National Institute of Weather and Atmospheric Research scientists reported that the ocean around the Chatham Islands to the east of New Zealand has warmed at around 5 times the global rate.
This is a concern for fisheries because 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 the Ministry for Primary Industries, over 200,000 tons of fish are caught from New Zealand’s deepwater fisheries each year.
NIWA physical oceanographer Dr. Phil Sutton said in an article on NIWA’s site that the Subtropical Front is a unique area where cold, fresh water from the Southern Ocean meets warm, salty water from the subtropics. 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,” he said, “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,” said Dr. Sutton.
NIWA principal scientist for fisheries Dr. Matt Dunn said, “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. 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.”
Dr. Dunn said 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. However, it might take a few years, or a few fish generations, for the scale of the changes to become clear.”
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. 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. Global climate models predict that the conditions leading to this system-wide change will strengthen and persist until at least the end of this century.
