Human impacts on the Bay of Plenty

Coastal marine ecosystems balance on a fine ecological edge. They are dynamic and can easily be disturbed by human impact such as contamination from pollution or excessive harvesting. They can also be affected by alterations of physical habitats, invasions of exotic species (often introduced by shipping in ballast water and fouled hulls) and global climate change. Many areas in New Zealand are seriously affected by such disturbances, including parts of the Bay of Plenty.

Impacts of fishing

There are many examples of direct damage to marine life from fishing, such as the entanglement of dolphins in set nets. Indirect damage to marine ecology may be less obvious but can be widespread. For example, snapper and crayfish are important predators of kina. They effectively control kina numbers. In the Bay of Plenty, overfishing of snapper and crayfish in the last 40 years has allowed kina numbers to go unchecked. Kina graze on seaweeds and have increased their occupation of the rocky bottom, annihilating kelp beds that provided shelter for many fish and invertebrates. There are now large barren areas abundant with kina where kelp forests once existed.

The good news is that marine life can recover when it’s undisturbed. If there is no interference, there can be a gradual shift back to the natural balance of kina, kelp, snapper and crayfish. Establishing marine reserves (where fishing is prohibited) allows this to happen. Providing breeding individuals to help restock depleted areas also helps with recovery.

Estuaries under threat

The Bay of Plenty is surrounded by agricultural, horticultural and industrial activity – all with catchments emptying into estuaries. Although estuaries are of significant conservation value (biodiversity and nurseries for many fish species), life in estuaries is under threat due to catchment activity such as increased rates of sedimentation, inflows of nutrientsheavy metals and polycyclic aromatic hydrocarbons (PAHs) and dredging.

Sedimentation occurs when rain, rivers and streams wash sediment off the land and into estuaries. The sediment smothers the seabed, killing organisms such as shellfish and worms that can’t burrow up quickly.

Nutrients are carried into estuaries with run-off from neighbouring farms and orchards. These nutrients cause algal blooms or other aquatic plant growth. The excessive growth disturbs the habitat balance for marine life. There’s little room for fish as mangroves and/or algae expand across the estuary. As algae die, they decay and rob the water of oxygen, affecting fish and shellfish that require oxygen. Algae also limit sunlight from penetrating the water, meaning underwater sea grasses are deprived of light and can die. Animals dependent on sea grasses for food or shelter leave the area or die.

Heavy metals and PAHs come from industrial zones. Zinc and lead, for example, are washed off the roads. Zinc is deposited by braking cars, and lead still persists on roads from when it used to be added to petrol. Copper is washed off buildings in residential areas. Heavy metals are toxic. PAHs occur in oil, coal and tar and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As pollutants, they are of concern because some compounds have been identified as carcinogenicmutagenic and teratogenic.

Heavy metals and PAHs stress an ecosystem. The resilience of organisms is lowered. Their health is affected, they don’t reproduce well, the mortality rate increases and numbers are reduced. Organisms such as pipi (they suck water through their gills to feed) and mud snails (they suck in and filter mud) are particularly sensitive to this pollution.

Other threats to estuaries come from coastal development such as marinas, subdivisions, aquaculture, reclamation and filling, roading, stock grazing, foreshore structures and flood control works. The Bay of Plenty Regional Council is working to improve catchments (minimising erosion, reducing run-off and nutrient leaching) and is responsible for managing effects of coastal development.

Pollution of beaches and rocky shores

More people use beaches than other types of seashore. Consequently, the beaches are subject to pollution from litter and other human interferences. Coastal seawalls (structures to prevent erosion of coastline) interfere with habitats and impede the dynamic (changing) nature of the coast. Rocky shores are often exposed to many forms of pollution – in particular, oil spills from ships.

Making ecosystems sustainable

To help protect our marine ecosystems and make them sustainable, the New Zealand Government made commitments under the international Convention on Biological Diversity (ratified in 1993) to prepare national strategies to conserve and sustainably use our plants and animals.

For example, the New Zealand Fisheries Act 1996 was designed to set up a sustainable use of fisheries resources. Consideration is given to the following environmental principles:

• Associated or dependent species should be maintained above a level that ensures their long-term viability.
• Biological diversity of the aquatic environment should be maintained.
• Habitat of particular significance for fisheries management should be protected.