Ahhhh, interesting. There were several individuals per colony that were dead or had obviously been removed. (Which I’d attributed to fishermen wanting bait) but perhaps they could’ve been predated.
Unfortunately you were probably right about the fishers. They are allowed to take some in the General Use Zones of the Park. The Tritons just put their proboscis down one of the existing holes in the cunjevoi and eat out the inside. If enough is left the cunjevois can regenerate the missing bits.