Naso caesius 

Scope: Global
Language: English

Translate page into:

Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Actinopterygii Perciformes Acanthuridae

Scientific Name: Naso caesius
Species Authority: Randall & Bell, 1992
Common Name(s):
English Gray Unicornfish

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2012
Date Assessed: 2010-05-06
Assessor(s): Choat, J.H., Abesamis, R., Clements, K.D., McIlwain, J., Myers, R., Nanola, C., Rocha, L.A., Russell, B. & Stockwell, B.
Reviewer(s): Edgar, G. & Kulbicki, M.
Naso caesius is widespread throughout the west and south central Pacific but not yet reported from the rest of the Coral Triangle Region. It is harvested in the Guam fishery and in Papua New Guinea. There is no evidence of population declines from harvesting. It occurs in marine reserves in parts of its range. It is therefore listed as Least Concern.

Geographic Range [top]

Range Description:Naso caesius is known from Palau, Mariana Islands, northern Marshall Islands, Hawaiian Islands, Great Barrier Reef, Osprey Reef, and Chesterfield Islands in the Coral Sea, New Caledonia, Fiji, Tuvalu, Society Islands and Pitcairn Islands. It is also recorded from Kavieng, Papua New Guinea (Hamilton et al. 2004), Christmas Island (Hobbs et al. 2010) and Cocos Keeling.
Countries occurrence:
American Samoa (American Samoa); Australia; Christmas Island; Cocos (Keeling) Islands; Cook Islands; Fiji; French Polynesia; Guam; Kiribati (Gilbert Is., Phoenix Is.); Marshall Islands; Micronesia, Federated States of ; Nauru; New Caledonia; Niue; Northern Mariana Islands; Palau; Pitcairn; Samoa; Tokelau; Tonga; Tuvalu; United States (Hawaiian Is.); United States Minor Outlying Islands (Howland-Baker Is., Johnston I.); Vanuatu; Wallis and Futuna
FAO Marine Fishing Areas:
Indian Ocean – eastern; Pacific – eastern central; Pacific – northwest; Pacific – southwest; Pacific – western central
Additional data:
Lower depth limit (metres):50
Upper depth limit (metres):3
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Naso caesius is uncommon in the American Samoa National Park (National Park of Samoa Checklist of Fishes accessed 21 April 2010). In Christmas Island, this species is common (Hobbs et al. 2010) and is the dominant Naso sp. (J.H. Choat pers. comm. 2010). It accounts for 2% of the Acanthurid fishery in Guam (Division of Aquatic and Wildlife Resources unpub. data) but is absent in the Saipan fishery in 2008 (P. Houk unpub. data).
Current Population Trend:Stable
Additional data:
Population severely fragmented:No

Habitat and Ecology [top]

Habitat and Ecology:Naso caesius is usually seen in aggregations on drop-offs, sometimes in mixed schools with N. hexacanthus. A few schools were observed in Christmas Island, all the N. hexacanthus seen were with N. caesius (J.H. Choat pers. comm. 2010). This species prefers oceanic conditions - clear water along steep dropoffs, offshore reefs and pinnacles (R.F. Myers pers. comm. 2010).


The sexes are separate among the acanthurids (Reeson 1983). Sexual dimorphism is differentiated with males having larger caudal spines (J.H. Choat pers comm. 2010). Nuptial males were observed to flash different colours (R.F. Myers pers. comm. 2010).
In Papua New Guinea, it is known to form spawning aggregations in open water above the reef every month of the year during the first and third quarter moon phase. It was reported to spawn early morning and late afternoon with group and pair spawning observed. N. caesius is reported to aggregate bimonthly, just prior to the new and full moons. Several hundred species were observed to spawn  (Hamilton et al. 2004).

Use and Trade [top]

Use and Trade: Naso caesius is harvested for food. In a spawning aggregation site in Papua New Guinea, fishers interviewed stated that catch rates have not changed in the years that they have targeted it. It was first exploited in 1999 (Hamilton et al. 2004).

Threats [top]

Major Threat(s): There are no major threats known for this species.

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.

Citation: Choat, J.H., Abesamis, R., Clements, K.D., McIlwain, J., Myers, R., Nanola, C., Rocha, L.A., Russell, B. & Stockwell, B. 2012. Naso caesius. The IUCN Red List of Threatened Species 2012: e.T178013A1521341. . Downloaded on 29 May 2017.
Disclaimer: To make use of this information, please check the <Terms of Use>.
Feedback: If you see any errors or have any questions or suggestions on what is shown on this page, please provide us with feedback so that we can correct or extend the information provided