These data were collected as part of a long-term monitoring study of 12 patch reefs off Lizard Island Research Station, GBR (14°40′S, 145°28′E), from July 2016 to October 2018. These reefs were part of a cleaner wrasse (Labroides dimidiatus) removal study initiated in September 2000. For these data, only the Lagoon site was used, and L. dimidiatus were removed from six reefs (removals), approximately every three months, and left undisturbed on six (control) reefs.

Gnathiid abundance was estimated using 1-meter squared (m2) emergence traps. During all years, traps were set for three consecutive days at the Lagoon site, except for August 2017 (1 day). Traps were moved every 24 hours. Sampling was conducted around the full moon because gnathiid emergence at this site is highest at times of full and new moon. When an additional site (Casuarina Beach) was sampled as part of another study, the sampling order of each site was determined randomly (4 days before or after the full moon) when only the Lagoon reefs were sampled, this occurred after the full moon. In this study, only the Lagoon site was analyzed, with a maximum of 180 samples per sampling trip.

We obtained both a day sample and a night sample from each trap within a 24-hour period. Traps were positioned such that they would not cause damage to or be damaged by the reef. Thirty traps were positioned haphazardly across all reefs beginning at sunrise, with 2–3 per reef and the number depending on reef size. Sampling bottles were replaced with new ones at sunset each day to obtain daytime and nighttime samples. Samples from traps that were damaged or dislodged were excluded. Samples were processed by filtering the contents of the sampling jar through a mesh-lined funnel. Contents were sorted and gnathiids were counted under a dissecting scope.

The data presented here are part of a long-term study that began before the funding period of this project. During 2016 and 2017, sites experienced mass-bleaching events.