Tex. Parks & Wild. Code § 66.022

A person authorized by the department who, for the purpose of establishing testimony, purchases or sells any aquatic life the purchase or sale of which is prohibited or regulated by this code is immune from prosecution for the purchases or sales. A conviction for the unlawful purchase or sale of any aquatic animal may be sustained on the uncorroborated testimony of the person authorized by the department to purchase or sell aquatic life.