2011 SOUTHEASTERN NATURALIST 10(1):145–154 Larval Fish Dynamics in Spring Pools in Middle Tennessee Phillip W. Bettoli1,* and Cory Goldsworthy2 Abstract - We used lighted larval traps to assess reproduction by fishes inhabiting nine spring pools in the Barrens Plateau region of middle Tennessee between May and September 2004. The traps (n = 162 deployments) captured the larval or juvenile forms of Etheostoma crossopterum (Fringed Darter) (n = 188), Gambusia affinis (Western Mosquitofish) (n = 139), Hemitremia flammea (Flame Chub) (n = 55), the imperiled Fundulus julisia (Barrens Topminnow) (n = 10), and Forbesichthys agassizii (Spring Cavefish) (n = 1). The larval forms of four other species (Families Centrarchidae, Cyprinidae, and Cottidae) were not collected, despite the presence of adults. Larval Barrens Topminnow hatched over a protracted period (early June through late September); in contrast, hatching intervals were much shorter for Fringed Darter (mid-May through early June). Flame Chub hatching began before our first samples in early May and concluded by late-May. Juvenile Western Mosquitofish were collected between early June and late August. Our sampling revealed that at least two species (Flame Chub and Fringed Darter) were able to reproduce and recruit in habitats harboring the invasive Western Mosquitofish, while Barrens Topminnow could not. Introduction Numerous springs dot the landscape of the Barrens Plateau region in middle Tennessee and provide habitat for rare spring-associated fishes including Forbesichthys agassizii (Putnam) (Spring Cavefish), Fundulus julisia Williams and Etnier (Barrens Topminnow), and Hemitremia flammea (Jordan and Gilbert) (Flame Chub) (Etnier and Starnes 2001). Spring habitats throughout the Barrens Plateau region have been altered during development of farmlands and nurseries, which has limited already rare spring-dwelling fishes to a few locales (Rakes 1989). Another anthropogenic manipulation of the ecosystems in the Barrens Plateau region is the liberal transplanting of Gambusia affinis (Baird and Girard) (Western Mosquitofish). The negative effects of introduced Western Mosquitofish on native fish assemblages (especially larvae) have been well documented (Belk and Lydeard 1994, Courtenay and Meffe 1989, Lydeard and Belk 1993, Myers 1967, Rincon et al. 2002). Reproduction (i.e., producing larval offspring) by Barrens Topminnows in the presence of Western Mosquitofish has been documented, but subsequent recruitment of juveniles to the spawning population is rarely observed (Goldsworthy and Bettoli 2006, Watts 2009). During the course of evaluating the efficacy of a Barrens Topminnow reintroduction program using hatchery-reared fish, larval fishes were sampled in a variety of habitats stocked with topminnows in the Barrens Plateau region in 1US Geological Survey, Tennessee Cooperative Fishery Research Unit, Box 5114, Tennessee Technological University, Cookeville, TN 38505. 2Minnesota Department of Natural Resources, 5351 North Shore Drive, Duluth, MN 55804. *Corresponding author - pbettoli@tntech.edu. 146 Southeastern Naturalist Vol. 10, No. 1 middle Tennessee. Goldsworthy and Bettoli (2006) reported where they caught larval Barrens Topminnow using light traps and noted the presence (or absence) of Western Mosquitofish. However, a depauperate literature exists on the biology of spring-associated fish species in Tennessee, and few larval fish studies exist for these fishes. In this paper, we report on the catches of larval and juvenile forms of five species we encountered in nine spring pools in the Barrens Plateau region and comment on the use of lighted larval traps in detecting reproduction by those fishes. Lighted traps have been developed and used for decades to sample larval fishes in freshwater and marine habitats (e.g., Bryan and Scarnecchia 1992, Floyd et al. 1984a, Sponaugle and Cowen 1996). However, larval fish studies of spring-associated fishes, especially troglophilic species, are rare using any approach or gear. We also describe patterns of emergence for four species captured in enough numbers (i.e., ≥10) to draw inferences regarding when and how often they spawned. Finally, we comment on the absence in our catches of larval forms of several species present as adults in the sites we sampled. Study Area Larval sampling was conducted at nine sites. Four sites were part of a small complex of seven natural and artificial spring pools (designated Clayborne 1 through 7) located on private property (the Clayborne complex; 35°30.351'N, 85°54.757'W) in the headwaters of the West Fork of Hickory Creek watershed in Coffee County, TN. The four pools that we sampled in this complex (Clayborne 1, 4, 6, and 7) ranged in size from 16 to 59 m2. One of the artificial pools (Clayborne 4) was isolated from the other pools by virtue of its elevation. Four pools in the Clayborne complex (Clayborne 3, 4, 6, and 7) were stocked with a total of 388 Barrens Topminnow between 2001 and 2003. The resident fishes in this complex of pools were Etheostoma crossopterum Braasch and Mayden (Fringed Darter), Semotilus atromaculatus (Mitchill) (Creek Chub), Spring Cavefish, Western Mosquitofish, and Flame Chub (Johnson 2004). Clayborne 4 only contained stocked Barrens Topminnow. Water temperatures in the springhead at this complex of pools between June 2002 and March 2003 (measured every three hours by a HOBO® recording data logger) ranged from 9.0 to 18.79 °C and averaged 14.9 °C (SE = 0.11). Submersed aquatic vegetation was scarce at all of these pools. Adjacent to and immediately downstream of the Clayborne Complex were two small (≈50 m2 each) connected pools on the Sain property (35o30.359'N, 85o54.714'W) that had been stocked with Barrens Topminnow. The resident fishes in these two pools were Flame Chub, Western Mosquitofish, and Fringed Darter (Johnson 2004). Aquatic vegetation was scarce at both of these pools. These two pools were stocked annually between 2002 and 2004 with a total of 267 Barrens Topminnow. We also sampled two sites that drained into Little Hickory Creek in Coffee County, TN. The most upstream site (Cunningham Barn; 35°29.569'N, 85°55.309'W) was a spring-influenced, excavated pool with a surface area of 191 m2. Water temperatures at this upstream site averaged 16.2 °C (SE = 0.34; range = 6.6–27.7) between August 2002 and April 2003. Resident fish 2011 P.W. Bettoli and C. Goldsworthy 147 fauna included Lepomis cyanellus Rafinesque (Green Sunfish), L. macrochirus Rafinesque (Bluegill), Western Mosquitofish, and Fringed Darter. The upper site was densely vegetated with Lemna spp. (duckweed) and Potamogeton spp. (pondweed). The downstream site (Cunningham Lower; 35°29.992'N, 85°54.426'W) consisted of two small (56–68 m2), excavated pools that were connected by a spring run. The two sites on the Cunningham property were stocked with 635 Barrens Topminnow between 2002 and 2004. The type locale (35°32.968'N, 85°59.015'W; Williams and Etnier 1982) for the Barrens Topminnow is a spring pond emanating from a cave located in the headwaters of the West Fork of Hickory Creek in the Caney Fork River watershed. A previous landowner constructed a small dam in the late 1970s or early 1980s to create a pool (approximately 150 m2) for Oncorhynchus mykiss Walbaum (Rainbow Trout). The trout dispersed downstream, but the pool acted as a refuge for Barrens Topminnow, and the dam (and subsequent modifications to it) prevented Western Mosquitofish invasion (Williams and Etnier 1982). A seine survey in April 2004 collected only Barrens Topminnow and Cottus carolinae (Gill) (Banded Sculpin). From 18 May 2003 to 21 April 2004, water temperatures at the type locale averaged 14.0 °C (range = 8.8–16.8 °C). Excluding the type locale, none of the sites we sampled harbored wild Barrens Topminnow before they were stocked with hatchery-reared fish in 2001–2004. We are unaware of any historical records of Barrens Topminnow at those stocked sites before Mosquitofish colonized the Barrens Plateau region. Methods The light traps we used were a modified design of the light traps used by Hartman (1994) which were based on a Tennessee Valley Authority design, as reported in Goldsworthy and Bettoli (2006). The main body of the trap consisted of a 10-cm long section of 28-cm (inside diameter) PVC pipe with about 1/3 of the circumference removed for an entrance. The entrance consisted of two pieces of clear acrylic plastic angled inward to create a 3-mm wide opening. The main body was glued to a 46- x 46-cm square piece of black plastic with holes drilled in each corner through which 25-cm bolts were passed to affix a similar piece of plastic, which acted as the removable top. A matching piece of 25-mm thick foam was attached to the removable top to make the traps float. With the exception of the Cunningham Barn site and type locale, the traps floated in shallow water, with the bottom of the traps less than 0.5 m off the bottom. At the two larger sites, the traps floated in water ≈1 m deep. The light source was a 12-hour photochemical stick that produced green light. In a laboratory trial, six of nine captive-reared Barrens Topminnows (20–25 mm TL) placed into a 720-l tank entered a lighted trap suspended in the tank, and in a second trial, none entered an unlit trap. Thus, we assumed that the trap would be sufficient for determining the presence/absence of larval topminnows, and we also assumed that the traps would capture other phototropic larval fishes. Floyd et al. (1984b) considered light traps semi-quantitative samplers at best because the frequencies (and intensity) of light that larvae respond to are unknown and turbidity in their study stream affected efficacy. Although we did not measure water 148 Southeastern Naturalist Vol. 10, No. 1 transparency or turbidity, water clarity at each site was high, which is characteristic of small, spring-influenced pools. Traps were deployed overnight at nine sites (one trap per site) at approximately weekly intervals between May and August 2004 and every two weeks in August and September 2004 (18 total trap-nights per site). Traps were deployed adjacent to the margin of each pool, retrieved the following morning, and the contents of each trap was washed through a 100-micron mesh net and fixed in 10% formalin solution. In the laboratory, samples were transferred to ethanol before processing. All larvae were subsequently counted and measured for total length; the total lengths of captured fishes were plotted over time for those sites where sufficient numbers of fish were collected in order to discern the duration of larval emergence. We obtained hatchery-propagated larval Barrens Topminnow to serve as a reference and used larval fish keys (Heufelder and Fuiman 1982, Hogue et al. 1976) to identify other species. Sampling at these sites using seines and backpack DC electrofishing gear to meet other project objectives during (and in the months and years preceding) our larval fish sampling yielded species inventories of adult fish species known to inhabit each site (Johnson 2004; P.W. Bettoli and C. Goldsworthy, unpubl. data), which aided in the identifications of larval forms. Results and Discussion The lighted traps (n = 162 deployments) captured a total of 393 larval or juvenile fishes, representing five species, in spring pools in the Barrens Plateau region. Those species were Flame Chub, Spring Cavefish, Barrens Topminnow, Western Mosquitofish, and Fringed Darter (Table 1). Flame Chub Flame Chub larvae and juveniles (n = 55) were collected at five of the six sites adults were known to inhabit (Table 2). The first larval Flame Chub were collected on 7 May 2004 at the Clayborne 7 spring pool (Fig. 1) and ranged from 6.2 mm to 15.0 mm TL; similar-sized Flame Chub were collected in late May. Progressively larger fish were collected through June, and the largest Flame Chub (29.7 mm TL) collected was also the last one observed on 19 June 2004. Flame Chub in an east Tennessee spring creek spawned between late January and May, with a peak in March (Sossamon 1990). The capture of a 15.0-mm TL larvae in early May suggests that Flame Chub were already spawning (and hatching) when Table 1. Species collected by larval light traps deployed at nine spring pools in the Barrens Plateau region of Tennessee, May–September 2004. Species n Range in total length (mm) Flame Chub 55 6.2–29.7 Spring Cavefish 1 8.0 Barrens Topminnow 10 7.2–24.0 Mosquitofish 139 9.5–32.0 Fringed Darter 188 5.8–21.5 2011 P.W. Bettoli and C. Goldsworthy 149 we commenced sampling. The absence of small (less than 10 mm TL) larvae after 27 May indicates that no spawning or hatching occurred in June; Sossamon (1990) also noted a cessation of spawning activity in June of her study years. Spring Cavefish Adult Spring Cavefish are a troglophilic species associated with, but not restricted to, caves (Etnier and Starnes 2001). In a Missouri study, adults appeared to occupy spring pools at night and retreat to subterranean cover near sunrise (Adams et al. 2002). We collected adult Spring Cavefish at three pools in the Clayborne complex (Table 2). However, only one larval Spring Cavefish (8.0 mm TL) was collected, and it was caught at Clayborne 1, the springhead pool, on 9 June 2004. Adams et al. (2002) caught no larval Spring Cavefish in light traps deployed at a Missouri spring site. Adult Spring Cavefish are negatively phototactic (Poulson 1963), and low catches of larvae in light traps might be expected. However, Adams et al. (2001) caught more than 100 larval and juvenile Spring Cavefish using lighted traps at an Illinois spring. In previous studies, Spring Cavefish fry, which hatch underground (Smith and Welch 1978), appeared in the epigean portions of their spring habitats between January and April (Adams et al. 2001, Poulson 1963); we did not begin deploying traps until May. However, our catch of a larval individual in early June indicates that spawning and hatching can occur much later than previously reported. Barrens Topminnow All nine sites harbored adult Barrens Topminnow through either stocking or natural reproduction (i.e., the type locale), but larval or juvenile Barrens Topminnow (n = 10) were collected at only three of those sites that we sampled with light traps. The first three Barrens Topminnow observed were larval forms (<10 mm Table 2. Number of individuals of nine fish species caught in lighted larval fish traps at nine spring sites in the Barrens Plateau region of middle Tennessee, May–September 2004. All sites where young-of-year fish were collected in the traps also harbored adults of those species. Open circles indicate that adults were present at those sites, but no young-of-year fish were collected in the fish traps. Flame Creek Spring Mosquito- Barrens Green Banded Fringed Site Chub Chub Cavefish fish Topminnow Sunfish Bluegill Sculpin Darter Clayborne 1 3 ○ 1 ○ ○ 2 Clayborne 4A 2 Clayborne 6 3 ○ 9 1 ○ Clayborne 7 44 ○ 8 ○ ○ Upper Sain 3 1 ○ 128 Lower Sain ○ 9 ○ 56 Cunningham 92 ○ ○ ○ 2 Barn Cunningham 2 ○ 20 ○ ○ Lower Type localeB 7 ○ AThis small, artificial pool was isolated from all nearby pools. BA concrete barrier erected at the outfall of this spring pool prevented colonization by stream fishes residing in downstream reaches. 150 Southeastern Naturalist Vol. 10, No. 1 TL) collected between 9 June 2004 and 8 July 2004 at two reintroduction sites stocked with propagated adults (Fig. 1), one of which also contained Western Mosquitofish. The low catches (or absence) of larval Barrens Topminnow was probably due to the presence of Western Mosquitofish at most of the sites (Goldsworthy and Bettoli 2006), given that they will readily prey upon larval Barrens Topminnow ex situ (Laha and Mattingly 2007). Etnier and Starnes (2001) noted that Barrens Topminnow can continue spawning into August and September, and the capture of the fourth larval topminnow (10.0 mm TL) at the type locale on 24 September 2004 would support their observation that the spawning period is protracted. That 10.0-mm TL individual was the only larval topminnow caught at the type locale. The six juvenile Barrens Topminnow (17–24 mm TL) were all collected at the type locale between 5 July and 24 September 2004. Figure 1. Total lengths of larval and juvenile fish captured in lighted traps deployed in nine spring pools in the Barrens Plateau region of middle Tennessee, May–September 2004. 2011 P.W. Bettoli and C. Goldsworthy 151 Western Mosquitofish Western Mosquitofish (n = 139) were the most common species captured in the light traps. Western Mosquitofish are livebearers; hence, no true larval Western Mosquitofish were collected. Young specimens (41% were less than 20 mm) were collected from the beginning of May to the end of September (Fig. 1). The smallest Western Mosquitofish collected was 9.5 mm TL, and the largest was 32 mm TL. No noticeable growth trend was evident, which is typical for a species that spawns multiple times between May and September (Vargas and de Sostoa 1996); those authors and others have noted that viviparity, multiple broods per female, and large larvae provide Western Mosquitofish with a competitive advantage over oviparous species. Adult Western Mosquitofish were present at seven of the nine sites we sampled, and juveniles were captured in light traps at six of those seven sites (Table 2). Fringed Darter Fringed Darter larvae (n = 188) were collected at four of the seven sites inhabited by adults (Table 2). Yolk-sac larvae appeared in light-trap samples from 13 May to 9 June 2004 (Fig. 1). The smallest Fringed Darter was 5.8 mm TL, the largest was 21.5 mm TL (17 June), and the last date of capture of any Fringed Darter was 8 July 2004. Poly (2000) observed Fringed Darter nests from mid-April to late May in a variety of degraded habitats in southern Illinois streams. Given the changes in lengths of larvae collected over time, Fringed Darter at the upper Sain site hatched over about 30 days (Fig. 1). Gregory and Powles (1985) noted that larval E. exile (Girard) (Iowa Darter) collected by light traps emerged over 22 days in an Ontario lake. Other Etheostoma species (e.g., E. simoterum [Cope] [Snubnose Darter]; E. blennoides [Rafinsesque] [Greenside Darter]) were among the most common larvae captured in light traps deployed by Hartman (1994) in a small Tennessee stream and Farmer (2001) in the French Broad River of east Tennessee. Floyd et al. (1984b) also reported large catches of four Etheostoma species in light traps deployed in a small Kentucky stream. Thus, members of this genus are phototaxic and readily susceptible to capture using lighted larval traps, as Gregory and Powles (1985) also noted. The absence of larval Fringed Darter in traps deployed at three sites where adults were collected (Table 2) suggests those particular sites provided suitable habitat for adults, but not good spawning or juvenile nursery habitat (i.e., they were probably sink populations). Species that were not collected in the larval traps Adult Creek Chub were observed at two of the nine sites, and none of their larvae were collected by the light traps (Table 2). Floyd et al. (1984b) collected larval Creek Chub in light traps set in a Kentucky stream; peak catches occurred between mid-April and mid-May in that system. The absence of larval Creek Chub in our samples, despite the presence of adults, indicates that either the adults were transients and unable to find suitable spawning habitat or we sampled too late in the season to collect larval Creek Chub. Adult Bluegill and Green Sunfish were observed at one site (the Cunningham Barn site), but no larval sunfish were collected at that site. Larval sunfish are susceptible to light traps (e.g., Gregory and 152 Southeastern Naturalist Vol. 10, No. 1 Powles 1985, Killgore and Baker 1996, Niles and Hartman 2007), including larval Bluegill (Farmer 2001, Floyd et al. 1984b, Kissick 1993) and larval Green Sunfish (Floyd et al. 1984b, Muth and Haynes 1984). Their absence in light-trap samples at the Cunningham Barn site suggests that there was no spawning habitat in that pond, and the adult sunfish were transients. Finally, Banded Sculpin were common at the type locale (the only species other than Barrens Topminnow at that site), but no larval sculpins were collected. Hartman (1994) did not report capturing Banded Sculpin larvae in light traps set in a small Tennessee stream, but noted that adults were common. Likewise, Floyd et al. (1984b) also failed to capture Banded Sculpin larvae in light traps in a stream where juvenile and adults were present. These observations might suggest that larval sculpins are not phototaxic. Another possible explanation for their absence in our samples is that Banded Sculpin spawn in late winter or early spring (Etnier and Starnes 2001); larvae may have grown and assumed a demersal existence (and been less likely to swim into a floating trap) by the time we deployed our traps in early May. Conclusions Sponaugle and Cowen (1996) and others have noted the taxonomic biases associated with using larval light traps; namely, a species must be positively phototaxic and display directional swimming at the larval stage in order to be sampled. However, all gears have biases associated with them, and lighted traps provide important information on the production and presence of larval forms for those species susceptible to capture in that type of gear (Thorrold 1992). Some of the spring pools we sampled were too small or harbored too much vegetation to be effectively sampled with other gear such as tow nets; the advantages of using light traps in shallow systems, despite their shortcomings, were also noted by Marchetti et al. (2004). The presence of larval Barrens Topminnow at some sites where subsequent recruitment could not be documented provided an important clue regarding the critical period in the early life history of that imperiled species (Goldsworthy and Bettoli 2006). The capture of numerous larval Flame Chub and Fringed Darter in the light traps (and adults of both species in other gears) indicated that recruitment was occurring in those populations despite the presence of numerous Western Mosquitofish juveniles and adults. Sossamon (1990) also noted that Western Mosquitofish were abundant at her study site in the upper Tennessee River watershed that supported a robust population of Flame Chub. Fringed Darter and Flame Chub have long inhabited springs in the Barrens Plateau region (Etnier and Starnes 2001) and persisted in many habitats during our study, which cannot be said of the Barrens Topminnow. The population dynamics and behavior of Western Mosquitofish and native fishes, especially at the early life-history stage, need to be examined more closely to determine the mechanisms that allow some species (e.g., Flame Chub and Fringed Darter), but not others (Barrens Topminnow), to persist in the presence of dense populations of invasive Western Mosquitofish. 2011 P.W. Bettoli and C. Goldsworthy 153 Acknowledgments Funding and support for this project was provided by the Tennessee Wildlife Resources Agency, the US Fish and Wildlife Service, and The Nature Conservancy of Tennessee. Additional support was provided by the Center for the Management, Utilization, and Protection of Water Resources at Tennessee Technological University, and the US Geological Survey. 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