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Life-history Aspects of the Cherokee Darter, Etheostoma scotti (Actinopterygii: Percidae), an Imperiled Species in Northern Georgia
Stephanie D. Barton and Steven L. Powers

Southeastern Naturalist, Volume 9, Issue 4 (2010): 687–698

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2010 SOUTHEASTERN NATURALIST 9(4):687–698 Life-history Aspects of the Cherokee Darter, Etheostoma scotti (Actinopterygii: Percidae), an Imperiled Species in Northern Georgia Stephanie D. Barton1 and Steven L. Powers2,* Abstract - Aspects of the life-history of Etheostoma scotti (Cherokee Darter) were investigated using 12 monthly collections from Hickory Log Creek (Etowah River Drainage) in Cherokee County, GA. Specimens were collected from riffles, runs, and pools with slow current and examined to illuminate age, growth, food habits, and reproductive characters. The bulk of the diet consisted of Chironomidae larvae, with mollusks, detritus, branchiopods, and other aquatic insects as smaller components. Peak feeding occurred in late winter and spring and immediately preceded gamete production for a single spring spawning season peaking in April. Gravid females, collected from March to June, contained 2–256 mature oocytes, ranging from 0.7 to 1.2 mm in diameter. Sexual maturity occurred at age 1, and maximum age was 2 years. The largest specimen collected was a male 49.1 mm SL and 2.0 g total weight. Males were larger than females and were outnumbered 1:1.56. Only about half of the adult-sized males appeared to be in nuptial condition during the spawning season in the study population. These findings provide a greater understanding of the biology of this imperiled species and may allow for more focused and effective conservation efforts. Introduction Etheostoma scotti Bauer, Etnier, and Burkhead (Cherokee Darter) is a member of the subgenus Ulocentra (Bauer et al. 1995) and sister to the more widespread E. coosae (Fowler) (Coosa Darter; Porter et al. 2002). As a narrow endemic restricted to small tributaries of the Etowah River Drainage of northern Georgia, E. scotti is listed as a threatened species (United States Fish and Wildlife Service 1994). Jelks et al. (2008) recognized three distinct populations of E. scotti, considering each “endangered”. The Etowah River drainage is located on the northern periphery of the Atlanta metropolitan area. Despite the well-documented association of urbanization and declining populations of aquatic species (Anderson et al. 1995, Onorato et al. 2000, Weaver and Garman 1994), and the potential impacts to E. scotti presented by expanding Atlanta suburbs, little is known of this at-risk species’ biology. Protection as a threatened species under the Endangered Species Act has prohibited traditional life-history research, leaving non-lethal methods of study (Storey et al. 2006) as the basis for our understanding of the biology of this species. The primary objective of this study was to document selected aspects of the life-history of this imperiled species. 1Department of Biology, Reinhardt College, Waleska, GA 30183-2981. 2Department of Biology, Roanoke College, 221 College Lane, Salem, VA 24153-3794. *Corresponding author - powers@roanoke.edu. 688 Southeastern Naturalist Vol. 9, No. 4 Study Area Fishes were collected from Hickory Log Creek at Fate Conn Road (34°17.579'N, 84°27.899'W) near Canton in Cherokee County, GA (Fig. 1). This study area provided a unique situation in which a population of E. scotti was to be sacrificed for the construction of a water storage reservoir. Hickory Log Creek is an upland second-order tributary of the Etowah River and one of the streams studied by Storey et al. (2006). Stream width at the study site varies from 2.9 to 6.1 m wide, and base flows are less than 1.0 m deep in the study area. Substrate is primarily gravel to cobble in riffles, gravel to sand in runs, and sand to silt in pools. Upstream of the study area, the Hickory Log Creek watershed is mostly forested with moderate residential development. Water temperatures during times of collection ranged from 6 °C in January 2008 to 24 °C in July and August 2007. A total of 12 fish species were collected during this study. Methods Etheostoma scotti and vouchers of associated species were collected monthly (near mid-month) over a one-year period from February 2007 to January 2008 with a 3.3-m x 1.3-m, 9.5-mm mesh seine and a Smith-Root model 24 backpack electrofisher. A total of 226 specimens was collected, preserved in 10% formalin, rinsed with water, and transferred into 70% ethyl alcohol (EtOH) for long-term storage. Specimens were accessioned into the University of Alabama Ichthyological Collection (UAIC 15015–15026). Standard length (SL) of preserved E. scotti was measured with digital calipers and recorded to the nearest 0.01 mm. Sexual size dimorphism was Figure 1. Map of Etheostoma scotti (Cherokee Darter) study area in Hickory Log Creek (34°17.579'N, 84°27.899'W), near Canton in Cherokee County, GA. 2010 S.D. Barton and S.L. Powers 689 detected through use of a two-sample t-test of SL; therefore, all age and growth analyses were performed separately for sexes. Specimens were blotted dry, and total weight (TW), eviscerated weight (EW), and gonad weight (GW) were measured using a digital analytical balance and recorded to the nearest 0.001 g. All statistical analyses were executed with Data Desk 6.0 (Data Description, Inc., Ithaca, NY) at a significance level of alpha equal to 0.05. In reference to regressions, independent variables are listed first and dependent variables second unless otherwise noted. Scales (n = 3) from each specimen were examined by S.L. Powers for annuli to provide an estimate of age class following O’Neil (1981), who noted that annuli are established in early to middle spring in E. coosae. Annuli appear to form in E. scotti during late winter to early spring as evidenced by their placement near the edge of scales from specimens collected in March. If the three scales did not display the same number of annuli, then additional scales were examined until a clear majority displayed the same number of annuli. Scales removed were discarded after examination. Corroboration of hypothesized age class was conducted by plotting month against SL for each sex independently (Figs. 2, 3). Gaps of 3 mm or more in the SL of specimens from a single month contiguous with similar gaps from adjacent months were considered indicative of different age groups. If 3-mm gaps in SL did not occur in a particular month, age groups were delineated by extrapolating lines from gaps in adjacent months. While both of these methods may be imperfect, together they provide a clearer picture of fish biology (see Summerfelt and Hall 1987). Due to high gonadosomatic index (GSI) values found in specimens collected in April and the collection of some females from May that Figure 2. Scatter plot of standard length (SL) by month of collection for male Etheostoma scotti (Cherokee Darter) from Hickory Log Creek, GA, between February 2007 and January 2008. 690 Southeastern Naturalist Vol. 9, No. 4 appeared to be spent, we used April as the month of spawning for estimating age of individuals. Specimens less than 12 months of age were counted as age 0, specimens 12–23 months were counted as age 1, and specimens 24 months or greater were counted as age 2. Proportion of total specimens collected represented by each age group was calculated to approximate the age class distribution of the population. A chi-square goodness-of-fit test of age groups was used to test differences in lifespan among sexes. Regression by least sum of squares was performed for the natural log of EW and SL. We opened the anterior third of the gastrointestinal track and removed its contents, and weighed them to the nearest 0.001 g using a digital analytical balance. Weight of gut contents for specimens with empty guts was recorded as “0”. We counted and identified food items to the lowest taxonomic category possible following Thorp and Covich (1991) and Merritt and Cummins (1996). Most food items were not identifiable below the level of family, order, or class. Taxa richness of gut contents was the total number of different food items in each specimen. We determined proportion of food items as Chironomidae by dividing total number of Chironomidae by total number of food items in each specimen. We performed a one-way analysis of variance on weight of gut contents/EW, taxa richness of food items, and proportion of diet as Chironomidae to test differences in feeding among different months. We performed regressions by least sum of squares for EW and weight of gut contents, EW and taxa richness of gut contents, and EW and proportion of diet as Chironomidae to test influence of size on feeding. Figure 3. Scatter plot of standard length (SL) by month of collection for female E. scotti (Cherokee Darter) from Hickory Log Creek, GA, between February 2007 and January 2008. 2010 S.D. Barton and S.L. Powers 691 We calculated GSI by dividing GW by EW. We performed a one-way analysis of variance to test mean differences in GSI among months. We counted greatly enlarged (≈1 mm in diameter), fully yolked, mature oocytes from gravid females and measured five representative oocytes to approximate ova size and number (see Heins and Baker 1988). Smaller oocytes (<0.5 mm in diameter) were not counted or measured. We performed regression of SL as a predictor of number of mature oocytes to test the influence of size on fecundity. Results The largest specimen collected was a 49.1-mm SL, 2.0-g TW male taken in January (Fig. 2). The smallest specimen collected was a 25.2-mm SL, 0.24-g TW female taken in February (Fig. 3). The August collection provided the earliest capture of age-0 specimens, ranging from 27.4–30.3 mm SL (mean = 28.8, SD = 2.03). For all collections, females outnumbered males 1.56:1. In age-0 specimens, males slightly outnumbered females 1.23:1, while age-1 females outnumbered males 1.85:1, and the only two age-2 specimens were males. Sexual size dimorphism was detected, with mean SL for females and males 35.9 (SD = 4.48) and 38.4 (SD = 6.05) mm, respectively (P < 0.001). Due to this sexual size dimorphism, the following results are presented for females and males, respectively, unless otherwise noted. Standard length increased with age in months (r2 = 0.61, P < 0.001; r2 = 0.72, P < 0.001) (Figs. 2, 3). Visual inspection of the data suggested a curvilinear relationship between SL and EW, so we ln-transformed EW and SL before regressing them (r2 = 0.976, coefficient = 0.306, P < 0.001; r2 = 0.963, coefficient = 0.300, P < 0.001). Eviscerated weight appeared to increase from 10–20 months of age (Fig. 4), which coincided with increases in SL from June to October (Figs. 2, 3). Of the 228 specimens collected, 16.7% were age 0 and 82.5% were age 1, and 1% were age 2. Age in months was not different among sexes (P = 0.97), with a median age of 14 months (SD = 4.42) for males and 16 months (SD = 3.82) for females. Maximum age of specimens captured was 24 months for males and 23 months for females. Chironomidae made up 79.5% of all food items in E. scotti examined (Table 1). Branchiopoda and Ancylidae each made up 3.1% of all food items, while Ephemeroptera and Trichoptera made up 2.5% and 2.3%, respectively. Of all specimens examined, only 4.9% of GI tracks were empty. The amount of food eaten by E. scotti was not uniform across all months (F = 4.04, df = 11, P < 0.001), with weight of gut contents being highest in February (mean = 1.4% of EW, SD = 0.01) and lowest in November (mean = 0.3% of EW, SD = 0.01). Specimens from each month from February through August had a mean of 10 or more food items per individual compared to fewer than 10 in the other months. Composition of diet also appeared to vary among months (F = 2.7, df = 11, P = 0.004), as taxa richness of food items was greatest in August (mean = 2.7, SD = 1.50) and lowest in November (mean = 0.8, SD = 0.96). Proportion of food items as Chironomidae (F = 7.98, df = 11, 692 Southeastern Naturalist Vol. 9, No. 4 P < 0.001) was greatest in April (mean 95%, SD = 0.05) and lowest in November (mean 25%, SD = 0.50). Feeding on other food items also appeared to vary seasonally, as mollusks were a relatively large component of summer and early fall food items, but only 1.8% of mollusks consumed were eaten outside of the period from June to November (Table 1). Similarly, 52% of the total number of Branchiopoda consumed was from December and January when consumption of chironomids appeared to be near its lowest point. Weight of gut contents increased with EW (r2 = 0.14, P < 0.001) as did taxa richness of food items (r2 = 0.04, P = 0.020). Proportion of diet as Chironomidae decreased with EW (r2 = 0.09, P < 0.001). Mean GSI was not uniform among months for females (F = 58.1, df = 11, P < 0.001) or males (F = 5.1, df = 11, P < 0.001), as April had the highest mean GSI for both sexes, with values of 0.12 (SD = 0.043) for females and 0.007 (SD = 0.005) for males. All females from March and April had GSI greater than 0.04 (Fig. 5); however, males from these months showed a bimodal distribution with only some males having GSI greater than values commonly found in “non-spawning months” (Figure 6). As this may suggest that males do not spawn until their second year, ANOVA of GSI with age class was performed, but was not significant (F = 2.5, df = 2, P = 0.10). Minimum GSI values were found in June for both sexes (females mean 0.003, SD = 0.001, males mean 0.001, SD = 0.0001). Water temperature during the April collection was 14 °C. Mature oocytes were found in females collected from Figure 4. Eviscerated weight (EW) ± one standard deviation by age in months for Etheostoma scotti (Cherokee Darter) from Hickory Log Creek, GA, between February 2007 and January 2008. 2010 S.D. Barton and S.L. Powers 693 Table 1. Gut Contents of Etheostoma scotti (Cherokee Darter) from Hickory Log Creek, GA, collected from February 2007 to January 2008. Numbers for each food item indicates total number of individuals for that item. Detritus and unidentified insect parts are exceptions due to the difficulty quantifying them. These two items are noted by occurrence within a single gut (e.g., the occurrence of detritus in two guts from a month is denoted as “2”). Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total % Total # of specimens 20 20 20 20 20 20 20 20 20 20 6 22 228 Sand 3 1 2 1 0 3 4 1 3 6 0 1 25 0.9 Detritus 7 9 3 4 11 11 16 9 11 13 1 13 108 4.0 Nematomorpha 0 0 0 2 0 0 0 0 0 0 0 0 2 0.1 Nematoda 0 0 1 0 0 0 0 0 0 0 0 0 1 0.0 Mollusca Helisoma 0 0 0 0 0 0 0 0 0 1 0 0 1 0.0 Elimia 0 0 0 0 0 1 5 4 2 0 0 0 12 0.4 Ancylidae 0 0 0 0 0 8 13 17 18 28 1 0 85 3.1 Physa 0 2 0 0 0 0 2 9 1 1 0 0 15 0.6 Annelida Oligochaeta 0 1 0 0 0 0 0 0 0 0 0 0 1 0.0 Arachnida Acarina 0 0 0 0 0 0 0 0 0 1 0 0 1 0.0 Crustacea Branchiopoda 17 3 0 8 2 9 0 2 1 9 6 27 84 3.1 Insecta Tipulidae 0 1 1 0 1 0 2 1 1 3 0 1 11 0.4 Simulidae 0 3 3 2 0 0 0 0 0 0 0 0 8 0.3 Megaloptera 0 0 0 0 0 0 0 1 0 0 0 0 1 0.0 Unidentified parts 5 4 3 4 2 2 3 4 6 2 1 2 38 1.4 Ephemeroptera 9 11 4 3 4 4 14 6 5 3 1 5 69 2.5 Plecoptera 0 4 1 4 0 0 0 0 0 4 0 5 18 0.7 Coleoptera 0 0 0 0 0 0 0 2 0 0 0 0 2 0.1 Colepotera larvae 0 0 0 0 0 0 0 0 8 0 0 0 8 0.3 Trichoptera 1 12 4 1 1 6 6 15 9 4 0 4 63 2.3 Diptera Chironomidae larvae 29 328 202 505 253 219 300 128 86 69 6 41 2166 79.5 Unidentified pupae 1 0 3 1 0 0 0 0 0 0 0 0 5 0.2 Unidentified adults 0 0 0 0 0 0 2 0 0 0 0 0 2 0.1 Empty 5 0 1 0 0 1 0 0 0 0 3 1 11 Total items 72 379 227 535 274 263 367 199 151 144 16 99 2726 Items/specimen 3.6 19.0 11.4 26.8 13.7 13.2 18.4 10.0 7.6 7.2 2.7 4.5 12.0 Empty % 25 0 5 0 0 5 0 0 0 0 50 4.5 4.9 694 Southeastern Naturalist Vol. 9, No. 4 March to May. Adult females contained between 2 and 256 (mean = 69.0, SD = 63.2) mature oocytes, with the maximum value found in a specimen 11 months old and 33.25 mm SL. Mature oocytes ranged from 0.7 to 1.2 mm Figure 6. Gonadosomatic index (GSI) by month of the year for male Etheostoma scotti (Cherokee Darter) collected from Hickory Log Creek, GA between February 2007 and January 2008. Figure 5. Gonadosomatic index (GSI) by month of the year for female Etheostoma scotti (Cherokee Darter) collected from Hickory Log Creek, GA, between February 2007 and January 2008. 2010 S.D. Barton and S.L. Powers 695 in diameter. Ovaries of gravid females appeared to contain a single group of mature oocytes. Standard length was not a significant predictor of number of mature oocytes in gravid females (r2 = 0.00, P = 0.98), and the youngest specimens approaching sexual maturity, as indicated by increased GSI, appeared to be 11 months of age. Discussion The above results indicate that E. scotti live to a maximum age of approximately two years, attain sexual maturity at approximately one year of age, feed primarily on chironomid larvae and other invertebrates, increase feeding during spring and summer, and have a single spawning season that peaks in April. Further, only about half of the males in a population reach peak spawning condition during spawning. Understanding aspects of the life history of this imperiled species allows for the composition and implementation of more effective conservation and management strategies that ensure its survival. The hypothesized maximum age of approximately 2 years is consistent with that of Etheostoma atripinne (Page and Mayden 1981), but a year shorter than that of E. coosae, E. pyrrhogaster, and E. zonistium (Carney and Burr 1989, O’Neil 1981). As the largest specimen captured during this study was 49.1 mm SL, and the largest specimen examined by Bauer et al. (1995) was 59.1 mm SL, it is possible that other populations may live to 3 years or grow faster than those in Hickory Log Creek. Much smaller specimens (as small as 15 mm SL) examined by Bauer et al. (1995) from summer months are likely age-0 specimens growing rapidly from larvae. The growth of age-0 individuals from only a few mm TL to greater than 30 mm SL in 6 months is well documented for other Ulocentra (Carney and Burr 1989, O’Neil 1981, Page and Mayden 1981). Also illustrated in these other studies of Ulocentra is how variable growth can be within an age class. O’Neil (1981) noted a specimen 10 months of age that was less than 20 mm SL and one 7 months of age that was greater than 34 mm SL. This variation in growth may explain the relatively low r2 for regression of SL with age in months in specimens from this study, and specimens less than 20 mm SL from throughout the year examined by Bauer et al. (1995). Maximum age does not appear to be different among sexes (24 and 23 months for males and females collected in this study), but is one year less than maximum age of E. coosae (O’Neil 1981). Median age was also not significantly different among sexes; however, overall females outnumbered males 1.56:1. Age-0 males outnumbered like females 1.23:1, while age-1 females outnumbered males 1.85:1. This skewed sex ratio in older specimens is consistent with other Ulocentra (Carney and Burr 1989, Page and Mayden 1981) including E. coosae (O’Neil 1981). Carney and Burr (1989) suggested the skewed sex ratio was due to increased predation on brightly colored males. The failure to capture age-0 specimens until August and their low proportion of the total specimens collected (17.7%) is likely due to the 696 Southeastern Naturalist Vol. 9, No. 4 ease at which small specimens pass through the 9.5-mm mesh of the 3.3-m x 1.3-m seine. Increased feeding in late winter through summer appears to coincide with increased energetic requirements associated with gamete production, spawning, and increased growth. The large proportion of Chironomidae larvae in the diet is consistent with other Ulocentra (Carney and Burr 1989, Page and Mayden 1981) including E. coosae (O’Neil 1981). Both E. scotti and E. coosae appear to have a greater proportion of their diet as Chironomidae in spring and Branchiopoda in winter. The importance of mollusks in the diet of E. scotti during summer and fall appears to be unique within Ulocentra that have been studied, as E. coosae appear to feed on mollusks only during summer months. This dietary reliance on mollusks coincides with a period of increasing EW following spawning season at roughly 12 months of age (Fig. 4) suggesting mollusks are a high quality food source during a critical growth period. The significant regression between EW and weight of gut contents indicates that quantity of food consumed increases with size. The increase in taxa richness of food items with EW also suggests that the diet becomes more varied as E. scotti become larger. The inverse relationship between proportion of food items as Chironomidae and EW also suggests that E. scotti rely less exclusively on chironomids for food as they get larger. As the r2 values supporting the hypothesized shifts in diet are low, these shifts may be slight. The diet of large E. coosae (>41 mm SL) also appears to include a greater taxa richness of food items, but the reliance on Diptera larvae appears to be least in individuals 21–30 mm due to a large number of crustaceans in the diet of individuals that size (O’Neil 1981). The reliance on mollusks in fall coincides with a period of increased EW. Along with the decreased reliance on Chironomidae in larger, older specimens, these findings suggest the maintenance of a diverse invertebrate community may be a key component of E. scotti conservation. Maximum GSI values in specimens collected in April suggest a single spawning season peaking in April. Elevated GSI from adjacent months suggests spawning may extend from March to May. Low values in specimens from June and July (Figs. 5, 6) suggest spawning activity is finished by these months. Storey et al. (2006) also observed spawning behavior from March to May with a peak in April. Similarly, E. coosae appears to spawn from March to May (O’Neil 1981). Sexual maturity in specimens approximately 11 months of age indicates E. scotti spawn in their first full spawning season. The bimodal distribution of male GSI could be interpreted that some males do not spawn in their first full spawning season, but the lack of significant differences between GSI in males of different age classes suggests similar levels of sexual maturity for all age classes of E. scotti males during spawning season. The bimodal GSI may suggest that only a portion of males in a population participate in spawning. Elevated GSI in nearly all females from April and May indicates 2010 S.D. Barton and S.L. Powers 697 that nearly all females spawn within a spawning season. The lack of relationship between SL and number of mature oocytes also suggests that females approaching their first full spawning season produce as many eggs as females approaching their second full spawning season. The relatively even contribution to the next generation by females and relatively uneven contribution to the next generation by males does appear to fit previous hypotheses regarding strongly sexually dimorphic species, but this uneven contribution by males is expectedly due to female choice, not lack of gonadal development (Bateman 1948). This leaves the uneven bimodal development of gonads in males a mystery. This uneven contribution to spawning by male E. scotti was not documented by Storey et al. (2006) and is unknown for E. coosae (O’Neil 1981). Mature oocytes from 0.7–1.2 mm in diameter in E. scotti appear similar to those for E. coosae, as O’Neil (1981) reported mature oocytes reaching a maximum size slightly greater than 1 mm in diameter. Reported fecundity is dramatically different among the two species, as the maximum number of mature oocytes observed in E. scotti was 256 (mean = 69) whereas O’Neil (1981) reported 288–496 oocytes in gravid female E. coosae. Much of this variation may be explained by all oocytes greater than 0.2 mm in diameter being counted by O’Neil (1981) versus only those greater than 0.7 mm in diameter being counted as mature oocytes in this study. Of all the oocytes reported by O’Neil (1981), 67% were less than 0.5 mm in diameter, leaving a mean of 126 oocytes greater than 0.5 mm in diameter per gravid female E. coosae—still approximately double the number of large oocytes in E. coosae that we found in E. scotti (mean = 69). The contrasting longer lifespan for E. coosae and strong relationship of number of eggs and SL reported by O’Neil (1981), and shorter lifespan and lack of relationship between oocytes and SL for E. scotti in this study suggest that the noted differences among fecundity are not simply differences in methodology of investigators. Rather, these findings suggest that E. coosae, with greater fecundity and proportion of lifespan in sexual maturity have a higher reproductive potential than E. scotti. This higher reproductive capacity would allow for a greater ability to reestablish populations following a disturbance and likely makes E. coosae less susceptible to local extirpation than E. scotti. Acknowledgments We thank C. Fortenberry and D. Holder for assistance with field work. We thank Reinhardt College for equipment used for field work and collection of data, and Roanoke College for facilities and equipment used during data collection, data analyses, and manuscript preparation. Fishes were collected under Georgia Scientific Collecting Permit number 16494 and US Fish and Wildlife Service Federal Fish and Wildlife Permit number TE136747-0 issued to S.L. Powers. This study was in part conducted as an undergraduate research project by S.D. Barton at Reinhardt College and did not violate Institutional Animal Care Protocol at Reinhardt. 698 Southeastern Naturalist Vol. 9, No. 4 Literature Cited Anderson, A., A.C. Hubbs, K.O. Winemiller, and R.J. Edwards. 1995. 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