Warmer waters are due this spring and summer for NZ – which might be great for swimmers but not for species that must flee, and those restricted by being part of marine farms

New Zealand has one of the largest exclusive economic zones in the world, supporting exceptional biodiversity and providing billions of dollars of benefit to our economy. Despite this, we are still struggling to reduce or mitigate many human impacts on our important marine ecosystems.

The recent occurrence of marine heat waves in New Zealand waters, and NIWA predictions of heat waves this spring and summer, highlight the need for urgent climate action. Impacts from these events are happening now and have the potential to negatively affect and disrupt marine ecosystems and the industries they support.

The latest report on our marine environment, released this week, highlights the main pressures as climate change, plastics and other land-sourced pollution, and the arrival of exotic species. While climate change impacts are widely recognised in the report, it’s generally focused on gradual (over decades) increases in average sea surface temperature.

Emerging evidence suggests we could be in for far more abrupt changes as a result of marine heat waves (MHWs)—periods of abnormally high water temperatures relative to the average seasonal temperature.

On average, sea-surface temperatures in New Zealand increased by between 0.1 and 0.2ºC per decade between 1981 and 2018. This increase seems small compared with the most recent heat waves, which saw temperature spikes of 4 to 5ºC in Fiordland and 3 to 4ºC in northern New Zealand, compared with average temperatures.

While we often focus on the maximum temperature of MHWs, it is also the duration of the event that is important. Some MHWs last months, while others last only weeks. Most impacts of recent MHWs in New Zealand remain anecdotal at present, largely because they were unexpected and few research groups are studying them.

One example is the mass bleaching and necrosis of sponges in Fiordland and the north of New Zealand, affecting tens of millions of sponges. Mass mortalities of farmed salmon in the Marlborough Sounds, linked to higher water temperatures, have also resulted in significant economic and job losses.

While impacts on organisms such as sponges might not seem economically important, what if these sponges are just the most susceptible species, and a degree or two more, or a longer duration MHW, has the potential to affect a much wider range of species? The effects could be very rapid and devastating and come at significant economic cost.

Marine organisms have evolved over many millions of years and have an optimum thermal range. Increases in temperature outside an organism’s optimal range typically result in stress on the organism, which can reduce growth rates, increase bacterial infections, and ultimately lead to death.

Mobile organisms, such as fish, have the potential to migrate south to colder water or into deeper water to escape the heat. However, their survival will depend on whether they can find food in their new environments and compete with the organisms already there.

Many of these species will be those we eat and sell on overseas markets. Their migration to colder waters will disrupt fisheries and could make fish harder to catch. Extreme temperate events also have the potential for devastating impacts on species grown in marine farm settings, such as salmon, as they are unable to migrate away from the heat.

The first part of dealing with an issue is recognising its scale. Our ability to predict the occurrence of MHWs has increased substantially in recent years, but our understanding of the impacts on plants and animals, and on the relationships between species, is comparatively poor.

Few New Zealand species have been specifically studied with respect to their response to MHWs. However, this information would support a range of marine management and economic activities. For example, MHW prediction and monitoring can inform the location of future aquaculture activity and determine the distribution of fished species under specific environmental conditions.

Such data would also support marine conservation by, for example, informing the placement of new marine reserves in areas that are less susceptible to MHW formation.

Marine heat waves need to be recognised for the major impact they are having now, otherwise future reports on the state of New Zealand’s marine environment will make even grimmer reading.

James Bell is professor of marine biology at Te Herenga Waka - Victoria University of Wellington

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