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Biodiversity Effects

A mussel farm may alter an area’s marine habitats and ecosystems through a variety of ways. The Resource Management Act, however, requires both central and local government to ensure that aquaculture does not occur in areas of high ecological sensitivity.

Farms are generally not approved if they are proposed over or very close to reefs or complex habitats with a high diversity or abundance of species.  Planning for new aquaculture sites takes into account the likely ecological effects on the environment, including effects on fisheries resources (particularly important spawning or nursery habitats) as well as general biodiversity.

Mussel farms can potentially increase the biodiversity in an area, through the marine life attracted to farm structures and the shells and live mussels that drop off the longlines.

Shells and mussels falling to the seafloor can provide hard surfaces for species like sponges to attach to. High diversity reef-like communities have been observed under existing farms including species such as sea squirts, sponges, sea cucumbers, worms, and mobile predatory species such as starfish, crabs and fish.  In other situations, mussel clumps and shell litter can remain relatively clear of reef-type communities. 

Mussel farming can also reduce the abundance of some species.  For example, several Marlborough Sounds studies record a decline in heart urchins and brittle stars where mussel farms have been established.  Although these large and ubiquitous mobile deposit feeders probably play a role in processing organic matter in sediments, this role may be taken up by other more tolerant deposit feeders such as sea cucumbers.
 
Another potential effect of mussel farming is changes to predator-prey interactions.

Concentrations of predatory species such as spotties, leatherjackets and eleven-armed starfish have been seen within marine farms in New Zealand. The impact of these concentrations of predators is uncertain. However, they are presumably feeding on organisms living on or fallen from the farm structures.  If this is the case, they are not likely to be creating additional predation pressure on the natural fauna of the area.

Fish

Although a limited number of scientific studies have investigated whether mussel farms cause enhancement, recruitment, or displacement of fish populations, the actual effects remain uncertain.  

Recreational fisherman and boaters have observed certain fish species such as snapper and kingfish congregating around mussel farms, attracted to the food supply provided by the mussel stock, especially during harvesting.  In addition to the food source, various fish species are attracted to the shelter and habitat created by farm structures and the shell drop-off beneath farm sites.  This may change the abundance and diversity of species locally in mussel farming areas.  No scientific studies have yet been done to confirm this effect. 

There is very little known about the potential effect of farmed mussels filtering zooplankton (including fish eggs) from the water column.  Based on the information available from regional scientific studies on plankton uptake and nutrient input, it is unlikely the current level of mussel farming in New Zealand is having significant flow-on effects on the sustainability of the wider ecosystem.

Marine mammals

The welfare of dolphin, whale and seal species is carefully considered when assessing marine farm applications.  And offshore farms are often required to monitor any interactions with marine mammals as part of their resource consent and/or marine farm permit conditions.  

There is a concern that mussel farms may adversely affect mammals like dolphins if structures block access to food sources or interfere with feeding and foraging habits, and social behaviour. To date there have been no reports of dolphin or seal entanglement in mussel farm lines in New Zealand, but there is a record of a brydes whale becoming entangled in a mussel farm line in New Zealand.

Information on locations important for marine mammals in New Zealand is limited.  Some studies have been undertaken in specific areas to monitor use by marine mammals.  For example, one study of a bay mostly lined with mussel farms in the Marlborough Sounds has shown the presence of mussel farms can limit the space available for the seasonal hunting of fish schools by dusky dolphins (Markowitz & Wursig 2004).

Marine mammals are protected by a number of regulations and closed or restricted areas around New Zealand’s coasts.  It is an offence under the Marine Mammals Protection Act to intentionally take or harass any marine mammals in New Zealand waters, whether alive or dead, without a permit from the Department of Conservation.

Seabirds

There is a concern that seabirds (e.g. shags, gulls, terns, gannets, shearwaters, fairy prions and oystercatchers) could be affected by mussel farms if farm structures interfere with feeding activities, particularly for birds that forage in open water environments. So the effects of a proposed mussel farm on seabirds are also considered in the farm planning process.

There is very limited information about the effects of mussel farms on seabirds. What little information there is revolves largely around king shags and suggests there could be both positive and potentially negative effects.