Habitat and Ecology
Home range estimates of N. unicornis from a study on Guam ranged from 51 m2 to 100,045 m2 (mean = 30,227 m2) almost ten times that for the same species in Hawaii (mean = 3,172 m2) (Meyer et al. 2000, Marshall unpub thesis). These differences may reflect differences in methods with the former employing a remote array deployed over five months and the latter a hand-held hydrophone used to track individual fish for a maximum of 21 days. There was a strong ontogenetic shift in both home range size and habitat preference at both locations. On Guam, large individuals (26 cm) use not only the shallow reef flats but range over deeper, more exposed habitat on the reef slope
The sexes are separate and there is evidence of sexual dimorphism in the caudal knives which are relatively larger in males (J.H. Choat pers. comm. 2010). Size at first reproduction (L50) for females collected from the fishery in Guam (2008-2009) was 328 mm (J. McIlwain unpub.data). In Hawaii the L50 for reproductive females collected only in June was 378 mm (Eble et al. unpub report). The smallest females with vitellogenic or hydrated oocytes were 286 mm and 342 mm respectively. The L50 for males from Hawaii was 286 mm, and the smallest male with sperm present was 266 mm. For the Hawaiian fishery, two thirds of females enter the fishery before maturation (Eble et al. 2009). In Guam, mature or spent females were only recorded from August to October during 2008. This species exhibits high recruitment rates (J. McIlwain unpub. data).
In Hawaii, mean age from a sample of 197 fish was 12.8 yrs (range of 1-58 yrs). They found no evidence of sexual dimorphism in size or differences in growth among locations. Males reached age at first maturity at 4.5 yrs, 7.5 years for females at 37.8 cm (TL) (J. Eble unpub. report). In the Great Barrier Reef females reach first maturity at four years 30-35 cm (TL) (J.H. Choat pers comm. 2010).
It was observed to form spawning aggregations on the Great Barrier Reef (Johannes 1981, Squire and Samoilys unpub.). The large pelagic larvae persist in the pelagic environment for approximately 90 days (B.Victor pers comm. in Horne et al. 2008).
The number of sagital increments in a study done by Choat and Axe (1996) suggest that members of the genus Naso attain the same maximum ages as the other Acanthurids, in excess of 20 years for this species. Maximum age was 30 years in the Great Barrier Reef (Choat and Robertson 2002a), 58 years in Hawaii (Eble 2009).
A study by Meyer and Holland (2005) revealed the first nocturnal movements in surgeonfishes. Naso unicornis moved up to 170 m on nights where moonlight was completely absent. The study also showed that this species was site-attached to home ranges situated within the boundaries of the study area and their movements were aligned with topographical features.
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