Summary of the Ecological Effects of Farming Shellfish and Other Non-finfish Species
This information sheet is based on Review of the Ecological Effects of Farming Shellfish and Other Non-finfish Species, Cawthron Report No. 1476, which was produced for the Ministry of Fisheries and published in August 2009.
The report was commissioned by the Ministry of Fisheries to address concerns raised by regional councils and the marine farming industry regarding a lack of publicly available information summarising the ecological effects of farming shellfish and other non-finfish species. The outcomes of this review are expected to assist regional councils, communities and the marine farming sector in planning for and developing sustainable aquaculture in New Zealand.
The report looks at the ecological effects of Greenshell™ mussel farming, intertidal Pacific oyster cultivation, and developing and potential non-finfish species. It also gives an evaluation of ecological risks from non-finfish aquaculture, considers future developments in New Zealand aquaculture, and discusses management and mitigation of ecological effects.
What are the ecological effects?
Like other farming activities, marine farming does have an impact on the immediate environment. The review considered ecological effects of farming shellfish and other non-finfish species on the seabed and on the water column. It also considered wider ecological issues, such as the effects of marine farming on fish, marine mammals, and the spread of invasive species or disease.
Seabed effects
The main ecological effects on the seabed from farming mussels, oysters and other filter-feeding bivalves comes from organic matter, e.g. faeces and shells that fall to the seabed. In most cases, the severity of seabed effects has been assessed as low to moderate.
Seabed effects are most pronounced directly beneath farm sites and these effects reduce rapidly with distance. In general, the effects are usually difficult to detect within 20-50 m of the farm site. Water depth and current speeds are the two most important factors in determining the effect of a farm on the seabed.
Water column effects
The effects of farming of mussels, oysters and other bivalve species on the water column are less understood than for the seabed, because water column characteristics are more dynamic and harder to quantify. Three main water column effects were considered: the presence of physical structures, additional nutrients, and filtration pressures by bivalves like oysters and mussels.
The physical presence of marine farms can alter and reduce current speeds, which may impact biological processes. Farm structures can also reduce short-period waves, which can affect inshore ecology. At the present scale of development in New Zealand these issues are not considered significant. The intertidal culture of oysters is thought to have a slightly more profound effect on hydrodynamics than mussel farming due to the structures occupying the full cross section of the water column and being in contact with the bottom. Species that require atypical cage structures or additional feed (e.g. crayfish and pāua) will likely interact with the water differently; the extent to which their effects will differ remains undetermined.
Farmed animals release dissolved nitrogen (e.g. ammonium) directly into the water column, which can cause localised enrichment and stimulate phytoplankton growth.
In terms of mussel farming, this growth could lead to toxic microalga blooms, although there is no evidence, to date, of this being an issue in New Zealand waters.
Another effect of mussel farming, or farming of other bivalves, is an increase in the filtration of plankton from the water. This interaction is poorly understood, although no significant water column related issues have been documented to date which may suggest that effects associated with traditional inshore farming practices are relatively minor.
Wider ecological issues
The wider ecological issues that were considered in the review include: habitat creation and alteration, effects on fish, seabirds and mammals, biosecurity risks, disease, and genetics. These issues are generally less well studied than seabed and water column effects, either because of logistical difficulties in obtaining quantitative data, lack of awareness, or because the need has not arisen, i.e. potential for adverse effects is generally perceived to be low.
Habitat creation
Because marine farms function as mid-water artificial reefs they provide a food source, breeding habitat, and refuge from predators for some species. Marine farms can also affect fish populations through changing fishing pressures and fish behaviour.
Seabirds and marine mammals
Potential effects on seabirds and marine mammals (seals, dolphins and whales) relate mainly to habitat modification, entanglement in structures and gear, and habitat exclusion. For seabirds, a range of potential effects are recognised, but these are generally not well understood. The few overseas studies describing seabird interaction with oyster culture sites provide no evidence of adverse effects.
The researchers found only one reported case of marine mammal entanglement in New Zealand’s mussel (spat) farm structures in an industry that now comprises ~900 farms, and there remains some uncertainty over whether the death was the result of entanglement. To date, the risk to marine mammals appears to have been low in New Zealand, but this may change with the advent of large offshore mussel farming sites. The potential for adverse interaction between intertidal oyster culture and marine mammals is minor in New Zealand, as there is probably minimal overlap between sites of intertidal cultivation and typical marine mammal habitat.
Biosecurity
It is known that aquaculture structures are reservoirs for the establishment of pest organisms. This means that aquaculture development in New Zealand has the potential to increase the spread of pest organisms. Through this review, in terms of ecosystem effects, biosecurity issues relating to the spread of pest organisms received the highest risk ranking.
To tackle this issue, various management approaches have been implemented to reduce this risk. These approaches include codes of practice, and treatments for infected seed stock.
Spread of disease
Overall, disease has not been a significant issue with New Zealand aquaculture, but disease propensity is species-specific and therefore needs careful consideration with the introduction of other new culture species.
Genetic diversity
One of the less recognised effects of non-finfish aquaculture concerns maintenance of genetic diversity. At this time, this is not considered an issue within Greenshell™ mussels in part because the industry is based on wild-sourced spat. This could change if the mussel industry were to increase its dependence on hatchery-supplied spat.
In the case of Pacific oyster cultivation, ecological effects on wild populations are not as relevant since Pacific oysters are non-indigenous to New Zealand. Also, recent advances in breeding and the future production of triploid oyster spat that are sterile will likely eliminate the potential for genetic interactions.
Is there a ‘tipping point’ with the amount of aquaculture development our marine environment can support?
There are no definitive studies that provide a clear answer to the question of carrying capacity in relation to New Zealand mussel farming.
Spatial modelling tools offer a way of estimating the extent to which the cumulative effects of mussel farming may be approaching ecological carrying capacity on local or regional scales. Knowledge gaps are still evident in these models, however, especially in terms of impacts on ecosystems.
The present farming intensity in New Zealand is still considered low to moderate in an international context.
What about future management?
A key management and mitigation measure identified throughout the review is the importance of careful site selection. This includes sites that have strong currents that flush out the site, a good food supply in the case of extractive culture, i.e. suspension feeding bivalves, and are removed from sensitive habitats. It is also recognised, however, that in near-shore regions, areas that have strong current tend to coincide with ecologically significant or sensitive habitats, creating a trade-off between waste-dispersion potential and localised effects on such areas.
Effects can also be mitigated through adopting management strategies for pest species, disease and genetic diversity. Approaches to managing pest species are likely to be transferable across different forms of aquaculture. In the cases of disease and genetics, management will differ between species and more research may be required before appropriate protocols can be developed.
Possible effects associated with likely future developments in the aquaculture industry, such as the move to larger offshore areas and the conceptual implementation of integrated (multispecies) culture, were considered in the review. While many of the management and mitigation techniques that work currently for small sites and single species farms may be transferred to these developments, there are still some knowledge gaps around the impacts of large scale sites on the marine environment.
As the review suggests, the management responses to marine farm developments should take into account information about other sources of environmental risk to estuarine systems at a bay-wide or regional scale, so that the effects of aquaculture are placed in context. Such an approach can be applied in defined regions or across multiple regions, providing a robust basis for developing plans for research and prioritising management according to the greatest sources of risk.
