Environmental monitoring at Pohatu Marine Reserve, Akaroa Harbour
Image: Dave Bradshaw | ©


Human activities are threatening the health of our marine environment. Learn more about them and how you can help.

Our actions, at sea and on land, are putting pressure the ocean. This is threatening many species with extinction. About 22% of marine mammals and 90% of seabirds are at risk of extinction. This is in part because many of the habitats they depend on are decreasing in size or being damaged.

Understanding how to reduce and manage threats is one of the most important steps needed to protect marine wildlife. Some habitats and species can recover quickly once threats are managed. However, other threats can cause permanent damage, so we need ways to prevent them happening.

We can manage some of these threats through marine protected areas (MPAs). But others need us to work together across our communities and industries to protect the biodiversity and taonga species of our oceans.

Learn about threats caused by:

Activities on land

Poor land management

Poor land management practices can speed up the amount of small particles, such as silt and mud, entering the ocean. They build up in a process known as sedimentation.

Ocean habitats can deal with sediments that are created slowly by natural processes. But too much causes problems as it blocks incoming sunlight and makes the water murky. This can choke animals, affect their ability to feed, and damage or kill plants. This can radically affect the wildlife living throughout our oceans.

How poor land management affects animals

Some animals depend on filtering small food particles from water to feed. They can be choked if there is too much sediment in the water. Sediments can also carry toxins and heavy metals from the land which can be poisonous.

Many other animals, such as penguins, are underwater hunters and need clear water to see their prey. When waters are murky with sediments this can affect their ability to hunt and feed their offspring.

How poor land management changes habitats

An example of how this can radically change ocean habitats is the effect on seagrasses. When excess sediments arrive in seagrass habitats, they cannot survive because of the reduced light and die quickly.

Although they only cover 0.1% of the ocean floor, seagrasses may store up to 12% of the ocean’s carbon. Yet they are declining 4 to 10 times faster than tropical forests. There are also not very well represented  within protected areas.

Nutrients run off developed land

Nutrients arrive in the ocean as they are carried in water running off the land. Measuring the amount of nutrients in seawater is often used to monitor the health of the wildlife there. They are essential for life to flourish and contribute to healthy, productive ecosystems.

But too many nutrients cause harmful effects such as algae blooms. These blooms use up the oxygen available in the water. This takes oxygen away from other organisms and can cause some areas to become known as dead zones, where nothing grows.

This state of excess nutrients is called eutrophication. It has been shown to disrupt the balance of ecosystems. This is because it can change the types of plants and animals found in a habitat and reduce their diversity.

Activities at sea

Overfishing and ecosystem balance

Overfishing occurs when fishing changes the balance of the ecosystem. When fishing leads to a decline or loss of the largest predators, it alters the predator-prey balance. This disrupts the health of the ecosystem.

Healthy ecosystems are important to preserve biodiversity and support thriving fisheries.

But overfishing can change  the balance of wildlife so it is unable to thrive. An example of this is the kina barrens found when tamure (snapper) and kōura (crayfish) are overfished.

The large, mature fish targeted by fishing are more efficient predators, helping tokeep this balance. They also produce many more eggs than smaller fish. This means the loss of the largest predators also reduces the number of young fish. 

Pollution and microplastics

Plastic pollution is becoming one of the largest threats to marine wildlife. Plastics affect animals more than oil spills, heavy metals, or toxins. In New Zealand, 61% of all marine litter is plastic.

Large plastics cause harm when they are ingested or when animals become entangled. Microplastics are pieces less than 5 mm in size and are significantly more difficult to remove from the environment.

When animals eat plastic pieces, this reduces their intake of nutrients they need. This can affect tiny zooplankton up to larger animals, such as  fish and shellfish. Tiny plastic fragments also absorb harmful chemicals, which can accumulate in the larger species who consume them. This build-up of plastic and toxins can also put human health at risk.

Physical damage to the seafloor

Physical damage to the seafloor and the delicate wildlife there can occur in many ways. These include boat anchors, trawling, fishing and mining.

This means such practices must be carefully managed and monitored. This is because once damaged, wildlife and habitats on the seafloor are not always able to recover. After being damaged, some habitats also become more vulnerable to other threats such as sedimentation and disease. They may also be unable to support other wildlife.

An example of this is bryozoan thickets. These thickets have a role as a nursery for shellfish larvae, but once damaged they cannot support them.

Effects from climate change

Heatwaves and our warming ocean

Globally, oceans absorb about 90% of the heat that we add to the atmosphere. This warming isn’t evenly spread throughout the oceans but arrives in ‘blobs’ which affect specific areas, similar to heat waves in the weather. Marine heatwaves can be caused for a variety of reasons, such as the ocean cooling less during warm winters.

This localised warming of the ocean has severe effects on marine life. Zooplankton (the small animals that are a large part of the food chain) will grow smaller and have less fat. Fish and marine mammals that eat the zooplankton will have less calories when feeding. Warmer waters can mean more frequent harmful algal blooms and a better environment for bacteria and parasites to thrive.

As ocean temperatures continue to rise, tropical species will move. They move further from the warmest parts of the globe around the equators and towards the cooler areas around the poles. As they do so, they displace native species such as bull kelp, which are less adaptable to rising temperatures.

Sea level rise and loss of breeding grounds

Rising sea temperatures cause the sea levels to rise in two ways. First,melting ice caps and glaciers add more water to the oceans. Second, water expands as it gets warmer.

As sea levels rise, they cover shoreline nesting and breeding grounds for many species, such as seals, sea lions, and shore birds. This “coastal squeeze” is made worse by what we do to protect ourselves. Sea walls may temporarily protect our homes and communities from the rising waters, but they also stop sandy beaches and habitats such as mangroves from retreating up the land. 

Acidification and animals who grow shells

Ocean acidification is ranked the highest threat to marine habitats. It occurs when carbon dioxide is absorbed from the atmosphere. Once in water, it is converted to carbonic acid, raising the acidity of the ocean.

Naturally, the ocean is neutral or slightly basic, and the organisms within it are suited to these conditions. Scientists think that ocean acidity has increased by about 30% in the last 100 years.

Shell-forming animals such as shellfish and corals are highly affected by acidification. The carbonate that many use to build their shells is less available in acidic waters. This means many will struggle to grow their protective shells. Their young may be deformed and unable to swim as they should, meaning that they cannot survive or settle in their habitat.

Changes to the balance of life in our seas

Introduced species

The ecosystems in the ocean evolved over millions of years. As a result, they have done so in careful balance.

New species arriving from overseas can quickly adapt to local conditions and settle in new areas. But this can mean they will threaten native species. The new animals that settle into these areas can compete with native species for space, shelter, or food. Others can cause physical damage. For example, an invasive tubeworm is known for blocking pipes in the Hauraki Gulf.

Once a species has been introduced to our waters, it can be extremely difficult to remove them. This is why biosecurity must be diligent in checking ships hulls and ballast waters for unwanted guests.

Undaria is an invasive species of seaweed which was first found in New Zealand in 1987 and has become established in some areas in Southland. One mature plant can produce up to 700 million spores, which can live in the ocean for up to 2.5 years, making them very difficult to remove.

Losing species and habitats permanently

Extinction is the worst-case scenario for a threatened species, but it is also a threat. Extinction can be which are global, localised, or functional. All can affect marine wildlife.

Global extinction is when a species is completely gone from the planet. Localised extinction is when a species dies off in a specific area but is still present in other places.

The third type of extinction is functional extinction. This is when a species or habitat is found in such low numbers they cannot fulfil their role in the ecosystem.

An example of localised and functional extinction is the kōura (crayfish or rock lobster). Kōura can still be found at places in New Zealand. But in some areas, such as the Hauraki Gulf, there are not enough breeding adults to maintain the population. There is also not enough for them to play their important role as predators on shallow rocky reefs.

The benefit of managing threats to our oceans

None of the threats that face our oceans can be understood in isolation from each other. Threats do not have distinct boundaries and effects. For many organisms these sources of stress can overlap. Most often, threats multiply each other, with far worse outcomes than if they were considered separately.

It can be very difficult to predict how a species or ecosystem will react when threats accumulate. This means that it is necessary to act with caution and minimize threats wherever we can, to give species the best chance at survival.

How reducing individual threats supports ecosystems

Unfortunately, we cannot protect every species or habitat from every threat. But if we can reduce or remove one threat it can make a vast difference for the survival of a species.

Managing the many threats that face our marine life is a hard task. But nature is resilient and has a natural capacity to respond to challenges.

By reducing one or more of our effects on the ocean, we may avoid pushing an ecosystem or species beyond their ability to recover.

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