Changes to seabed
The principle ecological effect of mussel and oyster farming is localised changes to the seabed caused by shells, shellfish, faeces and pseudofaeces (particles which are not digested bound in mucous) dropping from the mussel lines and oyster racks. These items can accumulate on the seabed and alter its physical, chemical and biological nature. The waste products are nutrient-rich and thus increase microbial activity, which can lead to oxygen depletion within the sediment, affecting species that live there and reducing biodiversity.
Changes to the seabed are particularly pronounced where the seabed is sand or pebble, rather than mud. In these areas, benthic species are less able to cope with smothering by fine material from the farm. The extent of these effects will largely depend on the type of substrate under the farm, the flushing capacity of the site, and the depth between farm structure and substrate. Shellfish farms are less likely to be situated over or near to rocky reefs where such effects could be even more pronounced.
The creation of a nutrient-rich environment may give rise to future opportunities to farm other species (polyculture) underneath such farms. Species such as sea cucumbers have been the subject of trials. A community-based aquaculture research project between Hongoeka Development Trust and NIWA focused on applying waste product from one species to become the food input for another, thereby reducing waste and maximising energy flows throughout the integrated system. In this particular project, the polyculture was investigated to see whether the waste produced by a pāua farm could be recycled to produce other high value foodstuffs relevant to Māori.
Farmed shellfish extract phytoplankton and organic particulates from the water column, and can thereby cause their depletion, particularly in sheltered bays. This depletion tends to be localised and its significance will depend on the density of the farms and the level of flushing in the area. Phytoplankton are the building-blocks of the marine food web, and the impact of their depletion on the broader marine ecosystem is poorly understood. 1701 Farmed shellfish release ammonia and organic particles into the water column which promote some additional phytoplankton and macra-algal growth. In addition, these shellfish eat fish eggs so could potentially have an impact on fish stocks if very large areas of shellfish farms are established.
The structures required for mussel farming create obstructions in the water column that can alter the direction and speed of currents in the area, which in turn can alter the rate at which plankton and nutrients are replenished and waste dispersed. The extent of these effects will depend on the particular characteristics of the area, the scale and density of the farm, and the orientation of the lines relative to the predominant direction of the currents. The effects can be minimised through good design, with larger, denser and poorly orientated farms having greater effects. Research suggests that dispersed development of marine farms in New Zealand is unlikely to significantly impact on the hydrodynamics over a wider area.
Work has been underway for a number of years looking at the pelagic impacts from marine farms in the Firth of Thames. An adaptive management framework was recommended in 2005 whereby Limits of Acceptable Change would be developed and agreed upon. The limits agreed to were that phytoplankton depletion shall not exceed 20 per cent over 10 per cent or more of the Firth of Thames and phytoplankton depletion shall not exceed 25 per cent over an area twice the size of the Aquaculture Management Area
‘Trigger points’ were used to determine when limits have been reached. These can then be reviewed and modified as necessary. In addition, these trigger points enable marine farm managers to respond with a range of management actions.
Effects on wild species
Material falling from mussel farms can attract small fish and encourage the development of reef-type communities including sponges, crabs, starfish and tube worms. Juvenile fish commonly use artificial structures placed in the sea for shelter, which can lead to the creation of new communities and increased biodiversity.
The overall effect of marine farms on fish abundance is not well understood. Research on farms in the Marlborough Sounds indicates that there is more abundance and diversity of fish adjacent to the farms than in surrounding areas. Anecdotal evidence from recreational fishers indicates that species such as snapper and kingfish can be found in higher numbers around marine farms, attracted to the food supply provided by the mussels and juvenile fish, especially during harvesting. An increasingly popular charter fishing industry is developing around the Wilson Bay mussel farms in the Firth of Thames.
Shellfish farms can also change the community structure in their vicinity because of the way they change the seabed substrate, increasing the amount of shell and nutrients. This results in a shift towards species better adapted to the new conditions. For example, studies of farms in the Marlborough Sounds found that heart urchins and brittle stars declined in the locality of mussel farms and were replaced by other species. 1706
Spat for mussel farms is predominantly sourced from Ninety Mile Beach in Northland and then moved to the rest of New Zealand to seed the farms. There is currently no evidence that wild greenlipped mussels are declining in population fitness, although mixing with farmed mussels has probably already occurred. This could be due to the long-lived dispersive larval stage of greenlipped mussels, which facilitates wide genetic mixing.
Pacific oyster farming in areas where they don’t already exist could result in the establishment of populations of a non-native oyster in the wild. However, wild populations of Pacific oysters are already widespread around the coastline, and are present in areas where there are no marine farms.
Increasingly, selective breeding is being used to supply spat for oyster farms and, with new research underway, is likely to become more common for mussel farms. Trials have started where selectively bred hatchery-produced spat mussels have been transferred to a marine farm. 1707 There is little information available about the impact of selective breeding on wild populations of oysters or mussels.
Abandonment of farm structures
Abandonment of marine farms can result in degrading structures being left in the marine area. The structures required for finfish and mussel farming are generally of a high enough value to incentivise farmers to remove them, whereas oyster racks are not. If oysters are left behind on the racks they can outgrow the structures and cause the farms to collapse into the mud and build up on the sea floor. The abandoned structures can be very visible for many years. This problem can be addressed by conditions of resource consents and bonds to ensure structures are removed on the cessation of farming, or by the requirement for reasonable assurance (such as a fidelity fund).
Siltation and alteration of natural currents
Oyster farms can cause siltation on the seabed by disturbing the natural flow of water in the area, and changing the height of the seabed beneath and around the farm, through scouring or silt build-up. These effects can be minimised by ensuring that oyster racks are oriented parallel with, rather than at right angles to, currents and wave action. The growing of oysters in baskets, which is becoming more common, minimises the trapping of sediment.
Ministry for Primary Industries, 2013c, 13
Plew et al., 2005
Aquaculture Management Areas were introduced in 2005 as a tool to address cumulative effects of aquaculture by including in planning documents specific zones for aquaculture. The 2011 aquaculture law reforms removed the requirement for these to be established.
Zeldis J R, Felsing M and J Wilson, 2005
Last updated at 1:31PM on February 25, 2015