Tuckey and Dehaven Fish assemblages found in tidal-creek and seagrass habitats in the Suwannee River estuary 



115 



fish assemblages found in seagrass habitats had greater 

 variability in the species present and in the abundance 

 of those species. The more consistent assemblage of 

 fishes found in tidal creeks could be explained by the 

 persistence of vegetation throughout the year in tidal 

 creeks, which may have contributed to reduced preda- 

 tion and may have provided direct or indirect sources 

 of food. Vegetation coverage in seagrass habitats was 

 seasonal and Strawn (1961) found that above-ground 

 seagrass biomass declined during winter and increased 

 during summer and fall. The increased complexity re- 

 sulting from blade density and seagrass species hetero- 

 geneity offered by the growing seagrasses is known to 

 affect fish abundance and composition (Stoner, 1983). 

 The fish community structure in our study reflected 

 this seasonal change; fewer fish species were present 

 during winter and spring than in summer and fall. As 

 seagrass biomass increased, fish species composition 

 and total numbers also increased, resulting in greater 

 variability within seagrass fish assemblages. 



We found that the combination of water tempera- 

 ture, salinity, and water depth, more than any other 

 combination of abiotic variables, helped to explain the 

 fish community structure found in the Suwannee River 

 estuary. Although water temperatures between the two 

 habitats were similar, tidal creeks typically had soft 

 mud sediments instead of sand and mud, marsh-grass 

 species instead of submerged aquatic vegetation, deeper 

 average depths, and lower salinity values. Water tem- 

 perature has been shown to correlate with timing of 

 recruitment for YOY fishes, which is ultimately re- 

 lated to adult spawning patterns (Subrahmanyam and 

 Coultas, 1980; Nelson, 1998; Paperno, 2002). Because 

 water temperatures were similar in each habitat, dif- 

 ferences in fish-community structures were more likely 

 related to salinity tolerances, factors that correlate 

 with salinity and water depth. Water depth in the Su- 

 wannee River estuary varies seasonally; lowest water 

 levels occur during winter (Strawn, 1961). The result 

 is a confounding effect of water temperature and water 

 depth that probably act in concert to limit distribution 

 of fishes. A strong indicator that salinity may be the 

 major abiotic factor that determines fish distributions, 

 and ultimately species assemblages, was the low-salin- 

 ity event during March 1998 that changed the seagrass 

 fish assemblage to one more closely resembling a tidal- 

 creek assemblage. If vegetation type were the primary 

 factor controlling species assemblages in these habitats, 

 tidal-creek species would remain in tidal-creeks and not 

 invade seagrass habitats when salinity values changed 

 to more favorable conditions. Therefore, varying salini- 

 ties allowed different groups of fishes to use habitats 

 according to their salinity tolerance (Wagner, 1999). 



Nordlie (2003) examined 20 studies of estuarine salt 

 marsh fish communities in eastern North America and 

 characterized communities based on the life history 

 patterns exhibited by the species. General life history 

 categories were originally established by McHugh (1967) 

 and included permanent residents, marine nursery, ma- 

 rine transients, diadromous, and freshwater transients. 



The 45 species that had overlapping distributions among 

 habitats in our study were consistent with the classifi- 

 cations for marine nursery or marine transient species. 

 Marine transient species do not require estuarine habi- 

 tats for development, but venture into estuaries during 

 periods of low rainfall, whereas marine nursery species 

 require estuarine conditions for development. The two 

 exceptions in our study (Gobionellus bolesoma and M. 

 gulosus) were considered primary residents of saltmarsh 

 communities, but were frequently found in estuaries. 



We collected 80 fish species in tidal creeks in the 

 Suwannee River estuary — more species than have been 

 found in most other studies of tidal creeks — and this 

 number could be related to the long-term duration of 

 sampling. For example, Peterson and Turner (1994) 

 observed 29 fish species inhabiting Louisiana marshes 

 in a one-year study, whereas we found 51 additional 

 species in our tidal-creek habitats. Similarly, Hettler 

 (1989) found 35 species in a one-year study of saltmarsh 

 fishes in North Carolina, and Weinstein (1979) recorded 

 61 species from his one-year study of the Cape Fear 

 River, North Carolina. Furthermore, Cain and Dean 

 (1976) found 51 species in a one-year examination of 

 fishes in an intertidal creek in South Carolina. The 

 first year of our study resulted in the collection of 61 

 species from tidal-creek habitats. It is likely that three 

 years of sampling in our study increased our chances 

 of collecting rare species, which resulted in the higher 

 level of species richness. 



Another reason for the high species diversity and 

 abundance of fishes that we found in tidal creeks could 

 be attributed to our sampling along the tidal-creek edge, 

 which is known for its structural complexity (Montague 

 and Wiegert, 1990) and importance as a foraging and 

 refuge area (Baltz et al., 1993; Kneib and Wagner, 1994; 

 Peterson and Turner, 1994). For example, Baltz et al. 

 (1993) collected fishes in Louisiana marsh edges to look 

 at the importance of the marsh-edge microhabitat and 

 found that the 15 most abundant fishes were concentrat- 

 ed near the marsh edge and consisted mostly of early- 

 life-history stages. They hypothesized that the fishes 

 aggregated near the marsh edge to take advantage of 

 the protection provided by the vegetation and the avail- 

 able food resources. Our sampling targeted the tidal- 

 creek edge, and the gear we used selected for juveniles 

 and small-adult species, which could explain the higher 

 diversity than that seen in other studies. Another pos- 

 sibility is that our randomly chosen sampling sites cov- 

 ered a greater variety of microhabitats along tidal-creek 

 shorelines than did the sampling of Weinstein (1979), 

 Hettler (1989), and Peterson and Turner (1994), which 

 could also explain the higher species richness. Despite 

 differences in sampling methods, the collection of 80 fish 

 species in tidal creeks appears to be unusual. 



The withdrawal of fresh water from the Suwannee 

 River would likely change the salinity regime in the 

 Suwannee River estuary, which may in turn reduce spe- 

 cies diversity in the region by reducing habitat availabil- 

 ity to groups tolerant of low salinity. Furthermore, the 

 high abundance of juvenile fishes that use low-salinity 



