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Oil and its impact on the marine environment

Oil spills can have severe and long-term biological, economic, political, cultural, and social impacts. In some cases however, impacts may be relatively minor simply due to the conditions prevailing at the time of the spill.

Oil spills – the threat to the environment

Oil and its impact at sea

Oil and its impact on the shore

Oil and its impact on fish

Oil and its impact on wildlife


Oil spills – the threat to the environment

The initial impact from an oil spill can vary from minimal impact to large scale mortality within a particular biological community.

Spilled oil poses serious threats to fresh water and marine environments, affecting surface resources and a wide range of subsurface species that are linked in a complex food chain that includes human food resources.

Spilled oil can harm the environment in several ways, including:

  • the physical damages that directly impact wildlife and their habitats (such as coating birds or mammals with a layer of oil)
  • the toxicity of the oil itself, which can poison exposed organisms.

The actions of winds, waves, and currents (called weathering) causes spilled oil to break down and become distributed throughout the water.

Oil toxicity is reduced as the oil weathers. An oil spill that reaches a shore quickly will be more toxic to the shore life than if the slick has been weathering at sea for several days before stranding.

The greatest toxic damage is caused by spills of lighter oil such as diesel, particularly when confined in a small area.

The various types of freshwater and marine habitats that exist in nature have different sensitivities to the harmful effects of oil contamination, as well as different abilities to recover.

Recovery times following spills can vary from a few days to decades; depending on the type of environment and the size and type of the spill and conditions at the time. There is no clear-cut relationship between the size of a spill and the extent of the damage caused.

The goal of any shoreline clean up is to clean only to the extent that will speed up the recovery and use of the area. Any activity has to be carefully measured against allowing natural recovery. In many cases intervention can do more harm than good. Sometimes leaving the oil to breakdown naturally is preferable and more effective.

Oil and its impact at sea

In the open sea there is a chance for oil spills to disperse naturally and cause
little ecological damage.

The ocean contains micro-organisms, such as bacteria that break down the molecular structure of oil into less complex substances that are not hazardous. Sunlight, wind and wave action will also help speed the breakdown of the oil.

Oil and its impact on the shore

The impact of oil depends on:

  • the properties and amount of oil spilled on the shore
  • the sensitivity of the spill area
  • the length of contact time.

Long-term damage on a rocky shore is generally rare, and recovery over two or three years is common. This is because oil is not normally retained on rocky shores in a form or quantity that causes long-term impacts and because rocky shore species can quickly re-establish their populations.

The cobble and pebbles contained on a sedimentary shore are usually unstable, drain quickly and support fewer organisms. Oil can however penetrate these beaches and float out on each high tide, causing more problems.

Sand and mud hold water after the tide has retreated, providing a habitat for many burrowing species and for micro-organisms.

The greatest concern is likely to be the effects of oil on sheltered environments such as mangroves and estuaries. These retain oil and many worms, molluscs and crustaceans may be killed.

Long-term depletion of sediment life could have an adverse effect on birds or fish that use tidal flats as feeding grounds.

A mangrove swamp that has trapped crude oil, leading to death of the mangrove trees and associated fauna, presents a particularly serious scenario in New Zealand.

Recovery depends on the sensitivity of the species concerned and the persistence of oil in the sediments.

Oil and its impact on fish

Wild fish in open waters are likely to swim away from oil spills and long-term effects on local populations are usually avoided. Fish populations moving back into an area following a spill may however, take some time to recover.

Fisheries can also be disrupted if migration routes are changed as a result of an oil spill. Spills that affect spawning migration into rivers can affect fisheries in subsequent years.

Spills can result in loss of fishing opportunities, with fishermen unable or unwilling to fish due to the risks present. Exclusion zones can be imposed until the target species has been declared safe. This may cause temporary financial loss to commercial fishermen and disruption to recreational fishermen.

The effects of oil on inshore shellfish beds, and fish or shellfish in aquaculture units, present a major risk.

Oil may contaminate aquaculture equipment and intertidal shellfish, and will damage stock in tanks or ponds if there is an intake of contaminated seawater.

Fish exposed to oil may become tainted. Tainting is of particular concern in caged fish and immobile shellfish that cannot swim away. Commercial catches may also become contaminated from contact with oil-fouled fishing gear. Taint is usually lost over time through natural processes once the oil source has gone. Testing is carried out to determine when fish are fit to eat.

Oil and its impact on wildlife

One of the most visible and distressing effects of an oil spill can be the suffering of oiled wildlife.

Birds can be smothered by oil or drown when oil prevents them floating and/or flying. Toxic components of oil can render birds unconscious and cause serious or fatal illness. Coastal seabirds are particularly vulnerable either through direct contact with the oil or through spoiling of their habitat.

All national and regional oil spill response contingency plans contain environmental and wildlife preservation goals. Oiled wildlife coordinators are based throughout New Zealand.

An oiled wildlife response unit has been developed within the New Zealand Wildlife Health Centre at Massey Universities' veterinary school. This facility has portable bird capture and washing equipment that can be rapidly deployed anywhere in New Zealand.

Massey Universities' oiled wildlife response unit mobilises with the Maritime New Zealand team during an oil spill incident. Wildlife and habitat are protected as much as possible during containment and clean-up activities.

Sometimes it may be appropriate to capture wildlife under imminent threat before they become oiled. Birds that have been oiled can be caught, given intensive care, cleaned, rehabilitated and re-released when the oil has been cleaned up from their habitat.

Marine mammals and coastal reptiles may also be threatened by oil spills and experts are on hand to protect and/or administer to them.

Oiled animals should never be approached or handled by people without specific training and authorisation as both injury to the handler and unnecessary stress to the animal can result. If people find oiled wildlife they should contact their regional council.