Written by Sarah Wilcox.
The sex life of shortjaw kōkopu – a species of fish that’s only found in Aotearoa – is shrouded in mystery. We know they’re doing it, but even today we know almost nothing about when or where. DOC freshwater ranger Suze Harris has been watching them closely in a small stream near Hokitika. And in the process she’s fallen just a little bit in love.
What we know about their reproduction (spoiler, it’s not much)
Like other whitebait species, shortjaw kōkopu (or ‘jaws’ as Suze calls them) reproduce by spawning – a process where females lay eggs and males squirt milt (that’s fish-speak for sperm) onto them.
“Egg-laying seems to be triggered by flooding in a stream. Jaws are experts at picking the peak of a flood to lay their eggs on the water’s edge. We think the eggs sit up on the bank above normal water level for a few weeks, grow into larvae and hatch when the next flood washes over them,” says Suze.
Apart from one study in Taranaki in 2003, there are no reports of spawning or shortjaw eggs in the scientific literature.
“If we knew when they spawned and what kind of material they spawned in, we’d be able to protect those areas and help their populations recover.”
Late-night dates at the stream
In the last year, Suze has dated jaws at the creek around midnight every week or two. Dress code is strict, but it’s not black tie or heels. Neoprene waders, boots, beanie, plus a spotlight and net are essential.
“They’re nocturnal so I have to visit when it’s dark. If I sit around for 10 minutes, they pop up. Some fish show scarring patterns and because I see them often I can recognise the individuals.”
Suze has measured and checked the condition of 16 large fish in the stream over the last year. She has named her fave fish couple Brad Pitt and Jennifer Anniston.
“Last summer Brad was consistently 195 mm and Jennifer more like 205 mm – she’s a bit bigger and harder to catch, whereas Brad reliably hangs out by the same rock. He’s a simple guy.”
“I’ve got to know their personalities – it’s nice to have a relationship with them.”
Lost, found and pretty done fish
Suze says the condition of the fish can show whether they are ripe (close to spawning) or spent (it’s all over). Carefully observing their condition provides clues about the timing of spawning and how events, such as floods and shifting seasonal temperatures, might trigger it.
“Many individuals looked hollowed-out and spent by May 2022. Over winter, these large fish essentially vanished from the stream. The ones I caught looked tired, and discoloured. It was a long winter.”
“I was getting a bit worried that they had got too knocked around with the flooding last season. But the last time I went out and caught Jennifer and Brad again – they’re still in the big pool! That was exciting.”
The cute fish with an overbite
Shortjaw kōkopu are descriptively named – they have a blunt snout that hangs over the mouth and creates an adorable overbite. They’re members of the galaxiid family and have silky skin rather than scales. The normal lifespan is 7–9 years but they can live for up to 20.
Most fish have a dark blotch behind their gills in a half-moon shape. Their eyes are a brilliant blue-green colour. The adults are usually a dullish brown colour with a faint, velvety sheen that glitters in torch light.
“Jaws are one of the species caught as whitebait. If you had a cup of whitebait from the West Coast, about 5 of the estimated total of 500 fish would be shortjaws. You might have eaten one in a fritter!”
Their conservation status is Threatened – Nationally Vulnerable, which is the same as whio/blue duck and tāiko/black petrel. They need rocks with hidey holes, deep pools and steep forested terrain at different times in their lifecycle.
“Jaws used to be much more widespread so it’s really important to look after those populations we still have. What’s not to love about these cool fish? Habitat loss and disconnected habitat because of barriers (like dams and weirs) are arguably their greatest threats, but there’s still a lot to learn about them.”
Where we know they hang out
The West Coast and Tasman regions are South Island hotspots. There’s a small population near Kaikoura and historical records from Bluff and Southland. The fish have been recorded throughout the North Island but in generally lower numbers.
“Last year we searched really hard for eggs and we think we found two nests in the study stream, but we didn’t nail the timing. Unfortunately there weren’t enough decent samples to confirm their identity with genetic analysis, so the mystery continues! I am optimistic we’ll find eggs from the stream this autumn. That would be fin-tastic!”
Suze’s intensive research is part of DOC’s Ngā Ika e Heke project, which is working to secure populations of four migratory fish species (shortjaw kōkopu, īnanga, longfin eel/tuna and lamprey/kanakana/piharau) across Aotearoa New Zealand.
Ngā Ika e Heke enables work at a catchment scale across the country with mana whenua, regional councils and others. Once known populations have been surveyed, what we learn can be applied to aid the recovery of populations around the country.
Read more about this work here.