Current Zoology 56 (1): 118−133, 2010
An examination of the population dynamics of syngnathid fishes within Tampa Bay, Florida, USA
Heather D. MASONJONES1*, Emily ROSE1,2, Lori Benson McRAE1, Danielle L. DIXSON1,3
1 Biology Department, University of Tampa, Tampa, FL 33606, USA 2 Biology Department, Texas A & M University, College Station, TX 77843, USA 3 School of Marine and Tropical Biology, James Cook University, Townsville QLD 4811, AU
Abstract Seagrass ecosystems worldwide have been declining, leading to a decrease in associated fish populations, especially those with low mobility such as syngnathids (pipefish and seahorses). This two-year pilot study investigated seasonal patterns in density, growth, site fidelity, and population dynamics of Tampa Bay (FL) syngnathid fishes at a site adjacent to two marinas un- der construction. Using a modified mark-recapture technique, fish were collected periodically from three closely located sites that varied in seagrass species (Thalassia spp., Syringodium spp., and mixed-grass sites) and their distance from open water, but had consistent physical/chemical environmental characteristics. Fish were marked, photographed for body size and gender measure- ments, and released the same day at the capture site. Of the 5695 individuals surveyed, 49 individuals were recaptured, indicating a large, flexible population. Population density peaks were observed in July of both years, with low densities in late winter and late summer. Spatially, syngnathid densities were highest closest to the mouth of the bay and lowest near the shoreline. Seven species of syngnathid fishes were observed, and species-specific patterns of seagrass use emerged during the study. However, only two species, Syngnathus scovelli and Hippocampus zosterae, were observed at high frequencies. For these two species, body size decreased across the study period, but while S. scovelli’s population density decreased, H. zosterae’s increased. Across six of the seven species, population size declined over the course of this preliminary study; however, seasonal shifts were impossible to distinguish from potential anthropogenic effects of construction [Current Zoology 56 (1): 118–133 2010]. Key words Syngnathids, Habitat-use, Seahorse, Pipefish, Population density, Mark-recapture
Seagrass ecosystems worldwide are under threat juvenile fish and invertebrates, and structurally complex from various anthropogenic stressors, including eutro- habitats for a diverse array of fauna (Sogard et al., 1987; phication leading to phytoplankton and macroalgal Jenkins and Wheatley, 1998; Boström et al., 2006; Car- blooms, increased sedimentation leading to light limita- ruthers et al., 2007). tion, rising temperatures, dredging, coastal construction, Before seagrasses are lost from the system, succes- and boat propeller scarring (Short et al., 1995; Thrush sional changes have been observed as one grass type is and Dayton, 2002; Campbell et al., 2003; Davenport replaced with another, either in isolated locations or in and Davenport, 2006; Orth et al., 2006; Fyfe and Davis, large areas of habitat previously occupied by another 2007). Decline of this vital habitat is devastating to the seagrass species (Williams, 1990; Fourqurean et al., local marine environment, with increases in nutrients 1995; Robbins and Bell, 2000). This has been attributed due to both the loss of seagrasses as a nutrient filter as to differences in the competitive abilities and physio- well as the breakdown of seagrass into organic compo- logical tolerances to disturbance, nutrients, and in- nents (Wang et al., 1999). In addition, seagrasses stabi- creased sedimentation of the seagrass species involved. lize sediments, and their absence leads to increased tur- These types of interactions between seagrasses have bidity in the water column and altered water flow even led to changes in the morphological structure of (Fonseca and Fisher, 1986). Parallel with the weakening the seagrass itself; in Halodule wrightii, for example, of these extensive ecosystem functions is the loss of reduced branching has been observed under the canopy habitat, as seagrasses play a critical role in providing cover of Thalassia testudinum, leading to a less com- food resources for herbivorous species, nursery areas for plex growth form (Tomasko, 1992). Both drift macroal-
Received Oct. 30, 2009; accepted Dec. 08, 2009 ∗ Corresponding author. E-mail: [email protected] © 2010 Current Zoology H. D. MASONJONES et al.: An examination of the population dynamics 119 gae and epiphytic algae have the potential to alter sea- fishing pressure, habitat destruction, and other unknown grass habitat complexity. In small amounts, growth of causes (Baum et al., 2003; Martin-Smith and Vincent, these algae can increase habitat complexity; however, 2005). moderate to large amounts of drift or epiphytic algae Few field studies on the distribution of syngnathid can lead to seagrass loss over time (Cebrián et al., 2001; fishes have investigated microhabitat preferences, but in Hauxwell et al., 2001; Havens et al., 2001; Balata et al., those that have, some species have demonstrated a con- 2008). As a result, broad-scale seagrass surveys would sistent association with a limited array of seagrass spe- not record an overall loss of seagrass area, but given the cies and/or preference for specific locations within the structural differences between seagrass species, these seagrass landscape. In studies of the diverse assemblage shifts have the potential to have dramatic effects on of Australian syngnathids, the most common species habitat complexity. In addition, many animal and fish found, pipefish Stigmatopora argus and S. nigra, were species are commonly associated with specific types both more often associated with the seaward edges of and/or densities of seagrass for foraging or reproductive seagrass beds (Smith et al., 2008), and S. nigra was habitat (Irlandi et al., 1995; Jenkins and Wheatley, 1998; predominantly found in Posidonia coriacea seagrass Matheson et al., 1999; Boström et al., 2006; Horinouchi, (Kendrick and Hyndes, 2003). In preference tests in the 2007; Malavasi et al., 2007); thus, shifts in seagrass laboratory, male Nerophis ophidian preferred long over species during successional events can influence the short seagrass blades, and both N. ophidian and Syng- faunal diversity in a seagrass ecosystem. Even grass nathus typhle showed preferences for specific micro- beds with the same species but different timelines of habitat locations within an artificial grass bed, findings establishment (natural vs. well-established recolonized) that correlated with observations from the field have different fish species complexes (Brown-Peterson (Malavasi et al., 2007). However, although clearly asso- et al., 1993). ciated with seagrass ecosystems (Diaz-Ruiz et al., 2000), Seagrass-dependent organisms that rely on particular the gulf pipefish Syngnathus scovelli was found to in- seagrass species or the entire seagrass ecosystem are crease in abundance in areas scarred with boat propel- expected to be most affected by seagrass loss or succes- lers, indicating that, although seagrass may be important, sional events. Syngnathid fishes (pipefish and sea- moderate fragmentation of the landscape does not in- horses), are such a group, because numerous species hibit retention or recruitment in this species (Bell et al., that have been described are indeed associated with 2002). Seahorses have also been shown to exhibit habi- seagrass landscapes (Matlock, 1992; Vincent et al., 1995; tat preferences (Hippocampus reidi, Dias and Rosa, Lourie et al., 2004; Kuiter, 2003; Foster and Vincent, 2003; H. breviceps, Moreau and Vincent, 2004). How- 2004; Monteiro et al., 2006). More important, one pipe- ever, two seahorse species found in Portugal (H. hippo- fish species Solegnathus hardwickii and the entire genus campus and H. guttulatus) are the only species for Hippocampus have been listed in CITES Appendix II, which seagrass preference has been investigated in de- with at least one species identified as endangered, nine tail (Curtis and Vincent, 2005). others as vulnerable, and the rest of the 33 species as In this pilot study, our aim was to determine if, in a data-deficient (H. capensis, IUCN 2008). In addition to continuous seagrass landscape composed of a complex their limited mobility and association with threatened of seagrass species distributed in monospecific stands, seagrass ecosystems, it has been suggested that other mixed-species stands, and with varying amounts of drift aspects of their biology may play a role in their eco- macroalgae, we could detect differences in the species logical vulnerability. For example, patchy spatial distri- diversity and abundance of syngnathid fishes between butions, low population density, and monogamous mat- these types of seagrass areas over time. Seahorses, due ing systems (seahorses and some pipefish) make them to their lower mobility and high site fidelity found in particularly susceptible to habitat change or loss (Jones previous field studies (H. whitei, Vincent and Sadler, and Avise, 2001; Foster and Vincent, 2004; Lourie et al., 1995), are expected to exhibit consistent population 2004). With their complex reproductive behaviors, even densities across season. Furthermore, a study of two an increase in turbidity could affect the ability of indi- other species suggested a preference for specific hold- viduals to locate each other and receive visual cues as- fasts (Curtis and Vincent, 2005); thus, we hypothesize sociated with courtship and mating (Gronell, 1984; stronger preferences for seagrass type than found in Vincent, 1994; Masonjones and Lewis, 1996). World- pipefish. Based on work with Australian pipefish and a wide, many syngnathid populations are declining due to pilot study investigating the potential for migration in 120 Current Zoology Vol. 56 No. 1
Syngnathus fuscus (Lazzari and Able, 1990; Kendrick inland of the three sites. It was dominated by Thalassia and Hyndes, 2003), we predict that pipefish will be testudinum (turtle grass) and had moderate macroalgal found in all habitats independent of seagrass species and cover (although seasonally high in the late fall and early macroalgal cover, but with higher population densities winter) and small bare sand patches (0.5 – 2 m in di- closer to open water. We also predict that pipefish den- ameter) interspersed with the seagrass patches. Site M3 sities will fluctuate across seasons as they move to had a moderate depth but was the site with the most deeper locations during the colder months. contact with open water. It was dominated by T. testu- 1 Materials and Methods dinum but had some S. filiforme and Halodule wrightii (shoal grass). In both M1 and M3, Ruppia maratima 1.1 Study region was occasionally observed during sampling. Each site The research site was located in the southeast end of was roughly circular (shape determined by both capture Old Tampa Bay, which is positioned on the western up- and return GPS coordinates) with a 100-m diameter, and per lobe of Tampa Bay (N27º52.34’, W82º32.19’; Fig. the centers of the sites were located roughly 170 – 200 1). This site was selected because of its robust syngnathid m from each other. populations relative to other locations in Tampa Bay (Masonjones, unpublished data). The tidal regime in the area is mixed semi-diurnal, and to access the site for sampling and to standardize measurements, all sampling events took place during either primary or secondary low tide events. The area selected is a shallow seagrass sys- tem, sections of which are exposed at extreme low tides. The immediate western edge of the study site is bounded by a sandbar, which is exposed during the majority of primary low tides. As a result of the sandbar, access to the site is restricted to wading during sampling. The sandbar serves as a partial barrier to water flow and therefore animal movement, at least during low tide events.
Within the region, three seagrass-covered sites were chosen that varied in seagrass type, macroalgal cover, and distance from open water (Table 1; for measurement Fig. 1 Research site located at the south end of Tampa Bay (inset 1), on the southeast side of the Gandy Bridge (inset 2) information, see section 1.3). Site M1, dominated by Animal collection and return locations are indicated in symbols; trian- Syringodium filiforme (manatee grass), was the deepest gles denote site M1, circles M2, pentagons/hexagons M3. Dotted of the three sites and had the most macroalgal cover symbols denote samples in 2006, and open symbols 2007. The two (both drift and epiphytic). In addition, M1 was a moder- contiguous marina construction sites are A) Westshore Yacht Club (WCI Communities) and B) Tampa Harbour Yacht Club (Tampa Ma- ate distance from the sandbar at the western edge of the rina Investments LLC). SAP indicates NOAA buoy used to obtain site, and thus had more access to open water than M2 temperature data. Aerials provided by USGS through Microsoft and less than M3. Site M2 was the shallowest and most Terraserver.
Table 1 Mean (± 1 SE) environmental data collected intermittently near the sites between 2005 and 2007
** Environmental factors 2005 n 2006 n 2007 n F-value 2,14 P-value Depth (cm) 86.9 (5.45) 7 78.4 (7.21) 5 97.5 (6.81) 5 0.584 0.057 Salinity (ppt) 24.1 (0.34) 7 27.4 (0.50) 5 25.8 (1.34) 5 3.301 0.067 Turbidity (ntu) 2.05 (0.44) 7 2.14 (0.32) 5 2.78 (0.48) 5 0.888 0.433 Water temp. (C°)* 27.1 (1.99) 7 24.5 (2.47) 5 31.3 (0.79) 5 2.758 0.098 Blade height (cm) 35.0 (2.35) 7 27.6 (2.23) 5 38.0 (2.18) 5 2.563 0.113 Blade density (per 0.1 m2)* 185 (42.4) 7 53.2 (5.12) 5 177 (24.8) 5 3.723 0.051 Faunal species diversity (H’) 0.62 (0.04) 7 0.68 (0.07) 5 0.53 (0.07) 5 0.944 0.412 * Data double-log transformed before use in MANOVA.** Univariate statistical results obtained after calculating the MANOVA.
Differences between years was investigated with a MANOVA, which showed a significant effect of year (Wilkes λ14,16 = 0.051, F14,16 = 3.913, P = 0.005). H. D. MASONJONES et al.: An examination of the population dynamics 121
1.2 History of site and land use measure seagrass cover because, for seahorses espe- Tampa Bay has had a long history of water quality cially, blades are used as attachment sites and thus rep- issues that have devastated seagrass communities resent habitat better than do short shoots. Because of the throughout the estuary, but the area has been steadily subtropical climate observed in central Florida, tradi- making progress in decreasing nutrient sources and aid- tional seasons were not used for analysis. Instead, the ing in seagrass recovery (Wang et al., 1999; Tomasko et distinction of the “wet season”, typically May 1 through al., 2005). In the research site being investigated in this October 31 in Florida, and “dry season”, from Novem- study, until roughly 1950, the seagrass was continuous ber 1 through April 31, was used. up to the Gandy Bridge, at the top edge of inset 2 (Fig- To distinguish between years a MANOVA was con- ure 1; Pirello, personal communication). In the 1950s, ducted, with year used as the explanatory variable, and dredging occurred at the north edge of the site to create salinity, temperature, depth, turbidity, total faunal diver- the Tyson Street peninsula, the location of one of the sity, seagrass blade density, and seagrass blade height construction projects that occurred during the study (B, used as response variables. Both temperature and blade inset 2). During the late 1960s, the Westinghouse sub- density were double-log transformed to fit the assump- marine engine facility was constructed (A, inset 2), tions of parametric statistical tests. MANOVA results which led to the dredging of an 11-ft channel stretching indicated that although a significant shift across years from A to B and then out into Old Tampa Bay, and the was detected (Wilkes λ14,16 = 0.051, F14,16 = 3.913, P = creation of two spoil islands seen just west of “A” in Fig. 0.005; Table 1), pairwise comparisons did not indicate 1. This channel was the main method of export of sub- that any of the years were significantly different from one another (Hotelling’s T2, all P > 0.05), and univari- marine engines out of the site from the early 1970s until ate tests did not indicate which environmental vari- the early 1990s. To the east of the research site, an ables contributed to the overall differences detected apartment complex with limited water access was con- among years (ANOVA, all P > 0.05). As a result, no structed in the 1980s. To the south, a channel was detectable shifts in the environment occurred during dredged to a boat-launching facility some time in the the study period. mid-1930s, and is still periodically dredged to maintain Second, environmental variables were collected dur- channel depth (shown at the south edge of inset 2). At ing the wet season at each site (Table 2), to distinguish locations A and B, demolition of existing structures and the sites from each other. Salinity, temperature, turbidity, construction of new buildings and marine facilities oc- and dissolved oxygen were determined using a YSI curred for the two marina complexes from February Multiprobe (MPS 556). Depth (in cm) was measured at 2004 to October 2007. the location of each environmental sample. Seagrass 1.3 Measurement of environmental variables variables were also measured, including the height of a and distinguishing between sites random sample of blades (in cm) and the density of Environmental variables were tracked in two con- blades in five replicate 0.1-m2 quadrats. Also, a texts during the course of the study. First, environmental N,N,-dimethyl formamide chlorophyll extraction was data were monitored to identify potential environmental taken of five randomly sampled blades at each replicate changes over time. Just outside the area of the three location to measure seagrass health, following the research sites, environmental samples were obtained methods of Dunton and Tomasko (1994) and using the through an existing research protocol using a 100-m equations for chlorophyll content from Porra et al. transect with five replicate samples collected along its (1989). During each fish collection, seagrass type was length. At each site, ecological variables such as water scored every 20 m, and these tallies were used to de- turbidity (Oakton T-100 Turbidimeter, measured in scribe the dominant seagrass species at the site. Macro- nephritic turbidity units, ntu), temperature (ºC), salinity algal cover was approximated based on an estimate of (Extech refractometer, Model RF20, ppt), seagrass spe- the blotted, uncompacted volume of algae that was in cies, seagrass blade density (total number of blades in a the net after each fish collection, as 0 (less than 1 l), low 2 0.1-m quadrat), seagrass blade heights (cm), and epi- (1–5 l), moderate (5–10 l) or high (> 10 l). These scores faunal and fish diversity (collected with pushnet and were then used to provide the macroalgal cover descrip- analyzed as Shannon-Weiner diversity, H’) were deter- tion seen in Table 2. mined for each sample (Table 1). We used seagrass To determine the most important environmental blade density rather than the density of short shoots to components distinguishing sites from one another, two 122 Current Zoology Vol. 56 No. 1
Table 2 General research site characteristics; environmental samples presented for late summer samples only
** Environmental characteristics M1 M2 M3 F-value2,12 P-value Distance from open water (m) 137.5 236.6 94.9 -- -- N27º52.33’ N27º52.39’ N27º52.32’ Location -- -- W82º32.14’ W82º32.19’ W82º32.26’ Salinity (ppt) * 28.6 (0.36) 28.5 (0.35) 28.6 (0.38) 0.029 0.971 Mean water temp. (C°) by site 27.6 (1.4) 27.7 (1.3) 27.7 (1.3) 0.001 0.999 Mean turbidity (ntu) * 6.92 (1.4) 5.85 (1.2) 6.26 (0.8) 0.299 0.747 Mean dissolved oxygen (mg O2/L) * 5.68 (1.9) 6.1 (2.1) 7.68 (2.1) 0.741 0.497 Mean depth (cm) 87.6 (3.5) 74.2 (9.8) 82.0 (5.8) 0.951 0.414 Overall faunal diversity (H’) 0.663 (0.05) 0.51 (0.03) 0.67 (0.02) 5.36 0.022 Mean seagrass blade count (per 0.1 m2) 218.7 (12.5) 145.1 (19.3) 223.2 (16.4) 7.22 0.009 Mean blade height (cm) * 66.5 (1.74) 29.8 (0.33) 54.3 (4.0) 32.5 < 0.0001 Total chlorophyll a/b content (mg/l) * 23.4 (3.6) 26.1 (4.0) 15.7 (0.7) 2.67 0.110 Dominant seagrass species Syringodium Thalassia, sand patches Thalassia, Halodule Syringodium -- -- Macroalgal level High Moderate/seasonal Low -- -- * Values were transformed for normality and equal variances before use in MANOVA (log-log; power1.5; log-log; power2.2; log transformed, respec- tively). **Univariate statistical results obtained after calculating the MANOVA. Mean (± 1 SE) provided. Two MANOVAs were conducted; one including only physical/chemical variables (listed above the line) to determine if they alone could distinguish between sites, and one including biological variables (below the line).
MANOVAs were conducted. Variables were trans- nant seagrass species observed in M1 (S. filiforme) and formed to fit parametric test assumptions (transforma- the slightly higher average depth in the site. Blade den- tions listed in Table 2). The first MANOVA was to de- sity also varied with grass type, with M2 having fewer termine if sites were similar with respect to physical and blades and dominated by Thalassia seagrass, and M3 chemical characteristics (abiotic factors), with site as the having the highest blade count, reflecting the mixture of explanatory variable and salinity, temperature, depth, Thalassia with Syringodium and Halodule, which both turbidity, dissolved oxygen, and depth the response have much higher blade densities than Thalassia alone, variables. Sites were indistinguishable from one another and which led to M3 most likely having the most spa- based on physical/chemical characteristics (Wilkes λ10,16 tially complex habitat.
= 0.310, F10,16 = 1.275, P = 0.321), with no individual 1.4 Fish collection and return to sites environmental characteristics significantly different Organisms were collected at three sites (M1, M2, M3) across sites (Univariate ANOVA, all P > 0.05; Table 2). using 1.3 m-wide pushnets with 6 mm pore size over a To determine if the seagrass or biological variables linear distance target of 100 m, as described by Strawn differed among sites, a separate MANOVA was con- (1958). The net was modified to include cable ties ducted for site by total faunal diversity, seagrass blade wrapped around the bottom edge to form soft teeth with density, seagrass blade height, and total chlorophyll which to “comb” the seagrass and encourage seahorses content. Based on these biological variables, significant to leave their holdfasts and swim up into the net. Sam- differences were detected among sites (Wilkes λ8,18 = pling distances were carefully measured to obtain an
0.023, F8,18 = 12.624, P < 0.0001), with pairwise com- accurate density estimate, and samples were collected in parisons indicating that sites M1 and M2, and M2 and random directions within each site in short (20 – 30 m) M3 were significantly different from one another, and segments to avoid stressing the animals from being in sites M1 and M3 approached significance (M1:M2, the net too long. Mean distance surveyed at each site Hotelling’s T2 = 221.7, P = 0.001; M2:M3, Hotelling’s was 114.0 m (standard error, 39.6), and total distance T2 = 122.4, P = 0.003; M1:M3, Hotelling’s T2 = 29.3, P covered was approximately 10,833 m. During collection, = 0.06). Faunal diversity, seagrass blade density, and GPS points for the start and end of each sampling drag seagrass blade height all significantly contributed to the were collected to map collection history at each site. distinction among sites (Table 2). M1 seagrass had The mean area surveyed at each site on each sampling longer blades than M3, while M2 seagrass blades had day was 148.24 m (standard error, 51.4), and mean area the shortest length. This corresponds with the predomi- of sites did not differ. Organisms were released the same H. D. MASONJONES et al.: An examination of the population dynamics 123 day to their original collection sites, which were tracked the snout to the middle of the operculum, and then down and mapped for each site and day using ArcGIS 9.2 (Fig. the axis of the body to the end of the tail, following the 1). Collections occurred between December 2005 and measurement protocol from Lourie (2003). December 2007, with an average sampling interval of 1.7 Statistical analysis 22.7 days (standard error, 2.73; range, 5 – 56 days). Upon Data were analyzed to identify patterns across site, subsequent sampling days, the starting point within the year, and species using queries written in Microsoft Ac- site was randomized to avoid pseudoreplication. cess (2003), with more advanced statistical analysis 1.5 Marking protocol conducted in JMP 5.1.2 and Systat 12.0. All study val- To determine independence of samples across the ues are provided as means plus one standard error, dis- study, fish were anesthetized with clove oil and then played in parentheses after the mean value. marked intradermally using visible implant fluorescent Fish counts were obtained across sampling days and elastomer (VIFE; Northwest Marine Technology, Inc.) were analyzed with a chi-squared goodness-of-fit test. on the ventral surface at the base of the tail, following Because density data with small numbers of organisms the protocol of Woods and Martin-Smith (2004) for the observed were right-skewed, a 0.33 power transforma- larger seahorse species Hippocampus abdominalis. tion was applied. In addition, data sets with high num- During marking, fish were examined to determine if bers of zero densities were excluded from ANOVA sta- they had previous marks, which would indicate a recap- tistical analysis (S. louisianae and S. floridae density ture event. In this study, marking was used specifically data) and analyzed using one-way Wilcoxon or to determine if we had previously collected individuals Kruskal-Wallis tests (χ2 approximation). When standard and not as a technique for population estimation. length data violated the assumptions of heteroscedastic- VIFE marking has been used successfully on many ity (Bartlett’s test) and/or normality (Shapiro-Wilk’s species of small organisms (juvenile kingfish Menti- test), data were transformed by calculating the lengths cirrhus saxatilis, Miller et al., 2002; larval salamanders to the 1.2 power. In some cases of body length calcula- Eurycea bislineata, Campbell and Grant, 2008; rainbow tions, transformation did not completely normalize the darters Etheostoma caeruleum, Weston and Johnson, data sets; as a result, an alpha of 0.01 was applied to 2008). In addition, a laboratory-based pilot study was those parametric statistical analyses following the pro- run during the summer of 2004 on H. zosterae seahorses, cedure of Underwood (1997). Multiple comparisons selected because they are smaller and more difficult to were conducted for ANOVA comparisons using Tukey’s mark due to their more ossified surface than the pipefish HSD values. species commonly seen in Tampa Bay. In the pilot, 24 Because recapture samples were small in number and individuals (12 males and 12 females) were selected for inconsistent in frequency over time, population esti- a range of body sizes from a seagrass sampling event mates are unlikely to be accurate (Krebs, 1999). In addi- near Ft. DeSoto, Florida. All 24 animals were anesthe- tion, the population is only intermittently geographically tized with clove oil (GNC incorporated), and 6 males closed at low tide. Other research in our laboratory in- and 6 females were then selected at random for VIFE dicates that, although H. zosterae are relatively sessile marking. All 24 animals were housed in a common and might be candidates for closed population estima- 30-gal tank for 12 weeks post-marking. We observed tion techniques such as the Schnabel method (Krebs, 100% survivorship and complete mark retention (Ma- 1999), in S. scovelli pipefish, up to 20% of recaptures sonjones and Curtis, unpublished data). have changed location (Masonjones and Rose, unpub- 1.6 Fish data collection lished data). These factors, combined with recapture Once marked, fish were photographed using a numbers too small for open population estimates, mean high-resolution digital camera with a measurement scale that we did not use recaptures to estimate population included in the frame. Seahorses and female pipefish size of fish in the study. Marking instead was used to were photographed laterally and male pipefish ventrally verify that fish captured during each sampling event had to identify reproductive stage and increase reliability of not been sampled in a previous event, thus verifying gender determination from photos. Photos were later sample independence. analyzed to determine the sex, species, and standard 2 Results length of syngnathids using the program ImageJ 1.40g (NIH). Due to their unique body shape, seahorse stan- 2.1 Syngnathid density shifts across site and season dard length was measured as the linear distance from Seven syngnathid species were observed in the study 124 Current Zoology Vol. 56 No. 1 region across the two-year sampling period (Table 3). of the study was investigated by examining independent Anarchopterus criniger (fringed pipefish), H. erectus effects of site, season, and year. Differences in density (lined seahorse), and S. springeri (bull pipefish) were all for both season and year were significant (Table 4), in- seen at frequencies well below 1% during the study. dicating that, overall, the wet season and 2006 both had However, four species (H. zosterae, S. scovelli, S. lou- the highest population densities observed in the study isianae, S. floridae) were observed at frequencies of (Tukey HSD, both P < 0.05). However, this does not 1.3% – 82% across sites and species. By far, the most consider the timeline of the study and does not give a common species during all sampling events across all measure, other than year, that the overall population has sites was the gulf pipefish S. scovelli, seen at frequen- changed. As a result, a concatenated variable of season cies roughly five times higher than the next most com- and year was created to quantify shifts in population mon species, the dwarf seahorse H. zosterae. density (Fig. 2). It appears that the population is large in No observed pattern in distribution relative to site the wet season of 2006 and then declines through the location was found in rare species (> 1%). Anarchop- other two seasons. Analyzed for season-year combined, terus criniger was most common in M1, H. erectus was significant relationships were detected (One-Way most common in M2, and S. springeri was only ob- ANOVA, F3,95 = 12.74, P < 0.0001). Most recently, the served in M3. Site M2 had a 26.8% lower observed population density at the end of 2007 was significantly count of H. zosterae than was expected given the fish lower than all other groups, and the population density counts at the other two sites (χ2 = 20.24, df = 2, P < in the 2007 wet season was much lower than in the 2006 0.001). A similar pattern was observed with S. scovelli, wet season (Tukey HSD, both P < 0.05). S. louisianae, and S. floridae with M2, a site dominated H. zosterae showed even densities across sites when by Thalassia, sand patches, and moderate algae (Table 2) separated across seasons (Fig. 3), with overall mean that exhibited the smallest number of fish. M1 had the density of 0.08 (0.009) animals/m2, and a range of 0.02 next highest fish count, and M3 had the highest fish animals/m2 (M2, 2006 dry season) to 0.18 animals/m2 counts and was clearly preferred by far over the other (M3, 2007 wet season). This may seem contradictory two sites (S. scovelli: χ2 = 193.17, df = 2, P < 0.001; S. considering that M2 had the lowest fish count observed louisianae: χ2 = 47.52, df = 2, P < 0.001; S. floridae: χ2 across the four most common species above. However, = 42.02, df = 2, P < 0.001). the pattern is clearly driven by the three times higher Total density of all syngnathid fishes over the course densities of seahorses in both M1 and M3 in the 2006
Table 3 Syngnathid species observed between December 2005 and December 2007, providing both counts and frequencies of occurrence of species by research site (M1, M2, M3) listed in order of abundance
Species M1 f M2 f M3 f Total count Total-f
Syngnathus scovelli 1310 75.42% 1240 81.69% 1930 79.10% 4480 78.66%
Hippocampus zosterae 318 18.31% 247 16.27% 357 14.63% 922 16.19% Syngnathus louisianae 76 4.38% 18 1.19% 90 3.69% 184 3.23% male 3 0 3 female 65 15 73 Syngnathus floridae 22 1.27% 9 0.59% 57 2.33% 88 1.54% male 0 2 11 female 19 6 39 Anarchopterus criniger 10 0.58% 1 0.07% 3 0.12% 14 0.25% male 3 0 2 female 7 1 1 Hippocampus erectus 1 0.06% 3 0.20% 1 0.04% 5 0.09%
Syngnathus springeri 0 0.00% 0 0.00% 2 0.08% 2 0.03%
Total observed 1737 30.50% 1518 26.65% 2440 42.84% 5695
For species of moderate abundance, gender of collected individuals is noted. See Table 6 for gender abundance data for most abundant species. H. D. MASONJONES et al.: An examination of the population dynamics 125
Table 4 ANOVA (H. zosterae, S. scovelli) results and Wilcoxon/Kruskal Wallis results (χ2 approximation; S. louisianae, S. floridae) for shifts in fish density (per m2) with site (M1, M2, M3), season (wet/dry), and year (2006 – 2007)
Total density H. zosterae S. scovelli S. louisianae S. floridae Sample size 96 95 95 95 95
2 2 Source of variation df F11,84 P F11,83 P F11,83 P χ P χ P Main effects Site 2 2.44 0.093 0.37 0.692 2.67 0.075 6.23 0.044 13.49 0.001 Wet/dry season 1 4.32 0.041 5.78 0.018 1.85 0.177 6.51 0.011 11.53 0.0007 Year 1 8.47 0.005 6.75 0.011 9.25 0.003 2.44 0.119 0.91 0.340 Two-way interactions Site*season 2 1.69 0.191 0.78 0.462 6.44 0.002 ------Site*year 2 1.35 0.265 0.76 0.473 0.96 0.387 ------Season*year 1 17.90 < .0001 6.06 0.016 34.57 < .0001 ------Three-way interaction Site*season*year 2 0.31 0.737 3.79 0.027 0.141 0.869 ------Missing data indicates samples sizes too small for meaningful results.
Fig. 2 Total syngnathid fish density (number of fish/m2) over time (late December 2005 – December 2007) separated by site (M1, M2, M3) and season (bar across top of figure) Dry season 2006 (December 2005 – April 2006), Wet season 2006 (May 1 – November 1, 2006), Dry season 2007 (November 1, 2006 – April 2007), Wet season 2007 (May 1 – November 1, 2007). wet season. Statistically, no effect of site was observed density then the other three seasons (Tukey HSD, P < for density, but both season and year had moderately 0.05). Juvenile density was investigated to determine if significant effects. Higher densities were found both in some sites harbored more offspring than others, but sta- the wet season and in 2007 (Table 4; both P < 0.02), tistically there was no difference across sites [M1: and a significant interaction was found among them, 0.0076 (0.0048), M2: 0.0046 (0.0048), M3: 0.0108 with the 2006 dry season having a significantly lower (0.0049); χ2 = 2.33, df = 2, P = 0.31]. 126 Current Zoology Vol. 56 No. 1
Densities of S. scovelli, on the other hand, were quite both P < 0.05). Similar to H. zosterae, S. scovelli juve- variable, with values over three times higher than densi- nile density across sites did not differ (M1: 0.046 ties seen in any other season or site (1.04 animals/m2 in (0.0066), M2: 0.029 (0.0068), M3: 0.038 (0.0067); χ2 = M3, 2006 wet season; Fig. 3), dropping to a low of 0.14 3.12, df = 2, P = 0.21). animals/m2 observed in M1 during the 2007 wet season, Data could not be transformed satisfactorily for S. with an overall mean density of 0.34 (0.019) animals/m2. louisianae and S. floridae, so non-parametric tests were Although neither site nor season were significant on used. Site played a significant role in describing density their own, year had a highly significant effect on density, differences (Table 4), with both species of pipefish with values falling between 2006 and 2007 (Table 4). In rarely observed in site M2 (Fig. 3). In addition, season addition, there were significant interactions, with site was an important factor explaining variation in both M3 during the wet season having significantly higher species’ density, with much higher values during the densities than all other sites under wet or dry conditions 2006 wet season than in all other seasons (Fig. 3). (Tukey HSD, P < 0.05). Also, density significantly in- However, year did not significantly differ in either spe- creased in dry seasons between 2006 and 2007, but de- cies. What is most interesting about the distribution of creased across years in the wet seasons (Tukey HSD, densities across time and site in both species is that
Fig. 3 Syngnathid density (number of fish/m2) separated for the four most common species in the study (H. zosterae, S. scovelli, S. louisianae, S. floridae) and organized by site (A: M1, B: M2, C: M3) Changes across time are indicated as follows: Dry season 2006 (De- cember 2005 – April 2006), Wet season 2006 (May 1 – November 1, 2006), Dry season 2007 (November 1, 2006 – April 2007), Wet sea- son 2007 (May 1 – November 1 2007). H. D. MASONJONES et al.: An examination of the population dynamics 127 males were rarely observed. Male S. louisianae were 6% smaller overall then all other combinations of site observed only a few times during the study, and not at and year, with sites in 2007 far more homogeneous in all in site M2 or during the 2007 wet season (Table 3). size overall than sites in 2006 (Tukey HSD, P < 0.05). Male S. floridae comprised a larger percentage of the Finally, although there was no size difference across population than did male S. louisianae, but male S. year between dry seasons, there was a significant in- floridae were not observed in site M1 and were also not crease in body size among fish collected in the 2007 wet seen during the 2007 dry season. season (Tukey HSD, P < 0.05). 2.2 Body sizes and variation across species, gen- For S. scovelli, site, season, and gender all contrib- der, site, and season uted to describing variation in body size among pipefish Body size varied substantially across factors investi- (Table 5, all P < 0.001, α = 0.01). Animals were sig- gated for both H. zosterae and S. scovelli, but not for S. nificantly larger in the site closest to open water with louisianae or S. floridae (Fig. 4). For H. zosterae, sea- the least macroalgal cover and a mixed seagrass species son played a highly significant role in describing varia- assemblage (M3). Animals were also larger in the wet tion in body size, with body size higher during wet sea- season but significantly smaller overall in 2007 than in sons than dry seasons (Table 5). Body size did not vary 2006 (Fig. 4). In addition, there was a significant inter- by site, year, or gender. However, there was a significant action effect between site and season in S. scovelli, with interaction between site and season: Dwarf seahorses small and extremely homogeneous body sizes in the dry from the M1 site during the dry season were signifi- season but larger and more variable body sizes in the cantly smaller than all other groups (Tukey HSD, P < wet season (Tukey HSD, all P < 0.05). Sexual dimor- 0.05). M1 dwarf seahorses collected in all of 2006 were phism was also observed in this species.
Fig. 4 Mean (± 1 SE) syngnathid body sizes (measured as standard length, mm) for the four most common species in this study (H. zosterae, S. scovelli, S. floridae, S. louisianae) organized by site (M1, M2, M3) within species Counts of number of fish included within each mean are included above each bar. Changes in body size across both year and season are indicated as follows: A. Dry season 2006 (late December 2005 – April 2006), B. Wet season 2006 (May 1 – November 1, 2006), C. Dry season 2007 (No- vember 1, 2006 – April 2007), D. Wet season 2007 (May 1 – November 1, 2007). 128 Current Zoology Vol. 56 No. 1
Table 5 ANOVA results for shifts in body sizes of syngnathid fishes with site (M1, M2, M3), season (wet/dry), year (2006-2007), and gender (M, F; where enough data available for gender analysis)
** * H. zosterae S. scovelli S. louisianae S. floridae Sample size (n**) 842 4252 160 77
Source of variation df F15,826 P F15,3962 P F7,152 P F8,66 P Main effects Site 2 2.71 0.067 27.40 < 0.0001 0.45 0.636 0.15 0.861 Wet/dry season 1 75.82 < 0.0001 175.52 < 0.0001 0.28 0.596 0.59 0.446 Year 1 4.91 0.027 2.39 0.122 0.036 0.850 3.97 0.051 Gender 1 4.93 0.027 126.06 < 0.0001 1.04 0.309 0.99 0.322 Two-way interactions Site*season 2 6.34 0.002 12.18 < 0.0001 1.02 0.364 0.51 0.560 Site*year 2 7.55 0.001 4.28 0.014 -- -- 1.22 0.303 Season*year 1 11.27 0.001 0.16 0.685 2.78 0.098 3.87 0.053 Site*gender 2 1.10 0.332 5.55 0.004 ------Season*gender 1 0.09 0.766 4.36 0.037 ------Three-way interactions Site*season*gender 2 0.34 0.712 2.45 0.086 ------
* Because of minor violations of assumptions, according to Underwood (1997), α = 0.01. ** Juveniles omitted from size analysis because they contributed to the violation of parametric assumptions and were unevenly distributed across sampling days. Missing data indicates samples sizes too small for meaningful results.
Table 6 Recaptured animals organized by species, gender and research site. No juveniles were recaptured, but their numbers are included in the totals Species M1 M1 recaps M2 M2 recaps M3 M3 recaps Total count Recap. totals Recap. freq. H. zosterae 318 0 247 3 357 0 922 3 0.325% male 110 0 103 0 128 0 341 0 0 female 184 0 126 3 192 0 502 3 0.598% S. scovelli 1310 12 1240 7 1930 27 4480 46 1.027% male 456 6 373 2 850 9 1679 17 1.012% female 651 6 740 5 919 18 2300 29 1.261% Total count 1628 12 1487 17 2287 27 5402 49 0.907% Recap. freq. 0.737% 1.143% 1.180%
2.3 Hippocampus zosterae and Syngnathus mals were recovered in 2007, and two of those recap- scovelli recapture rates tures were two of the only three remarked seahorses in All animals that were collected were tagged, but only the study. No male seahorses were recaptured, and dwarf seahorses and gulf pipefish were recaptured. Re- among pipefish, females were more frequently recap- capture rates for both H. zosterae and S. scovelli were tured, corresponding with their higher catch rate overall. extremely low, at 0.33% and 1.0% respectively, making 3 Discussion population estimates unreliable (Table 6). However, we used marking as a technique to verify whether or not we 3.1 Syngnathid density shifts across site and season had observed individuals during a prior collection event In investigating the patterns of syngnathid distribu- and not to estimate populations, and our sampling tech- tion by site, H. zosterae appear to be generalists in the niques reflected the goal of obtaining samples that were seagrass landscape. This result is contrary to our expec- independent from one another over time. tations, based on their limited mobility and previous Recapture rates reflected the overall density of ani- research showing that some seahorse species display mals in sites the first year, with extremely high popula- particular habitat preferences. We observed low popula- tions in the M3 site during the 2006 wet season, when tion densities overall, with a consistent distribution most of the recaptures occurred. In fact, only six ani- across seagrass sites and seasons. There were, however, H. D. MASONJONES et al.: An examination of the population dynamics 129 significant density increases during the wet season and mixed Thalassia, Syringodium, and Halodule seagrasses, from 2006 to 2007. The density increase during the wet but with the lowest observed levels of macroalgae, season cannot be explained by increases in the number likely due to the close proximity to open water and as- of juveniles, as juvenile densities were consistent across sociated wave action, leading to more flushing. Con- site and across years. During collection, some seahorses trary to what was observed for the dwarf seahorse, the were observed clinging to drift macroalgae in sampling overall trend for S. scovelli was of decreasing densities nets, and this could serve as a method of dispersal for H. from 2006 to 2007. As we hypothesized, S. scovelli zosterae similar to that seen in the predominant distri- were observed along a gradient relative to the open bution of Syngnathus fuscus in drift algae in a Cape Cod ocean, with higher densities in M3, followed by M1 and estuary (Able et al., 2002). In other studies of seahorse then M2, which was the site farthest away from open habitat distribution, seahorses were also found at mean water. This pattern was also observed for both Stigma- densities at or below 0.06 seahorses/m2, but locally high topora nigra and S. argus populations in southeast Aus- densities were observed around preferred habitat (1 tralia, where syngnathids were more common on the seahorse/m2 in H. whitei, Vincent et al., 2004; 0.70/m2, seaward edge of grass beds than the shoreward edge H. breviceps, Moreau and Vincent, 2004; 0.51/m2, H. (Smith et al., 2008), and which may reflect a general guttulatus, Curtis and Vincent, 2005). However, these preference for access to open water in pipefish species. studies were all focused on larger species of seahorse This association of S. scovelli with site M3 could be that could be visually located in a complex landscape, explained in many ways. First, it is possible that the allowing assessment of local high densities. In the pre- seaward edge allows better access to open water for sent study, lower densities are a function of a general foraging, although in other studies, pipefish have rarely survey approach. In previous seahorse studies con- been associated with open sand sites, and 20 – 30 me- ducted with visual surveys, seahorses were found asso- ters of sand separates M3 from open-water seagrass ciated with structures that could be used as holdfasts, beds (Kendrick and Hyndes, 2003; Smith et al., 2008). although substantial variation existed in those structures, Gulf pipefish and dwarf seahorses feed on harpacticoid including macroalgae, bryozoans, tunicates, polychaetes, copepods and amphipods, and due to changes in the and seagrass. Across species investigated, few prefer- velocity of the water as it moves over the seagrass, these ences were observed, and no preferences for specific organisms may be more abundant along the edges of seagrass species as holdfasts were noted. seagrass beds (Huh and Kitting, 1985; Fonseca and All previous studies observed seahorse residents re- Fisher, 1986; Tipton and Bell, 1988; Smith et al., 2008). maining in a small area for a given period of time, indi- Because M3 is at the outer edge of this seagrass system, cating small home range size. However, H. breviceps, these food sources are likely to be more abundant, thus the species closest in size to the dwarf seahorse ob- potentially explaining the higher abundance of pipefish served in this study, had a high turnover rate in these and their larger size in this site as well. high-occupancy sites, with adults observed on average Another reason S. scovelli may be abundant at this only 20 days (range 1 – 37 day residency time) out of site is the combination of the diversity of seagrass the 37-day study (Moreau and Vincent, 2004). This in- blades and their length. Gulf pipefish, like other pipefish, dicates that at least some fraction of the adult population rely on crypsis to avoid predation, and their body shape is mobile, and it differs from other studies of seahorses aids in the process of hiding among the seagrass blades. that have indicated high site fidelity and low mobility In M3, the mixture of broad-bladed Thalassia spp. with (H. whitei, Vincent et al., 2004; H. subelongatus, the thinner-bladed Syringodium, both with long blades, Kvarnemo et al., 2000). Our data suggest that H. zoste- could potentially provide the best cover for the rae may be seagrass generalists, found across a gradient broad-bodied females and thinner-bodied males. In of seagrass species, macroalgal abundance, and distance other species, seagrass preference followed these types to open water. Low recapture rates may indicate that this of body dimension patterns. The larger Syngnathus ty- species is more mobile and more similar to H. breviceps phle and Nerophis ophidian preferred long over short than to other seahorse species. blades in the laboratory and were more often associated For S. scovelli, the most common species in this with broader-bladed Cymnodocea nodosa; the smaller S. study, site M3 during the 2006 wet season exhibited abaster preferred Zostera marina (Malavasi et al., 2007). three times higher densities than any other combination In another study, both Stigmatopora nigris and S. argus of sites, seasons, and year. Site M3 was composed of preferred the strap-like Posidonia spp., which matches 130 Current Zoology Vol. 56 No. 1 their body morphology more closely than the branching population study for the purpose of distinguishing natu- Amphibolis griffithii (Kendrick and Hyndes, 2003). Al- ral fluctuations from anthropogenic effects on the popu- though only correlations, the distribution of pipefish lation (Estacio et al., 1999). These issues notwithstand- body shapes with seagrass morphology is an interesting ing, a measured decrease in syngnathid population size pattern observed across studies and might explain some occurred in the same time frame as the demolition and of the variation in this study. construction phases of two marina projects adjacent to Although there were no differences in density be- the research site. Dramatic construction projects in tween years in S. louisianae and S. floridae, these spe- aquatic systems, like building a dam, can have immedi- cies had significantly higher densities during the wet ate and obvious impacts on resident fish communities season overall. Site was significant, as they were rarely (Habit et al., 2007), but slower changes such as the observed in M2, which had a mix of Thalassia spp., gradual loss of seagrass and succession to less optimal sand, and algae and had significantly lower blade height species can also cause dramatic shifts in resident fish and blade densities. We expected that pipefish in general distribution and community composition (Thayer et al., would be found in all seagrass locations due to their 1999). Although we did not observe a shift in species greater mobility; however, these two species were ob- composition during the study, population density de- served at extremely low levels at the most inland site creased from one year to the next. This could indicate a and not at all there in some seasons. This can potentially disturbance in the natural population cycle of wet and be explained by the size difference in these two species dry seasons at the site, indicating the potential for shifts from the smaller S. scovelli. With a longer body size, the to have occurred in the environment that we are not yet two larger species may prefer a deeper habitat, which able to detect. Only continued post-construction allows for longer grass to create more protection from population monitoring in the site will help resolve these predators and closer access to open water to facilitate questions. possible migrations (Lazzari and Able, 1990; Power and 3.2 Body sizes and variation across species, gen- Attrill, 2003; Malavasi et al., 2007). Further evidence of der, site, and season migration for the two larger species is the lack of males We observed an increase in body size for dwarf sea- in the populations. Previous research shows that, within horses overall and for gulf pipefish in M3 during the pipefish populations of Syngnathus fuscus, males are the wet season. Increasing body size through increased limiting factor and do not tend to migrate and risk pre- growth in wet seasons could certainly be an explanation, dation (Roelke and Sogard, 1993). In our study, this because increased rains and longer days increase pho- could explain the high number of females relative to tosynthetic rates and therefore food availability. How- males; females may migrate to search for mates in a ever, this may indicate also that larger individuals mi- new habitat or use the current habitat as foraging space grate into the site from deeper-water seagrasses not in- independent of males. cluded in the study area. Residents are smaller and During the course of the two-year research study, we theoretically poorer swimmers, and larger individuals observed a substantial decrease in overall syngnathid may shift habitats seasonally to broaden their range of population size. It is possible but unlikely that our sam- resources. Predation could also play a role; because of pling techniques had a deleterious effect on the popula- the protected nature of the site, there may be fewer re- tion, as our pilot study indicated 100% laboratory sur- sources and predators, and the larger individuals might vivorship after marking, and another more recent study seasonally risk leaving in order to gain more food in showed significantly higher recapture rates of all syng- another habitat. nathids and much more consistent numbers of syng- Evidence from some species of pipefish indicates that nathid fish across seasons (see section 3.3 below; Ma- seasonal migrations occur based on size frequency data sonjones and Rose, unpublished). This study did not (S. fuscus, Lazzari and Able, 1990; S. rostellatus, Power follow the proper BACI study design for detecting the and Attrill, 2003; Nerophis ophidian, Monteiro et al., effects of anthropomorphic stressors on ecological sys- 2006), but in other species studied, the evidence is tems, with a clear before and after and an appropriate equivocal (Stigmatopora nigra, Kendrick and Hyndes, control (Underwood, 1994); however, there is still the 2003). In our studies, the increase in density during the potential to glean insight from the changes we observed wet season did not appear to simply be a factor of in- during the study. Another limitation in this context is the creases in juveniles, because very few were observed in study period, as two years is a short time to conduct a the study, and their numbers were consistent throughout H. D. MASONJONES et al.: An examination of the population dynamics 131 the year for both of our most common species. Even higher recapture rate with all species was obtained though juveniles were excluded from the body size (overall recapture rate 4.7%, pipefish recapture rate analyses to increase the normality of the data, a separate 5.0%, seahorse recapture rate 3.1%; Masonjones and analysis was conducted including them, and patterns in Rose, unpublished data). Theoretically with the same the body size data sets did not change. This is most sampling and marking methods, survivorship would be likely because, in both H. zosterae and S. scovelli, juve- the same for both studies, indicating that low recapture nile density did not differ with site, and juveniles com- rates in the present study were most likely associated prised roughly the same percentage of the population at with sampling times longer than two weeks (the mean each site. Based on the observation that animals gener- difference in collection times between the two studies ally became larger and more numerous in the wet season, was 11.5 days) and a sampling design that included it is likely that at least some portion of these populations random recapture locations relative to the previous re- migrate out of these shallow seagrass beds during the lease site. This is also supported by the recapture rates dry season. between 2006 and 2007. In 2007, we only recaptured Among the four most common syngnathids, real pat- six animals, compared with 43 in 2006. This is corre- terns in body size were most likely observed for the lated with more frequent sampling in 2006, with about a groups with larger sample sizes, H. zosterae and S. week’s difference in average resampling time between scovelli. The other two species appear to be migrants in 2006 and 2007. In mobile populations, sampling inter- the system, with breeding habitat elsewhere. More evi- val is critical to accurate population estimates and is a dence of this is that few males were observed at any of key assumption of many population estimation calcula- the sites. Female S. scovelli also outnumbered males by tions (Krebs, 1999). Based on these arguments, we are almost 1.4 to 1, but the sex ratios observed in samples confident that sampling problems did not significantly of S. louisianae (19:1) and S. floridae (4.8:1) may indi- alter the populations under study. cate that the sites surveyed in this project represent a Our low recapture rate could also indicate that the potential sink habitat and do not support breeding species are highly mobile, with large and flexible popu- populations. Other data indicate a sex ratio of 1.36 to 1 lations. These recapture rates (0.33% for H. zosterae in Florida populations of S. floridae (Mobley and Jones, and 1% for S. scovelli) are not unheard of for estuarine 2007), suggesting that the population in this study may fishes. In a small, intermittently open estuary in South not be typical for this species. Africa, of the 12 fish species tracked, across a range of 3.3 Hippocampus zosterae and Syngnathus scovelli body sizes, six had recapture rates that were 1% or be- recapture rates low (Lukey et al., 2006). In other species that move Although a large number of animals were captured in through the habitat, where they are collected and this study, few were recaptured. This could have been marked has a dramatic effect on recapture rates. In a caused by a number of factors, including: 1) tag loss, 2) study of juvenile northern kingfish, recapture rates var- sampling mortality, 3) too-long sampling intervals, 4) a ied from 0% when tagged on ocean beaches to 14% large and flexible population, 5) highly mobile animals, when tagged in an estuary (Miller et al., 2002). By re- 6) naturally high mortality, or some combination of ducing the resampling interval in future studies, we these factors. These possibilities fall into two groups hope to increase recapture rates and thus make popula- spanning sampling problems and natural population tion estimates possible for the four most common spe- variation. From the perspective of sampling problems, cies in the region. This change in design should also we have investigated these three issues and are confi- allow better determination of the use of the site as habi- dent the first two are not factors. First, in our pilot study, tat by S. floridae and S. louisianae, since the 0% recap- we verified that tags were not lost during three months, ture rates in this study cannot be clearly attributed to longer than our resampling intervals. Second, neither their migratory status. Even if we were to obtain the anesthesia nor marking represented survivorship haz- same 1% recapture rate with these two species as we did ards for the animals used in the study. Temperatures with S. scovelli, with our very small sample size over remained constant during marking, and animals were two years, their absence in our recaptures could be due acclimated to water conditions at the release site, swam solely to random sampling bias. We plan to further in- upon release, and returned to the seagrass below. In a vestigate the movement patterns of these fish through more recent study using shorter sampling intervals the seagrass landscape, attempt to estimate population (mean± SE sampling interval was 11.2±1.60), a much size, elucidate gender differences in seagrass habitat use, 132 Current Zoology Vol. 56 No. 1 and further resolve the conservation status of these (Syngnathidae) in Tamiahua Lagoon, Gulf of Mexico. Ciencias CITES Appendix II listed species. Marinas 26(1): 125–143. Dunton KH, Tomasko DA, 1994. In situ photosynthesis in the seagrass Halodule wrightii in a hypersaline subtropical lagoon. Mar. Ecol. Acknowledgements Field work for this project was con- Prog. Ser. 107: 281–293. ducted under State of Florida Special Activities License Estacio FJ, Garcia-Adiego EM, Carballo JL, Sanchez-Moyano JE, Number 05SR-902. We would like to thank the many UT stu- Izquierdo JJ et al., 1999. Interpreting temporal disturbances in an dents who volunteered hundreds of hours in total on the field estuarine benthic community under combined anthropogenic and crew for the project. In addition, Sawyer, Kellerin, and Grae- climatic effects. J. Coast. Res. 15(1): 155–167. lyn Masonjones provided a great deal of field and data proc- Fonseca MS, Fisher JS, 1986. A comparison of canopy friction and essing assistance. Thanks to Dr. Michael Masonjones for pro- sediment movement between four species of seagrass with refer- viding Access database and statistical assistance. Dr. Mark ence to their ecology and restoration. Mar. Ecol. Prog. 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