Seabed Effects
One of the most common effects reported from oyster farms world-wide are changes to the seabed beneath the farm. These largely occur through the deposition of organic material, sedimentation, and accumulation of shell litter.
The effects tend to be in the immediate vicinity of the site and typically extend no further than a few tens of metres or less from the edge of the culture area.
Biodeposition
Pacific oysters filter out plankton, detritus and inorganic particles (mud and silt) from sea water as it flows through the farm. The oysters then produce waste products – faeces and ‘pseudofaeces’ – that settle to the seabed beneath the culture areas and can alter the physical, chemical and biological nature of the seabed.
The deposited particles (called ‘biodeposits’) are organic-rich and so increase microbial activity. This can lead to oxygen depletion in the sediment.
How great these effects are depends mainly on oyster stocking density and biomass, and the flushing characteristics of the environment. Descriptions of oyster cultivation overseas and in New Zealand suggest these effects are usually relatively mild.
Seabed communities are likely to recover relatively rapidly from these effects if farming ceases. This could take anything from a few months - in well-flushed areas where effects are minor - to a few years in poorly flushed areas where moderate/strong enrichment effects occur. Most New Zealand oyster farms are sited in well-flushed areas.
Accumulation of shell litter, debris and associated organisms
The accumulation of shell litter, organisms and other material that can fall and accumulate beneath oyster farms is one of the most visible effects.
The range of material beneath intertidal racks reflects drop-off of cultured oysters, wild oysters and fouling organisms that have established on racks. These are defouled or slough off during normal farm operation, maintenance or harvesting. Other organisms (e.g. starfish, crabs and fish) can be attracted to the accumulated material, as it poses a potential food source. .
The New Zealand Oyster Industry Code of Practice states that marine farming structures and gear should be kept in good order and repair and operated to minimise gear and oysters dropped onto the seabed.
The code states that waste shall be disposed of at an approved disposal site on land. This includes discards of stock, shell and materials used on the farm.
Siltation
Intertidal oyster farming structures – racks and frames etc – can disturb the natural flow of water in an estuary or coastal situation. This can change the height of the seabed beneath or around the farms, through either scouring or build-up of silt.
These effects can be managed by making sure any intertidal farm racks are oriented parallel to (rather than at right-angles to) currents and wave action.
The New Zealand Oyster Industry Code of Practice sets out guidelines for rack orientation, size and other details to minimise these water flow effects. The code requires that farmers should have regard to these guidelines.
Physical disturbance
Although there may be a mild enrichment effect beneath oyster cultivation areas, physical disturbance (for example from barges and from farm workers walking along racks) is also an important consideration
Time scales of recovery from physical disturbance are likely to be a matter of months to a few years.
Shading effects
Oyster farming structures can potentially reduce the light reaching seabed habitats and communities directly beneath them. Shading effects such as these have not been conclusively described in New Zealand.
Such effects would conceivably be most important where farms are located in places of high water clarity and above seagrass or algal beds. Marine farm applicants tend to not apply for farms in locations such as these. Site location considerations, including avoiding sensitive areas and habitats, are an important part of the application process for a new oyster farm.
Contaminant inputs
Wooden oyster racks are often constructed from treated timber, so can potentially leach trace treatment contaminants like copper, chromium and arsenic into sediments around the treated timber. These elements have not been found to contaminate farmed oysters.
