The largest parrotfish in the world: up to 1.3 m (4.3 ft) and 75 kg (165 lb), blue-green, with a squared bony forehead and a beak worn pale by use. Schools of a dozen adults moving along a reef edge sound like construction work, because that is effectively what it is.
It eats live coral, and it uses its head to get it. Adults ram coral heads to break off pieces before biting them, and the forehead bulge is the tool - the only fish known to headbutt its food. Males also ram each other head-on in disputes, at speed, which was filmed for the first time only in 2012.
The throughput is enormous. A single adult processes on the order of five tonnes of reef structure a year, grinding it in a throat mill and passing it out as sand, so a school of them is a geological agent as much as an animal - they are among the largest producers of coral sand anywhere.
They sleep in caves in groups and are therefore trivially easy to spear at night, which is why a fish that was once common across the Indo-Pacific is now scarce almost everywhere and gone from many reefs. It is Vulnerable, slow-growing and late to mature, and a sighting here is genuinely rare.
The genus.Bolbometopon Smith 1956 - bolbos (swelling) + metopon (forehead). ETYFish
How to recognise it
huge blue-green fish with a vertical bony bulge on the forehead
beak of fused teeth, often pale and visibly worn
moves in schools of a dozen or more, all of similar size
The major extrinsic threats to this species are from uncontrolled fishing for both local and export uses and the destruction of or damage to inshore habitats and coral reef decline due to coral bleaching. Juveniles depend on healthy lagoon habitat that is vulnerable to sedimentation or other disturbances. In the Solomon Islands, for example, a 24-fold decline in juvenile abundance was noted due to habitat loss relating to logging operations (Hamilton et al. 2017).
The species is highly vulnerable to night spearfishing and the very distinctive recruitment requirements may have a role in adult distributions (which are very patchy at geographic scales) and abundance. This latter is important because the placement of marine protected areas as conservation measures would depend heavily on where these are placed and would need to include key habitat particularly inshore recruitment sites.
Fishery based models of demographic processes, such as those applied by NOAA (Kobayashi et al. 2011) rather than direct examination of life history parameters and key life history processes may not produce meaningful stock/status assessments of this species.
Intrinsic threats include its long life span and aggregate behaviour, particularly during nocturnal resting periods. These aggregations are easily located by fishers and can be quickly depleted. This creates a hyperstable fishery in which catch-per-unit-effort (CPUE) may remain high despite large declines in abundance (Hamilton et al. 2016).
Threat classification from the IUCN Red List.
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