How will fish cope in marine heatwave?
A rare marine heatwave is under way in places around New Zealand, with fish and other marine life likely to spend the next couple of months swimming south to find cooler water.
The National Institute of Water and Atmospheric Research (Niwa) has confirmed marine heatwave conditions in parts of the Tasman Sea and the seas east of the country, for the second consecutive summer.
Sea-surface temperatures are now up to 4 degrees Celsius warmer than average in the Tasman and up to 3C hotter than normal in places off Hawke’s Bay, Marlborough and Canterbury. Some eastern hot spots are even 1C or so warmer than at the same time during last summer’s marine heatwave.
The warmer than normal waters increase the chance of subtropical sea life appearing around New Zealand. It also means fish that thrive in cooler water will have to drift further south to find it.
Marine experts say there is a pressing need to better understand how our fish stocks and the distribution of different species are affected by sea temperature.
Niwa principal scientist and marine biogeochemist Professor Cliff Law said there was ocean acidification research – ‘‘but less in the field of temperature change’’. ‘‘It’s very hard to put your hand on it and say, for snapper or hoki for example, a temperature change of 3C by the end of the century will mean X or Y.’’ Scientists needed more data from catches and observations, plus better models to link these together, Law said.
Otago University geneticist and conservation biologist Professor Neil Gemmell is also concerned at the potential disruption to fish from temperature changes.
‘‘I have a reasonably weather-eye on what is going on around New Zealand and I don’t see any major progress when it comes to research on changing distribution of fish.’’
Niwa figures show average seasurface temperatures in the first two weeks of the month varied from about 21C north of Auckland and around the Bay of Plenty, to about 17C to 18C across the centre of the country and down to 15C around the south of the South Island.
Gemmell’s work includes environmental DNA sampling, which has been deployed recently in Scotland to see if there is any evidence supporting the existence of the Loch Ness monster.
Gemmell took to Twitter recently and said: ‘‘THIS IS HUGE! Sea temps in NZ continue to be much higher than average – up to +6 C. Fish distributions will alter (far south may see snapper again this year); reproduction and recruitment will suffer for many species (mussels, salmon, paua, cod); some stocks may just collapse.’’
He told Stuff warmer seas could have repercussions on the distribution of fish stocks, the strength and location of marine currents, and on ocean chemistry.
‘‘The warming looks to be an emergent trend. If it is getting warmer, what is happening to our marine systems? And who is looking for the evidence to see if there are changes?
‘‘It could be good news for snapper – they will come further south – but what about cold-temperature species, like cod? If they disappear from the south of the South Island there will be a lot of sad people.’’
Law said marine species either swam further south or deeper to find cooler water.
Warming seas could also affect fish by changing ocean current strength and location, altering plankton levels and affecting marine species’ feeding habits, and leading to further ocean acidification.
The world’s oceans were alkaline but becoming more acidic, he said.