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New Habitats in Sarasota BayHarbors, bays and estuaries are critical to the lives of many species of marine fish, shellfish, and bird life. NOAA's National Marine Fisheries Service estimates that 70% of all commercially-harvested fish spend a significant stage of their lives in an estuary. Humans also use these water bodies heavily--and these two uses often come in conflict, often with detrimental effects to the natural resources. The Sarasota Bay National Estuary Program, concerned with the impacts to living resources, has developed a series of mitigation approaches.
In its level of development and historic disruption of natural resources, Sarasota Bay is similar to many other harbors and embayments around the country. Nearly 39% of its tidal wetlands and 43% of its seagrass meadows have been lost, largely to dredging for the intracoastal waterway and filling for development of canal communities. More than 75% of the original shoreline has been modified, mostly due to seawalls. These changes resulted in major losses of suitable habitats for shorebirds, juvenile fish, shellfish, and other organisms. The resulting impact to the ecosystem has been felt by the commercial and recreational industries of the region.
While there may be no real substitute for natural habitats, some engineering options are available to partially mitigate the losses. The Sarasota Bay National Estuary Program (the NEP) is looking at four approaches.
Reforming SeawallsWidespread construction of seawalls and bulkheads meant the loss of large amounts of shallow subtidal and intertidal areas where fish spawn and live a critical part of their juvenile lives. Because they are basically smooth, vertical slabs of concrete or steel, seawalls provide very little in the way of protection or habitat for fish.
The NEP, with funding from the US EPA and the Florida Department of Environmental Protection, sponsored the development of inexpensive seawall modules to attract larval, juvenile and adult fish. In a pilot project, these were deployed and monitored over a period of almost two years. The results showed fish abundance over 100 times greater at the modules compared with equal lengths of typical seawall, suggesting that artificial habitat modules have great potential for improving fisheries habitat value.
Shoreline SofteningThe City of Sarasota is planning the development of a bay-side community park on a site presently delineated by a seawall. The original plans for the park called for reinforcement of the seawall and construction of a boat pier. The NEP suggested to the city that additional environmental, recreational, and aesthetic benefits could be achieved if the seawall was broken up and a tidal pool established in order to integrate habitat restoration with the other concerns.
The conceptual design now calls for cutting approximately 300 feet of seawall at the mean tide level and distributing the material near shore in formations which both mitigate wave energy and foster shellfish habitat. At one end of the seawall, an opening will be made to allow tidal flow into an excavated pool. Native plants will be used to protect the shoreline from erosion, provide wading bird habitat, and mitigate stormwater runoff.
Bay Bottom ImprovementsMore than 4,500 acres of Sarasota Bay bottom have been damaged through dredging for coastal development. In the 1950s and 1960s, dredge and fill operations were conducted in order to build waterfront property. This often resulted in "pockets" with water depths of 12 to 18 feet in otherwise shallow bay areas. Many of these areas were originally productive seagrass beds providing living areas for fish and shellfish.
Because such holes have low velocity currents or poor circulation, over time they have collected fine sediments and organic material--sometimes up to four or more feet thick. The net result is a bottom habitat far different from the natural bay bottom; one with anoxic sediments which support very little plant or animal life. These areas contribute to bay water quality problems by acting as a source of turbidity during storm events or when prop wash from boats re-suspends fine-grained materials.
To help mitigate these problems the NEP has designed a program to carefully fill the holes (while avoiding harm to adjacent seagrass beds). Around the holes, artificial reef structures will be placed to provide substrate for epiphytes and other organisms, habitat for larger animals, and to control erosion. The artificial reef structures consist of PVC posts driven into the sand, steel piles connected with old fishing net, or concrete block piles. These techniques are expected to improve water transparency, prevent further losses of seagrasses and shoreline habitat, and restore lost habitats.
Seawall modules to increase fish habitat
Channel Markers as HabitatMany areas in Sarasota Bay are identified by regulatory or navigation markers. The NEP is testing the use of artificial habitat structures such as "reef balls" which allow the markers to be used as anchors and locators for small scale habitat enhancement projects. If proven to be a viable approach, it may be possible to establish a policy calling for the placement of these structures when new markers are installed or old ones are replaced.
The Big Picture
Significant questions remain about the effectiveness of wide-scale use of such habitat enhancement. Past projects have shown dramatic increases in abundance of fish around individual artificial structures. A critical question is whether the very high abundance observed reflects a general increase in fish populations or merely the result of attraction and concentration of fish from nearby areas. If substantially larger amounts of artificial habitat were to be deployed, would fish density at artificial habitats remain as great, would the same number of fish be distributed at lower density over the additional artificial habitat, or would an intermediate condition of increased number, but decreased density occur? Quantitative answers to these questions will provide information valuable in understanding fish/habitat relationships in bays and estuaries. From a practical standpoint, these answers are essential for evaluating the cost effectiveness of artificial habitat approaches.
For Further Information
For more information on these projects, contact Susan Walker or Jaime Doubek at the Sarasota Bay National Estuary Program, 5333 N Tamiami Trail, Suite 104, Sarasota, Florida 34234, (941)359-5841.
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