Abstract
Reefs formed by the eastern oyster Crassostrea virginica are ecologically and economically important in the estuaries along the East and Gulf coasts of the United States. Crassostrea virginica is a sessile benthic organism that plays a vital role in improving water quality in estuaries by filtering nutrients, sedimentary fines (microscopic sand particles), phytoplankton and pollutants. The resulting increase of sunlight penetration into the water column also helps promote seagrass growth. However, environmental value increases further when oysters and the reefs they create provide feeding stations for many species of predatory fish. These reef structures provide refugia for mobile benthic organisms that utilize the reef's interstitial spaces for protection, feeding and reproductive activities. These organisms are important as a food source for higher trophic level organisms, such as recreationally and commercially important fishes, that use reefs as feeding stations. Previous studies have shown that unique organisms are associated with specific types of benthic habitats, such as oyster reefs, seagrass meadows, mangrove root structure systems and mudflats. Habitats for particular organisms encompass certain characteristics such as physical structure, provision of food, substrate, hydrodynamics and hydrology that, together, determine their utilization by organisms. For example, species of mud crabs in southwest Florida belonging to the family Xanthidae are found predominantly on oyster reefs because of their ability to find protection inside the interstitial spaces between and beneath oyster valves, where they can endure the harsh environment created by inter-tidal exposure, and wave energy. For each of the benthic habitats previously mentioned, there are unique parameters that allow certain organisms to thrive, while others may struggle to survive. Unique organisms that have evolved to exist in a particular environment as their primary residence are usually found in the greatest numbers in those habitats. This presence makes them an indicator species for specific benthic habitats. However, many of these same organisms can be found in lesser numbers in several types of habitat as they could transition between habitats via tidal currents, swimming or hitching a ride on floating algae. This project investigated the link between oyster reef communities and the trophic transfer of biomass to tertiary-level predator fishes that were captured in proximity to isolated reef structures. A previous study using stable isotopes (Abeels et al. 2012), demonstrated trophic transfer of biomass from nutrients in the water column through oysters and several organisms that reside on the oyster reefs. By examining gut contents and identifying prey items classified as oyster reef indicator species, this extension of the trophic link can be established. From January 2006 through September 2006, sampling of fishes around oyster reefs was conducted using an entanglement net (also referred to as a gill net). A total of 294 fishes were captured for analysis of stomach contents. Of those fishes, 106 stomachs contained identifiable prey items. Within the gut contents, a total of 26 different prey item categories were identified. These prey items were then characterized using the Lima-Junior and Goitein (2001) Importance Index method of analysis. This method serves to rank predator diet composition of stomach contents to community prey assemblages of oyster reefs and of those from other benthic habitats. These fishes were captured in the Horseshoe Keys area of Estero Bay, Lee County, Florida from a specified area measuring 4 km2. This site was chosen because of the nearly complete isolation of oyster reef habitat from that of other habitats, such as seagrass. Results showed that prey items indicative of oyster reef residency belonging to the family Xanthidae (mudcrabs) occurred in the greatest number of stomachs (44%). Xanthids found in the stomachs of predator fish include Panopeus spp. and Eurypanopeus depressus. Other prey items deemed to be indicator species of oyster reefs, including Palaemonetes pugio (daggerblade grass shrimp), Alpheus heterochaelis (big claw snapping shrimp), Petrolisthes armatus (green porcelain crab), Opsanus beta (gulf toad fish) and Gobiesox strumosus (skillet fish), occurred in the majority of stomachs, contributing 53% of the overall Importance Index (AI). These indicator species contributed 48% of the dry season diet and 58% of the wet season diet for cumulative prey items of all predators. Results suggest that while the diversity of predatory fish caught in wet and dry seasons did vary, the diets of those fish did not differ significantly. These results demonstrate that several key prey items identified as oyster reef indicator species and found in the stomachs of predatory fishes contribute to oyster reef biomass transfer. Therefore these results identify an important ecological service provided by oyster reefs as feeding sites for a variety of transient fishes. Further studies could show that expanded oyster reef restoration efforts can lead directly to enhanced fisheries production and, peripherally with clearer bay water, to greater production of seagrass meadows. This, in turn, could greatly enhance the overall ecological production of estuaries such as Estero Bay.