Friends of the Environment

Abaco, Bahamas

Cross Harbour Creek Restoration

After months of planning, a series of community meetings and a lot of hard work, Friends of the Environment announced that the Cross Harbour tidal creek restoration was a wonderful success, with more than 50 acres of wetlands reclaimed and put back into use as nursery habitat for a variety of marine life.

The causeway to Cross Harbour was pulled out on Monday, April 3, 2006 and five culverts were installed so that the water could flow under the road. After some creative engineering, overseen by FRIENDS President Michael Albury and lead scientist Dr. Craig Layman of Yale University, the culverts were weighted down and the road was replaced over them.

* Dr. Craig Layman, Ph.D. - Donnelley Environmental Fellow, Department of Ecology and Evolutionary Biology, Yale University

But the work wasn't done yet. A huge amount of volunteer work was needed to help recreate tidal channels that had overgrown due to the decrease in water flow caused by the road. During the course of a week, over 200 students from seven local Abaco schools and 70 interested residents and visitors came to Cross Harbour to get involved.

Volunteers waded through mud and muck to remove over-grown mangroves, re-creating natural tidal channels and monitoring over 400 tagged fish as they moved into the restored area. Students from Sandy Point volunteered on two Saturdays, assisted by Earth Watch intern Monica Arso. Thanks to Ms. Mills and Ms. Campbell over 60 6th graders from Abaco Central Primary traveled down to volunteer on Friday, April 7th. Friends Board Member and Forest Heights teacher, Jim Richard, took a group of students to volunteer and camp for the weekend.

Science teachers Ms. Bailey and Ms. Wedgewood led their science students from Abaco Central High on Monday, April 10th. Hope Town All Ages School also made the trek to Cross Harbour on Monday led by Principal Candace Key and teacher Nancy Burnett. Man-O-War All Ages School traveled to Cross Harbour on Tuesday led by Principal Kelli Janes.

Once the volunteers overcame their initial fears about wading through the muck they all seemed to enjoy their visit. By the end of each day everyone was covered in mud, but pleased with the work that had been accomplished. A number of the visitors had never been in a mangrove area and their importance as a nursery for fish, conch and crawfish was news to many.

Partners in this initiative include The Nature Conservancy - Bahamas Program, the Bahamas National Trust, the Bahamas Ministry of Works, Acorn Alcinda Foundation and Yale University.

When the dust had settled on the project it was easy to declare it a success. Already a huge difference could be seen in the speed of the water flowing through the area. It is now possible to kayak from the causeway all the way to the ocean. The water clarity in area that had been blocked by the road is already improving. Even more exciting, according to Layman, was the fact that within a few days fish that had been tagged before the restoration were spotted swimming through the culverts into the restored area.

Tidal creeks, one particular type of estuary, are integral components of the coastal zone of the Bahamas. In general, estuaries are semi-enclosed water bodies where fresh and salt water mix; however, due to limited input of surface freshwater, Bahamian estuaries largely are marine-dominated. Most often, these systems are lined by stands of mangrove trees (primarily red mangrove Rhizophora mangle and black mangrove Avincennia germinans), and also often support extensive seagrass beds (Thalassia testudium), macroalgal flats, as well as a mosaic of other habitat types (e.g., rocky reefs, sand flats, etc.).

These systems are critical because of the wealth of resources they support internally, as well as for their important linkages with adjacent back reef and marine ecosystems. For example, tidal creeks serve as nursery areas for Nassau grouper, queen conch, and spiny lobster.

In numerous places throughout the Bahamas, roads have been built across tidal creeks, thereby constraining or totally obstructing tidal flow (i.e. fragmenting the ecosystem). Such fragmentation creates a physical barrier to movement for aquatic organisms like fish, but also initiates processes that render remaining aquatic habitat inhospitable for many marine species. In fragmented Bahamian creeks, obstruction of tidal flow leads to increased sediment accumulation. Sediment buildup facilitates red mangrove encroachment into creek channels. Mangroves further slow water velocities, increasing sediment deposition, and leading to additional losses of aquatic habitat. This process results in shallower water depths, a decrease in available aquatic habitat, lower dissolved oxygen, and changes in other physiochemical parameters.

Over 60 years ago, a road was built across the middle of Cross Harbour Creek blocking the majority of tidal flow between the ocean and upstream aquatic habitats.

More than 40 acres of tidal creek (wetland) habitat were blocked from tidal influence, resulting in an isolated wetland with limited marine input (only seepage through the limestone rocks under the road, and on extremely high tides). The road facilitated mangrove encroachment into the tidal channel both above and below the road, further increasing sediment deposition and decreasing channel depth. Above the road, little obvious channel remains, but downstream, the channel remains largely intact (although shallow).

Physical and biological characteristics of the wetland area above the road is substantially different than would expected if the road did not block tidal flow. Above the road low salinities are indicative of the lack of tidal flow. Six fish species still inhabit the upstream area: mojarra spp. (Eucinostomus spp.), yellowfin mojarra (Gerres cinereus), gray snapper (Lutjanus griseus), barracuda (Sphyraena barracuda), mosquitofish (Gambusia hubbsi), and sheepshead minnow (Cyprinodon variegatus). The first four of these species ( mojarra spp. yellowfin mojarra, gray snapper and the barracuda) are all relatively small in size compared to the largest individuals found downstream and the last two species (mosquitofish and the sheephead minnow) are typically representative of degraded tidal creek conditions.

Gray Snapper

Yellowfin Mojarra

Barracuda

Mosquitofish

Sheephead Minnow

We suggest that restoration of tidal flow to Cross Harbour Creek will result in a more functional ecosystem with greater biodiversity (e.g., more fish species will use the area), more natural food web structure, and greater production of ecologically and economically important fishes. Additionally, during high tides, many transient fishes likely move up to the road to forage on the abundant prey in tidal creeks (e.g. large barracuda, bonefish, needlefish, sharks), as we have observed repeatedly in tidal creeks on Andros. Installing large culverts in the road will allow these fish to pass into the large upstream area, taking advantage of extensive habitat and feeding areas. Following restoration, there is up to 0.5m of sediment overlying the bedrock in the channel which likely will be relocated from the main channel to natural sand and mud flats by the physical process of tidal flushing. This will provide a larger channel for upstream movement by transient fishes, as well as additional habitat for resident fishes.

A unique characteristic of this tidal creek, and proposed restoration project, relates to the blue holes that are situated just downstream of the road. These blue holes still provide habitat for dozens of snapper, a source pool which likely will facilitate colonization of the restored area upstream of the present road. Furthermore, restoration is expected to increase the number of fish species that use these blue holes, because increased tidal flushing of this system should improve overall habitat quality.

Proposed Methodology

The basic concept will be to re-establish flow to the creek channel by: (1) removing a section of the road and installing culverts to restore tidal flow to upstream areas and (2) clearing mangrove trees to re-establish the channel. This general protocol will follow that established in the restoration of the Adrosian creeks Man-O-War Sound (May 2004) and Conch Sound (May 2005).

The first step in the project will be to remove the section of the causeway at its intersection with the historic tidal channel. Six 36” culverts will be installed in the channel and fill replaced around the culverts. The design will significantly increase tidal flow in the area, while still allowing vehicles to pass over the road and reach the Cross Harbour beach. This work will be carried out using a small back hoe has been donated from local business for use in the project.

The second half of the project will involve removing mangroves from the main channel. These are all mangroves that were not historically in the creek channel, i.e. tress that encroached into the historic creek channel following the causeway construction. Less than 0.01% of all mangroves in the system will be removed. All mangroves will be removed using hand tools such as shovels, saws, and brush clippers. Numerous volunteers will be involved in this work, ranging from elementary school children to local fisherman. Removed mangrove debris will be transported away from the site using pickups trucks

Proposed site for the culvert at Cross Harbour Creek. Note the remnant channel that remains, necessitating that few mangroves will have to removed in the project

Scientific Monitoring

An essential part of any restoration project is scientific monitoring of pre- and post-restoration conditions. Especially as multiple stakeholders and interested parties look to larger restoration projects in The Bahamas, there is a need to document specific successes and difficulties, so larger projects can be optimally designed. Dr. Craig Layman will oversee scientific monitoring, which will consist of three components::

Food web structure We also propose to analyze the structure of the creek food web before and after restoration using stable isotope techniques. Stable isotope techniques are important tools in analyzing food web structure, as they can reveal both the basal resources that ultimately support secondary consumers and top predators, as well provide information as to the trophic position of all members of the food web. Following our protocols established in creeks on Andros Island, we will collect key members of the food web and seek to establish general food web attributes before and after restoration. Particular attention will be given to identifying the “resident” food web, i.e. feeding patterns for fishes which remain in the creek, and how these feeding relationships are impacted by transient members that enter the creek at high tides. Samples will be transported to, and analyzed at, either Yale or Florida International University.

Physical Monitoring

Water level recorders and temperature recorders will be installed before restoration to provide a continuous measurement record and to document the extent to which these critical physical parameters change. Salinity will also be monitored before and after restoration. Using a hand held GPS and measuring tapes, the total spatial extent of all habitat types (mangrove, rocky areas, blue holes, etc.) will be estimated. Average and maximum water depths will be determined above the blockage before and after restoration. Sediment depth (to the bedrock) in the main tidal channel also will be tracked through time.

Fish movement patterns One of the most important purposes of tidal creek restoration is to improve quality of, and access to, important habitat that may serve as “nurseries” for ecologically and economically important fish species. By examining fish distribution and movements following the restoration, we will be able to evaluate the extent to which the project increases net secondary production (i.e. how many grams of fish per year are produced in the system). Tracking fish movement will be carried out in two phases.

In Phase 1, we will tag individuals (n >>500)) of abundant species (gray snapper, schoolmaster snapper, blue striped grunt) that reside downstream of the road, using uniquely numbered external dart tags. We hypothesize that many of these fish will move into upstream areas quickly after restoration of tidal flow, as suggested by our preliminary data on Andros Island.

Monitoring of these fishes can be carried out by snorkeling, and provides an opportunity to train local volunteers to assist with project assessment. We will quantify tagged fish that move into upstream areas and remain (i.e. residents), as well as those that move into the upstream areas only at high tides (i.e. transients).

In Phase 2, ultrasonic transmitters will be inserted into a sub-set of the resident upstream snappers, and we will use a spatial array of acoustic receivers at the creek mouth to see if these fish ultimately move back into the coastal marine environment. Thus, we will quantify the amount of fish biomass exported to the marine environment following restoration of the tidal creek.

Outreach And Education

Every stage of the project will involve extensive outreach and educational efforts, involving primary to high school students and numerous local community members. High school students have already played a substantial role in preliminary data collection, and these efforts will be substantially expanded in the coming weeks.

We expect hundreds of Abaconians to assist with the mangrove removal in April, and many of these people will remain involved in post-restoration monitoring. This project should provide an ideal opportunity to educate local people about the importance of conservation of Abaco’s natural resources.

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