Bluespine unicornfish (Naso unicornis) are both natural control agents and mobile vectors for invasive algae in a Hawaiian Marine Reserve

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It has been hypothesized that protecting herbivorous fishes within Marine Reserves (MRs) will help these areas to resist algal overgrowth of corals. However, we lack empirical studies demonstrating the validity of key assumptions underpinning this concept, including that herbivorous fishes (1) are permanently resident within MR boundaries, (2) routinely consume the invasive algae of concern at significant levels and (3) are not significant mobile vectors for propagules of the invasive algae. A 46-year-old MR (Hawaii Marine Laboratory Refuge, 21°27′35″N, 157°48′15″W) in waters off Oahu provided a natural setting to examine these factors with the herbivorous bluespine unicornfish (Naso unicornis) and the invasive rhodophyte, Gracilaria salicornia. We acoustically tracked five unicornfish to quantify their residency and habitat use within the MR, conducted field and laboratory observations to confirm N. unicornis consume G. salicornia, and tested the viability of G. salicornia fragments recovered from unicornfish feces. Unicornfish were resident within the MR where they spent most time in reef crest habitat with occasional, brief forays into reef flat habitat. We confirmed unicornfish consume G. salicornia and found a significant positive correlation between algal canopy height and distance from the reef crest, presumably because grazing intensity is lower in less frequently utilized reef flat habitat. We demonstrated that unicornfish egest viable fragments of G. salicornia that resume vegetative growth after several weeks. We conclude that N. unicornis may act as both a natural control agent and a mobile vector for invasive alien algae.