In the Waikato Region water quality is suitable for shellfish gathering at most open sites. It is not suitable in many estuaries and harbours. Most shellfish are safe to eat except after rain, when runoff containing bacteria is washed into estuaries.
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Why it’s important
Many shellfish feed by filtering particles from the water. Shellfish that feed this way include:
- scallop (tipa)
- mussel (kuku)
- pipi
- oyster (tio repe and tio para).
When water quality is poor the filtered particles can include contaminants such as:
- bacteria
- toxins from marine phytoplankton (tiny plants that float around in the sea)
- heavy metals (for example, cadmium, zinc, mercury, and lead).
These contaminants can build up in shellfish guts or flesh, causing health problems for people who eat them. This means that water quality standards for shellfish gathering need to be higher than the standards for swimming.
Bacteria and shellfish
There is limited information available on the levels of bacteria found in shellfish, as only two sites in the Region have been tested – Whitianga and Whaingaroa Harbour (Raglan).
Shellfish collected at Whitianga had low levels of bacteria, met the health standards and were considered safe for eating.
Most of the Whaingaroa samples also met health standards. All of the samples that did not meet the health standard had been collected following a period of rain. Bacteria levels in water are often higher after rain. When it rains, runoff containing bacteria is washed into estuaries and harbours. The bacteria come from animal manure or leaking septic tanks on surrounding land.
Health Waikato recommends that shellfish should not be collected from Whaingaroa Harbour on rainy days or for a few days after rain.
Heavy metals
During the summer of 1996, shellfish were collected from 15 sites (four West Coast sites, three Hauraki Gulf sites, and eight East Coast sites). In these shellfish:
- no lead was found
- copper and mercury levels were well within health standards
- zinc levels were less than half of health standards
- cadmium levels at some open coast and offshore island sites off the East Coast were above the health standard.
The source of cadmium at the East Coast sites is not known. But there are similar reports of high cadmium levels at other apparently uncontaminated sites in New Zealand.
Marine biotoxins
Some marine phytoplankton produce chemicals called biotoxins, which can build up in shellfish and make people who eat them sick.
In 1993, toxin producing phytoplankton bloomed around the coast and caused at least 180 cases of human illness. In late January 1993, shellfish harvesting was temporarily banned all around New Zealand’s coast.
After the 1993 phytoplankton bloom, a national marine biotoxin monitoring network was established. Health Waikato monitors biotoxins weekly at five coastal sites (Kennedy Bay, Tairua, Mercury Island, Slipper Island and Raglan). The shellfish industry monitors water conditions at Coromandel and Port Charles.
In late 2000 and early 2001, there was another bloom of toxin producing phytoplankton and harvest was temporarily banned on the Region's West Coast. For current information on shellfish harvest bans ask Health Waikato’s Public Health Unit, phone 0800 800 977.
Is it safe to eat?
Water quality for shellfish gathering was monitored at 35 locations in the Region between the early 1970s and 1995. Slightly more than half of the sites were in estuaries or harbours (enclosed waters), with the rest on the open coast.
- Water quality is suitable for shellfish gathering at most open coast sites.
- Many estuaries or harbours are not suitable for shellfish gathering.
There are often more bacteria in estuaries and harbours. These enclosed waters are more likely to receive contaminated runoff from surrounding land and are not flushed out as well as the open coast.
Health Waikato and the shellfish industry monitor for the potential presence of marine biotoxins. For current information on shellfish harvest bans ask Health Waikato’s Public Health Unit, phone 0800 800 977.
Heavy metals in shellfish are well below levels of health concern, except for cadmium in the Mercury Islands.
Want to know more?
Find out what you can do to help improve coastal water quality.