by Amanda Tyler-Jedlund and Tim MacDonald
Routine maintenance and dredging of boat basins and shipping channels by the U.S. Army Corps of Engineers (USACE) generates dredge material. Typically these materials are disposed of in spoil areas, but recently the USACE has been aggressively pursuing options for beneficial use of dredge materials. The Tampa Bay estuary has numerous subtidal borrow pits from which material was typically been removed for non-navigational purposes (dredge holes). It has been proposed that materials from routine maintenance dredging could be used to fill these dredge holes and restore shallow-water habitat that might be colonized by seagrasses
and serve as critical nursery areas for nekton (fish and macro-invertebrates). A 2004 study funded by the Tampa Bay Estuary Program (TBEP) and conducted by FWRI’s FIM program and the Hillsborough County Environmental Protection Commission found that the majority of the studied dredge holes represented habitats unique from adjacent shallow- and deep-water habitats.restore shallow-water habitat that might be colonized by seagrasses
Before a dredge hole can be selected as a sight for beneficial disposal of dredge material, its existing condition should be evaluated to define its ecological function and to determine the appropriateness of restoration. The goal of the current study is to determine available habitats and describe nekton distribution and abundance in eleven Tampa Bay dredge holes, ten of which have not previously been studied. The dredge holes being studied are distributed from the mouth of Tampa Bay into Hillsborough and Old Tampa bays, and have a high probability of appropriate dredge materials becoming available to fill them within the next ten years. Options for dredge hole restoration include retaining in current condition, filling with appropriate dredge material, or restoring in some other beneficial manner (i.e., artificial reef materials). In conjunction with TBEP’s Dredge Hole Advisory Committee, an implementation plan for restoration of the eleven dredge holes will be proposed.
Sampling with 6.1-m otter trawls and 21.3-m seines began in May 2016 and will continue through April 2017. Monthly samples are being collected at randomly selected sites within and adjacent to the eleven dredge holes. During the first month of sampling, it became apparent that the South Skyway dredge hole contained large concrete slabs and pilings. This previously undocumented artificial reef material prevented deployment of our standard gears; a challenge that led to a great opportunity to assess hard bottom habitats within Tampa Bay. We constructed a small, easily deployable underwater camera stand with which we record GoPro camera video each month near the structural habitat within this hole. The GoPro camera records 30 minutes of video with each deployment and two camera deployments are completed each month. Twenty minutes of each video are read back at the lab during which nekton are identified and counted.
So far we have had relatively good visibility overall and have seen large schools of Gray Snapper, Black Drum, Jacks, and baitfish. Other economically important species (Red Grouper, Common Snook, and juvenile Lane Snapper) have also frequently been identified. As confirmed in our earlier study dredge holes can provide valuable habitat and refuge.