How do you catch a scorpion?

You can get a Nookington by leting someone  in your Town when you get the Threed shop and if the person shops there 2.

From what I remember, you can only give fish to residents if they start a little competition with you. You might be able to send them fish in the mail. I've not tried that.

Other than those two things, you can sell the fish in the shop or give them to the mayor during a fishing competition or add them to the museum if you haven't already.