As far as fishing goes in Texas, you can only go after non-game fish with a blow gun, and you still have to have a fishing license. As Parks and Wildlife puts it:
“Spear:
Any shaft with single or multiple points, barbed or barbless, which may be propelled by any means, but does not include arrows.
* May be used to take NONGAME fish only.
Speargun:
Any hand operated device designed and used for propelling a spear, but does not include the crossbow.
* May be used to take NONGAME fish only.”
(From http://www.tpwd.state.tx.us/publications/annual/fish/legal_devices/ )
Nongame fish are basically anything that isn’t a game fish or an endangered or threatened species.
Game fish in Texas include:
Game Fish (includes hybrids or subspecies of fish on this list):
* Bass: Guadalupe, largemouth, smallmouth, spotted, striped, white, yellow
* Catfish: blue, channel, flathead
* Cobia
* Crappie: black, white
* Mackerel: king, Spanish
* Marlin: blue, white
* Pickerel
* Red drum
* Sailfish
* Sauger
* Seatrout, spotted
* Sharks
* Snook
* Spearfish, longbill
* Swordfish, broadbill
* Tarpon
* Tripletail
* Trout: brown, rainbow
* Wahoo
* Walleye
And here is a link to endangered and threatened species: http://www.tpwd.state.tx.us/huntwild/wild/species/endang/animals/fish/
Basically the same thing holds for hunting: nongame animals only. These include
* Armadillos*
* Bobcats*
* Coyotes*
* Flying squirrels
* Frogs
* Ground squirrels
* Mountain lions
* Porcupines
* Prairie dogs
* Rabbits
* Turtles
http://www.tpwd.state.tx.us/publications/annual/hunt/nongame/ (by Turtles, they mean freshwater turtles, not terrapins or sea turtles).
I am sure your local game warden can clarify this further.