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Assessment of fish and decapod distributions between mangrove and seagrass habitats in St. John, U.S.V.I.

ABSTRACT: Fish and decapod community structure and catch per unit effort (CPUE) were assessed within a mangrove previously impacted by a hurricane and an adjacent seagrass bed in St. John, United States Virgin Islands. Diel sampling utilizing minnow traps showed significantly greater total fish CPUE in the seagrass habitat with few individuals using the mangrove regardless of the time of day. Decapods showed no difference in CPUE and were equally distributed between mangrove and seagrass habitats. When individual species were analyzed, Haemulon flavolineatum, H. sciurus, Calcinus elegans and Panulirus argus had significantly higher CPUEs in seagrass beds compared to the mangrove, while Panopeus occidentalis showed the opposite distribution. Panulirus argus and C. elegans had significantly higher CPUEs during the night, while Eurypanopeus abbreviatus was significantly more abundant during the day. A Similarity of Percentages analysis showed low species similarity among individual habitat-diel samples (<25%), but strong dissimilarities among all sampling combinations (>90%) and an Analysis of Similarity showed significant differences m the fauna among each habitat-diel configuration. These analyses reflect the minimal use of the mangrove by fish and the strong did habitat differences for individual decapod species including an example of potential niche partitioning between two mud crabs species in the mangrove.

KEYWORDS: mangrove, Rhizophora mangle, seagrass, Thalassia testudinum, niche partitioning

INTRODUCTION

Coastal tropical systems are often dominated by a mix of mangrove, seagrass and coral reef habitats. Each of these habitats is important as refuge and feeding regions for a variety of fish (Nagelkerken et al. 2000, Dorenbosch et al. 2004) and invertebrates and frequently they are linked through trophic transfer (Baelde 1990). As coastal systems are impacted by natural and anthropogenic disturbances, the value and linkages among these habitats may often be altered or degraded. Human disturbances, such as over-fishing, habitat destruction, and eutrophication, generally lead to significant or permanent ecosystem damage (Fondo and Martens 1998), while natural disturbances, such as hurricanes, are generally followed by natural recovery.

The value of seagrasses and mangroves as nursery habitat has been well documented (see Heck et al. 1997, Faunce and Serafy 2006, and references within). Primary productivity in these systems is directly consumed (Kirsch et al. 2002, Feller and Chamberlain 2007), but a substantial portion of this production enters detrital pathways (Cebrian et al. 1997, Lee 1999). High levels of secondary production lead to substantial trophic transfer to higher level consumers (Wolff et al. 2000). Often, movement among these habitats leads to energy export to coral reef communities; as juveniles take up residence on the reefs and also through daily foraging of adults into mangroves and seagrass beds (Nagelkerken et al. 2000). Consequently, community disturbance may reduce overall productivity of a coastal system (Wolff et al. 2000). This is especially evident when mangroves are destroyed for aquaculture, because the resultant loss of habitat leads to increased erosion and turbidity, loss of essential habitat for all fish and invertebrates previously utilizing this habitat and subsequent declines in adjacent seagrass and coral reef communities (Primavera 2006). Less understood is how the faunal communities change and recover following a natural disturbance, such as a hurricane.

This research focused on a mangrove-seagrass habitat complex in the northern part of Great Lameshur Bay, U.S. Virgin Islands that had been severely impacted by two hurricanes (Hugo 1989, Marilyn 1995). While the seagrass beds have recovered (Kendall et al. 2001), the mangrove has seen very limited natural recovery (Nemeth et al. 2004). Our objective was to investigate the diel usage patterns of the mangrove and adjacent seagrass habitat by fish and decapods to determine whether the limited recovery of the mangrove was providing usable habitat for these organisms. Little was known regarding the use and value of this particular mangrove community prior to the hurricane, but currently this system remains substantially degraded with diminutive re-growth of the mangrove interior (Kendall et al. 2001).

MATERIALS AND METHODS

Great Lameshur Bay is inside the boundaries of Virgin Islands National Park, a United Nations Biosphere Reserve. The region is characterized by several communities including seagrass beds, coral reefs, mangrove forests, and unvegetated zones covering a total area of roughly 50 acres (Kendall et al. 2001). Our experiment was conducted among the prop roots of a red mangrove (Rhizophora mangle) forest and the adjacent seagrass (Thalassia testudinum) habitats within the bay. The mangrove forest was damaged during Hurricanes Hugo (1995) and Marilyn (1995) causing severe internal damage resulting in piles of dead mangrove trees and large regions of unvegetated mudflat (Nemeth et al. 2004, Bologna pers. obs.).

Minnow traps were used to sample juvenile fish and decapods in each habitat. The traps were constructed of 3.2 mm seine net and a steel wire frame with two 5,1 cm diameter door openings at each end. The traps measured 25.4 cm x 25.4 cm x 43.2 cm and had two zippered pockets for bait and emptying catch along with a 3.7 m drop cord. Six traps were placed within the red mangrove prop roots by kayaking along the mangrove channel and six traps were placed into the adjacent Thalassia testudinum bed. Squid was used to bait traps during both day and night sampling events. Traps were set in the morning (sunrise) and retrieved approximately 11-11.5 hours later (sunset) and then reset to assess nighttime utilization of the two habitats. Traps were deployed for 3 daily cycles in January 2006. Fish and decapods from retrieved traps were enumerated, measured, and identified to their lowest taxonomic level and then released. Data were standardized for trap fishing time to generate a catch per unit effort (CPUE) by dividing the abundance of organisms by trap collection time (# fish, decapods/ # hours sampled). To assess habitat use and diel differences among the common fish and decapods, CPUE was analyzed using a 2-way ANOVA with habitat (i.e., seagrass, mangrove) and diel stage (day, night) as independent variables with an [alpha] = 0.05. The four habitat-did stage combinations were further assessed by conducting an Analysis of Similarities (ANOSIM, Bray-Curtis matrix on non-transformed data) and Similarity of Percentages (SIMPER, non-transformed data) analysis using the PRIMER software package (Clarke and Warwick 1994) on the species specific CPUE to determine how related the sampled faunal communities were.

RESULTS

Fish

Traps collected eight fish species: French grunt (Haemulon flavolineatum), schoolmaster (Lutjanus apodus), bluestriped grunt (Haemulon sciurus), yellowtail snapper (Ocyurus chrysurus), slippery dick (Halichoeres bivittatus), marked goby ( Ctenogobius stigmaticus), squirrelfish (Holocentrus adscensionis), and damselfish (Stegastes adustus) (Table 1). Significantly more fish used seagrass habitat than mangrove ([F.sub.1,64] = 13.04, P <0.0006, Fig. 1a), but utilization was similar between day and night. Haemulon flavolineatum were significantly more abundant in seagrass beds ([F.sub.1,64] = 15.8, P <0.002) as were H. sciurus ([F.sub.1,64] = 5.92, P <0.02). No other fish showed significant affiliations with either habitat. Average fish length indicated that primarily juvenile H. flavolineatum, H. sciurus, and O. chrysurus were using these habitats (Table 1).

Decapods

Decapods were represented by ten species including common mud crab (Panopeus occidentalis), lobate mud crab (Eurypanopeus abbreviatus), blue crab (Callinectes sapidus), spiny lobster (Panulirus argus), ocellate swimming crab (Portunus sebae), peppermint shrimp (Lysmata wurdemanni), grass shrimp (Palaemonetespugio), snapping shrimp (Alpheus heterochadis), sponge decorator crab (Stenocionops furcata), and blue-legged hermit crabs (Calcinus degans) (Table 1). Total decapod CPUE was not different between habitats ([F.sub.1,64] = 0.08, P <0.77) or time of collection ([F.sub.1,64] = 1.32, P <0.26; Fig. 1b). Despite this lack of overall habitat or time effect at the decapod community level, several individual species exhibited habitat or diel preferences. Specifically, significantly more P. occidentalis ([F.sub.1.64] = 4.3, P <0.04) were collected in mangroves compared to seagrass beds, while C. elegans and P. argus were significantly more abundant in seagrass habitat ([F.sub.1,64] = 13.28, P <0.0005, F=9.56, P <0.0061, respectively). Eurypanopeus abbreviatus were significantly more abundant during the day than at night ([F.sub.1,64]=6.86, P <0.01), while C elegans and P. argus were significantly more abundant at night ([F.sub.1.64]=12.53, P <0.0008, F=8.05, P <0.0061, respectively).

Community Response

Fish and decapod communities varied substantially in the use of each of the habitat-diel category (Table 2). Two important features were apparent. First, the low similarity percentage for each habitat-did category (8-24% SIMPER) suggests that there is substantial variability among the replicates present in the data set, but also that only a few species contributed greatly to the overall response. Second, the very high dissimilarity percentages (90-99.8%) demonstrate that the faunal communities using each habitat-did configuration are substantially different (Table 3).

DISCUSSION

It is well known that diel changes in species composition occur in reef associated fish populations such as Haemulidae, Lutjanidae, and Apogonidae as a result of inter-habitat movement and foraging (Weinstein and Heck 1979, Nagelkerken et al. 2000, Unsworth et al. 2007). Additionally, fish species assemblages within seagrasses contain similar families throughout the world (Pollard 1984). While higher abundance and diversity of Lutjanids (snappers) have been found in Thalassia testudinum habitats near coral reefs than near mangroves (Baelde 1990), we found that Lutjanus apodus was found only in mangroves, but Ocyurus chrysurus was found in both habitats (Table 1). More juvenile Haemtdon flavolineatum and H. sciurus utilized seagrass habitats and their presence dominated the fish fauna captured. Juveniles of these two grunts species are also dominant in barrier reef lagoons in Belize (Sedberry and Carter 1993), while newly settled Haemulidae are among the most abundant species found in the backreef embayments of nearby St. Croix, U.S.V.I. (Mateo 2002). The migration of juvenile grunts is photomechanically attuned to light intensities suggesting that they are crepuscular (McFarland et al. 1979). In the backreef areas of Belize, H. flavolineatum were found mostly in sand flats at night, while H. sciurus were found in seagrass beds at night indicating that these fish may be leaving the reef at dusk to forage in these adjacent habitats (Burke 1995). It is interesting to note that H. flavolineatum and H. sciurus were found almost exclusively in the seagrass, but distributed equally in the day and night (Table 1). This suggests that without the mangrove habitat as an alternate, these species utilized the seagrass bed to a much higher extent than expected.

[FIGURE 1 OMITTED]

While the value of mangroves as fish habitat has been demonstrated (Faunce and Serafy 2006), our results showed significantly more fish were collected within seagrass compared to mangrove habitat during both day and night sampling periods (Fig. 1a). This finding may largely be attributed to the reduced structural habitat and the highly turbid, warm, and hypoxic conditions present within the hurricane impacted mangrove. In fact, temperature exceeding 34[degrees]C and dissolved oxygen concentrations below 5 mg/l during the day were observed (Bologna unpubl, data). These environmental conditions could certainly curtail the use of the mangrove, especially for vertebrates with higher respiration demands. Furthermore, unlike decapods, many reef-associated fish species may be unable to tolerate the higher temperatures and hypoxic conditions of the hurricane-impacted mangrove, especially during the day, and our results support the near absence of juvenile fish use of the mangrove during that time period (Fig. 1a). Schools of large tarpon were also observed feeding along the edge of the turbidity plume exiting the mangroves during daytime ebbing tide (Bologna pers. obs.) potentially influencing the presence of fish in this area indicating that anti-predator behavior may be affecting the use of the mangrove (sensu Abrahams and Kattenfield 1997).

Decapod species composition, however, varied extensively based on habitat type and diel sampling period with more decapods being collected at night, but with similar distributions between mangrove and seagrass habitats (Fig. 1b). The most abundant decapod species, Palaemonetes pugio, utilized predominantly mangrove habitat at night, but it was present in seagrass habitat during the day (Table 1). This suggests a diel migration into the mangrove to feed and active refuge seeking during the day within the seagrasses. The second most abundant species, Calcinus elegans, preferred nocturnal foraging within seagrass habitat with 96% of the individuals being collected during this time frame. Juvenile Panulirus argus showed a similar pattern of being only collected at night, within the seagrass beds (Table 1). Panulirus argus is strongly nocturnal (Cox et al. 1997) especially in regards to juvenile movement and feeding.

One unique finding relates to the potential habitat segregation or niche partitioning between two species of mud crabs in the mangrove. Specifically, Panopeus occidentalis was significantly more abundant in mangroves and dominated the mud crab abundance at night there (80% CPUE, mud crabs), while Eurypanopeus abbreviatus dominated the mangrove during the day (70% CPUE). This apparent diel segregation of the mangrove may limit interspecific competition between them. Other species of these genera exhibit a similar relationship. Specifically, P. herbstii and E. depressus appear to have different life histories with respect to size of individual and diet, which allows them to co-occur in intertidal oyster reefs (McDonald 1982) and Meyer (1994) demonstrated that they partitioned the intertidal oyster reef habitat in North Carolina. Our results indicate a similar niche partitioning potential between P. occidentalis and E. abbreviatus, but further research is needed to ascertain this.

Species assemblages of tropical marine ecosystems can be indicative of changing or altered environmental conditions. In this study, several fish and decapod species varied considerably based on time of day and habitat type. Considering the degraded condition of the mangrove habitat, the reduced abundance and species composition detected during our sampling period compared to other studies (see Pinto and Punchihewa 1996, Dorenbosch et al. 2004) was not unexpected. Our community assessment indicated that significant differences existed in the fauna using the mangrove and seagrass habitat as well as diel usage (Table 2). While we have been able to document a divergence in faunal utilization of these habitats, more research into how these differences impact communities and trophic transfer is necessary to assess the recovery and functioning of the mangrove. For each habitat-time configuration, we documented low similarity among replicates and these relationships were dominated by two or three species (Table 3). Not surprisingly, each habitat configuration showed extremely high dissimilarities (90-99.8%), with the greatest dissimilarity being seen between mangrove day and seagrass night, where nocturnal foraging of C. elegans in seagrass and the lack of H. flavolineatum in mangroves dominated the relationship (Table 3). The mangrove is functioning as a habitat for decapods, but it is providing little value to the juvenile fish in this bay. Given the original impact year of 1985 and the lack of interior recovery, it may be many more years before this mangrove is contributing to the fish abundance and productivity of Great Lameshur Bay.

One primary issue in coastal systems is understanding how the loss of habitats impacts the communities of organisms previously using them. While it is clear that human impacts on systems like mangroves detrimentally affect the associated fauna (Wolff et al. 2000), our understanding of how natural disturbances influence the fauna and the potential route toward recovery is still unclear. Long-term monitoring of these sites is necessary to resolve the return to functionality of these valuable communities. Unfortunately, many of these communities are also under threat from anthropogenic pressures and recovery may never occur naturally. Due to the protected nature of this Biosphere Reserve, long-term monitoring and assessment on this site are possible and we can evaluate the potential recovery trajectory for use of the mangroves by fish and decapods.

ACKNOWLEDGMENTS

We would like to acknowledge Daniel Ward for his instrumental role throughout the field portion of this study; without his perseverance, effort, and expertise this project could not have been completed. We would also like to thank Cathleen Dale, Rita Papagian, Suzann Regetz, and Taryn Townsend for their assistance and recommendations which have proved especially valuable in the completion of this manuscript. We would like to thank Rafe Boulon with the Virgin Islands National Park for administrative support of this project. Last but not least, we would like to thank the volunteers and staff at the Virgin Islands Environmental Resource Station for their jovial support during this field study.

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CHARLES C. KONTOS (1) AND PAUL A. X. BOLOGNA (2)

(1) DEPARTMENT OF BIOLOGY AND MOLECULAR BIOLOGY, MONTCLAIR STATE UNIVERSITY, MONTCLAIR, NJ 07043

(2) AQUATIC AND COASTAL SCIENCES PROGRAM, DEPARTMENT OF BIOLOGY AND MOLECULAR BIOLOGY, MONTCLAIR STATE UNIVERSITY, MONTCLAIR, NJ 07043, EMAIL: BOLOGNAP@MAIL.MONTCLAIR.EDU
Table 1. Species Specific Catch Per Unit Effort (CPUE). Values
represent the mean CPUE [+ or -] 1 SE for all species identified from
this study. Mean size values provided for fish are standard length
values [+ or -] 1 SE, crabs carapace width [+ or -] SE, and shrimp/
lobsters carapace length [+ or -] 1 SE. NR represent values not
recorded.

 MANGROVE

FISH SPECIES NIGHT DAY

Ctenogobius stigmaticus 0.007 [+ or -] 0.005 0
Haemulon flavolineatum 0.014 [+ or -] 0.014 0
Haemulon sciurus 0 0
Halichoeres bivittatus 0 0
Holocentrus adscensionis 0 0
Lutjanus apodus 0.027 [+ or -] 0.015 0.006 [+ or -] 0.006
Ocyurus chrysurus 0.003 [+ or -] 0.003 0
Stegastes adustus 0.003 [+ or -] 0.003 0
DECAPOD SPECIES
Alpheus heterochaelis 0.003 [+ or -] 0.003 0
Calcinus elegans 0.003 [+ or -] 0.003 0
Callinectes sapidus 0.007 [+ or -] 0.005 0.003 [+ or -] 0.157
Eurypanopeus abbreviatus 0.014 [+ or -] 0.014 0.076 [+ or -] 0.021
Lysmata wurdemanni 0 0
Palaemonetes pugio 0.151 [+ or -] 0.082 0.02 [+ or -] 0.015
Panopeus occidentales 0.051 [+ or -] 0.019 0.03 [+ or -] 0.018
Panulirus argus 0 0
Portunus sebae 0 0
Stenocionops furcata 0 0

 SEAGRASS

FISH SPECIES NIGHT DAY

Ctenogobius stigmaticus 0 0
Haemulon flavolineatum 0.265 [+ or -] 0.105 0.212 [+ or -] 0.098
Haemulon sciurus 0.015 [+ or -] 0.01 0.019 [+ or -] 0.014
Halichoeres bivittatus 0.023 [+ or -] 0.16 0
Holocentrus adscensionis 0 0.006 [+ or -] 0.006
Lutjanus apodus 0 0
Ocyurus chrysurus 0 0.012 [+ or -] 0.008
Stegastes adustus 0 0
DECAPOD SPECIES
Alpheus heterochaelis 0 0
Calcinus elegans 0.182 [+ or -] 0.062 0
Callinectes sapidus 0 0
Eurypanopeus abbreviatus 0 0.033 [+ or -] 0.027
Lysmata wurdemanni 0.008 [+ or -] 0.008 0
Palaemonetes pugio 0.008 [+ or -] 0.008 0.076 [+ or -] 0.069
Panopeus occidentales 0.015 [+ or -] 0.01 0
Panulirus argus 0.03 [+ or -] 0.013 0
Portunus sebae 0 0.013 [+ or -] 0.009
Stenocionops furcata 0.008 [+ or -] 0.008 0

FISH SPECIES MEAN SIZE (CM)

Ctenogobius stigmaticus 3.60 [+ or -] 1.27
Haemulon flavolineatum 8.78 [+ or -] 1.95
Haemulon sciurus 10.18 [+ or -] 0.95
Halichoeres bivittatus 9.97 [+ or -] 0.40
Holocentrus adscensionis 14.60
Lutjanus apodus 13.59 [+ or -] 2.39
Ocyurus chrysurus 6.77 [+ or -] 4.22
Stegastes adustus 9.40
DECAPOD SPECIES
Alpheus heterochaelis 3.90
Calcinus elegans NR
Callinectes sapidus 6.6 [+ or -] 2.26
Eurypanopeus abbreviatus 2.23 [+ or -] 0.79
Lysmata wurdemanni 3.30
Palaemonetes pugio NR
Panopeus occidentales 2.60 [+ or -] 0.55
Panulirus argus 8.92 [+ or -] 1.10
Portunus sebae 4.47 [+ or -] 0.93
Stenocionops furcata 0.70

Table 2. ANOSIM comparisons among the habitat-diurnal treatments.

 SIGNIFICANCE
COMPARISON PAIR GROUPS R STATISTIC LEVEL

Seagrass Day vs. Seagrass Night 0.16 0.028
Seagrass Day vs. Mangrove Day 0.29 0.001
Seagrass Day vs. Mangrove Night 0.296 0.001
Seagrass Night vs. Mangrove Day 0.507 0.001
Seagrass Night vs. Mangrove Night 0.449 0.001
Mangrove Day vs. Mangrove Night 0.238 0.002

Table 3. SIMPER Community Comparisons among the habitat-diurnal
treatments. Values represent the individual percent contribution
and the cumulative contribution to defining the fauna responsible
for the relationship.

 SEAGRASS DAY (SD)

 Average SD Similarity 9.99

 Ind.% Cum.%

SD H. favolineatum 84.9 84.9
 P. sebae 9.4 94.3

 Average Dissimilarity 90.4%

 Ind.% Cum.%

SN H. favolineatum 38.6 38.6
 C. elegans 25.9 64.5
 P. pugio 7.1 71.6
 P. argus 6.8 78.4
 F. abbreviatus 4.4 82.7
 P. sebae 4.4 87.1
 H. sciurus 4.3 91.4

 Average Dissimilarity 97.5

 Ind.% Cum%

MD H. favolineatum 29.1 29.1
 E. abbreviatus 25.2 54.2
 P pugio 13.1 67.3
 C. sapidus 9.4 76.6
 P. occidentalis 7.9 84.6
 P. sebae 6.4 91.0

 Average Dissimilarity 97.3%

 Ind.% Cum%

MN H. favolineatum 31.6 31.6
 P. Pugio 22.9 54.6
 P. occidentalis 12.3 66.8
 P. sebae 7.7 74.5
 E. abbreviatus 6.2 80.8
 O. chrysurus 4.3 85.1
 L. apodus 4.2 89.3
 H. sciurus 3.5 92.9

 SEAGRASS NIGHT (SN)

SD

SN Average SN Similarity 23.9

 Ind.% Cum.%

 C. elegans 51.5 51.5
 H. flavolineatum 39.6 91.1

MD Average Dissimilarity 99.8%

 Ind.% Cum%

 C. elegans 26.3 26.3
 H. flavolineatum 25.9 52.3
 E. abbreviatus 15.8 68.1
 P. argus 6.8 74.9
 C. sapidus 6.8 81.7
 P occidentalis 6.1 87.8
 P pugio 3.7 91.5

MN Average Dissimilarity 98.2

 Ind.% Cum.%

 C. elegans 27.4 27.4
 H. flavolineatum 26.7 54.1
 P. pugio 12.4 66.6
 P. occidentalis 9.5 76.0
 P. argus 8.0 84.0
 L. apodus 3.3 87.3
 S. furcata 3.2 90.5

 MANGROVE DAY (MD)

SD

SN

MD Average MD Similarity 18.1

 Ind.% Cum.%

 E. abbreviatus 79.3 79.3
 C. sapidus 12.9 92.3

MN Average Dissimilarity 94.2

 Ind.% Cum.%

 E. abbreviatus 28.5 28.5
 l? Pugio 22.2 50.7
 P occidentalis 21.4 72.1
 C. sapidus 13.1 85.1
 L. apodus 6.6 91.7

 MANGROVE NIGHT (MN)

SD

SN

MD

MN Average MN Similarity 8.5%

 Ind.% Cum.%

 P. occidentalis 48.4 48.4
 P. pugio 41.3 89.7
 L. apodus 6.8 96.5
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Author:Kontos, Charles C.; Bologna, Paul A.X.
Publication:Bulletin of the New Jersey Academy of Science
Date:Jun 22, 2008
Words:4543
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