I investigated dietary differences, overlap and shifts for two atherinid fish species inhabiting the highly modified Tennessee-Tombigbee Waterway (TTW) in Mississippi, U.S.A. The Mississippi silverside (Menidia audens) is now common throughout the waterway and apparently invaded from the Tennessee River after man-made connection with the Tombigbee River in the mid 1980s. A subsequent decline in the distribution and abundance of the native brook silverside (Labidesthes sicculus) led to speculation that dietary competition might be involved. I used extant data to test the null hypotheses that diets are not different between the two silverside species in TTW, that dietary overlap between them is random and that no dietary niche shift occurs for either species in the presence or absence of its putative competitor. I accomplished this by quantifying the diet of both species from sympatric and allopatric collections, then using multivariate statistics and null models to test for significant dietary differences, overlap, and niche shifts. The analysis indicated that diets differed significantly between the two species. However, dietary overlap at sites of co-occurrence was greater than expected under conditions of incipient competitive exclusion; and, dietary niche shifts were not found when the diet of either species in allopatric collections was compared to its diet in sympatric collections. Thus, there was no evidence for direct involvement of dietary competition in the population trends of silversides in the Tennessee-Tombigbee Waterway. Yet, it appears that M. audens is replacing L. sicculus in dominant habitats of TTW, just as has happened in other, similar systems. Simulations with STELLA� were used to estimate the degree of inter-specific competition that might account for observed rates of replacement. Results implied a 10 % ecological advantage of M. audens over L. sicculus.

I investigated dietary differences, overlap and shifts for two atherinid fish species inhabiting the highly modified Tennessee-Tombigbee Waterway (TTW) in Mississippi, U.S.A. The Mississippi silverside (Menidia audens) is now common throughout the waterway and apparently invaded from the Tennessee River after man-made connection with the Tombigbee River in the mid 1980s. A subsequent decline in the distribution and abundance of the native brook silverside (Labidesthes sicculus) led to speculation that dietary competition might be involved. I used extant data to test the null hypotheses that diets are not different between the two silverside species in TTW, that dietary overlap between them is random and that no dietary niche shift occurs for either species in the presence or absence of its putative competitor. I accomplished this by quantifying the diet of both species from sympatric and allopatric collections, then using multivariate statistics and null models to test for significant dietary differences, overlap, and niche shifts. The analysis indicated that diets differed significantly between the two species. However, dietary overlap at sites of co-occurrence was greater than expected under conditions of incipient competitive exclusion; and, dietary niche shifts were not found when the diet of either species in allopatric collections was compared to its diet in sympatric collections. Thus, there was no evidence for direct involvement of dietary competition in the population trends of silversides in the Tennessee-Tombigbee Waterway. Yet, it appears that M. audens is replacing L. sicculus in dominant habitats of TTW, just as has happened in other, similar systems. Simulations with STELLA� were used to estimate the degree of inter-specific competition that might account for observed rates of replacement. Results implied a 10 % ecological advantage of M. audens over L. sicculus.

Thesis (M. Sc. (Zoology and Entomology))--Rhodes University, 2006.

Phylogeographic patterns of coastal organisms with different life histories and breeding strategies may reveal patterns not consistent with the current delineation of the biogeographic provinces around South Africa. The subdivision of the South African coastline into these three main climatological or biogeographic regions: namely the cool temperate west coast, the warm temperate south coast and the subtropical east coast, is based on average seawater temperatures and hydrological conditions. Genealogies of two estuarine fish species Atherina breviceps, a marine breeder, and Gilchristella aestuaria, an estuarine spawner, were reconstructed using mitochondrial DNA (mtDNA) control region sequences. The study comprised two components, an assessment of a small dataset of both fish species to compare their population structure along the South African coastline and a more comprehensive investigation of the phylogeography of G. aestuaria collected from 21 estuaries around the coast. The comparative study of A. breviceps and G. aestuaria indicate different population distribution patterns along the South African coastline. Results of the A. breviceps analysis demonstrate substantial gene flow due to the random mixing of alleles, while the comparative G. aestuaria dataset indicates a more structured population and considerably less gene flow. The G. aestuaria population demonstrates geographic separation into four groups, namely the west coast (Great Berg), Bot (south coast), Seekoei (south coast) and east coast (Bushmans, Kasouga and Cefane). Results from the larger G. aestuaria dataset indicate that the phylogeographic patterns observed during this study do not conform to existing biogeographic boundaries along the southern African coastline. The delineation identified during this study between the warm temperate and subtropical regions is further south than originally perceived and this southward extension can be ascribed to the prevailing hydrology. The life history patterns and ecology of these two estuarine fish species appears key to understanding their population structure. These factors interact with environmental characteristics such as physical oceanography and the distribution of estuaries (along the coastline) to explain the observed distribution patterns and population structure of A. breviceps and G. aestuaria.