Decapterus koheru
Slim silvery schoolers logged at 27 North Island sites in 17.3-18.3C water, uncommon but worth watching for on New Zealand reefs.
Slender, torpedo-shaped body around 28cm long with a silvery-blue back and a distinct yellow stripe running along the lateral line. Look for the small forked tail and the tight, fast-moving schools it forms, which is often the giveaway before you can pick out individual fish. The finlets near the tail and the deeply forked caudal fin help separate it from bulkier jacks.
Typically seen moving in tight, fast schools over rocky reef rather than sitting still, often mid-water above the bottom structure. It tends to keep some distance from divers and reacts to sudden movement by tightening the school and peeling away rather than scattering individually. Most logs describe it as an uncommon background sighting rather than a targeted encounter, so it is often noticed while divers are focused on something else.
Schooling itself is the main defence for this species, with fish packing tightly when threatened so predators struggle to single one out.
Sightings show no detected seasonal pattern, so it is logged across the year rather than in a tight window. Water temperatures where it is recorded sit in a cool 17.3 to 18.3C band, consistent with temperate North Island conditions rather than a warm-season spike.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Every logged sighting comes from New Zealand, with the North Island the clear stronghold for this species. Divers looking for koheru should focus on North Island rocky reef sites, where it turns up as an uncommon to occasionally common schooling fish.
Mapping where Koheru is logged…
Koheru is a New Zealand species, with sightings concentrated at North Island rocky reef sites. It has not been logged in significant numbers outside New Zealand waters.
There is no strong seasonal pattern in the sighting data, so koheru is logged year-round at North Island sites. Expect the water to sit in a cool 17.3 to 18.3C range when you encounter it.
Look for a slim, silvery-blue fish around 28cm long with a yellow lateral stripe, moving in fast, tight schools over rocky reef. The forked tail and schooling behaviour are the quickest cues.
Koheru typically reach around 28cm in length, making them a small to mid-sized member of the jack and trevally family.
No, koheru pose no danger to divers. They are a schooling fish that keeps its distance rather than approaching or interacting with people underwater.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Rainbow Runners
1,438 sites
