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The pressures on our marine environment

In this section, we describe the key pressures on our marine environment. We provide information about long-term climate change, and marine- and land-based activities that affect the marine environment.

Changes associated with greenhouse gas emissions are likely to have the greatest long-term impact on the marine environment (MacDiarmid et al, 2012b). Commercial fishing is an ongoing pressure on marine life. In particular, bycatch injures or kills marine animals, and trawling damages the seafloor. Land use can also affect the marine environment, particularly in coastal waters.

Long-term climate change

Global increases in greenhouse gases in the atmosphere have led to changes in the climate. In New Zealand, the concentration of carbon dioxide (one of the main greenhouse gases) has increased 21 percent since observations began in 1972 (see also Atmosphere and climate chapter). The increase in greenhouse gases, and especially carbon dioxide, in our atmosphere is changing our marine environment in three ways: it is leading to increased ocean acidification, rising sea levels, and increased sea temperatures over the long term.

Our oceans are becoming more acidic

The subantarctic waters east of New Zealand have become more acidic. Ocean acidification poses the greatest threat to our marine habitats, by directly affecting marine species and ecosystem processes (MacDiarmid et al, 2012b).

The acidity of New Zealand’s oceans is measured in the subantarctic ocean off the Otago coast. Since 1998, the acidity has increased by a statistically significantly amount – an average decrease of 0.0015 units a year in the seawater’s pH (a measure of acidity and alkalinity) (see figure 38). The increase in acidity measured at this site is consistent with changes measured elsewhere in the world (Bates et al, 2014). Acidity in our oceans is predicted to continue to rise as a result of carbon dioxide emissions (Ciais et al, 2013).

Figure 38:


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This graph shows how the pH of subantartic waters east of New Zealand has fluctuated between 1998 and 2014. Visit the MfE data service for the full breakdown of the data.

Ocean acidification directly affects marine species as well as ecosystem processes. With increasing acidification, plants and animals with calcareous shells (composed of or containing calcium or calcium carbonate), such as some plankton, snails, and corals, will find it harder to extract calcium carbonate from the ocean to build their shells (Secretariat of the Convention on Biological Diversity, 2014). Increased ocean acidity may slow the development of plankton species that have calcareous shells and reduce their survival rate. Plankton forms the base of the food chain and is a direct or indirect source of food for almost all marine animals. For this reason, acidification has potential to have widespread effects on marine ecosystems (Fabry et al, 2008).

Increased acidification may also affect our economic and social well-being, because it is likely to affect some of the species we harvest for customary, commercial, or recreational purposes, such as pāua, mussels, and oysters.

For more detail see Environmental indicators Te taiao AotearoaOcean acidification.

Our coastal sea levels are rising

Increased greenhouse gas concentrations in our atmosphere are also causing coastal sea levels to rise. This is probably due to the expansion of warming waters, and ice sheets or glaciers melting as sea temperatures rise and release water. Rising sea levels increase the likelihood of coastal erosion, a potential risk for New Zealand’s cities, coastal communities, and infrastructure. Sea-level rise can lead to the loss of coastal habitats and species.

Between 1900 and 2013, sea levels rose (relative to land) between 1.31 millimetres and 2.14 millimetres annually at observation sites around New Zealand (see figure 39). These changes are consistent with the sea-level rise observed worldwide (Church & White, 2011).

For more detail see Environmental indicators Te taiao AotearoaCoastal sea-level rise.

Figure 39:


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Note: Data were not available for some periods.

This graphs shows the sea level relative to land for five monitored sites (Auckland, New Plymouth, Dunedin, Lyttelton, and Wellington) between 1900 and 2013. Visit the MfE data service for the full breakdown of the data.

Sea temperature and natural climatic variations also affect the marine environment

Other climatic pressures with potential to change the marine environment are rising sea temperatures and natural cyclical changes known as climate oscillations. Sea temperatures are expected to continue to rise as a result of climate change, while climate oscillations are part of a natural climatic cycle.

New Zealand’s annual average sea surface temperatures measured by satellite do not show a significant trend over the past 20 years. However, long-term surface temperature measurements taken using ships, buoys, and satellites indicate an increase of about 0.71 degrees Celsius over the period 1909–2009 (Mullan et al, 2010). In comparison, global sea temperatures in the upper 75 metres have increased about 0.1 degrees Celsius every decade since 1971 (Rhein et al, 2013).

Change and variability in ocean temperatures can affect marine processes, habitats, and species. Depending on their tolerance for changing environmental conditions, some species may find it hard to survive in areas where waters are warming.

The oceans around New Zealand are influenced by natural cyclical changes (climate oscillations). During certain phases, these oscillations can lead to warmer seas or more stormy weather.

For more detail see Environmental indicators Te taiao AotearoaOceanic sea-surface temperatureCoastal sea-surface temperature, and Climate oscillations.

Marine-based activities

Many of the activities people undertake in our marine environment can affect its productivity and biodiversity, and our ability to enjoy it in other ways, such as for recreation or customary purposes. A quota management system is used to control commercial fishing in New Zealand, to ensure our fisheries are sustainable. However, some aspects of commercial fishing continue to put pressure on marine life and habitats. The most significant of these are bycatch and trawling.

This section covers the impacts of commercial fishing (bycatch and trawling), aquaculture, and extraction of oil, gas, and minerals. It also covers exotic species introduced through shipping, and other human activities in the marine environment.

Bycatch of protected species is decreasing but still a threat

‘Bycatch’ occurs when fish or other animals are unintentionally caught and injured, or killed, during fishing operations. This is particularly concerning when it involves species that are threatened or at risk of extinction. Seabirds, sea lions, fur seals, and dolphins are the main protected species caught as bycatch. Protected sharks, such as basking sharks, are also caught as bycatch.

Fishing bycatch is the main pressure on seabirds. An estimated 55,000 seabirds were caught in fishing gear between 2001/02 and 2013/14. Over this period, the estimated number of seabirds caught each year fell, from 7,280 to 4,380. This decrease may be partly due to the fishing industry using bird-scaring devices and other measures to prevent bycatch.

Bycatch is a factor leading to the decline in the sea lion population in New Zealand waters (Baker et al, 2010; Robertson & Chilvers, 2011). The sea lion population is small (estimated at fewer than 3,000 mature individuals) and estimated to have decreased about 70 percent over three generations (about 32 years). The species is listed by the New Zealand Threat Classification System as nationally critical, meaning it has the highest risk of extinction (Baker et al, 2010). The number of sea lions estimated to have been caught as bycatch decreased from 59 in 2003/04 to 33 in 2012/13. This decrease may be partly due to the use of devices that help sea lions escape from nets, making them less likely to drown.

An estimated 508 fur seals were caught in 2012/13, a substantial decrease from the estimated 1,509 caught in 2004/05. Fur seal populations are now considered to be healthy, although they remain a protected species.

Hector’s dolphins have an estimated population of between 7,000 and 9,000, while the population of Māui’s dolphins aged over one year is estimated to be 55 (Baker et al. 2010; Hamner et al, 2012). The most common cause of unnatural death for Hector’s and Māui’s dolphins is entanglement in nets and other fishing gear (Currey et al, 2012). Of the deaths for which a cause could be determined, 42 percent are attributed to entanglement in fishing gear (Department of Conservation, 2013).

Bycatch of species that are not protected occurs in most of our commercial fisheries. From 2001/02 to 2011/12, the estimated bycatch of fish and invertebrates such as sponges, crustaceans, and cold-water corals fell 72 percent, to 32,098 tonnes. This decrease may be partly due to a decrease in overall catch.

For more detail see Environmental indicators Te taiao AotearoaBycatch of protected species: sea lion and fur sealBycatch of protected species: seabirds, and Bycatch of fish and invertebrates.

Seabed trawling is decreasing

Seabed trawling has a major impact on seafloor habitats and species. When trawls are dragged along the seabed, they disturb sediment, damage corals, and scoop up seabed species such as crustaceans and brittle stars.

The number of trawl tows and dredge tows in New Zealand waters has fallen (Ministry for Primary Industries, 2014; Black & Tilney, 2015). From 1997 to 2014, the number of trawl tows reported each year decreased more than 50 percent. From 1996 to 2014, the number of dredge tows reported in New Zealand waters decreased 83 percent.

In 2010/11, deepwater fishing operators trawled 1.3 percent (53,031 square kilometres) of the territorial sea and EEZ. Trawling mainly occurs in the same areas each year, which limits the extent of newly affected habitat and species (Black & Tilney, 2015).

The extent of seabed trawled for the first time, where the potential for damage is greatest, has been decreasing each year since 2007. Similarly, the overall area trawled each year has been decreasing since 2002/03, except in 2010/11 when a larger area was trawled than in the previous year (Black & Tilney, 2015).

For more detail see Environmental indicators Te taiao AotearoaCommercial seabed trawling and dredging and Commercial coastal seabed trawling.

Aquaculture is increasing

Aquaculture is an expanding industry, and there is limited information about how aquaculture affects the marine environment. Operations are concentrated in particular areas (mainly around the top of the North and South islands), and the potential impacts from aquaculture are also likely to be concentrated in those environments.

Shellfish farms contain a high density of animals that filter the water to feed, and this can reduce the amount of phytoplankton (the base of the marine food chain) available for other species. Aquaculture operations can also increase nutrient enrichment of the surrounding seabed, which affects nearby habitats. They can deposit live animals, shells, and faeces (from shellfish farms), or uneaten food and faeces (from finfish farms), which can smother seabed species and habitats. Marine mammals and seabirds can also be displaced by aquaculture, and some species become entangled in the fish-farm structures. In addition, aquaculture may increase the risk of pests and diseases spreading or becoming established (Ministry for Primary Industries, 2013).

Extraction of oil and gas and minerals poses risks

About 197 offshore oil and gas wells have been drilled in New Zealand waters, most of them (176) in the Taranaki region (Petroleum Exploration and Production Association New Zealand, 2015). While there is no mineral extraction in New Zealand waters, possible sites are currently being surveyed and explored.

Oil and gas extraction can adversely affect the marine environment. Extraction operations directly affect seafloor habitats and species, although the effects are localised. Sediment plumes produced by the extraction process can have effects over an extensive area, as the suspended sediment spreads. They reduce food availability for some species and smother seafloor species such as corals. Discharge of tailings (residues from extraction) and effluent can have a wide range of impacts on plankton and fish species, including reducing primary productivity where suspended sediment shades phytoplankton (Chung et al, 2002; MacDiarmid et al, 2012a).

As seen in the Gulf of Mexico in 2010, the risk of an oil spill may be low, but the consequences for the marine environment can be catastrophic.

For more detail see Environmental indicators Te taiao AotearoaOil and gas and minerals extraction.

Exotic species can affect indigenous biodiversity

Once exotic species become established, they can adversely affect indigenous species by competing for food and habitat. Examples of exotic species that have become pests in New Zealand’s marine environment are the Asian paddle crab and didemnum (Whangamata sea squirt).

The Asian paddle crab is native to South-East Asia and was first reported in Auckland in late 2000. It is now widespread in the Waitematā Harbour and wider Hauraki Gulf. This aggressive species could compete with indigenous crabs for space and food. It is a potential threat to marine farming because it preys on shellfish and other aquaculture species. Didemnum has become a problem in Marlborough. It is a threat to marine farming because it can smother human-made structures such as mussel lines (Ministry for Primary Industries, 2010).

Of the 339 exotic species recorded as being present in New Zealand waters, 182 have established populations. The remainder have been found on boats or floating structures (Graeme Inglis, NIWA, personal communication, 2015), and have not become established. We do not have adequate information about the impact of exotic species on indigenous species in our marine environment.

For more detail see Environmental indicators Te taiao AotearoaMarine pests.

Land-based activities

The marine environment is also affected by activities on land. For example, excess nutrients and sediment from farmland can flow into waterways and eventually out to sea. Stormwater discharges and pollution also contribute to degrading our oceans.

Run-off from land affects the coastal and marine environment

The transfer of sediment from land into waterways, and ultimately out to sea, is a natural process. It brings sand to our beaches and sediment to estuaries, creating habitats for wildlife. However, this natural process is disrupted when excess sediment enters waterways from eroding land. New Zealand loses about 192 million tonnes of soil into waterways and the ocean every year. This is estimated to contribute about 1.5 percent to global sediment loss, despite New Zealand making up only 0.2 percent of the global land area (Syvitski et al, 2005; Walling, 2008). Erosion is a particular problem when erosion-prone hill country is farmed (see Land chapter).

Excess sediment in estuaries can smother habitats such as seagrass meadows and mussel beds. It can also have a detrimental effect on water clarity, reducing the ability of phytoplankton and plants to turn carbon dioxide into energy through photosynthesis.

Land-based activities, and agriculture in particular, can cause an excess of nutrients – especially nitrogen and phosphorus – to enter waterways, estuaries, and coastal waters. Having too many nutrients in the sea promotes the growth of algae and can lead to harmful algal blooms that can affect habitats and species.

In 2013, monitoring found the levels of nutrients and turbidity (murkiness) caused by sediment were higher in estuaries than in other coastal environments. This highlights the impact of run-off from land and sediment being washed down waterways. Levels of dissolved oxygen in estuaries were lower than in other coastal environments. Nutrients and organic matter reduce oxygen in water, which can affect fish and other marine animals that depend on dissolved oxygen to survive.

Run-off from roads and other human-made surfaces contains heavy metals such as lead, zinc, copper, and cadmium. Heavy metals are toxic to both animals and humans, even at low concentrations. They wash into waterways, estuaries, and harbours, especially at times of heavy rain. Cadmium is also a component of some fertilisers, and can be contained in run-off from farms.

Heavy metals from run-off accumulate in the sediments of estuaries and can be taken up by organisms living in the sediment. They also build up in higher concentrations in species further up the food chain (bio-accumulate).

For more detail see Environmental indicators Te taiao AotearoaHeavy metal load in sediment and Coastal and estuarine water quality.

Large amounts of waste go into our oceans

Waste, mainly from urban areas, is a further pressure on our marine environment. Large amounts of waste is carried along waterways and washed out to sea. While some of the waste collects on beaches, most remains in the ocean. Plastics are a particular problem because they break down over time into small particles and enter the food chain.

Although we have no data for waste in New Zealand’s marine environment, globally it is estimated that more than 5 million tonnes of plastic waste entered the world’s oceans in 2010 (Jambeck, 2015). An estimated 250,000 tonnes of plastic is afloat on the ocean surface (Eriksen et al, 2014), forming huge drifting patches such as the Great Pacific Garbage Patch in the North Pacific Ocean.

The build-up of debris in the marine environment has a range of effects: marine animals and seabirds can become entangled in it and be injured or killed; seabirds and turtles can swallow plastic debris they mistake for food; and debris drifting in the ocean can damage fragile marine habitats (Baird et al, 2012; Derraik, 2002).

We have only limited information about the effects of marine debris in New Zealand, but one study identified that between 1995 and 2005, the incidence of fur seals and sea lions being entangled in plastic and other debris in Kaikoura was one of the highest rates reported in the world (Boren et al, 2006).

For more detail see Environmental indicators Te taiao AotearoaEffects of marine debris on marine life.