The Fish-Wyvern is an aquatic variant of wyverns. They fly (or swim through the air) above traditional defenses towards the Eternia Crystals. Often, a map will give a warning before they start to spawn.
Fish-Wyverns behave identically to regular wyverns. But instead of fireballs, they shoot poison projectiles.
Beware of fish wyverns encountered late into Nightmare Aquanos Survival, as they are able to destroy an Eternia Crystal in two shots (one hit if buffed by a Djinn). To counter this, kill off any ogres holding up defenses as fast as possible.