Naso unicornis 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Actinopterygii Perciformes Acanthuridae

Scientific Name: Naso unicornis (Forsskål, 1775)
Common Name(s):
English Bluespine Unicornfish, Brown Unicornfish, Humphead Unicornfish, Longhorn Unicornfish, Longnose Unicornfish, Long-snouted Unicornfish, Unicorn Tang
French Corne, Dawa, Licorne, Nason à Eperons Bleus, Nason Brun, Nason Vert
Spanish Barbero de Aguijón Azul, Berbero de Aguijon Azul
Acanthurus unicornis (Forsskål, 1775)
Chaetodon unicornis Forsskål, 1775
Taxonomic Notes: Naso unicornis is widespread throughout the Indo-Pacific region, genetic studies conducted by Horne et al. (2008) did not find any evidence of geographic population structure even at the largest spatial scales.

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2012
Date Assessed: 2010-05-07
Assessor(s): McIlwain, J., Choat, J.H., Abesamis, R., Clements, K.D., Myers, R., Nanola, C., Rocha, L.A., Russell, B. & Stockwell, B.
Reviewer(s): Edgar, G. & Kulbicki, M.
Naso unicornis is widespread in the Indo-Pacific. It is common and can achieve high abundances in parts of its range. It is a targeted fish species in most of its distribution. There have been signs of catch declines - however, with no corresponding effort data population declines cannot be easily quantified. There is no clear trend of population declines. It is found in a number of well-managed protected areas and has shown positive responses to protection. It is therefore listed as Least Concern. We recommend continued monitoring of the harvest levels and population status of this species.

Geographic Range [top]

Range Description:Naso unicornis is found from the Red Sea and East Africa to the Hawaiian Islands and Pitcairn Islands, northwards to southern Japan, southwards to New South Wales, Lore Howe Island, and Norfolk Island. It is also reported from Western Australia. It was recently recorded from Easter Island.
Countries occurrence:
American Samoa; Australia; British Indian Ocean Territory; Brunei Darussalam; Cambodia; Chile (Easter Is.); China; Christmas Island; Cocos (Keeling) Islands; Comoros; Cook Islands; Disputed Territory (Paracel Is., Spratly Is.); Djibouti; Egypt; Eritrea; Fiji; French Polynesia; French Southern Territories (Mozambique Channel Is.); Guam; India (Andaman Is., Nicobar Is.); Indonesia; Israel; Japan; Jordan; Kenya; Kiribati (Gilbert Is., Kiribati Line Is., Phoenix Is.); Madagascar; Malaysia; Maldives; Marshall Islands; Mauritius; Mayotte; Micronesia, Federated States of ; Mozambique; Myanmar; Nauru; New Caledonia; New Zealand; Niue; Norfolk Island; Northern Mariana Islands; Oman; Palau; Papua New Guinea; Philippines; Pitcairn; Réunion; Samoa; Saudi Arabia; Seychelles; Singapore; Solomon Islands; Somalia; South Africa; Sri Lanka; Sudan; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Timor-Leste; Tokelau; Tonga; Tuvalu; United States (Hawaiian Is.); United States Minor Outlying Islands (Howland-Baker Is., Johnston I., Midway Is., US Line Is., Wake Is.); Vanuatu; Viet Nam; Wallis and Futuna; Yemen
FAO Marine Fishing Areas:
Indian Ocean – western; Indian Ocean – eastern; Pacific – northwest; Pacific – southwest; Pacific – western central; Pacific – southeast; Pacific – eastern central
Additional data:
Lower depth limit (metres):80
Range Map:Click here to open the map viewer and explore range.

Population [top]


FAO capture production for Naso unicornis from Saudi Arabia recorded 126 tonnes in 2000 to 191 tonnes in 2001, catch decreased from 2002-2004 with 133, 145 and 113 tonnes respectively; it has increased since then with 195 tonnes recorded in 2007. There are no capture production records prior to 2000. Density of N. unicornis was 3-10 times greater after marine reserves around five islands in New Caledonia were protected from fishing for 4 yrs (Wantiez et al. 1997). In the central Philippines, there were two orders of magnitude difference in mean biomass between fished areas and marine reserves (0.5 to 11 years protection) (Stockwell et al. 2009).

In Saipan, this species was the most abundant fish in the surveys with landings accounting for 14% of the total fish landings, and 50% of the acanthurids landings (P. Houk unpublished data).  Census data from seven sites around Saipan using timed visual surveys on SCUBA revealed N. unicornis  was extremely rare at 0.05% of the total acanthurid abundance.  A similar result was found on Guam, where this species made up 17% of the acanthurid fishery but was rarely recorded during visual surveys.  In these and other locations where spearfishing is the primary fishing method, daytime visual surveys using fixed length transects might be inappropriate.

In the Federal States of Micronesia, particularly Yap, N. unicornis accounts for approximately 12% of the total catch landed from the sheltered coastal reefs and lagoons at two sites Yyin and Riiken (Kronen et al. 2006). On Pohnpei, this species is the second most abundant acanthurid in the fishery (26% of all acanthurids) (Rhodes et al. 2008). Analysis of longterm fishery data (1984-2007) from Guam reveal no decline in mean individual weight for this species (Division of Aquatic and Wildlife Resources unpub. data). An average of 7,700 kg/year is landed in Hawaii (Division of Aquatic Resources unpub. data). This species is collected as an aquarium fish in West Hawaii. The total number of individuals caught from FY 2005-2009 was 24 with a total value of $68 (Walsh et al. 2010).

This species was recorded as occasional in terms of relative abundance in Palawan Province, Philippines and the northern Bismarck Sea, Papua New Guinea (Werner and Allen 2000, Palawan Council for Sustainable Develeopment, Allen 2009).  It is moderately common in Milne Bay Province, Papua New Guinea and in Raja Ampat, Indonesia (Allen 2003, 2003b). In Moorea, French Polynesia, SPOT satellite images allowed estimation of the surface area of fringing reef (1,076 ha), barrier reef (3,788 ha) and outer slop (493 ha). A total of 21,346 individuals were recorded in this area in fish visual surveys conducted from 1990-1993 (Lecchini et al. 2006).

In the Nabq Managed Resource Protected Area, South Sinai, Egyptian Red Sea, mean abundance of this species was higher in the fished areas than in the no-take zones across 3 and 10 m depths. This can be attributed to a result of reduced predation or competition (Ashworth and Ormond 2005). In a stock assessment conducted at the northern province of New Caledonia by Letourneur et al. (2000), density and biomass was higher in the north zone and on barrier reefs (17.5 g m-2 and 129 ×10-4 individuals m-2) compared to other areas surveyed and other reef types.

Current Population Trend:Unknown
Additional data:
Population severely fragmented:No

Habitat and Ecology [top]

Habitat and Ecology:Naso unicornis is found in inshore coral reefs but will enter shallow water (Randall 2001a). It feeds on macroscopic algae, mainly larger brown algae (Turbinaria and Dictyota) (Choat et al. 2002, 2004). It undergoes ontogenetic habitat shifts from shallow, sheltered areas of the reef to deeper, more exposed habitats (Meyer and Holland 2005), juveniles move from shallow reef flats to the reef crest at approximately 12 cm (J. McIlwain pers. obv.). It typically occurs in small groups, migrates around the outer reef edge, and spawns around both new and full moons (Johannes 1981, Arai and Sato 2007).

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 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.

Use and Trade [top]

Use and Trade: Naso unicornis is an important food fish and is targeted by fishers. In Atafu, large netting drives with seines are occasionally employed as a community fishing actvity involving over 100 people. This species is one of the main species captured by this netting method (Ono and Addison 2009).  It is also an occasional component of the aquarium trade (Global Marine Aquarium Database accessed 19 March 2010).

Threats [top]

Major Threat(s): Naso unicornis is a targeted food fish. There have been significant reductions in biomass in parts of its range.

Surgeonfishes show varying degrees of habitat preference and utilization of coral reef habitats, with some species spending the majority of their life stages on coral reef while others primarily utilize seagrass beds, mangroves, algal beds, and /or rocky reefs. The majority of surgeonfishes are exclusively found on coral reef habitat, and of these, approximately 80% are experiencing a greater than 30% loss of coral reef area and degradation of coral reef habitat quality across their distributions. However, more research is needed to understand the long-term effects of coral reef habitat loss and degradation on these species' populations. Widespread coral reef loss and declining habitat conditions are particularly worrying for species that recruit into areas with live coral cover, especially as studies have shown that protection of pristine habitats facilitate the persistence of adult populations in species that have spatially separated adult and juvenile habitats (Comeros-Raynal et al. 2012).

Conservation Actions [top]

Conservation Actions: There are no species-specific conservation measures in place for this species. However, its distribution overlaps several marine protected areas within its range. In Queensland, Australia, there is a recreational catch limit of five per species and a minimum size limit of 25 cm (Department of Primary Industries accessed 8 April 2010).

Classifications [top]

9. Marine Neritic -> 9.2. Marine Neritic - Subtidal Rock and Rocky Reefs
9. Marine Neritic -> 9.8. Marine Neritic - Coral Reef -> 9.8.1. Outer Reef Channel
9. Marine Neritic -> 9.8. Marine Neritic - Coral Reef -> 9.8.2. Back Slope
9. Marine Neritic -> 9.8. Marine Neritic - Coral Reef -> 9.8.3. Foreslope (Outer Reef Slope)
9. Marine Neritic -> 9.8. Marine Neritic - Coral Reef -> 9.8.4. Lagoon
9. Marine Neritic -> 9.8. Marine Neritic - Coral Reef -> 9.8.6. Inter-Reef Rubble Substrate

In-Place Research, Monitoring and Planning
In-Place Land/Water Protection and Management
  Conservation sites identified:Yes, over part of range
  Occur in at least one PA:Yes
In-Place Species Management
  Harvest management plan:Yes
In-Place Education
5. Biological resource use -> 5.4. Fishing & harvesting aquatic resources -> 5.4.1. Intentional use: (subsistence/small scale) [harvest]
♦ timing:Ongoing ♦ scope:Unknown ♦ severity:Unknown ⇒ Impact score:Unknown 
→ Stresses
  • 2. Species Stresses -> 2.1. Species mortality

1. Research -> 1.2. Population size, distribution & trends
1. Research -> 1.3. Life history & ecology
3. Monitoring -> 3.1. Population trends
3. Monitoring -> 3.2. Harvest level trends

♦  Food - human
 Local : ✓   National : ✓ 

♦  Pets/display animals, horticulture
 International : ✓ 

Bibliography [top]

Allen, G.R. 2003. Appendix 5. List of the reef fishes of Milne Bay Province, Papua New Guinea. In: G.R. Allen, J. P. Kinch, S.A. McKenna, and P. Seeto (eds), A Rapid Marine Biodiversity Assessment of Milne Bay Province, Papua New Guinea–Survey II (2000), pp. 172. Conservation International, Washington, DC, USA.

Allen, G.R. 2003b. Appendix 1. List of the Reef Fishes of the Raja Ampat Islands. In: R. Donnelly, D. Neville and P.J. Mous (eds), Report on a rapid ecological assessment of the Raja Ampat Islands, Papua, Eastern Indonesia, held October 30 – November 22, 2002. The Nature Conservancy - Southeast Asia Center for Marine Protected Areas, Sanur, Bali.

Allen, G.R. 2009. Coral Reef Fish Diversity. In: R. Hamilton, A. Green and J. Almany (eds), Rapid Ecological Assessment: Northern Bismarck Sea, Papua New Guinea. Technical Report of survey conducted August 13 to September 7, 2006, The Nature Conservancy.

Arai, H. and Sato, T. 2007. Prominent ornaments and rapid color change: use of horns as a social and reporductive signal in unicornfish (Acanthuridae: Naso). Ichthyological Research 54: 49-54.

Ashworth, J.S. and Ormond, R.F.G. 2005. Effects of fishing pressure and trophic group on abundance and spillover across boundaries of a no-take zone. Biological Conservation 121: 333-344.

Choat, J.H. and Robertson, D.R. 2002a. Age-based studies on coral reef fishes. In: P.F. Sale (ed.), Coral reef fishes: dynamics and diversity in a complex ecosystem, pp. 57-80. Academic Press, Burlington, San Diego and London.

Choat, J.H., Clements, K.D. and Robbins, W.D. 2002b. The trophic status of herbivorous fishes on coral reefs. 1. Dietary analyses. Marine Biology 140: 613-623.

Choat, J.H., Robbins, W.D. and Clements, K.D. 2004. The trophic status of herbivorous fishes on coral reefs. Marine Biology 145: 445-454.

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Department of Primary Industries - Queensland Government. 2010. Surgeonfishes. Available at: (Accessed: 8 April).

Eble, J. 2009. How old is that fish? Journal of Micronesian Fishing Fall 2009(3): 5.

Eble, J.A., Langston, R. and Bowen, B.W. 2009. Growth and reproduction of Hawaiian Kala, Naso unicornis. Final Report prepared for Fisheries Local Action Strategy. Division of Aquatic Resources, Honolulu.

Global Marine Aquarium Database. 2010. Species Trade Details. Available at: (Accessed: March 19).

Horne, J.B., L. van Herwerden, J.H. Choat and D.R. Robertson. 2008. High population connectivity across the Indo-Pacific: Congruent lack of phylogeographic structure in three reef fish congeners. Molecular Phylogenetics and Evolution 49: 629-638.

IUCN. 2012. IUCN Red List of Threatened Species (ver. 2012.2). Available at: (Accessed: 17 October 2012).

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Citation: McIlwain, J., Choat, J.H., Abesamis, R., Clements, K.D., Myers, R., Nanola, C., Rocha, L.A., Russell, B. & Stockwell, B. 2012. Naso unicornis. The IUCN Red List of Threatened Species 2012: e.T177970A1506556. . Downloaded on 24 June 2018.
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