The resurgence of Texas' snook (Family Centropomidae) recreational fishery is coupled with an uncertainty as to what species occur in State waters, a limited understanding of life history, and habitat needs of its constituents. This study described species composition and early life history aspects of juvenile (< 100 mm SL) centropomids taken in bag seine collections in estuarine and freshwater habitats along the upper, middle, and lower coast of Texas during 2006 to 2010. Centropomus specimens (n = 548) captured from 41 locations across the Texas coast as well as congeners from Mexico (n = 24), Florida (n = 7), and Costa Rica (n = 3) were used in a genetics- and meristic-based determination of species composition, growth rates, range of hatching dates, geographic distribution, and habitat association. Genetic analyses of the mitochondrial DNA 16s ribosomal RNA gene and the mitochondrial control region (D - loop) validated the presence of smallscale fat snook (C. parallelus Poey, 1860, n = 333), common snook (C. undecimalis Bloch 1792, n = 212) and Mexican snook (C. poeyi Chavez, 1961, n = 3) in Texas, with the last of these validations representing the first known record of this species in Texas. AMOVA of 16s and D - loop sequences failed to detect genetic differentiation within Texas for C. parallelus and C. undecimalis. However, AMOVA for 16s and D - loop C. undecimalis sequences did yield significant genetic differences between Texas and Mexico against those from Florida and Costa Rica. Juvenile centropomids (< 100 mm SL) in Texas occupied backwater habitats with dissipated currents similar to those of Florida congeners (tidal sloughs, freshwater habitats, and structured shorelines). Coastal ranges of these species differed with C. parallelus taken from the Rio Grande to West Galveston Bay, whereas C. undecimalis was captured from the Rio Grande northward near Palacios. Three C. poeyi were captured at only two locations (Laguna Vista and Port Aransas). Daily growth rates varied between species and capture years, with these ranging from 0.22 to 0.97 mm d^-1. Analyses of hatch-date distribution suggest centropomids in Texas begin spawning in August and continue it through late September into mid-November.

Among the 155 species of fish recorded so far in the St Lucia estuarine lake, Oreochromis mossambicus and Ambassis ambassis are the two most prominent. Although originally endemic to southern Africa, O. mossambicus is now one of the most widely distributed exotic fish species worldwide. Together with A. ambassis, they have become the dominant fish species in the St Lucia estuarine lake since the closure of the mouth in 2002 and are, therefore, a crucial component of the food webs throughout the system. After a decade dominated by dry and hypersaline conditions, the St Lucia system has changed dramatically in terms of prevailing environmental conditions, as a result of higher than average rainfall at the end of 2011 and the onset of a new wet phase at the start of 2012. In response, A. ambassis, which prefers lower salinity regimes, has expanded its distribution range throughout the estuarine lake. Stable δ¹⁵N and δ¹³C isotope analysis was used in conjunction with gut content analysis to elucidate the diet of these species at sampling localities spanning the geographical range of the system and determine whether these species shift their diet in response to environmental or climatic shifts. From both studies it is evident that from a temporal and spatial scale these two species adopt similar, yet very different, dietary tactics. Oreochromis mossambicus was shown to adopt a generalist feeding strategy, opportunistically feeding on dietary items that are available thus allowing this species to alter its diet according to the environment that it inhabits. Trophic positioning of this species was found to be controlled by salinity in St Lucia as dietary composition differed greatly between sites. In contrast, Ambassis ambassis displayed a more specialist dietary composition, feeding predominantly on zooplankton. However, this species also opportunistically supplements its diet with additional sources when available. Trophic position of A. ambassis was higher in the dry season owing to the increased productivity of the system during the wet season. The success and dominance of both species in the St Lucia system can therefore be attributed to their dietary strategies. Under extreme environmental conditions, O. mossambicus has the added advantage of its wide tolerance of different environmental conditions, particularly salinity, thus allowing it to proliferate. / M.Sc. University of KwaZulu-Natal, Durban 2014.