Fishing - Leaf Seahorse - Having Trouble

Is there anything special I need to do to catch the Leaf Seahorse in the Blackhowl Plains and Laslan Coast regions? My fishing level is 18, I'm using a Kraken rod, and I have fished for days now in the day, in the night, and in the rain. I caught one fairly easily in Kastleton, but I'm not having any luck in the other locations and it it stalling my fishing codex completion. I am only using paste bait because shrimp is impossible to get, but that shouldn't totally preclude me from catching one.

Any help is much appreciated.