Characterization of the Ashepoo-Combahee-Edisto (ACE) Basin, South Carolina
The ACE Basin contains a diverse assemblage of fish species in its freshwater, estuarine, and coastal environments. Salinity, vegetation, and bottom substrate are some of the physical characteristics that affect fish species composition and distribution. Many of the fish species that characterize the ACE Basin move in response to environmental cues, and because their reaction to cues may change with growth and development, few species mature in the area where they were spawned. Thus, occurrence within a habitat or salinity regime may change with life history stage or season, making a static description of the fish community in the ACE Basin difficult to obtain.
Freshwater fishes occupy the uppermost portions of rivers in the ACE Basin study area, occurring in nontidal freshwater areas. In low salinity (brackish) areas of the estuary, freshwater and estuarine fishes co-occur, thereby creating a dynamic and diverse community. In recent years, research emphasis has been placed on characterizing the status of the fisheries resources and the structure and composition of the fish community in the Edisto River (Thomason et al. 1993), which is one of the most significant "blackwater streams" of the southeastern coastal plain (Marcy and O'Brien-White 1995). Since 1967, 87 species from 25 families have been identified from the freshwater portion of the Edisto River Basin. Although diversity is high, production is low in the Edisto River, especially in the area below Orangeburg (Tomason et al. 1993; Marcy and O'Brien-White 1995). High diversity in combination with low production suggests that there may be impacts on the macroinvertebrate food base in the system. Most species in the freshwater fish community utilize the shallow, palustrine emergent wetlands as foraging areas. Plankton, crustaceans, aquatic insects and small fishes are preferred prey. The introduction of the non-endemic flathead catfish to the Edisto River has had a profound effect on the resident freshwater fish community. Abundances of the native bullhead catfish are dramatically reduced in areas where flathead are abundant. Populations of the sportfish, redbreasted sunfish, have also been depleted by the presence of flathead catfish in the Edisto River. As the abundance of flathead catfish increases, certain native species appear to decline in abundance. This can lead to a significant alteration of the fish community in the Edisto River and possibly other areas of the ACE Basin study area.
Estuarine portions of the ACE Basin study area and adjacent rivers are typically important nursery areas for numerous fish species. These areas have been sampled extensively over the past 30 years. Most studies of the community ecology and life history of fishes from subtidal estuarine habitats have been based on sampling with otter trawl. A long-term trawl survey of the three rivers begun in 1993 collected a total of 54,714 individuals and 80 species of fish during the first five years. The greatest number of species was recorded from the Ashepoo (68) and Edisto (67) rivers, while the Combahee (49) yielded the fewest species. Within each river, the greatest number of species was recorded from stations nearest the mouth. The numerically dominant finfish species for all three rivers were star drum, Atlantic croaker , and bay anchovy. These species showed temporal regularity in their abundance in the three major rivers within the characterization study area. Although some competition for food undoubtedly occurs, preferences for different salinity regimes, substrates and bathymetric zones are adaptive means to reduce competition from co-occurring species.
Tidal creeks are a major feature of estuaries in the ACE Basin study area. Variable in size and water depth, they provide primary nursery habitat for larvae and juveniles of many fish species. Shallow water creek habitats in the ACE Basin study area have been sampled by trammel nets and rotenone. Both methods provide an effective way to sample shallow habitats and to capture fast-moving as well as demersal species. The spotted seatrout, striped mullet, spot, hardhead catfish, red drum, and southern flounder were the most abundant species collected in tidal creeks and together constituted over 80% of the total number of individuals. The fish community from other intertidal habitats of salt marsh, impoundments, and oyster reefs has not been specifically studied in the ACE Basin. Research on fishes from these habitats in other areas of the state indicates that they provide nursery grounds for larvae and juveniles of many fish species before they move into deeper subtidal habitats in the estuary. The shallow-water marsh and oyster reef habitats also serve as a refuge from predation by providing spatially complex habitat which predators have difficulty penetrating (Boesch and Turner 1984; Knott et al. 1996).
Fishes that occur in the coastal zone consist of year-round residents and migrant species that are in transit to or from spawning grounds or are using the coastal zone as spawning grounds (Wenner and Sedberry 1989; Beatty and Boylan 1997). An ongoing trawl survey of fishes in the nearshore coastal zone indicates numerical dominance by a number of species that occur within the estuaries of the ACE Basin during part of their life cycle. Since 1989, collections have been largely dominated by spot and Atlantic croaker, two of the common estuarine transient species found in trawl surveys of the ACE Basin study area. Other species of numerical importance have been the Atlantic bumper, the striped anchovy, the star drum, scup, and pinfish. The fish community from the coastal zone appears to be dominated by sciaenid fishes, many of which utilize estuaries for some part of their life cycle.
The extensive aquatic habitats of the ACE Basin study area that are used for spawning, nursery, and foraging areas support and maintain many fish populations (Beasley et al. 1996). Limiting development in the ACE Basin study area contributes to the health of fish populations by limiting impacts on water quality, hydrology, and vegetation. Because of dependence by fishes on the tidal wetlands in the ACE Basin as a source of food and protection from predators, it is essential that these areas remain intact. Destruction or disturbance of habitat will impact the community dynamics, resulting in decreased utilization by species and, in some cases, avoidance of the area. Habitats at risk from land-based impacts include oyster reefs, mudflats, and emergent tidal marshes. Degradation of water quality or hydrologic modifications can also affect habitat quality. Anthropogenic activities in the ACE Basin that can affect fish populations and the benthic invertebrates upon which they feed include clearcutting hardwood timber, heavy crop irrigation, and industrial/residential development. Cutting of bottomland hardwoods eliminates leaves and woody debris that are an important primary food source. Loss of canopy cover allows solar radiation to raise water temperature to high levels, especially in summer. Erosion and siltation resulting from logging operations can also have detrimental effects on water quality.
As the linkage between anthropogenic impacts and declining fish abundance, health, and quality becomes clearer, the implications of increased population growth in areas surrounding the ACE Basin study area raise concerns. Because the southeast coastal zone is one of the nation's fastest growing regions, it is especially critical that the monitoring of fishes that are dependent on rivers and estuaries for their survival continues. It is also important that effects at the population and community level be linked to physical and hydrologic alteration, as well as water quality modifications.