The greenbone — also called butterfish — is New Zealand's own kelp grazer, a beak-toothed odacid that changes sex, varies in colour, and moves kelp energy into fish.
Swim a shallow New Zealand kelp bed on a still morning and you will probably see greenbone (Odax pullus) before you notice them: a mid-size fish moving head-down through the fronds, nipping algae with a beak-like mouth, unhurried, unbothered. Kiwis call it by two names — greenbone, for the pale-green skeleton that shows when the fish is prepared, and butterfish, for a texture that cooks like its nickname suggests. On the reef, though, its more useful name would be "the grazer": this is New Zealand's counterpart to southern Australia's herring cale, a dedicated kelp browser that converts standing forest into fish flesh.
The Odacidae are one of the quiet wonders of southern-hemisphere reef ecology: a whole family of herbivorous, beak-toothed reef fishes that radiated only in the waters around Australia and New Zealand. Where a tropical reef has parrotfish grazing coral and algae, a southern-hemisphere temperate reef has odacids grazing kelp — same great idea, convergent hardware, different ancestors.
Greenbone carry the family kit. The mouth is a small cutting beak of fused teeth; the body is long and compressed for threading among fronds; the colour ranges from brilliant green-bronze in young, shallow fish to dark, mottled forest-green and grey-banded in mature males. Underwater, large fish can look almost black in poor light and suddenly shimmer blue-green when a shaft of sun reaches them. The species changes colour fast — minutes to hours — a useful trick for a fish whose bed and backdrop are both mobile algae.
From one end of New Zealand to the other. Greenbone are on the subtropical-temperate boundary reefs of the Poor Knights Islands, the storm-cut ledges of Wellington's south coast, the kelp gardens of the Akaroa harbour mouths, and south to the exposed reefs of Stewart Island. Wherever kelp — predominantly golden kelp (Ecklonia radiata) in the north, mixed kelp further south — covers rock, greenbone will find it.
They are shallow-water fish at heart. The densest concentrations are in the top fifteen metres, in the same band where snorkellers spend their time, in the fronds and the algal flats below the canopy. This makes greenbone one of New Zealand's most reliably encountered reef fish for anyone willing to put a face in cold water.
Most kelp energy reaches temperate food webs indirectly — as storm-torn drift, decomposed detritus, or dissolved organics released as blades wear. Greenbone flatten that chain. By taking living kelp in quantity, they convert canopy production into growth, movement and predation potential while the forest is still standing, and put reef herbivore biomass directly into the predator menu.
That matters in more than one direction. Where greenbone are abundant, other reef life gains a steady, local protein source: predators including banded morwong and blue cod, seals and shags, all cash in on odacid production. Downstream, greenbone faeces — rich in processed algal material — feed the small detritivores that in turn feed small fish. The effect is to keep more of the forest's productivity cycling within the forest, rather than exporting it on the tide.
For restoration work, that function is conspicuous in monitoring: a reef with standing kelp and visible grazing loops (herring cale, greenbone, marblefish, blennies) behaves differently from a reef with standing kelp and none. Tidewright's fictional restoration metrics include grazer abundance as a positive indicator of functional kelp food-web recovery; the science on why is exactly the logic above.
Greenbone are protogynous hermaphrodites — an individual matures first as a female and may, years later, transition to the dark-male phase. Courtship and spawning happen in open water above the weed: a male claims a patch of column, courts females from his group, and pairs rise together in a fast spiral to release eggs and sperm before separating. The eggs are planktonic, carried on the tide for weeks before larvae settle into shallow, algae-covered reef to start the whole cycle again.
Because the biggest, darkest fish are the males, and because males are the scarce sex in a protogynous system, a reef well-populated with greenbone big boys is usually a reef whose whole local social structure is working — accumulated age classes, successful recruitment years, enough food and enough structure to support decades of life.
Greenbone are easiest to identify by behaviour before detail: they graze. A New Zealand reef fish moving head-down through kelp fronds, nipping at blades, is very likely a greenbone or the superficially similar marblefish — which is rounder, more strongly patterned, and feeds with a rasp-and-suck motion rather than a clean clip.
Three colour phases to learn:
Yes — same fish, two common names. "Butterfish" speaks to the cooking texture; "greenbone" to the distinctly green skeleton visible when the fish is cleaned. Fisheries and menus use both; field workers and fishers usually pick whichever their family used.
Highly regarded: firm, white, mild, and famously buttery when gently cooked. The flesh takes marinade and smoking particularly well. As always, check the current New Zealand recreational fishing rules for size and bag limits before retaining a fish.
Mostly kelp and other brown algae, but the diet is broader: red algae, epiphytes growing on kelp blades, and small invertebrates taken incidentally with the weed. Greenbone are grazers — they eat the habitat's surface — rather than pure kelp specialists.
They are protogynous hermaphrodites born functionally female. The largest females in a local social group transition, over a season or more, to the male phase — darkening, changing shape in the head and fins, and taking over spawning duties. Not every female changes; it depends on the local group's composition.
Almost any accessible kelp bed within the recreational range. Poor Knights reefs, Wellington's south-coast reserve zones, the Banks Peninsula harbour mouths and the easier bays of Stewart Island are all dependable. Swim slowly, stay shallow, and look for the motion of fins in the fronds.
The herring cale is a cigar-shaped, beak-toothed herbivore that grazes kelp across the Great Southern Reef — and moves its energy up the food web.
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Golden kelp forms the backbone of Australia's Great Southern Reef. Learn to identify Ecklonia radiata and why this habitat-maker is on the move as oceans warm.
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Marbled, thick-lipped and built to crop kelp: the marblefish of New Zealand and southern Australia, a herbivore with the reef's strangest tools.
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