Ecological Repercussions of Ditching Wetlands for Mosquito Control in Tampa Bay, Florida, or "Fishes in Ditches"
Fish ecologists at the U.S. Geological Survey (USGS)'s Florida Integrated Science St. Petersburg Science Center in St. Petersburg, FL, have initiated a 5-year study at selected wetland sites in Tampa Bay, FL, to compare natural-wetland fish habitats with those altered by ditches dug for mosquito control.
Salt-marsh and mangrove wetlands serve as critical habitat for numerous fish species of ecologic and economic importance. Habitat modification through construction of mosquito-control ditches certainly alters the hydrologic and geomorphologic characteristics of these systems. Linear channels or ditches, with their steep, spoil-lined banks, create submerged habitats atypical of natural wetlands and may make parts of the marsh surface inaccessible to fishes.
The few previous studies of fishes in the altered wetlands of Tampa Bay are limited in scope, both spatially and temporally. Studies include assessing the effects of restoring mosquito-ditched wetlands on the fish communities at Feather Sound, Archie Creek, and Double Branch Bay.
The Double Branch Bay study examined fishes in a reconnected impoundment but did not include the complex of parallel-grid ditches adjacent to the study site. The most comprehensive study was a 2-year effort that examined fish communities in natural wetlands in comparison with those from altered and restored wetlands at several localities within Tampa Bay.
With the advent of insecticides and absence of maintenance, ditches are no longer required to control mosquitoes and could be filled or blocked to restore more natural wetland hydrology. The present condition of wetland ditches varies widely throughout Tampa Bay. Many ditches have become partly to completely filled with anoxic sediment and overgrown by mangroves and Brazilian pepper, a small, nonnative tree; other ditches remain open, with good flow, and structurally resemble natural wetland creeks.
Despite their altered condition, ditched wetlands provide habitat for various species of marine, estuarine, and freshwater fish speciesthus the problem: which ditches should be restored, and which are still viable habitat?
The 5-year USGS study aims to determine the magnitude of functional equivalency between natural wetland habitats and those altered by ditching. Specific questions to be addressed include
Sample sites were chosen randomly from accessible areas of several county preserves throughout the Tampa Bay estuary.
Sites in creeks and ditches will be isolated with block seines and sampled with a bag seine.
Ponds will be sampled with a bag seine, using the State of Florida's Fisheries-Independent Monitoring offshore-seining procedure.
Wetland habitat will be characterized by documenting width, length, and bottom profile of the ditch or creek site. Substrate type and depth, water depth, flow rate, and water quality will also be measured at ditch, creek, and pond sites, as will shoreline and bottom vegetation.
Differences in species abundance, composition, and diversity, as well as size structure and overall biomass, will be used to define community structure.
Finally, habitat characteristics influencing fish-community structure will be delineated by using multivariate statistics. In addition to enhancing our knowledge of habitat function for fish communities, study results will be applied to guide restoration within county preserves and to assist in construction of a model of ecosystem processes in the estuary.
in this issue: Seamount Environments off California
Ecological Repercussions of Mosquito Control