Spatial and seasonal abundance of sand seatrout (Cynoscion arenarius) and silver seatrout (C. nothus) off the coast of Texas, determined with twenty years of data (1987-2006).(Report)

Abstract--Sand seatrout (Cynoscion arenarius) and silver seatrout (C. nothus) are both found within the immediate offshore areas of the Gulf of Mexico, especially around Texas; however information is limited on how much distributional overlap really occurs between these species. In order to investigate spatial and seasonal differences between species, we analyzed twenty years of bay and offshore trawl data collected by biologists of the Coastal Fisheries Division, Texas Parks and Wildlife Department. Sand seatrout and silver seatrout were distributed differently among offshore sampling areas, and salinity and water depth appeared to correlate with their distribution. Additionally, within the northernmost sampling area of the gulf waters, water depth correlated significantly with the presence of silver seatrout, which were found at deeper depths than sand seatrout. There was also an overall significant decrease in silver seatrout abundance during the summer season, when temperatures were at their highest, and this decrease may have indicated a migration farther offshore. Sand seatrout abundance had an inverse relationship with salinity and water depth offshore. In addition, sand seatrout abundance was highest in bays with direct passes to the gulf and correlated with corresponding abundance in offshore areas. These data highlight the seasonal and spatial differences in abundance between sand and silver seatrout and relate these differences to the hydrological and geological features found along the Texas coastline.

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The sand seatrout (Cynoscion arenarius) and the silver seatrout (C. nothus) from the family Sciaenidae are sympatric species that co-occur within the Gulf of Mexico (GOM). In the literature, the co-occurrence and distribution of these species has been noted, particularly in offshore areas where feeding grounds overlap (Miller, 1965; Chittenden and McEachran, 1976). These studies, in concert with previous life history data from Texas (Shlossman and Chittenden, 1981; DeVries and Chittenden, 1982) have provided some insight into when and why any distributional variation occurs in the western GOM. Nevertheless, most studies of the abundance of these species have been limited on a spatial and temporal scale. Additionally, the spatial and temporal abundance of these species in relation to hydrological characteristics such as water temperature, salinity, depth, and bay access to the gulf through a channel or pass has not been thoroughly investigated.

Sand seatrout use inshore waters extensively but also move offshore seasonally to evade the temperature extremes of the inshore bays and to spawn (Shlossman and Chittenden, 1981; Vetter, 1982). In contrast silver seatrout, although on occasion they can be collected inshore, live their entire lives offshore (Gunter, 1945; Miller, 1965). In addition to their distributional difference, differences exist in their hydrological preference, particularly in salinity and water depth, adding to the complexity of the distributional preferences of these two species (Chittenden and McEachran, 1976). However, a great deal of distributional overlap of these species occurs within the immediate offshore area, year-round (Gunter, 1938; Sheridan et al., 1984), although the spatial and temporal dynamics of this overlap are poorly understood.

Furthermore, information is limited on whether the distribution of sand seatrout offshore correlates with what is found inshore (bays) (Shlossman and Chittenden, 1981). Each of the bays along the Texas coastline is unique in their geological and hydrological characteristics, giving rise to differences in species assemblages between the bays (Blackburn, 2004). Sand seatrout use functioning offshore passes to facilitate egg and larval transport from spawning areas (the immediate offshore) to nurseries (estuaries within bays) (Simmons and Hoese, 1959). However, the seasonal change in distribution of sand seatrout between these two locales has not been thoroughly examined. Furthermore, the differences in abundance of sand seatrout inhabiting bays with direct passes to the GOM and sand seatrout inhabiting bays with limited access to these passes have not been determined.

The purpose of this study was to expand current information regarding the distribution of sand and silver seatrout in the western GOM. To this end, two major objectives were identified 1) to compare the spatial and seasonal abundance of sand seatrout and silver seatrout within the immediate GOM, within the boundaries of Texas, and relate any distributional differences between the two species to specific hydrological variables (i.e., temperature, salinity, depth) and 2) to investigate the spatial and seasonal abundance of sand seatrout between the immediate offshore and the inshore areas (the bays) where there were direct passes to the gulf.

Materials and methods