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

Article Excerpt
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 ( 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...

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