Overcoming challenging telemetry data of giant sunfish Mola alexandrini (Molidae) in Bali, Indonesia

Abstract

The ocean sunfishes Mola spp. (Family Molidae) are charismatic marine megafauna that are difficult for nature-based tourism operators to target due to their sporadic occurrence. However, the giant sunfish M. alexandrini is seasonally important for the SCUBA tourism industry in Bali, Indonesia, where these molids seek cleaner-fish interactions on the reefs of the local Nusa Penida Marine Protected Area (MPA) in the Lombok Strait. Little is known of their movements, including their apparent seasonal affinity to the Nusa Penida MPA. We tagged 8 fish with Fastloc GPS tags; however, intermittent surface visits and crepuscular diving habits, combined with early tag dislodgment and tag transmission failures, yielded a challenging data set to analyze. We overcame this difficulty with a geolocation model (HMMoce) that was parameterized to leverage the complex topography of the study region, be robust to data sparsity, and accommodate switching between distinct resident and migrant behaviour states. The results revealed small-scale movements whereby the fish exhibited particular affinity to bathymetrically complex, intense upwelling areas around Nusa Penida, the east coast of Bali, and an inter-basin trough to the north. Dive patterns suggested that the molids were foraging in these oceanographically dynamic areas. Our findings suggest that the Lombok Strait is at least of seasonal importance to this species, and the local SCUBA tourism industry likely relies on individual fish that re-visit the reefs of the Nusa Penida MPA multiple times during the sunfish season.