you may also want to create a 'killing field' for them.
maybe try a large gently sloped terrain coming in from the sea to shore, and then from shore to sea, have it be a sheer cliff face like shape. Think fish-hook, terrain style. Then you drag it over the slope and it gets stuck in there for fighting?
you would have to go back to EXACTLY THERE when you catch one, so you need to find a consistent nightspawn serpent and a suitable place to kill it etc. Then you can return every few days and score at least 1. May have to repeat all this in a few places?

Disclaimer, I have not done this, its just the seed of an idea for you.