Report: TR2008/38
Author: Shane Kelly (Coast and Catchment Ltd.)

Abstract

Local and international literature was reviewed to identify the potential environmental effects of fish farming in the Firth of Thames (FoT), and the results of FoT-specific studies were summarized.  The key conclusions of this review were:

• Marine farms provide habitat for invasive species and the movement of farm stock and equipment provides a pathway for their transfer within and between regions.  Nutrients released from fish farms are likely to exacerbate the growth of some invasives already in the Firth of Thames, such as the Asian kelp Undaria pinifitada, and potentially increase their spread.  The potential consequences of invasive species could be very significant, and their scale of impact could extend well beyond the farm area.
• Interbreeding between farmed and wild stock has the potential to alter the genetic make-up of wild fish stocks, if: farmed fish are selectively bred; are grown to maturity; and/or have high escape rates.  The potential for genetic effects is also influenced by the size of the wild population and natural immigration rates.  Genetic impacts can be minimized by preventing fish escapes, using sterile fish or harvesting before maturity, avoiding selective breeding and maintaining large, natural populations of wild fish.
• Fish farming uses significant quantities of fishmeal, which is produced from fish obtained by wild capture.  Rapid growth in the fish farm industry has increased the demand for fishmeal and led to global concern about the sustainability of fish stocks used in its production.  Currently, all fishmeal used in New Zealand is sourced from overseas.
• There is a high probability that the deposition of waste food, faeces and chemical contaminants will led to degradation of the seabed directly beneath fish farms, and for a relatively small distance beyond (up to several hundred meters).  Benthic ecosystems are likely to be heavily impacted within the immediate deposition zone, but the level of impact will reduce toward the margin of the depositional footprint.
• The Firth of Thames currently receives relatively high nutrient loads from its river systems.  Nutrients released from fish food and metabolic wastes would add to the overall nitrogen budget of the Firth.  The influence of this could range from insignificant to significant relative to Firth-wide nitrogen-ecosystem processes, depending on scale of fish production and to a lesser extent fish-food conversion rates.  Local effects are likely to be greater than Firth-wide effects.
• Mussel culture has the potential to offset some nutrient effects.  At full production, Areas A and B in Wilson Bay, plus other mussel farms in the Firth could theoretically offset nitrogen released from 2900 tonnes of fish production.  In practice, the level of direct offsetting is likely to be less than this, because all of these mussels would have to be located in the area(s) directly influenced by farm nutrients.
• Infections of parasites and disease agents may be amplified within sea cages, but actual disease is only likely to occur in the cultured fishes.  This is because the mobility of the wild fishes tends to prevent hyperinfections from occurring, eliminating a necessary prerequisite for disease.  However, infection rates may increase slightly in wild fishes that have an association with the area surrounding sea cages. A high concentration of fish farms can act as a reservoir of parasites, such as sea lice and infect wild populations.
• The value of the southern Firth of Thames to waders is recognised through the designation of Ramsar status to intertidal areas.  135 bird species have been identified in the Ramsar site and around 35,000 waders use the southern Firth each year.  The only potential link between fish farms in Wilson Bay and waders in the Ramsar site appears to be via an indirect response to changes in food abundance or habitat modification, caused by nutrient enrichment.  However, it is unlikely that such indirect effects will have a significant impact on the Ramsar site.
• Fish farms can positively affect seabirds through the provision of new roosting sites and by attracting fish.  Conversely, they can negatively affect seabirds through entanglement, disturbance and loss of habitat.  However, the footprint of fish farms on seabird habitat would be very small, so any effects are likely to be minor.
• Fish farms can affect marine mammals through entanglement, habitat exclusion, and disturbance by vessel strikes and underwater noise.  However, available information suggests that the adoption of good farm management praccices should minimize the risk of these impacts actually occurring.
• Wild fish can be attracted to fish farms and this may have a beneficial effect on wild fish stocks if the area is protected from intensive fishing, or improve the recreational fishing resource if the area is left unprotected.
• Fish farms can also alter waves and current flows, attract wild fish and promote the settlement and growth of non-resident native species.  The (additional) impacts of these issues are considered to be relatively minor.

Potential Environmental Effects Associated with the Proposed Shift from Mussel to Finfish Farming in the Firth of Thames [PDF, 204 KB]