NIWA scientists say that unprecedented ocean changes may impact key New Zealand fisheries.

The scientists have seen substantial changes in the ocean to the east of New Zealand, with possible impacts for important fisheries.

Since 2006, strong, full-depth ocean warming has occurred south of the Chatham Islands at around five times the global rate because of the ocean currents moving 120km west.

NIWA physical oceanographer Dr Phil Sutton says it’s the first time they have seen change through the full depth of the ocean off New Zealand.

"The water that sits along the Chatham Rise is known as the Subtropical Front - a unique area where cold, fresh water from the Southern Ocean meets warm, salty water from the subtropics," says Dr Sutton.

"The front runs from west to east, before dipping south. Over the years, it has gone through periods of warming and cooling, always coming back to baseline.

"However, since 2006, an area south of Chatham Islands started warming and hasn’t stopped, and models predict this will continue until beyond 2100.

"This warming has resulted from the Subtropical Front moving to the west, a change observed in three different datasets that matches our modelling precisely - something that’s rare in oceanography."

NIWA used ocean measurements from satellites and Argo floats - a fleet of robotic instruments that move up and down through the water column and drift with the ocean currents.

Phil Sutton inspects a decommissioned Argo float. Photo: NIWA / Luke McPake.

They found that areas that once contained cooler, fresher water are now warmer and saltier. This was seen at all depths, from the ocean surface to the seafloor.

NIWA principal scientist - fisheries Dr Matt Dunn says the biological impacts are yet to be determined but several local species will likely be affected.

"There are animals adapted to live on the warm northern side and others adapted to the cold southern side, so when warm waters encroach, you’d expect the species that favour the warmer conditions to increase, and those that favour cold conditions to move away or disappear," says Dr Dunn.

"However, it might take a few years, or a few fish generations, for the scale of the changes to become clear."

An Argo float. Photo: NIWA / Luke McPake.

The Subtropical Front along Chatham Rise creates a unique habitat that supports some of the most productive deep-sea fisheries in the world, where important hoki and orange roughy fisheries take place, including the oldest and largest orange roughy fishery in the world.

This is of great importance to New Zealand’s economy. According to MPI, over 200,000 tonnes of fish are caught from New Zealand’s deepwater fisheries each year.

"I remember when I first started working at NIWA, I was told that this Subtropical Front doesn’t and wouldn’t ever move - that the geography of the seabed has locked it in place," says Dr Dunn.

"So, these changes are unexpected. Because we’re watching it happen for the first time and don’t have anything to compare it to, we will only know the biological and fisheries impacts as they happen."

Changes in marine heatwave number of days/year (late-early). b) change in windstress and MSLP (late-early), with zonal-average changes. Image: NIWA.

NIWA marine physics modeller Dr Graham Rickard says the changes have occurred because of two mechanisms.

"The oceans in mid latitudes have warmed more than the oceans further south, resulting in a stronger gradient across the Southern Ocean. Additionally, westerly winds over the Southern Ocean have strengthened," says Dr Rickard.

"These mechanisms have accelerated the surface flows in the Southern Ocean, which move from west to east. It’s not just an ‘event’ or something transient - it’s been driven by hemispheric changes in ocean heat content and winds that are embedded into the system."

Under future scenarios, global climate models predict that this system-wide change will strengthen and persist until at least the end of this century.

Challenges remain to maintain critical observational infrastructure to further understand how these physical changes impact the wider New Zealand marine ecosystem.