2009 SOUTHEASTERN NATURALIST 8(3):495–502 Abundance and Distribution of Larval and Juvenile Fundulus heteroclitus in Northeast Florida Marshes Stacy N. Galleher1,*, Iara Gonzalez1, Matthew R. Gilg1, and Kelly J. Smith1 Abstract - Larvae and juveniles of Fundulus heteroclitus (Mummichog), commonly occur in small, water-filled depressions on the intertidal marsh surface during low tide. Previous work has shown that larger juveniles are typically found at lower elevations on the marsh surface, while small larvae are more abundant in the high marsh. The present study compared the abundance and size distributions of larval and juvenile Mummichog between relatively low- and high-elevation sites on the marsh surface at three locations in Northeastern Florida. Fundulus heteroclitus were both more abundant and larger in size at low-elevation sites than at high-elevation sites following the size-selective marsh-use pattern shown in other locations. Introduction Fundulus heteroclitus (L.) (Mummichog), is an example of a resident salt marsh fish that spends all stages of its life history exposed to the widely fl uctuating abiotic conditions of the tidal marsh (Kneib 1986, Lipcius and Subrahmanyam 1986, Weisberg 1986). Fundulus spp. play an important role in salt marsh energetics, as both a predator and a source of food for other fish and commercially important invertebrates (Allen et al. 1994, Kneib 1986). Although Fundulus spp. are nearly ubiquitous in salt marshes along the US East coast, the marsh habitat is not a homogenous environment and patchy distributions of young Fundulus spp. and other nekton are commonly reported (Kneib 1984, 1987; Kneib and Wagner 1994; Rozas 1995). Extreme environmental conditions on the marsh surface prevent most teleosts, except for a few well-adapted genera such as Fundulus, from utilizing this habitat for reproduction. Adult killifish predominantly reside in tidal creeks and access the marsh surface during high tide to feed and reproduce (DiMichelle and Taylor 1980; Kneib 1984, 1997a). Larvae inhabit natural depressions on the marsh between culms of the emergent Spartina alternifl ora Loisel. (Smooth Cordgrass) (Kneib 1984). These shallow depressions are highly variable and susceptible to extreme abiotic conditions because they are only fl ushed during a subset of high tides. Northeast Florida, specifically the area south of St. Augustine, FL close to Indian River Lagoon, is the southern-most extent of the Mummichog’s range (Gonzalez et al. 2009). Previous studies on Sapelo Island, GA have shown differences in marsh use by larval and juvenile Mummichog related to intertidal elevation, with smaller fish inhabiting high-marsh areas and 1Department of Biology, University of North Florida, 1 UNF Drive, Jacksonville, FL 32224. *Corresponding author - s_galleher@yahoo.com. 496 Southeastern Naturalist Vol. 8, No. 3 larger fish residing closer to the marsh edge (Kneib, 1984, 1986, 1993, 1997a). A similar pattern of larval and young juvenile fish abundance on the high-marsh surface has also been shown in other locations such as New Jersey (Able and Hagan 2000, 2003; Talbot and Able 1984), Delaware (Taylor et al. 1977), and Connecticut (Fell et al. 2003). Some of these studies have found that abundance of Mummichog larvae may be an indicator of marsh health in areas with the introduced plant species such as Phragmites australis (Cav.) Trin. ex Steud. (Common Reed) (Able and Hagan 2000, 2003; Able et al. 2003). Kneib (1997b) hypothesized that the patterns of marsh use by larval and juvenile Mummichog may be a key factor in tidal marsh trophic dynamics, and reductions of viable larval habitat could have significant implications on energy transfer throughout the marsh ecosystem. Therefore, in the face of many changing abiotic and biotic factors on Florida marshes including climate and land-use changes, and the introduction of non-native species, we conducted a study to determine a current baseline for abundance and distribution of Mummichog larvae and juveniles at different intertidal elevations in salt marshes of northeastern Florida. Methods Distribution and abundance of larval and juvenile Mummichog were determined by collecting fish from pit traps at three intertidal marsh locations in northeastern Florida, including Nassau River (30.5209850°N, 81.4986218°W), Crescent Beach (29.9507020°N, 81.3109463°W), and Pellicer Creek (29.6797363°N, 81.2245700°W) (Fig. 1). The Nassau collection site was chosen based on abundance of adult Mummichog recorded by the Fish and Wildlife Research Institute (FWRI) Jacksonville field lab from 2001– 2004. All other sites were chosen because they had habitat characteristics similar to the Nassau site. Collection sites were situated adjacent to small intertidal creeks that fed into larger subtidal channels. Intertidal vegetation was predominantly Smooth Cordgrass with Juncus roemerianus Scheele (Black Needlerush) located nearby but not within sampling sites. At each location, traps (10-cm diameter x 5-cm height pyrex containers) were placed in two grids consisting of 9 traps each in a 3 x 3 formation. Each trap was buried to its top edge in the marsh substratum, and held fl ush to the surface by three metal stakes. One grid was placed at each of two elevations (low and high) within each location. Simple devices consisting of plastic mesh markers with fl oats were held in place by wooden dowels set at each corner of the sampling grids to measure maximum tidal inundation prior to each sampling event. Low- and high-elevation sites were characterized by a mean spring tidal inundation of 45.7 (±12.97) cm and 28.7 (±13.22) cm, respectively (Table 1). Grids were located within 30 m of each other near the spring high-tide mark and were completely inundated during spring high tides. All traps were sampled at least once after each semi-monthly set of spring tides from March–July in 2005 at the Nassau and Crescent sites and March–July 2006 at all three sites. Fish were identified, enumerated, and measured to the nearest mm standard length (i.e., 2009 S.N. Galleher, I. Gonzalez, M.R. Gilg, and K.J. Smith 497 tip of snout to end of vertebral column at the caudal peduncle). Mummichog was the dominant fish species at all sites (Gonzalez et al. 2009). Other species caught by pit traps (e.g., Poecilia latipinna (Lesueur) [Sailfin Molly] and Figure 1. Larval and juvenile pit-trap sampling locations in Northeast Florida. Table 1. Average maximum tidal inundation and standard deviations for all sites (cm) from March–July 2006. Sites Mean S.d. Low Nassau 44.70 14.15 Crescent 47.81 14.05 Pellicer Creek 44.46 10.72 High Nassau 29.54 12.58 Crescent 35.55 16.43 Pellicer Creek 21.12 10.64 498 Southeastern Naturalist Vol. 8, No. 3 F. confl uentus Goode and Bean in Goode [Marsh Killifish]) were not included in this study. Mean abundance of fish collected per sampling event was compared between high-and low-elevation sites using paired t-tests. Length frequencies were not normally distributed and so were compared between elevations within locations using non-parametric Mann-Whitney rank sum tests. A reciprocal transformation was applied prior to analysis in order to equalize variances. Size distributions among locations were normally distributed, and so mean standard lengths were compared using ANOVA and post hoc Tukey’s HSD applied separately to high- and low-elevation sites. Results Mean number of fish collected per sampling interval was significantly greater at low- than high-elevation sites at all locations (Nassau: t5df = 2.58, P = 0.0491; Crescent: t4df = 3.12, P = 0.0354; Pellicer: t5df = 3.38, P = 0.0196). When samples were pooled over the season, mean abundance was approximately two times greater at low-elevation sites than high-elevation sites (Table 2). Fish from low-elevation sites were significantly larger than fish collected from high-elevation sites at all locations and in both years (Table 2). Inequality of variances between years precluded the use of all data in comparisons of length distributions across locations. Consequently, comparisons were made only for 2006, when data were available from all three locations (Nassau River, Crescent Beach, and Pellicer Creek). Mean length at high-elevation sites did not differ significantly among locations (Table 3, Fig. 2), but mean length (reciprocally transformed) at low-elevation sites was greater at Nassau than at the other two locations (Table 4, Fig. 2). Table 2. A series of separate Mann-Whitney Rank Sum tests performed on standard length (SL) distributions for all locations for both sampling years. Reciprocal transformation of all data was performed to homogenize variances. * = significant comparisons marked in bold. Location/year n Mean SL (mm) Mean rank Sum of ranks Mann-Whitney U P Nassau 05 High 152 7.7090 253.48 38,529.00 Low 249 9.3092 168.96 42,072.00 10,947.000 <0.001* Nassau 06 High 61 9.1285 151.25 9226.50 Low 156 13.0641 92.48 14,426.50 2180.500 <0.001* Crescent 05 High 150 7.5024 276.29 41,444.00 Low 326 8.4407 221.11 72,082.00 18,781.000 <0.001* Crescent 06 High 50 8.2316 99.79 4989.50 Low 109 10.9900 70.92 7730.50 1735.500 <0.001* Pellicer 06 High 5 6.5240 10.30 51.50 Low 66 10.2326 37.95 2504.50 36.500 0.004* 2009 S.N. Galleher, I. Gonzalez, M.R. Gilg, and K.J. Smith 499 Discussion In all study locations, Mummichog displayed a pattern of size selectivity in marsh use, with larvae/juveniles less abundant in the high marsh, and smaller size classes found in higher elevation areas farther from the marsh Table 3. Comparison of mean standard length of Mummichog from high-elevation sites among locations from 2006. Source Sum of squares df Mean square F P Corrected model 45.272(a) 2 22.636 2.253 0.110 Intercept 2413.144 1 2413.144 240.198 <0.001 Location 45.272 2 22.636 2.253 0.110 Error 1135.253 113 10.046 Total 9819.155 116 Corrected total 1180.525 115 Figure 2. Mean larval length ± 1 s.d. from 2006. Significant differences among locations are indicated by different letters above the bars. Table 4. Comparison of reciprocally transformed mean standard length of Mummichog from low-elevation sites among locations from 2006. Source Sum of squares df Mean square F P Corrected model 0.035(a) 2 0.017 14.487 <0.001 Intercept 2.950 1 2.950 2461.606 <0.001 Location 0.035 2 0.017 14.487 <0.001 Error 0.393 328 0.001 Total 3.564 331 Corrected total 0.428 330 500 Southeastern Naturalist Vol. 8, No. 3 edge. These data show that Northeast Florida marshes display a pattern of marsh-surface use by young Mummichog similar to that reported in Georgia (Kneib 1984, 1986) and New Jersey (Talbot and Able 1984). Small fish may use high-marsh areas to avoid nektonic predators, including larger conspecifics that are too large to inhabit the shallow marsh-surface depressions (Kneib 1997a). It has been hypothesized that larger juveniles tend to reside closer to the marsh edge because: (1) there are typically more and deeper puddles of residual tidal water on the exposed marsh surface at lower elevations closer to creeks or rivulets (Able and Hagan 2003; Kneib 1984, 1997a), and (2) food resources may be re-distributed and replenished by more frequent tidal inundation (Kneib 1997b). Size-selective marsh use has been observed in previous studies; however, it is difficult to make direct comparisons across studies due to differences in methodology and the definitions of Mummichog larvae among researchers. Kneib (1984) defined larvae as fish less than 10 mm SL, the size at which they developed a full adult complement of fin rays. Able et al (2003) defined larvae as 20 mm TL (≈16 mm SL), the size at which scale formation was complete. The present study followed Able et al (2003) and defined the maximum size of larvae as 16 mm SL. Kneib’s (1984) study has the most in common with the present study, with greater fish abundance at low elevations (less than 10 m from marsh creek) than at high elevations (135 m from marsh creek). Talbot and Able (1984) found many larval fish at a high-elevation creek site in New Jersey and, in a later study conducted at low-elevation sites, Able and Hagan (2000, 2003) found a greater abundance of young fish between 5 and10 m from the marsh creek, with larger individuals found on the marsh edge. Although the size definition of larvae and juveniles may be different among studies, the general intertidal trend in size distribution, with smaller fish residing near the upland border of the marsh and larger, more abundant fish closer to the creek, is recognized across multiple geographic locales. Some differences between the present study and earlier work, including the relative abundance of fish sampled and the ratio of larvae to juveniles, may be a result of the sampling methods. In this study, collections occurred during the spring tide, where the maximum abundance of larvae is expected (Kneib 1997a), but these numbers do not account for high larval mortality or recruitment into other size classes. Several alternative hypotheses have been proposed to explain the intertidal distribution pattern of young Mummichog in intertidal marshes. These have included the occurrence and physical characteristics of aquatic refugia, frequency and duration of tidal inundation, and the distribution of prey, predator, or competitor fields (e.g., Able and Hagan 2003; Able et al. 2003, 2007; Kneib 1984, 1986, 1993, 1997a). It was beyond the scope of the present investigation to distinguish among these alternative explanations for the observed distribution of young Mummichogs in marshes of northeastern Florida, but the similarity of pattern in the intertidal distribution of larvae and juveniles from New Jersey to north Florida suggests that Mummichog 2009 S.N. Galleher, I. Gonzalez, M.R. Gilg, and K.J. Smith 501 may be an excellent model organism for future research aimed at exploring the ecological consequences of abiotic and biotic variability along intertidal gradients within salt marsh ecosystems. The size-selective pattern of marsh use by Mummichog observed throughout its range could be important in comparing functional aspects of different salt marshes. Able and Hagan (2000, 2003) reported a reduction in Mummichog larvae in marshes dominated by the invasive Common Reed when compared to adjacent Cordgrass-dominated marshes, suggesting that larval abundance may be an indicator of marsh nursery-habitat function. The high marsh can be susceptible to a variety of disturbances, such as non-native plant introductions or activities on uplands adjacent to marshes (e.g., dredging and residential or industrial development) and could similarly affect the presence or availability of intertidal aquatic microhabitats on the marsh. High marshes immediately adjacent to disturbed uplands may be the most vulnerable to these types of disturbances that could remove essential nursery habitat for the smallest larvae, forcing them to occupy lower marsh elevations where they are less protected from predation. Northeast Florida marks the southern extent of the Mummichog’s range, so it may be interesting to determine if marsh-use patterns differ in the zone of overlap with Fundulus grandis Baird and Girard (Gulf Killifish), a closely related species that occurs just south of Pellicer Creek (Gonzales et al. 2009). Gulf Killifish are generally larger as adults than Mummichog, and if they display similar marsh-use patterns, Gulf Killifish could preferentially inhabit low-marsh areas in greater numbers, possibly affecting the area available to each species. Predation by, or competition with, the larger Gulf Killifish in lower marsh habitat may be a factor that could affect Mummichog population densities. 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