Climate change could spell disaster for at-risk marine populations
Warming waters are changing the types of fish in our southernmost waters and marine mammal scientists tell Laura Hooper it will have an impact on other species.
Southland fishers are seeing larger and more exotic fish than ever before as the ocean’s temperature gauge ticks upwards, even in the deep south. For Southern Sport Fishing Club president Ian Carrick, it’s made trips out on the boat all the more exciting. Where blue cod was once the fish synonymous with Southland, snapper, kingfish and occasional marlin are becoming more commonplace in recreational catches.
Two decades years ago, snapper was considered a ‘‘North Island fish’’, Carrick said, while kingfish weren’t typically seen around Southland until six or seven years ago. Now, they’re increasing in size and could be caught all around Stewart Island and Fiordland, he said.
‘‘The fact that kingfish are becoming more resident means the water must be getting consistently warmer. We had two catches of striped marlin off Fiordland. To my knowledge [that] has never happened before... to get marlin, the waters have got to be warm, there’s no other reason for that. That shows that something different is happening out there.’’
It’s not just a hunch Carrick’s relying on, his temperature gauge – used to catch fish along temperature lines – has been lifting consistently in his 20 years fishing in Southland. Last summer, the gauge hit around 19C or 20C, while 10 or 15 years ago it would have been about 16C or 17C on average, he said.
‘‘This was quite an extreme year, but consistently over the past 10 years the water temperatures have just slowly lifted.’’
University of Otago marine biologist Dr Bridie Allan said kingfish and snapper were moving further south in search of cooler waters, and would be competing with resident fish like blue cod for food. For marine ecologists, it’s also a concerning threat to rare coral and marine life. ‘‘These larger fishes have very high metabolic rates, so they eat a lot of food.’’
Smaller fish, such as spotties, which are commonly caught around the south coast, would be facing higher predation rates as they struggled to respond to the new predators, she said.
The other major concern was that species could only move so far south before they hit latitudes where light would be limited. ‘‘Some species require sunlight, so they can’t move further south than that… you would hope that they would adapt, but with the rate that climate change is happening we just don’t know. That’s why these marine heatwaves are so significant, because they’re overlaid with global warming.’’
Marine heatwaves are when water temperatures stay in the warmest 10% of historical observations for at least five days.
Allan pointed to the mass bleaching of marine sponges in Fiordland this past summer as an example of the impact heatwaves could have when combined with climate change. The discovery was made by a Victoria University team in May after water temperatures sat about five degrees warmer than usual.
The project’s lead researcher Professor James Bell, at the time, said as far as he was aware, it was the largest-scale sponge bleaching event anywhere in the world. ‘‘The mass bleaching event highlights again how dramatically our oceans are changing due to global warming and climate change.’’
The heatwave may have also impacted other native marine species that had gone unnoticed, he said.
Marine mammal scientist and hector’s dolphin expert Gemma McGrath, who is based in Colac Bay in western Southland, said climate change combined with other environmental factors had stagnated dolphins’ population growth.
In the Catlins, the dolphins’ population had fluctuated around about 40 dolphins for years, indicating the population was not growing despite protection measures, she said.
The arrival of new fish to the area had also brought with it increasing amounts of predators, which made Hector’s vulnerable in their smaller pods, McGrath said.
There had been quite a few orca around Colac Bay during last summer, she said, with an increasing amount sighted during February.
A study by Massey University released last month detailed that as oceans warmed, sightings of whales around northern New Zealand would decrease, with the South Island and offshore islands becoming more suitable habitats.
‘‘They [dolphins] have to deal with a natural level of predators this whole time, which is just part of life. But this is a different kettle of fish for them. It’s a human-induced threat well and above what predators do,’’ McGrath said.
McGrath was also concerned about the impact warming oceans could have on the breeding habits of Hector’s dolphins.
‘‘A female will give birth every two to three years, and that’s when conditions are good. They don’t start breeding until they’re about six, and again that’s when conditions are good,’’ she said.
‘‘That’s the thing about climate change, it’s the added pressure... we don’t know what impact it could have.’’
NZ King Salmon reduced its forecast earnings for the 2022-year by $4m to $5m because of higher salmon losses caused by rising sea temperatures, while Sanford chief executive Peter Reidie said warmer-than-usual waters in its Big Glory Bay Salmon farm in January had some impact on salmon mortality.
CRA8 Rock Lobster Industry Association chief executive Malcolm Lawson said warming waters was an important factor it was monitoring.
Rock Lobster fisheries in New Zealand are divided into 10 different management areas.
CRA8 fisheries is the largest mainland fishery geographically, and extends from Long Point in Otago to Stewart Island, the Foveaux Strait and along the Fiordland coast to Bruce Bay.
The species of rock lobster in the CRA8 area were adaptable to changing temperatures, Lawson said, but they just required time to acclimatise.
The company had not seen any evidence of lobsters moving away from the area, as is currently happening in the Gulf of Maine in the United States, but had noticed some lobsters were coming up lethargic. This was because warmer waters carried less dissolved oxygen, which although not fatal, could compromise the survivability of live exports.
The industry had been adapting, using tanks with increased water flow to increase dissolved oxygen after lobsters had been caught, but Lawson flagged that as marine heatwaves extended, fishing in the summer may have to be curtailed.
‘‘That is if we got to an extreme… we’re not seeing anything like the Gulf of Maine just yet.’’
‘‘Yet’’ is the key word, with research from the Deep South Challenge and Niwa predicting that by 2100 marine heatwave days in New Zealand would increase from about 40 days a year currently to between 80 and 170.