2012 NORTHEASTERN NATURALIST 19(3):475–486 Tidal Migrations of Intertidal Salt Marsh Creek Nekton Examined with Underwater Video Matthew E. Kimball1,2, 3,* and Kenneth W. Able3 Abstract - Nekton tidal migration patterns were examined in oligo-mesohaline intertidal salt marsh creeks using underwater video observations collected throughout multiple tidal cycles (i.e., flood–ebb) during summer 2005–2006. Underwater video observations indicated that species composition and abundances varied with tide stage. Three intertidal salt marsh species (Fundulus heteroclitus, Morone americana, Menidia menidia) were the most abundant species observed. In general, resident species were most abundant in early flood and late ebb tide stages, whereas transient species were most abundant around slack high tide. F. heteroclitus displayed a consistent symmetrical tidal migration pattern and primarily occurred in early flood and late ebb tide stages. M. americana occurred throughout flood and high tides, but were largely absent from intertidal creeks during ebb tide. M. menidia was observed during all tide stages, but displayed no distinct migration patterns. The results of this study highlight the advantages and disadvantages of using underwater video for examining small-scale tidal migrations of nekton in intertidal salt marsh creeks. Introduction Salt marshes are a mosaic of multiple interconnected habitats that include the vegetated marsh surface, marsh ponds and pools, intertidal and subtidal creeks, and open-water habitats (Minello et al. 2003, Rountree and Able 2007). During periods of tidal inundation, intertidal creeks function as an important physical and biological corridor linking marsh surface with subtidal habitats (McIvor and Odum 1988, Rozas et al. 1988). Nekton utilization of salt marsh habitats varies temporally and spatially relative to physical and biological factors such as tidal cycle and species-specifi c migration patterns (Cattrijsse and Hampel 2006, Kneib 1997, Rozas 1995). In general, nekton migrate or follow the tide as it rises (i.e., floods), thereby gaining access to highly productive intertidal marsh surface areas, then similarly follow the ebbing tide as waters recede into subtidal habitats. Beyond this widely accepted general pattern, however, information is lacking to describe tidal migration patterns for many species commonly found in intertidal salt marsh creeks. The majority of studies on intertidal salt marsh creek nekton have focused on a single tide stage (e.g., high tide) or a large consolidated portion of the tidal cycle (e.g., ebb tide) (Bozeman and Dean 1980; Cain and Dean 1976; Desmond et al. 2000; Jin et al. 2007, 2010; Koutsogiannopoulou and Wilson 2007; LeQuesne 2000; Paterson and Whitfi eld 1996, 2000, 2003; Quan et al. 2009; Reis and Dean 1GTM National Estuarine Research Reserve, 505 Guana River Road, Ponte Vedra Beach, fl32082. 2Department of Biology, University of North Florida, 1 UNF Drive, Jacksonville, fl32224. 3Marine field Station, Institute of Marine and Coastal Sciences, Rutgers University, 800 c/o 132 Great Bay Boulevard, Tuckerton, NJ 08087. *Corresponding author - m.kimball@unf.edu. 476 Northeastern Naturalist Vol. 19, No. 3 1981; Shenker and Dean 1979). Relatively few studies have examined the distribution and habitat utilization of nekton in intertidal creeks on smaller temporal and spatial scales throughout tidal cycles (but see Bretsch and Allen 2006b, Cattrijsse et al. 1994, Hampel et al. 2003, Salgado et al. 2004). The diffi culties associated with sampling nekton in salt marshes (e.g., highly dynamic environmental conditions), along with the limitations of traditional sampling gears (e.g., block nets, seines, fyke nets), often limit the scale (both temporal and spatial) and scope of examination possible in various salt marsh habitats, including intertidal creeks (Connolly 1999, Rozas and Minello 1997). Recently, underwater video has been successfully used in shallow (less than 1.5 m) salt marsh habitats (e.g., fringing intertidal mangrove, shallow patches in open estuarine waters, salt marsh ditches) to examine fi sh behavior (Becker et al. 2010, Ellis and Bell 2008, James-Pirri et al. 2010). In this study, we used underwater video to examine small-scale tidal migration patterns of nekton in intertidal salt marsh creeks throughout the tidal cycle to elucidate nekton behavior and habitat use that may have heretofore been overlooked or unobservable using traditional sampling gears. Methods field-site description Sampling occurred in the oligo-mesohaline salt marshes in the Hog Islands area of the Mullica River–Great Bay estuary (39°34'46.35"N, 74°31'52.20"W), which is part of the Jacques Cousteau National Estuarine Research Reserve (JCNERR) located in southern New Jersey. Intertidal marsh areas were characterized by semi-diurnal tides and marsh vegetation consisting of a mix of species including (all species names from United States Department of Agriculture Plant Database, http://plants.usda.gov/java/): Atriplex patula L. (Spear Saltbush), Pluchea odorata (L.) Cass. var. succulenta (Fernald) Cronquist (Sweetscent), Schoenoplectus maritimus (L.) Lye (Cosmopolitan Bulrush), Spartina alterniflora Loisel (Smooth Cordgrass), S. patens (Aiton) Muhl. (Saltmeadow Cordgrass), and large interspersed stands of Phragmites australis (Cav.) Trin. ex Steud. (Common Reed). field sampling Intertidal creeks sampled (n = 4 creeks) had similar widths (mean width at mouth = 3.4 m, SE = 0.3) and lengths (mean length = 75.7 m, SE = 13.1), and all creeks had soft mud substrate bottoms with a few remaining pools of water at low tide. Average water depth at slack high tide in all creeks was <1 m. All creeks had a similar degree of sinuosity, terminated on the high marsh, and were unconnected to other creeks at high tide. To observe nekton, an underwater video sampling system was positioned upstream of the mouth of each creek (mean distance from creek mouth = 19.4 m, SE = 3.5), consisting of a flume (fig. 1), low-light black-and-white camera (2.8 mm, f2.8 wide-angle lens, 0.27 Lux), self-contained video recorder, and battery. To guide intertidal creek nekton past the camera, the flume was constructed in an "X" shape with a center channel (length x width x depth = 0.9 x 0.4 x 0.9 m) and four wings (1.2 x 0.9 m) extending onto the marsh surface at approximately 45° angles. The flume frame was constructed of PVC piping, and cloth mesh 2012 M.E. Kimball and K.W. Able 477 (3.2 mm) was used for the walls of the wings and one side of the center channel (i.e., the camera side). The wall opposite from the camera consisted of a solid sheet of white plastic to provide a contrasting background that enhanced nekton identifi cation (e.g., Daum 2005). The camera was placed 30 cm above the creek bottom with the lens inserted through the mesh and positioned flush with the channel wall. In this confi guration, the water column from approximately 15–50 cm could be observed (fi eld of view at distance of 40 cm: 45 x 35 x 52 cm). Once constructed, flumes were left in place in all creeks for the duration of the study, while video equipment was moved among sampling sites. In order to determine nekton abundance throughout the tidal cycle, each intertidal creek was sampled with underwater video during consecutive flood and ebb tides. To coincide with periods of high abundance for the dominant salt marsh nekton species in Mid-Atlantic Bight estuaries (Able and Fahay 2010), sampling occurred monthly in late summer: from August to September in 2005 (n = 8 tidal cycles) and from July through September in 2006 (n = 10 tidal cycles). During daylight hours of each sampling day, two creeks were sampled, flood through ebb tide, for a total of 18 tidal cycles observed. On each sampling day, prior to tidal inundation, the camera, recorder, and battery were integrated with the flume in each creek. Video recording began when the camera was totally submerged on the flood tide and continued uninterrupted until the camera emerged during ebb tide. Surface temperature and salinity were measured in individual creeks, figure 1. Overhead diagram of video sampling system flume in an intertidal creek (video recorder and battery are not pictured). The camera backdrop is indicated by the solid line. Dashed lines indicate mesh walls. Arrows indicate the creek center channel flow directions. 478 Northeastern Naturalist Vol. 19, No. 3 at slack high tide, with a handheld meter (YSI model 85): temperature (n = 18), but not salinity (n = 15), was recorded for each tidal cycle. Turbidity (NTU) was measured with a nearby datasonde (Lower Bank) deployed as part of the System- Wide Monitoring Program at JCNERR (data made available from the NERRS Central Data Management Offi ce). Data analysis Each video record was viewed completely, and all individuals observed in each 1-min increment were identifi ed and enumerated. Because of low abundances and patchiness of migrating individuals, sub-sampling at time increments >1 min was not feasible. To examine nekton tidal utilization patterns, video footage also was segmented by tidal stage. We divided each complete flood (beginning of recording to slack high tide) and ebb (slack high tide to end of recording) tide into 8 tide stages of equal length (flood: tide stages 1–8, mean duration = 25 min, SE = 1; ebb: tide stages 9–16, mean duration = 20 min, SE = 1). In each tide stage (1–16), the counts (i.e., number of individuals of a given species) from each 1-min increment were summed by species. This protocol removed the time component within the tidal cycle and provided species totals for each of the 16 tide stages. Because the purpose of this study was to examine fi sh behavior in intertidal creeks throughout the tidal cycle, we sought to minimize the influence of sometimes highly variable site-specifi c differences in nekton species abundances on statistical analyses. Therefore, species abundance for individual tide stages was expressed as a fraction of the maximum tide-stage abundance observed for that species during an entire tidal cycle (i.e., individual tide-stage abundance/ maximum tide-stage abundance) (e.g., Ellis and Bell 2008). Thus, regardless of the total number of individuals observed for a given species during any tidal cycle, abundance values for each tide stage ranged from 0 to 1 (Ellis and Bell 2008). Using this calculation allows us to treat each tidal cycle as independent, and therefore compare nekton tidal behavior among the observed tidal cycles to examine the consistency of migration patterns. For each tidal cycle, mean species tide-stage abundance values were calculated for three tide classes: flood, stages 1–6; high, 7–10; ebb, 11–16. Tide class abundance data for 18 tidal cycles was then analyzed with a repeated measures analysis of variance (PROC GLM; SAS 2003). Differences in tide class abundances (the repeated factor) were analyzed with individual contrasts (von Ende 2001). Only species with a total abundance ≥ 75 individuals for all tidal cycles combined were analyzed (n = 3 species; this analysis included 98% of all individuals observed with underwater video during this study; see Table 1). Individual species were assigned to an estuarine category (i.e., resident, transient, freshwater) (Able and Fahay 1998, 2010; Arndt 2004). Environmental and water quality data were examined with descriptive statistics. Results A total of 109.5 h of underwater video data from intertidal marsh creeks recorded over 18 tidal cycles was reviewed for analysis. During fi eld recordings, temperatures were similar between years (mean temperatures: 2005 = 26.4 °C, SE = 0.04; 2006 = 25.7 °C, SE = 1.0). Salinities were lower in 2006 (mean 2012 M.E. Kimball and K.W. Able 479 salinity = 5.6, SE = 1.3) than in 2005 (mean salinity = 11.9, SE = 0.6). Turbidities were lower in 2005 (mean turbidity = 5.1 NTU, SE = 1.8) than in 2006 (mean turbidity = 14.4, SE = 7.4). Based on underwater video data, fi shes dominated the intertidal creek nekton with 9 species and 5679 individuals out of a total of 11 species and 5722 individuals observed (Table 1). Callinectes sapidus Rathbun (Blue Crab) (n = 35) and Malaclemys terrapin Schoepff (Diamondback Terrapin) (n = 8) were also observed during the study. Four resident marsh nekton species accounted for 88% of the total, and most (87%) of these individuals were Fundulus heteroclitus L. (Mummichog). Other resident marsh nekton included Morone americana Gmelin (White Perch), Gobiosoma bosc Lacepède (Naked Goby), and M. terrapin. five transient marsh nekton species made up 11% of the total. These were primarily Menidia menidia L. (Atlantic Silverside; 9%), but also included C. sapidus, Brevoortia tyrannus Latrobe (Atlantic Menhaden), Pomatomus saltatrix L. (Bluefi sh), and Anguilla rostrata Lesueur (American Eel). Two freshwater species, Ameiurus nebulosus Lesueur (Brown Bullhead) and Notemigonus crysoleucas Mitchill (Golden Shiner), were infrequently observed and represented only 1% of the total. Shrimps were infrequently observed in video recordings and were not reported on in this study. Species composition and abundance, as observed by underwater video, were influenced by tide stage (fig. 2). The number of species observed during flood (n = 10 species) and ebb (n = 11 species) tide stages was similar, but resident nekton dominated (>75%) early flood tide stages (1–3) and most ebb tide stages (10–16). Transient nekton were most abundant in the late flood tide stages (4–8; Table 1. Intertidal creek nekton species composition and abundance (number of nekton per tidal cycle, expressed as catch-per-unit-effort, CPUE, with standard error, along with the maximum number of individuals per an entire tidal cycle and the overall total number of individuals) observed with underwater video during 18 tidal cycles. Each species was assigned to an estuarine category (EC): estuarine resident (R), estuarine transient (T), or freshwater (F). The relative abundance of estuarine resident, estuarine transient, and freshwater species is indicated as a percentage of the total abundance. Species EC CPUE SE Max Total Ameiurus nebulosus F 0.78 0.78 14 14 Anguilla rostrata T 0.17 0.12 2 3 Brevoortia tyrannus T 1.67 0.91 16 30 Callinectes sapidus T 1.94 0.43 6 35 Fundulus heteroclitus R 277.11 107.32 1513 4988 Gobiosoma bosc R 0.06 0.06 1 1 Malaclemys terrapin R 0.44 0.20 3 8 Menidia menidia T 28.89 13.41 170 520 Morone americana R 4.17 1.41 19 75 Notemigonus crysoleucas F 1.61 1.55 28 29 Pomatomus saltatrix T 1.06 0.48 8 19 All species combined 317.89 107.70 1517 5722 Resident species 88% Transient species 11% Freshwater species 1% 480 Northeastern Naturalist Vol. 19, No. 3 maximum = 82% at stage 8) and the earliest ebb tide stage (9), when water depths were greatest. Tidal migration patterns were determined for the three most abundant intertidal creek species: F. heteroclitus, M. menidia, and M. americana (fig. 3). F. heteroclitus abundance in intertidal creeks varied by tide stage (Wilks’ Lambda: F2,16 = 11.82, P = 0.0007), and generally followed a symmetrical tidal migration pattern with greater abundances in flood and ebb tides, and almost no individuals detected in the flume during high tide. Flood tide (F1,17 = 8.23, P = 0.0106) and ebb tide (F1,17 = 19.48, P = 0.0004) abundances were greater than high tide abundances, while flood and ebb tide abundances were not signifi cantly different (F1,17 = 2.50, P = 0.1324). M. menidia abundance was unaffected by tide stage (Wilks’ Lambda: F2,16 = 0.25, P = 0.7784) and displayed no distinct intertidal creek migration patterns throughout flood, high, and ebb tides. M. americana intertidal creek abundance varied by tide stage (Wilks’ Lambda: F2,16 = 5.59, P = 0.0144), with individuals occurring throughout flood and high tides, and largely absent from the area near the mouth of the intertidal creeks during ebb tide. Both flood (F1,17 = 5.09, P = 0.0375) and high (F1,17 = 11.54, P = 0.0034) tide abundances were greater than those observed for ebb tide, but M. americana abundance at high tide was not signifi cantly different from flood tide (F1,17 = 4.15, P = 0.0576). Discussion Underwater video has been used successfully in a variety of aquatic habitats (Becker et al. 2010, Burrows et al. 1999, Ellis and Bell 2008, James-Pirri et al. 2010, Jury et al. 2001, Mueller et al. 2006), and was an effective method for examining nekton behavior in these intertidal salt marsh creeks. Underwater video allowed the observation of small-scale nekton utilization patterns (e.g., multiple tide stages within flood and ebb tides) with minimal disturbance to the animals or habitat, much reduced labor intensity (e.g., 1 person), and minimal time constraints. Such observations are diffi cult (or impossible) to detect when sampling figure 2. Percentage of estuarine resident, estuarine transient, and freshwater species in relation to total abundance (all tidal cycles combined) per individual tide stage (1–16). Slack high tide occurred between tide stages 8 and 9 (indicated by vertical dashed line). 2012 M.E. Kimball and K.W. Able 481 figure 3. Tidal migration patterns (mean species abundance with standard error) for the three most abundant species observed with underwater video during 18 tidal cycles. Species abundance was calculated as a percentage of the maximum tide-stage abundance observed for that species during an entire tidal cycle: (individual tidestage abundance / max tide-stage abundance)*100. Tidal cycles were divided into 16 tide stages and grouped into three tide classes: flood, stages 1–6; high, 7–10; ebb, 11–16. Slack high tide occurred between tide stages 8 and 9. 482 Northeastern Naturalist Vol. 19, No. 3 with other commonly used traditional gears (e.g., block nets, fyke nets) over complete tide stages. Deployment of traditional sampling gears is often labor intensive and results in disturbance to the animals and possibly habitat alteration. Traditional sampling gears, however, are typically inexpensive (to purchase) and widely used (and therefore standardized), which facilitates data analyses and promotes multi-study comparisons. In contrast, initial costs are often relatively high for underwater video equipment (although rapidly becoming less expensive), and the specifi c operating requirements (i.e., good visibility) may preclude the use of underwater video in more characteristically turbid salt marshes; though it should be noted that turbidity was not a problem in this study despite observing higher turbidities than those reported for other studies using underwater video (e.g., Becker et al. 2010, Mueller et al. 2006; few studies reported turbidity levels). Analysis and interpretation of video data is diffi cult and, for example, initial attempts to sub-sample video footage (at time increments >1 min) in the present study were abandoned because several species that were present were not being detected. Because it is almost impossible to determine whether an individual fi sh left the fi eld of view then returned (Ellis and Bell 2008), there is a risk of individuals being counted multiple times, although this circumstance should be reduced by sub-sampling (i.e., sampling at a scale less than the full amount of data collected). In addition, while the present study focused only on the number of individuals, sampling and analysis protocols for the use of underwater video to observe nekton behavior could (and should) be designed to permit the collection of additional data, such as size and swimming direction for each individual. The arrangement of the underwater video sampling system, particularly the camera, influences data collection and may introduce sampling biases. In the present study, tidal migration patterns of nekton in intertidal creeks were observed in a fi xed portion of the water column throughout the tidal cycle. The portion of the water column examined in this study (i.e., 15–50 cm) coincides with the depths of maximum abundance for many common intertidal nekton species (see figs. 2–5 in Bretsch and Allen 2006b, fig. 5 in Ellis and Bell 2008); however, some individuals migrating above or below the view of the camera might not be detected. Therefore, species that migrate in surface waters at the highest tide stages, or those that migrate on or close to the creek bottom throughout all tide stages (e.g., shrimps or crabs) might be underrepresented. It should be noted, however, that most of the nekton species commonly found in oligo-mesohaline intertidal salt marsh habitats in Mid-Atlantic Bight estuaries (Able and Fahay 1998, 2010) were observed with underwater video in this study. Also, although nekton were not observed with underwater video at or around the entrance to the flume in this study (see James-Pirri et al. 2010), intertidal nekton appeared to be unaffected by the flume and camera set-up as individuals were observed swimming through the center channel of the flume in a direct manner and did not hesitate or change direction. As observed with underwater video, intertidal salt marsh creeks were characterized by a few abundant and ubiquitous intertidal salt marsh nekton species, primarily F. heteroclitus. Nekton species composition and abundance observed with underwater video was similar to those found in previous studies of intertidal habitats in these same salt marshes (Able and Hagan 2000, Hastings 1984). In 2012 M.E. Kimball and K.W. Able 483 general, the intertidal creek nekton tidal use patterns observed with underwater video in this study were similar to those reported from other studies in North America (Bretsch and Allen 2006b; Kimball and Able 2007a, b) and Europe (Cattrijsse et al. 1994, Hampel et al. 2003). Resident species were most abundant in early flood tide stages and were abundant in most ebb tide stages. Transient species were most abundant around slack high tide, and the greatest abundances occurred in late flood tide stages. The divergent tidal use patterns of resident and transient species in intertidal creeks may be due to a number of factors including refuge from predation, foraging behavior, and water-depth preferences (Ellis and Bell 2008, Rountree and Able 2007, Rypel et al. 2007, Salgado et al. 2004). Common intertidal marsh fi shes (i.e., F. heteroclitus, M. menidia, and M. americana) displayed unique, species-specifi c tidal migration patterns based on underwater video observations. Our results largely agreed with and expanded upon nekton tidal migration observations from intertidal creeks elsewhere. F. heteroclitus generally displayed a symmetrical tidal migration pattern. The abundance of this species peaked during the early flood and late ebb tide stages. This result is consistent with F. heteroclitus using marsh surface habitats near slack high tide as reported for a Georgia salt marsh with flume-weir sampling (Kneib and Wagner 1994) and a southern New Jersey salt marsh with flume-weir and pit-trap sampling (Able and Hagan 2000), then moving back into intertidal creeks as the water level dropped with the ebbing tide. This same tidal migration pattern for F. heteroclitus also was observed in South Carolina with a sweep flume (Bretsch and Allen 2006b) and Delaware Bay when sampled with seines (Kimball and Able 2007a). This consistency suggests that, for F. heteroclitus, variable habitat characteristics such as creek geomorphologies and environmental parameters that may be associated with marshes along a latitudinal range do not affect the timing of migration. The tidal migration pattern for M. americana of peak occurrence during high tide, which may be due to foraging strategies (McGrath and Austin 2009), was consistent with migration patterns observed in a previous study (using seines) in Delaware Bay salt marshes (Kimball and Able 2007a). This result suggests that the tidal migration pattern of M. americana also may be unaffected by intertidal creek characteristics that may be associated with marshes along a latitudinal range. While no migration patterns were observed with underwater video for M. menidia in the present study, studies employing seines in Delaware Bay salt marshes found M. menidia tidal migrations centered around slack high tide, with a peak occurrence during late flood tide (Kimball and Able 2007b). A different tidal migration pattern was observed with a sweep flume in South Carolina, however, where M. menidia peak occurrence was during mid-ebb tide (Bretsch and Allen 2006b). Such differences may be related to the sampling location in intertidal creeks. For instance, in the present study, if M. menidia preferred the lower portion of the intertidal creek, they would be more consistently detected with the underwater video camera positioned near the creek mouth. Biotic factors, such as species co-occurrence, can influence water-depth preferences of common salt marsh nekton in shallow waters (Bretsch and Allen 2006a), and may have influenced tidal migration patterns. Predation risk has been shown to differ with depth 484 Northeastern Naturalist Vol. 19, No. 3 for small prey fi shes in tidal creeks (Rypel et al. 2007), and therefore may influence the timing of migration for common prey species, such as F. heteroclitus (Kneib 1997, Nemerson and Able 2003). In summary, the tidal migration patterns of the dominant species in intertidal creeks, as detected near the creek mouth, were species-specifi c and probably due to feeding patterns (i.e., feeding on marsh surface vs. intertidal creeks) and water-depth preferences along the upper-to-lower creek gradient. Acknowledgments This project was funded by the National fish and Wildlife Foundation Budweiser Conservation Scholarship awarded to M. Kimball. Additional fi nancial support for M. 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