SpECIES-SpECIFIC pATTERNS OF FISh

BULLETIN OF MARINE SCIENCE, 80(3): 609–624, 2007

species-specific patterns of fish abundance and size along a subtropical mangrove shoreline: an application of the DELTA APPROACH Joseph E. Serafy, Monica Valle, Craig H. Faunce, and Jiangang Luo ABSTRACT

From 1998 to 2005, 537 visual fish surveys were conducted along a 50-km stretch of mangrove-lined shoreline in the vicinity of Biscayne Bay (southeastern Florida, USA). The shoreline lies directly downstream of a major wetlands restoration project that aims to return more natural salinity regimes to the western margin of the region’s coastal bays. As part of a “baseline” ecological assessment, we applied the delta approach to examine spatial and temporal patterns of mangrove habitat use by three fishes: Lutjanus griseus (Linnaeus, 1758), Sphyraena barracuda (Walbaum, 1792), and Floridichthys carpio (Gunther, 1866). Along a north-south gradient, seasonal variation in their size-composition and three abundance metrics (occurrence, concentration, and density) was quantified and overall correlations with water salinity, depth, and temperature were evaluated using multiple regression. Results indicated that the shoreline is used by subadult and adult L. griseus and by mostly juvenile S. barracuda; for both species, highest abundance metric values occurred during the wet season, generally increasing southwards. In contrast, F. carpio were almost exclusively of adult (mature) sizes, with greatest values during the dry season at the shoreline’s northern extent. For all species, water depth and/or temperature had a significant effect on abundance metrics. Positive correlations were found for L. griseus and S. barracuda abundance metrics, whereas the reverse was true for F. carpio. To date, most mangrove-fish studies have evaluated a single measure of fish abundance, usually density. We suggest consideration of occurrence and concentration, in addition to density, has value in an assessment and monitoring context as well as for gaining insight into how a given fish species disperses and clusters within habitats and across gradients.

A series of three subtropical bays are located on Florida’s southeastern coast. From north to south, these are Biscayne Bay, Card Sound, and Barnes Sound (Fig. 1). To varying degrees, all three are bordered by mangroves (primarily Rhizophora mangle Linneaus) and their substrates vegetated with marine seagrasses (mostly Thalassia testudinum Banks and Soland. ex Koenig). In general, anthropogenic impacts on the three bays, their watersheds, and their shorelines decrease from north to south (Snedaker and Biber, 1996; Browder et al., 2005). Encompassed by the highly urbanized metropolis of Miami, northern Biscayne Bay’s mangroves have been almost entirely replaced with concrete seawalls and limestone boulders (Teas et al., 1976). However, from central Biscayne Bay southward, human influence tends to lessen as the dominant watershed use grades from urban to suburban to agricultural, and finally to serve primarily as parkland (Roessler and Beardsley, 1974). Areal coverage by mangroves increases about 24-fold from central Biscayne Bay to the southwestern boundary of Barnes Sound (Teas, 1974; Ross et al., 2001). The western margins of central and southern Biscayne Bay, Card Sound, and Barnes Sound form a semi-continuous, 50-km stretch of mangrove-lined shoreline, which is interspersed with natural creeks, artificial channels, and freshwater canal mouths (Fig. 1). Restoring more natural freshwater flows and salinity regimes along the westBulletin of Marine Science

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ern margins of the three bays is a major objective of the Biscayne Bay Coastal Wetlands Project (BBCWP), a $300 million component of the Comprehensive Everglades Restoration Plan (USACE and SFWMD, 2002). The BBCWP comprises numerous large-scale engineering projects designed to re-hydrate existing coastal wetlands that are now drained by the canal system, and to redistribute freshwater flow to the Bay across a broad front using pump stations, spreader swales, culverts, and other means. These efforts will modify the timing, location, and volume of freshwater inflows to the three bays, and likely change salinity regimes, contaminant loads, and nutrient dynamics, which, in turn, could have consequences for the bays’ habitats, fishes, and fisheries. Therefore, an important element of the BBCWP is to gauge its impact on the fishes of the area, before, during, and after implementation, which is planned to begin during or before 2008. This paper represents part of a “baseline” assessment that focuses on fishes inhabiting one of the habitats mostly likely to be affected by BBCWP activities—i.e., the 50-km stretch of mangrove shoreline along the western perimeter of central Biscayne Bay, Card Sound, and Barnes Sound. To reveal seasonal and spatial patterns of fish abundance and size-composition along this shoreline, we draw on a portion of the data collected in an ongoing visual belt-transect fish survey. Our focus is on the analysis of abundance and size data for three species: gray snapper, Lutjanus griseus (Linnaeus, 1758), great barracuda Sphyraena barracuda (Walbaum, 1792), and goldspotted killifish, Floridichthys carpio (Gunther, 1866). These fishes were selected because: (1) they are among the most abundant fishes that are readily identified (visually) to the species level; (2) two have economic importance (i.e, gray snapper and great barracuda); and (3) together, they span at least two trophic levels. Conducting species-by-species analyses can be highly informative, but difficulties can arise. At the species-specific level, fish abundances are often positively skewed and dominated by zeros (Lo et al., 1992), reflecting patchiness in the environment and in the distribution of the species concerned (Gaston, 1994). Consequently, such data are usually inappropriate for conventional parametric statistical analyses. A recent paper by Fletcher et al. (2005) promoted wider application of the delta approach to ecological problems, which in our case was to gauge pre-restoration levels, patterns, and variability in fish abundance at the species-specific level. The delta approach is not new (Aitchison and Brown, 1957; Seber, 1982; Pennington, 1983) and is often used in fisheries applications (e.g., Lo et al., 1992) and in ecological studies on niche overlap (Krebs, 1999). It is virtually absent, however, in the mangrove-fish literature (Faunce and Serafy, 2006). In our study, the delta approach entailed analyzing a species’ frequency of occurrence over the transects surveyed (hereafter termed “occurrence”) and concentration (i.e., density where present, exclusive of zeros) separately and then in combination (as delta-densities). As an extension of the delta approach, we also examined intra-specific concentration-occurrence relationships (Gaston et al., 2000) to determine their utility for ecological assessment and monitoring. Methods Data Collection.—The present study focused on fish use of the mainland mangrove fringe between latitudes 25°16´N and 25°39´N. Portions of this stretch of habitat have been surveyed visually for fishes during daylight hours following a biannual, random stratified sampling design (Serafy et al., 2003; Faunce, 2005) for over 7 yrs. Specifically, from 1998 to

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2005, a total of 537 60-m2 visual belt-transect fish surveys were conducted over 15 consecutive wet (July–September) and dry (January–March) seasons (Table 1). Details of the visual fish survey and microhabitat measurement techniques employed are provided by Serafy et al. (2003). Briefly, the former entailed a trained observer, using mask, snorkel, and waterproof paper, recording the identity, number, and size-structure (minimum, mean, and maximum total length) of fishes observed within a 30 × 2 m belt of inundated prop-root habitat. Belt width was measured landward from the prop-root edge and belt length was measured by means of spooling pre-measured length of parachute cord during each fish survey. Care was taken to record all fishes from the top of the water column to the substrate. Upon completion of each fish survey, the microhabitat measurements of water temperature and salinity were made using a multi-probe water quality instrument and water depth using a 2 m-long polyvinyl chloride pole marked off every 2 cm. Survey data were subsequently transferred to computer spreadsheets prior to analysis using SAS (1990) statistical software. Study Design and Data Analysis.—The purpose of this analysis was to conduct a seasonally-resolved, spatial characterization of mangrove shoreline use by three fish species prior to implementation of the BBCWP. Because human impacts and mangrove coverage vary along a latitudinal gradient, we: (1) subdivided the mainland mangrove fringe into a north-south series of seven shoreline segments, each measuring approximately seven km in length (Fig. 1); (2) assigned each transect to its appropriate segment; and then (3) analyzed for seasonal abundance and size trends along north-south and microhabitat gradients. We arrived at the seven-segment scheme to balance the conflicting needs of having: (1) adequate sample sizes per season-segment combination; and (2) a sufficient number of segments to expose patterns (if any) of mangrove utilization along the north-south gradient. Data analysis consisted of several steps. Abundance-frequency plots were constructed as a basic data diagnostic step to judge the appropriateness of taking the delta approach, a method useful in situations where data are positively skewed and zero values predominate. As noted by Fletcher et al. (2005), the delta approach involves generating two data sets from the original: one indicating the species presence (occurrence, proportion of transects positive for the species in question) and the other abundance when present (concentration, numbers observed per transect, when present). The product of occurrence (O) and mean concentration values (C) yields an index of relative density, hereafter termed “delta-density” (D):

D = O ) C var ] D g = O 2 ) var ]C g

(1) (2)

where occurrence (O) is a constant variable. The variance of density can be estimated via the delta method, which uses a Taylor approximation to estimate variances of functions of random variables (Rice, 1995). This index is considered more representative of the data than a mean density estimate calculated in the conventional way, which generally has large variance due to the large number of zeros (Seber, 1982). Separate analysis of the two components results in more robust estimation and understanding of the variance associated with each, and often results in a greatly reduced estimate of the variability around the (composite) deltadensity value (Lo et al., 1992; Ortiz et al., 2000, 2001). Species-specific patterns in occurrence, concentration, and delta-density were examined at two levels. First, mean and standard error values of all three abundance metrics were generated for each season and shoreline segment. This was achieved by pooling across years (1998– 2005), and in the case of occurrence values, weighting each proportion by sampling effort (i.e., the number of visual transects). The year time-series was pooled because this project seeks for single reference, or “baseline”, values with variability. Results were then plotted against each shoreline segment’s (central) latitude and regression analysis was used to describe general north-south trends along the 50-km stretch. We also constructed concentration-occurrence relationship plots (termed “abundance-occupancy relationships” by Gaston et al., 2000) as a possible framework for describing pre-restoration abundance metric levels and variabil-

Shoreline segment 1 2 3 4 5 6 7 Totals:

‘98 W 3 2 5 5 – – – 15

D 4 4 5 3 – – – 16

W 4 3 3 3 2 – – 15

‘99 D 3 4 4 5 1 3 7 27

W 3 2 1 5 1 3 12 27

‘00 D 3 3 3 2 4 3 8 26

‘01

Year-Season ‘02 W D W 5 10 9 6 4 4 3 4 4 1 2 4 – 2 2 2 7 7 3 – – 20 29 30 D 9 4 4 4 1 6 – 28

W 9 4 3 10 5 7 – 38

‘03 D 9 4 3 10 5 7 – 38

W 9 4 4 7 6 8 – 38

‘04 D 10 8 11 20 13 18 14 94

‘05 W 10 8 11 20 13 18 16 96

Total wet Total dry Grand total 52 48 100 33 31 64 34 34 68 55 46 101 29 26 55 45 44 89 31 29 60 279 258 537

Table 1. Sampling effort (number of visual fish transects) conducted along the 50-km stretch of mainland mangrove habitat per year, season, and shoreline segment. Shoreline segments are numbered from north to south. W = wet season (July–September); D = dry season (January–March). Each visual transect entailed recording the identity and size structure of fishes within a 30 × 2 m belt transect.

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Figure 1. Map depicting Biscayne Bay, Card Sound, and Barnes Sound and the 50-km stretch of mangrove-lined shoreline subdivided into seven segments for analysis purposes. The gray, shaded area indicates coverage of adjacent mangrove-dominated wetlands. Black lines across the watershed indicate coastal canal system. ity. This involved plotting the mean concentration of each species per segment-year-season against its respective occurrence value and expressing variability in this space as an 80% confidence ellipse. The second level at which species-specific occurrences, concentrations, and delta-densities were examined aimed to reveal overall correlations, if any, with water salinity, depth and temperature. Following Hocking (1976) and Draper and Smith (1981) stepwise multiple regression analysis was performed. Specifically, a backwards elimination approach was taken whereby factors were removed sequentially if their P-values were > 0.05 and final model fit was judged from adjusted R 2-values. Prior to these regression analysis, occurrence data were arcsine-transformed; concentration and density data loge-transformed. Throughout, concentration and delta-density values are expressed on a per 60 m2 basis. Finally, following Serafy et al. (2003), species-specific (percent) length-frequency distribution plots were constructed using the method of Meester et al. (1999). These were generated with 1, 5 or 10 cm intervals (length bins) to infer life stage (mature/immature) and to examine for possible differences in size composition between northern (segments 1–4) vs southern (segments 5–7) shoreline segments. Kolmogorov-Smirnov two sample tests (Sokal and Rohlf,

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1981) were applied separately to dry and wet season data to determine whether north-south differences in cumulative size frequency distributions were statistically significant (i.e., P < 0.05). In the event that within-season, along-shore differences were found to be non-significant, northern and southern segment data were pooled and overall seasonal differences analyzed using the same Kolmogorov-Smirnov two sample tests.

Results General.—The realized distribution of sampling effort (i.e., number of belt-transect fish surveys) within each year, season, and shoreline segment reflects increasing effort along the entire 50-km stretch, as manpower and other resources became available, peaking in final year (2005) at 94 and 96 visual transects during the dry and wet seasons, respectively (Table 1). Pooling across years, effort per shoreline segment—season ranged from 26 to 52 visual transects. The water salinities, temperatures, and depths measured during fish surveys ranged 0–42, 15–36 °C, and 9–175 cm, respectively. Forty-seven fish taxa were observed along the 50-km stretch over the period of record. The 10 most abundant fish taxa were (in decreasing order): small, water column fishes (Atherinidae, Clupeidae, and Engraulidae), goldspotted killifish, small mojarras (Eucinostomus spp.), gray snapper, yellowfin mojarra, Gerres cinerus (Walbaum, 1792), sailfin molly, Poecilia latipinna (Lesueur, 1821) pinfish, Lagodon rhomboides (Linnaeus, 1766), rainwater killifish, Lucania parva (Baird and Girard, 1855), great barracuda, and bluestriped grunt, Haemulon sciurus (Shaw, 1803). Gray snapper, great barracuda, and goldspotted killifish were the 6th, 3rd, and 2nd most frequentlyobserved fishes, respectively. Of the total number of transects (537), 138 (26%) were positive for gray snapper, 194 (36%) were positive for great barracuda, and 257 (47%) were positive for goldspotted killifish. Abundance and size analyses were based on sightings of 2640 gray snapper ranging 5–51 cm (TL), 395 great barracuda ranging 5–76 cm and 12,507 goldspotted killifish ranging 1–6.5 cm. Spatiotemporal Abundance Patterns.—Spatial and seasonal similarities in the abundance metric values of gray snapper and great barracuda were evident along the 50-km stretch. Gray snapper mean occurrence values tended to be highest during the wet season, with peak values along the central shoreline segments (Fig. 2A). During the dry season, however, gray snapper occurrence was generally lower, increasing linearly from north to south. The mean concentration values of gray snapper also increased linearly from north to south during both seasons, with only minor differences in magnitude between seasons (Fig. 2B). Mean delta-density values for gray snapper (Fig. 2C) were consistently higher during the wet season vs the dry, increasing exponentially from north to south during both seasons, although one value (i.e., the southernmost shoreline segment 7) strayed from this general trend. Mean occurrence values for great barracuda showed no clear pattern along the north-south gradient during the dry season (Fig. 2D). However, as with gray snapper, mean values were higher during the wet season and peaked along the central shoreline segments. A slight linear increase in mean concentration was detected from north to south during the dry season, but not during the wet (Fig. 2E). Great barracuda delta-density means were all elevated during the wet season, but no clear north-south trends were evident (Fig. 2F).


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Figure 2. Seasonal abundance patterns of three species observed along the 50-km mainland mangrove fringe: (A–C) gray snapper, (D–F) great barracuda, and (G–I) goldspotted killifish. Shoreline segments 1–7 on the y-axis are numbered consecutively from north to south. Solid symbols indicate wet season and open symbols dry. Values are means per (± 1 standard error) per shoreline segment. Occurrence values are the proportion of transects positive for the species in question. Concentration and density values expressed on a per 60 m−2 basis (i.e., 30 × 2 m belt transect). Lines and their associated R2-values indicate significant (P < 0.05) north-south trends.

Mean abundance metric levels and trends of goldspotted killifish were the most distinct from the other two species. Mean occurrence values were uniformly low with no north-south trend during the wet season (Fig. 2G), but during the dry season were very high along the northernmost shoreline segments, and decreased sharply towards the south. Mean concentration values for goldspotted killifish followed the same north-south pattern as their occurrence values, with a sharp linear decline evident during the dry season, but not during the wet (Fig. 2H). Mean delta-density

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Figure 3. Concentration-occurrence plots with 80% confidence ellipses for each species by season. Shown are values and ellipses for (A) gray snapper, (B) great barracuda, and (C) goldspotted killifish. Thick lines and solid symbols indicate wet season; thin lines and open symbols indicate dry season. Note that fish concentration axis is in natural logarithm units. See text for details.

values for goldspotted killifish (Fig. 2I) increased exponentially from south to north during the dry season only. During the wet season, these values were uniformly low and no clear spatial trend was evident. Concentration-occurrence Confidence Ellipses.—Seasonal variability concentration-occurrence space is depicted in Figure 3 for each species along the entire 50-km stretch. Most ellipses indicated weakly positive correlations between occurrence and concentration. For gray snapper and great barracuda, dry season el-


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Table 2. Results of backwards elimination multiple regression analysis indicating strength and direction of relationships between species-specific abundance metrics and three habitat measures. Final model intercept values and coefficient estimates (± 1 standard error) associated with water salinity, depth, and temperature are shown with adjusted R2 values. (A) Gray snapper Occurrence (S.E.) Concentration (S.E.) Density (S.E.)

BSAL BDEP BTEMP B0 −1.559 (0.327) 0.011 (0.005) 0.014 (0.002) 0.0372 (0.009) −0.441 (0.427) ns 0.038 (0.008) ns −3.381 (0.901) 0.041 (0.015) 0.029 (0.006) 0.0648 (0.025)

Adj. R2 0.42 0.19 0.28

(B) Great barracuda Occurrence (S.E.) Concentration (SE) Density (S.E.)

B0 −1.046 (0.303) −0.054 (0.157) −0.996 (0.282)

BSAL ns ns ns

Adj. R2 0.32 0.24 0.29

(C) Goldspotted killifish B0 Occurrence (S.E.) 7.973 (1.157) Concentration (SE) 6.583 (1.085) Density (S.E.) −0.023 (0.011)

BSAL ns ns ns

BDEP BTEMP 0.012 (0.012) 0.0026 (0.010) 0.016 (0.003) ns 0.012 (0.002) 0.0324 (0.010)

BDEP BTEMP Adj. R2 −0.015 (0.003) −0.0532 (0.011) 0.37 ns −0.1484 (0.039) 0.13 −0.023 (0.011) −0.1726 (0.039) 0.22

lipses were smaller and more laterally compressed than wet season ellipses. The opposite was true for goldspotted killifish. Microhabitat Correlations.—Final regression models describe the strength and direction of correlations between each of the species-specific abundance metrics and water salinity, temperature and depth (Table 2). Adjusted R2 values of the final models ranged from 0.13 to 0.42. Best fits were for occurrences and the poorest for fish concentrations. In final models pertaining to gray snapper and great barracuda, all factor coefficients were positive, whereas those pertaining to goldspotted killifish were negative. Gray snapper occurrence and delta-density increased with salinity, depth, and temperature, whereas great barracuda occurrence and density increased with only depth and temperature. Concentrations of these species appeared unrelated to salinity or temperature, with a tendency for increase with depth alone. Goldspotted killifish occurrence and delta-density both decreased with increasing depth and temperature; however, their concentration values were only related to temperature, declining as it increased. Size-composition.—Given that minimum size at maturity for gray snapper is about 20 cm TL (Claro et al., 2001), a roughly equal percentage of immature (47%) and mature fish was observed. According to de Sylva (1963) minimum size at maturity for great barracuda is about 60 cm TL, suggesting 89% of individuals observed were immature. Published size-maturity information is lacking for goldspotted killifish, but assuming the same minimum size at maturity for sheepshead minnow, Cyprinodon variegates Lacépède, 1803, applies (i.e., 2 cm TL; Hardy, 1978), 95% of the individuals observed were sexually mature. Within the wet season, north-south differences in size composition were not statistically significant for any of the species examined (Fig. 4). During the dry season, however, significant north-south differences were detected for gray snapper (Fig. 4A,B) and goldspotted killifish (Fig. 4D,E). For gray snapper the greatest difference was in the predominance of 10–15 cm size class in the north versus the south. For goldspotted killifish, size distribution was unimodal in the north and bimodal in the

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south, with the greatest difference being in the predominance of the 4–5 cm size class in the north. Because no spatial differences in the size composition of great barracuda were detected during either season (data not shown), segments were pooled to test for strictly seasonal size composition differences. This analysis revealed that great barracuda size distribution was unimodal during the wet season and bimodal during the dry (Fig. 4C). During the wet season the 10–20 cm size class predominated, whereas during the dry season, the 30–40 and 60–70 cm size classes were best represented. Discussion The present study reports baseline information pertaining to the abundances and sizes of three commonly observed fishes along the mangrove-lined, western perimeter of central and southern Biscayne Bay, Barnes Sound, and Card Sound. Similar analyses have been performed for other fish species and at the entire assemblage level (Serafy et al., 2005); however, those results are not detailed here. A recent review of over 100 mangrove-fish studies (Faunce and Serafy, 2006) indicated most analyses to date have been conducted at the assemblage- rather than the species-level. We suggest both assemblage- and species-level analyses are required to improve our understanding of the roles that mangroves play in the lives of fishes. Our species selection (gray snapper, great barracuda, and goldspotted killifish) represents a balancing of practical, ecological, and economic considerations. From a practical standpoint, given our underwater visual methodology, these fishes pose far less of a challenge to accurately identify to species, count, and measure than, for example, the occasionally more numerous anchovies (Engraulidae), silversides (Atherinidae), herrings (Clupeidae), and mojarras (Gerridae), that share this habitat in highly variable, often mixed-species schools. Other, typically larger, mangrove fishes, such as the common snook, Centropomis undulates (Bloch, 1792), are readily identified and quantified visually, however, the relative rarity of their detection along the 50-km stretch precludes meaningful analyses, at least for this pre-restoration time period. An ecological basis for focusing on these species is that they span at least two trophic levels, as inferred from previous gut content, stable isotope, and energetic modeling studies conducted in mangrove habitats in the region (de Sylva, 1963; Starck and Schroeder, 1970; Odum, 1970; Harrigan et al,. 1989; Hettler, 1989; Schmidt, 1989). At the size ranges in which they occur, goldspotted killifish represent secondary consumers in the food web, whereas gray snapper and great barracuda, both predators of goldspotted killifish, represent tertiary consumers. Finally, gray snapper and great barracuda are targeted by both recreational and commercial fisheries; therefore, results pertaining to these species resonate with fishers and the greater public. Unlike the first quantitative mangrove-fish study conducted along this shoreline (see Serafy et al., 2003), here we incorporated more years of sampling, extended our view farther south, and provided greater detail, especially in terms of spatial variation in fish abundances. Our use of the delta approach also represents a substantial information increase for the species examined here. This was prompted by the preponderance of zero abundance values, a condition that appears to be more the rule than the exception when analyzing species-specific fish abundance data from field surveys. As noted by Fletcher et al. (2005), by separately considering the likelihood


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Figure 4. Size-compositions of the three focal species along the 50-km mainland mangrove fringe during the dry and wet seasons. Gray snapper (A–B), great barracuda (C) and goldspotted killifish (D–E). In all panels but (C), plots show within-season percent length-frequency distributions for northern shoreline segments (segments 1–4, thin lines) versus southern shoreline segments (segments 5-7, thick lines). In (C), plots show percent length-frequency distributions by season (i.e., spatial differences not significant, thus segments were pooled) with the thick line indicating the wet season and dotted line the dry. In all plots, the D- and P-values indicate Kolmogorov-Smirnov two sample test results: D = maximum difference between cumulative size frequency distributions; P = probability of obtaining the observed difference by chance alone. Vertical lines (with arrows) indicate minimum size at maturity values obtained from the literature.

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of presence (occurrence) and the abundance given presence (concentration), we have gained more insight into the system than had we only considered fish densities, computed either conventionally or as delta-densities (i.e., occurrence × concentration). From a monitoring perspective, the delta approach is somewhat precautionary in that it furnishes additional, biologically-meaningful abundance measurements that can each be tracked and evaluated as potential indicators of habitat status or change. Moreover, the concentration-occurrence framework has potential for visualizing and detecting simultaneous change in these abundance metrics. Certainly, as restoration and monitoring proceed, new fish data can be plotted in occurrence-concentration space to: (1) determine whether they fall inside or outside the pre-restoration (i.e., “baseline”) confidence ellipses; and (2) examine the degree to which density changes are driven by changes in concentration, occurrence, or both. With additional data, this framework may be applied at finer spatial levels than demonstrated here (e.g., shoreline segment). The numerous differences among the three species revealed here undoubtedly reflect their very different life history strategies, trophic positions, habitat requirements, and physiological tolerances as well as the disparate time-scales and mechanisms underlying the observed abundances and sizes of each species. Whereas gray snapper and great barracuda larvae originate from offshore waters, goldspotted killifish spend their entire life cycle within shallow, wetland and nearshore seagrass habitats. A valid argument against use of gray snapper and great barracuda abundances as indicators of wetland restoration impacts is that their “pathways” to mangrove shorelines depends on a series of unrelated, prior conditions and stochastic events. These include fishing pressure on their parental stocks (Ault et al., 1998; Harper et al., 2000) and the many biophysical processes and behavioral factors behind successful navigation from pelagic, offshore waters to the littoral zone. However, it is important to reiterate the primary objective of this paper: to characterize mean levels, broad patterns, and inter-annual variability of habitat use by three species that presently occur within our sampling domain. This objective differs from attempting to find and/or justify the monitoring of one species over another (Zacharias and Roff, 2001; Rice, 2003) as a “sentinel” of condition or change. Our results show that fish utilization of this 50-km stretch of red mangrove differs according to the latitude, season, species, and abundance metric under scrutiny. Based on our size composition analysis, the shoreline serves to varying degrees as habitat for mature and immature gray snapper, and mostly immature great barracuda. Conversely, the goldspotted killifish we observed were almost exclusively of adult (mature) sizes, with greater usage during the dry season as compared to the wet. Low detection of juvenile and subadult stages of this species (i.e., individuals < 2 cm TL), however, may have contributed to this result. Regardless of detection problems, our size composition results for all three species provide baseline (pre-restoration) size distributions by shoreline and season against which future comparisons can be made. Several species-specific differences were evident among the abundance metrics examined, along a north-south gradient, seasonally, and in terms of overall relationships with water quality and depth. Results for gray snapper and great barracuda were similar in that their occurrences were consistently elevated during the wet season, with no significant north-south trend. This may reflect a broadly spread initial ingress by both species to this shoreline during the wet season, a pattern that great barracuda retains during the dry season when they tend to be larger, and pre-


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sumably, older. The dry season trend of southerly increase for gray snapper, however, may reflect subsequent movement or poor habitat quality that only becomes clear when water depths and temperatures tend to be lower and salinity higher. Gray snapper and great barracuda also diverged in that the former displayed a strong tendency for increased concentrations from north to south, but the latter did not. The consistently low great barracuda concentration values may reflect the preference of individuals of this species to distance themselves from one another, possibly to reduce hunting interference and/or cannibalism among these ambush predators (de Sylva, 1963). In contrast, the seemingly more gregarious, gray snapper and goldspotted killifish, which tend to feed on benthic invertebrates (Hettler, 1989; Ley et al., 1994), occurred in much higher concentrations, depending on location and season. In keeping with the large southerly increase in mangrove-dominated wetlands along the 50-km stretch, the delta-density values for gray snapper tended to increase exponentially along this gradient. This was not the case for great barracuda, which showed either no, or very little, variation attributable to mangrove coverage. Spatially, temporally, and for all three of its abundance metrics, goldspotted killifish displayed opposite trends relative to the other species, with the tendency for sharp southerly decrease during the dry season. As noted by Serafy et al. (2003), this exclusively dry season pattern may reflect that wetland drainage, which serves to force fishes from the forest interior to the shoreline, declines in a southerly direction. Restoration and Conservation Considerations.—The BBCWP is at the beginning of its implementation period and so are several associated ecological monitoring projects put in place to gauge its impacts. The primary purpose of most of the monitoring efforts, including ours, is to establish present “baseline” levels and/or patterns of one or more biotic and abiotic measurements and then track changes in them as project-related modifications ensue. Under ideal circumstances, we would have reliable, prior knowledge regarding: (1) precisely when, where, and by how much freshwater flows and salinity regimes will change due to restoration activities; (2) what the relevant ecological metrics are; and (3) how they should be measured and interpreted. Unfortunately, due to the lack of quantitative, spatially-explicit historical data, the many uncertainties regarding the timing, magnitude, and form of restoration effects, and the limited understanding of community dynamics in general, it seems prudent to “hedge our bets” by monitoring multiple community components in multiple ways. While a recent surge of studies is revealing important and overwhelmingly positive connections among mangrove habitats, fishes, fisheries, and wider ecosystems (Faunce and Serafy, 2006), they are also demonstrating important differences in the roles that mangroves play, which vary over time, by location and on a species-by-species basis. Mangrove habitats continue to be destroyed or degraded in a “piecemeal” fashion. Therefore, from a conservation standpoint, the need for detailed and spatially-focused faunal utilization studies is particularly pressing. In practice, efforts to preserve or enhance the ecological integrity of mangrove habitats may require numerous, small scale mangrove-fish assessment studies, strategically placed in, or directly downstream of, areas slated for alteration. By virtue of its relatively narrow spatial focus, the present study provides such information. Although funded for their relevance to the BBCWP, the data may also have value as numerous other changes, ultimately driven by human population increase, ensue. In our particular case, these

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include threats of further development of the area’s watershed for housing and additional increases in fish exploitation, especially by the recreational fishing sector. Our results, therefore, may have utility beyond evaluating restoration impacts. In conclusion, we recommend that researchers complement their assemblage-level analyses with detailed, species-specific analyses of fish abundance and size. For the latter, we suggest consideration of the delta approach as a potentially useful and informative way of examining species-specific abundances. The approach may provide insight into how a given fish species disperses and clusters within habitats and across gradients that would not be gained by considering fish density (or biomass) alone (Fletcher et al., 2005). Moreover, each of the “triad” of abundance metrics, along with size-composition data, is simple to convey to scientists and non-scientists alike, and certainly more straightforward to compare among studies than, for example, most multivariate statistical results. We contend that the delta approach holds promise as means of refining our understanding of mangroves as fish habitat, and therefore, may be an important tool for conservation and management. Acknowledgments Financial support for this work was provided by the U.S. Army Corps of Engineers, South Florida Water Management District, National Oceanic and Atmospheric Administration, National Park Service, National Audubon Society, and U.S. Geographical Survey, Perry Institute for Marine Science Caribbean Marine Research Center, and the University of Miami’s Rosenstiel School of Marine and Atmospheric Science. We are indebted to the technical support provided by J. Barimo, E. D’Alessandro, D. Keickbusch, C. Peyer, E. Orbesen, D. Snodgrass, M. South, and P. B. Teare. This paper is Protected Resources Division Contribution PRD-06/07-05.

literature Cited Aitchison, J. and J. A. C. Brown. 1957. The lognormal distribution, with special reference to its uses in economics. Cambridge University Press, Cambridge. 176 p. Ault, J. S., J. A. Bohnsack, and G. A. Meester. 1998. A retrospective (1979–1996) multispecies assessment of coral reef fish stocks in the Florida Keys. Fish. Bull. 96: 395–414. Browder, J. A., R. Alleman, S. Markley, P. Ortner, and P. A. Pitts. 2005. Biscayne Bay conceptual ecological model. Wetlands 25: 854–869. Claro, R., K. C. Lindeman, and L. R. Parenti. 2001. Ecology of the Marine Fishes of Cuba. Smithsonian Instituion Press, Washington D.C. 253 p. de Sylva, D. P. 1963. Systematics and life history of the great barracuda Sphyraena barracuda (Walbaum). Studies in Tropical Oceanography No. 1. University of Miami, Coral Gables. 179 p. Draper, N. and H. Smith. 1981. Applied regression analysis. Wiley Interscience, New York. 709 p. Faunce, C. H. 2005. Reef fish utilization of mangrove shoreline habitats within southeastern Florida. Ph.D. Diss. University of Miami, Coral Gables, Florida. 146 p. ___________ and J. E. Serafy. 2006. Mangroves as fish habitat: fifty years of field studies. Mar. Ecol. Prog. Ser. 318: 1–18. Fletcher. D., D. Mackenzie, and E. Villouta. 2005. Modelling skewed data with many zeros: a simple approach combining ordinary and logistic regression. Environmental and Ecological Statistics 12: 45–54. Gaston, K. J. 1994. Rarity. Chapman & Hall. London. 205 p. __________, T. M. Blackburn, J. D. Greenwood, R. D. Gregory, R. M. Quinn, and J. H. Lawton. 2000. Abundance-occupancy relationship. J. Appl. Ecol. 37: 39–59.


623

Hardy, J. D. 1978. Development of the fishes of the mid-Atlantic Bight: an atlas of egg, larval and juvenile stages, vol. II, Anguillidae through Sygnathidae. Biological Services Program, Fish and Wildlife Service (FWS/OBS-78/12), U.S. Department of the Interior. 458 p. Harper, D. E., J. A. Bohnsack, and B. R. Lockwood. 2000. Recreational fisheries in Biscayne National Park, Florida, 1976–1991. Mar. Fish. Rev. 62: 8–26. Harrigan, P., J. C. Zieman, and S. A. Macko. 1989. The base of nutritional support for the gray snapper (Lutjanus griseus): an evaluation based on a combined stomach content and stable isotope analysis. Bull. Mar. Sci. 44: 65–77. Hettler, W. F. 1989. Food habits of spotted seatrout and gray snapper in western Florida Bay. Bull. Mar. Sci. 44:155–162. Hocking, R. R. 1976. The analysis and selection of variables in linear regression. Biometrics 32: 1–50. Krebs, C. J. 1999. Ecological methodology. 2nd Edition. Addison Wesley Longman Inc. Menlo Park, California. 620 p. Ley, J. A., C. L. Montague, and C. C. McIvor. 1994. Food habits of mangrove fishes: a comparison along estuarine gradients in northeastern Florida Bay. Bull. Mar. Sci. 54: 881–889. Lo, N. C., L. D. Jacobson, and J. L. Squire. 1992. Indices of relative abundance from fish spotter data based on delta-lognormal models. Can. J. Fish. Aquat. Sci. 49: 2515–2526. Meester, G. A., J. S. Ault, and J. A. Bohnsack. 1999. Visual censusing and the extraction of average length as an indicator of stock health. Naturalista Siciliana 23: 205–222. Odum, W. E. 1970. Pathways of energy flow in a south Florida estuary: Ph.D. Diss. University of Miami, Coral Gables, Florida. 180 p. Ortiz, M. and G. P. Scott. 2001. Standardized catch rates for white marlin (Tretapturus albidus) and blue marlin (Makaira nigricans) from the pelagic longline fishery in the Northwest Atlantic and the Gulf of Mexico. Col. Vol. Sci. Pap. ICCAT, 53: 231–248. ______, C. M. Legault, and G. P. Scott. 2000. Variance component estimation for standardized catch rates of king mackerel (Scomberomorus cavalle) from U.S. Gulf of Mexico recreational fisheries useful for inverse variance weighting techniques. NMFS SEFSC Sustainable Fisheries Division Contribution SFD-99/00-86. Mackerel Stock Assessment Panel Report MSAP/00/03. Pennington, M. 1983. Efficient estimators of abundance, for fish and plankton surveys. Biometrics 39: 281–286 Rice, J. 1995. Mathematical statistics and data analysis. 2nd ed. Duxbury. 597 p. Rice, J. 2003. Environmental health indicators. Ocean Coast. Manage. 46: 235–259. Roessler, M. A. and G. L. Beardsley. 1974. Biscayne Bay: its environment and problems. Fla. Sci. 37: 286–204. Ross, M. S., P. L. Ruiz, G. J. Telesnicki, and J. F. Meeder. 2001. Estimating above-ground biomass and production in mangrove communities of Biscayne National Park, Florida (U.S.A.) Wetl. Ecol. Manage. 9: 27–37. SAS. 1990. User’s Guide, Ver. 6., 4th ed. SAS Institute, Cary N.C. 584 p. Schmidt, T. W. 1989. Food habits, length-weight relationship and condition factor of young great barracuda, Sphyraena barracuda (Walbaum), from Florida Bay, Everglades National Park, Florida. Bull Mar. Sci. 44: 163–170. Seber, G. A. F. 1982. The estimation of animal abundance. Edward Arnold. London. 205 p. Serafy, J. E., C. H. Faunce, and J. J. Lorenz. 2003. Mangrove shoreline fishes of Biscayne Bay, Florida. Bull. Mar. Sci. 72: 161–180. _________, J. Luo, M. Valle, and C. H. Faunce. �� 2005. Shoreline fish community visual assessment. CERP Monitoring and Assessment Plan Component. Activity Number 3.2.3.6. First Cumulative Report. 48 p. Sokal, R. R. and F. J. Rohlf. 1981. Biometry. 2nd Edition. W. H. Freeman and Company. New York. 859 p.

624


Snedaker, S. C. and P. D. Biber. 1996. Restoration of mangroves in the United States of America: a case study in Florida. Pages 170–188 in C. D. Field, ed. 1996. Restoration of mangrove ecosystems. ISME, Okinawa, Japan. 304 p. Starck, W. A. and R. E. Schroeder. 1970. Investigations on the gray snapper, Lutjanus griseus. Stud. Trop. Oceanogr. (Miami) 10. 224 p. Teas, H. J. 1974. Mangroves of Biscayne Bay. Mimeo, Date County. 107 p. _______, H. R. Wanless, and R. Chardon. 1976. Effects of man on the shore vegetation of Biscayne Bay. Pages 133–156 in A. Thorhaug and A. Volker, eds. Biscayne Bay: past/present/ future: Papers presented for Biscayne Bay Symposium I, Coral Gables. Univ. Miami Sea Grant Program. 315 p. USACE and SFWMD (U.S. Army Corps of Engineers and South Florida Water Management District) 2002. Central and Southern Florida project, Comprehensive Everglades Restoration Plan, Project Management Plan, Biscayne Bay Coastal Wetlands. 126 p. with Appendices. Zacharias, M. A. and J. C. Roff. 2001. Use of focal species in marine conservation and management: a review and critique. Aquatic Conserv: Mar. Freshw. Ecosyst. 11: 59–76. Addresses: (J.E.S.) Southeast Fisheries Science Center, National Marine Fisheries Service, 75 Virginia Beach Drive, Miami, Florida 33149. (M.V., J.L.) Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami Florida, 33149. (C.H.F.) Florida Fish and Wildlife Conservation Commission, Florida Fish and Wildlife Research Institute, Tequesta Field Laboratory, P.O. Box 3478, Tequesta, Florida 33469. Corresponding Author: (J.E.S.) E-mail: .