According to what I read about Whiteleg Shrimp, the best choice is the one farmed in the U.S. using the method of indoor follow-through raceway, on ponds, or the one farmed worldwide in an indoor recirculating tank. Since the employee of the restaurant that you asked mentioned that the Whiteleg Shrimp they get might be coming from the Eastern Pacific Coast, it would be crucial to know the method the shrimp is being caught to verify that is a good choice or a good alternative, and not a kind of shrimp source that should be definitely avoided.
As pink shrimp grow, they migrate seaward to deeper, saltier water. They travel primarily at night, especially around dusk, and bury themselves in the bottom substrate during the day. Smaller pink shrimp remain in estuaries during winter and bury themselves deep in the sand or mud to protect themselves from the cold temperatures. Also, bycatch is extremely common with shrimp fishery and includes finfish, sea turtles, and overfished red snapper. By law, fish trawlers are required to use bycatch reduction devices that allow the non-target species to escape fishing nets. Additionally, they are required to use turtle extruder devices (TEDs), a grid of bars with an opening either at the top or the bottom of the trawl net designed to prevent turtle deaths. Fishermen tend to avoid trawling areas of high coral reef concentrations for ethical reasons but also to prevent any damage to their gear.