Hey guys, I'm going on the San Marcos River collecting trip and I want to share some info for everyone.  You need to bring your own net, bags, etc. for the collecting.  The following is a list of species that you may encounter:
Herichthys cyanoguttatus "Texas Cichlid"
Poecilia latipinna "Sailfin Molly"
Gambusia - used to be an endemic species (geiseri), but now replaced by the more aggressive invasive, affinis
Astyanax mexicana "Mexican Tetra"
Notropis amabilis "Texas Shiner"
Cyprinella lutrensis "Red Shiner"
Etheostoma fonticola "Fountain Darter" (in the shallow, calm waters - look, but don't touch!  They're endangered, and protected)
Percina apristis "Guadalupe Darter" (large and mostly green - in the rapids)
Lepomis megalotis "Longear sunfish"
Micropterus treculii "Guadalupe Bass"
Micropterus salmoides "Largemouth Bass"
Oreochromis/Tilapia species (introduced)
occasionally other released aquarium fish, like red zebras, oscars, various tetras, etc.