysis. Obtaining ripe eggs from field-caught samples
is rare, especially in minnows (Heins and Baker 1993, Machado et al. 2002),
and this generality holds true for this study.
Size estimates of eggs in each clutch were obtained by measuring
the diameter of 10 randomly chosen eggs from the clutch. Because eggs
were not spherical in preserved specimens, egg diameters were estimated
by averaging measurements to the nearest 0.05 mm of the maximum and
minimum dimensions. Size at maturity was determined for each sex by assessing
samples collected during the reproductive season. Sexually mature
females were those classified as EM, LM, MA, MR, and RE. Latent (LA)
individuals were considered sexually immature. Mature males were determined
by visual inspection of testes as noted earlier. Following Heins and
Baker (1993), the smallest individual that was EM was considered the size
of maturity. Mean size at maturity was calculated as the average standard
length of specimens in each gonad stage.
Sex ratio was determined for fish at each site by using pooled fish from
monthly collections. Deviations from the expected 1:1 sex ratio were determined
using a chi-square test. Analysis of variance (ANOVA) and analysis
of covariance (ANCOVA) were used to test for life-history parameter differences
among sites followed by Tukey post hoc tests for pair-wise comparison
between site means. Differences in SL of fish among sites were tested for
significance using ANOVA. Differences in clutch size, gonad mass, egg size,
egg mass, and GSI among sites were tested using ANCOVA with mean SL as
a covariate. Linear regression was conducted to determine the relationship
of life-history parameters to SL, and correlations in life-history parameters of
females were determined using Pearson’s correlation analysis.
684 Southeastern Naturalist Vol. 7, No. 4
All values were log10 transformed prior to analysis and tested for normality
and homogeneity of variances to satisfy the assumptions of statistical
analysis. All analyses were conducted using SAS (SAS Institute, Inc. 2003)
and SPSS 11.0 (SPSS, Inc. 2005) and considered significant at P ≤ 0.05.
All physical habitat parameters differed significantly among some sites
except turbidity (Table 2). Uchee 169 had the greatest water depth and the
most heterogeneity (highest CV) of depths (Table 2). This pattern was also
true for water velocity (fl ow). Uchee 39 had the highest water temperature
for the July samples compared in Table 2, and for the rest of the year as well.
This site also had a considerable amount of clay substrate, which was absent
at other sites. In addition, turbidity and water temperature showed the greatest
variability at Uchee 39.
The sex ratio (male:female) for fish from Uchee Creek 39 did not depart
from a 1:1 ratio (χ2 = 0.098, df = 1, P = 0.325), while deviations from a 1:1
ratio were found in the other three sites, with skewness towards females.
There was a significant difference in sex ratio of Blacktail Shiners at Uchee
Creek 169 (0.8:1) (χ2 = 6.556, df = 1, P = 0.010) and a highly significant
departure from the expected 1:1 sex ratio in fish from Moffits Mill (0.7:1)
(χ2 = 19.702, df = 1, P = 0.0001), and Wacoochee Creek (0.6:1) (χ2 = 17.376,
df = 1, P = 0.0001).
Female sizes differed significantly among sites (ANOVA: F = 22.88, df =
3, P = 0.0001; Table 3). Wacoochee Creek females were significantly larger
than females from the other three sites and were almost 10 mm larger than
the smallest mean female size from Uchee Creek 39. Male sizes also differed
among sites (ANOVA: F = 5.03, df = 3, P = 0.003). Males from Uchee Creek
169 were largest, while Uchee Creek 39 had the smallest males. There was
also a significant size difference of fish among sites when both males and
females were combined (ANOVA: F = 7.97, df = 3, P < 0.0001); fish from
Wacoochee Creek were largest, and Uchee Creek 39 fish were smallest.
Moffits Mill fish had the largest size at maturity for females, and males
from Wacoochee Creek had the largest size at maturity (Table 4). Females
from Uchee 39 reached maturity at the smallest size relative to fish from
other sites, and males from Moffits Mill were the smallest at maturity.
Table 3. Mean standard length (SL, in mm) of Cyprinella venusta (Blacktail Shiner) during the
spawning season from four sites in the Chattahoochee River Drainage. Numbers in parentheses
are standard deviations. Means with same superscripted letters are not significantly different.
Site Female Male Female/male combined
Uchee 39 61.7 (8.3)A, n = 119 80.1 (8.0)A, n = 38 65.7 (11.4)A, n = 157
Uchee 169 62.2 (7.6)AB, n = 93 88.0 (8.6)B, n = 31 67.9 (13.4)A, n = 124
Moffits Mill 64.6 (7.0)B, n = 114 82.5 (7.9)A, n = 45 69.2 (10.9)A, n = 159
Wacoochee 71.0 (11.5)C, n = 82 85.1 (11.4)7, n = 14 72.9 (12.4), n = 96
F value 22.8 5.03 7.97
P value 0.0001 0.003 <0.0001
2008 L.R. Casten and C.E. Johnston 685
Clutch size peaked in May and June and started to decrease as the spawning
season progressed and as eggs were being spawned. Clutch size increased
with female length when fish from all sites were considered together (F =
435.64, P < 0.0001; Table 5). Fish within sites had a significant correlation
between clutch size and SL for all sites (Pearson’s: P < 0.0001). Since female
SL was not uniform in all sites, we adjusted clutch size for SL in the analysis of
data (standard covariate analysis using average SL as the covariate). Among
sites, there were significant differences in SL-adjusted mean clutch size for
May (ANCOVA: F = 3.21, df = 3, P = 0.0266), June (ANCOVA: F = 4.30, df
= 3, P = 0.0069), and July (ANCOVA: F = 4.79, df = 3, P = 0.0035) (Table 6).
When all females were pooled for all months, there was a significant clutchsize
difference among sites (ANCOVA: F = 4.31, n = 408, P = 0.0053).
Mature-egg diameter of fish did not differ among sites (F = 0.71, df = 3,
P = 0.5467). For fish from Moffits Mill the range for mature-egg diameter
was 0.78–1.23 mm (mean = 0.94), 0.80–1.17 mm (mean = 0.96) for fish from
Uchee Creek 169, 0.79–1.24 mm (mean = 0.94) for fish from Uchee Creek
39, and 0.78–1.21 mm (mean = 0.94) for Wacoochee Creek fish. There was
no significant relationship between regressions of mature-egg diameter with
SL for all sites (P > 0.05); however, when all females were pooled, there
was a positive relationship between mature-egg size and SL (F = 123.00, P <
0.0001). Mature-egg diameters were not significantly correlated with SL for
all sites and were smaller than ripening eggs.
Ripening-egg diameter was not correlated with SL or clutch size among
females within sites. There was no significant relationship between regressions
of ripening-egg diameter with SL for sites considered separately (P >
0.05), but when all females were pooled, there was a positive relationship
between ripening-egg diameter and SL (F = 387.60, P < 0.0001). During
June and July, ova from the four sites differed in size (June ANCOVA:
Table 4. Sizes at maturity (SL in mm) for Cyprinella venusta (Blacktail Shiner) and mean temperature
for each site during spawning months. Means with the same superscripted letters are
not significantly different (P > 0.05).
Size at maturity (SL,mm)
Sites Female Male Temperature (°C)
n 2311 128
Uchee 39 38.2A 66.1A 26.2
Uchee 169 44.4BC 65.1A 24.1
Moffits Mill 46.4C 60.1B 23.3
Wacoochee 41.1AB 72.8C 24.0
Table 5. Statistics for clutch size-standardized length relationships of Cyprinella venusta
(Blacktail Shiner) for four sites. All values were log10 transformed prior to analysis.
Site n Intercept Slope R2 P value
Uchee 39 119 -2.881 2.947 0.522 <0.0001
Uchee 169 93 -3.330 3.171 0.506 <0.0001
Moffits Mill 114 -2.097 2.530 0.337 <0.0001
Wacoochee 82 -2.406 2.704 0.478 <0.0001
686 Southeastern Naturalist Vol. 7, No. 4
F = 3.87, n = 95, P = 0.0155; July ANCOVA: F = 5.91, n = 125, P = 0.0014;
Table 7). Pair-wise comparison between sites showed that Wacoochee Creek
fish had egg diameters that were significantly larger than for fish from
Uchee Creek 39 in June. Blacktail Shiner from Uchee Creek 169 had egg
diameters that were significantly larger than ripening-egg diameters from the
other three sites in July. There was a significant difference in ripening-egg
diameters when all females were pooled (ANCOVA: F = 3.07, n = 195, P =
0.0290). Uchee Creek 39 females had the smallest eggs, and Uchee Creek
169 females had the largest ripening eggs among sites.
GSI peaked in July for all sites (Fig. 2) and did not differ among sites.
Females from all sites became latent by September, indicating that spawning
had ended. Fish from Moffits Mill (mean = 5.8, range = 4.1–7.0) showed the
highest GSI, and Uchee Creek 169 females showed the lowest GSI (mean
= 4.9, range = 3.4–6.6). Fish from Uchee Creek 39 and Wacoochee Creek
had mean GSI values of 5.3 (range = 4.1–6.5) and 5.2 (range = 2.5–6.5),
respectively. For males, GSI peaked in July, and the values dropped after
the peak month, showing a pattern similar to female GSI (Fig. 3). Pair-wise
comparison for July showed that the GSI (0.8) for fish from Uchee Creek 169
Table 6. Monthly SL-adjusted mean clutch sizes for Cyprinella venusta (Blacktail Shiner) at
each site. Numbers in parentheses are standard deviations. Means with the same superscripted
letters are not significantly different.
Site May June July August combined
Uchee 39 366 (117)BC 388 (149)A 286 (101)AB 170 (64)A 281 (145)AB
n 25 24 32 38 119
Uchee 169 327 (107)B 318 (125)B 254 (110)A 136 (83)A 252 (124)A
n 22 28 24 19 93
Moffits Mill 412 (143)C 396 (133)A 316 (100)B 155 (70)A 300 (140)B
n 25 26 38 25 114
Wacoochee 345 (117)BC 390 (124)AB 324 (80)B 137 (87)A 302 (129)B
n 25 17 31 9 82
F value 3.21 4.30 4.79 1.54 4.31
P value 0.0266 0.0069 0.0035 0.2104 0.0053
Table 7. Mean ripening ova diameter (mm) of Cyprinella venusta (Blacktail Shiner) from four
sites in the Chattahoochee River drainage. Numbers in parentheses are standard deviations.
Means with the same superscripted letters are not significantly different.
Site May June July August Combined Range
Uchee 39 1.08 (0.08) 1.03 (0.06)A 1.02 (0.05)A 1.05 (0.08) 1.04 (0.06)A 0.94 – 1.18
n 9 14 16 18 57
Uchee 169 1.08 (0.06) 1.07 (0.04)AB 1.10 (0.09)B 1.06 (0.06) 1.08 (0.06)B 0.93 – 1.31
n 9 14 11 10 44
Moffits Mill 1.06 (0.06) 1.08 (0.06)AB 1.02 (0.05)A 1.08 (0.05) 1.06 (0.06)B 0.93 – 1.19
n 13 12 18 11 54
Wacoochee 1.09 (0.07) 1.11 (0.05)B 1.04 (0.04)A 1.02 (0.04) 1.06 (0.07)B 0.96 – 1.19
n 13 9 15 3 40
F value 0.52 3.87 5.91 0.84 3.07
P value 0.6732 0.0155 0.0014 0.4812 0.0290
2008 L.R. Casten and C.E. Johnston 687
was significantly higher than the GSI (0.5, 0.4) for fish from Wacoochee and
Uchee Creeks 39 (F = 3.24, df = 3, P = 0.0330). For August, fish from Moffits Mill had GSI values that were significantly higher compared to the other
three sites (F = 3.20, df = 3, P = 0.0455). Males did not differ in GSI over
Figure 2. Monthly variation in gonadosomatic index (GSI) of female Cyprinella venusta
(Blacktail Shiner) from four sites in the Chattahoochee River drainage. There
were no significant differences in monthly GSI for all sites in all spawning months
(F = 1.87, n = 408, P = 0.1339).
Figure 3. Monthly variation in gonadosomatic index (GSI) of male Cyprinella venusta
(Blacktail Shiner) from four sites in the Chattahoochee River drainage. Uchee
Creek 169 GSI was significantly higher than Wacoochee Creek and Uchee Creek
39 in July. GSI for Moffits Mill was significantly higher than the other three sites
in August. All other months showed no significant differences in GSI for all sites.
Wacoochee Creek males were latent by September. n = 128.
688 Southeastern Naturalist Vol. 7, No. 4
the four sites. The mean GSI for fish was 0.3 from Uchee Creek 39 (range =
0.1–0.4), and 0.3 (range = 0.1–0.8) for fish from Uchee Creek 169, 0.4 (range
= 0.1–0.6) for fish from Moffits Mill, and 0.4 (range = 0–0.5) for fish from
Wacoochee Creek. Reproductive males were still present during September
for three sites (Uchee Creek 39, Uchee Creek 169, Moffits Mill); however,
male fish from Wacoochee Creek had become latent a month earlier.
Blacktail Shiner showed significant differences in life-history parameters
among sites with varying physical properties. These differences included
body size (SL), size at maturity, sex ratio, clutch size, and egg size. The
smaller body size, small sizes at maturity, and smaller ova found at Uchee
39 follow a trajectory predicted by life-history theory as a response to
harsh environmental conditions (Roff 1992, Stearns 1992) or a decrease
in adult survivorship (Reznick et al. 1990). These variations in life history
parameters may be phenotypic responses to biotic or abiotic factors, as
the sites differed in both species richness (one of many biotic factors that
potentially differed among sites) and physical habitat (a measure of abiotic
factors). Alternatively, these differences could be due to genetic differentiation,
but the close proximity of the sites (<20 km within Uchee Creek) and
mobility of Cyprinella (Johnston 2000) makes this explanation less likely.
Females from Uchee Creek 39 had smaller sizes at maturity as well as
smaller body sizes than fish from the other three sites. This pattern is typical
of the trade off in somatic size at onset of reproduction seen in many
organisms, and may be a response to stress (Stearns 1992). For example, fish
mature earlier in response to predation pressure as a bet-hedging strategy
against early mortality (Reznick et al. 1990). There were few predators at
Uchee 39, as species richness was diminished (Johnston and Farmer 2004),
but it is possible that the relative habitat degradation has stimulated these
life-history shifts. Females from Uchee 39 also had the smallest ripening
ova, although this was not correlated with standard length or clutch size,
which is also in agreement with predictions from life-history theory as a
response to environmental stress (Roff 1992).
Previous studies of reproductive life-history parameters of Blacktail
Shiner found no difference in ova size and female size within sites, but ova
size was correlated with female standard length among sites, similar to our
findings for all sites pooled (Heins and Baker 1987, Machado et al. 2002).
Although earlier studies did not find a significant relationship between female
size and clutch size, our results indicated strong relationships between
these two variables at all sites. Earlier studies also found correlations between
ova size and mean annual runoff of streams, and proposed that larger
progeny may be less susceptible to mortality from high-fl ow events. Data for
size at maturity and body size were not presented in these previous studies
(Heins and Baker 1987, Machado et al. 2002).
Blacktail Shiner at our study sites differed in life-history traits associated
with reproductive effort, including ova size. At the site we perceive as
2008 L.R. Casten and C.E. Johnston 689
most degraded (no riparian zone, low species richness, highest variation in
temperature and turbidity), fish had the smallest body size, and females matured
earlier and had the smallest ripening ova as compared to other sites. It
is possible that Uchee 39 is a harsher environment for Blacktail Shiner than
sites of previous studies, producing stronger life-history shifts.
Other studies have found interpopulation differences in life histories of
fishes, and such differences may be ubiquitous, at least in fishes with inherent
plasticity (Baker 1994; Guill and Heins 1996; Heins 1991, 2000; Heins
and Baker 1987; Machado et al. 2002; Reznick and Miles 1989). Many studies
also correlate such differences with environmental factors. The effects of
these changes in life-history parameters on the ecology of the stream ecosystem
have yet to be addressed, but may have substantial consequences (Miner
et al. 2005). In particular, changes in reproductive timing may allow the offspring
of one species to out compete another for food or shelter, or to escape
predation and reach a larger size faster. These differences may help explain
why some species, such as Blacktail Shiner, persist while others are extirpated
as environmental conditions change. Blacktail Shiner is considered a generalist
species and has proliferated in the Uchee Creek system while other species,
such as Bluestripe Shiner, are extirpated in this system (Johnston and Farmer
2004). Our findings suggest that life-history plasticity may contribute to the
success of Blacktail Shiner.
Documentation of life-history plasticity is the first step towards understanding
the complex relationships between environmental change and species
persistence. These relationships should be taken into account as conservation
biologists strive to understand faunal change and habitat protection.
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