Seafood to Avoid Oysters - Bluff/Nelson Scientific name: Ostrea chilensis Other names: tio, tio para, tio repe (Maori), Bluff oyster, Foveaux Strait Oyster, Nelson Oyster, flat oyster Ranking: E (Red - Worst Choice)

Icon Explanations

The main concerns with this fishery are: the habitat damage caused by bottom dredging (especially from the use of heavy box dredgers) has removed bryozoan reef communities dominated by Cinctopora elegans from much of Foveaux Strait. This has exposed much biogenic material to movement by storm and tide action. There is a bycatch of a range of species and impacts on blue cod stocks. The fishery also has a substantial impact on ecological systems, exemplified in Tasman Bay and Golden Bay by a decline in fish stocks. There is also no management plan.

The fishery assessment plenary report states: "While uncertainty exists in levels of future recruitment and continued [Bonmia] related mortality, projections [for Foveaux Strait] indicate that current catch levels are unlikely to have a significant impact on future stock levels. Instead future disease mortality will determine future stock status." For Nelson/Marlborough stocks it states: "It is not known if recent catch levels or the current catch limit is sustainable or will allow the stock to move towards a size that will support the MSY [maximum sustainable yield]." (MFish, 2007, p164-165, 216)

Market: New Zealand for Foveaux Strait oysters, with some Nelson/Marlborough oysters exported (up to a value of $10 million per year).

Status and sustainable yield
Status: Uncertain after disease killed off two-thirds of the Foveaux Strait population in the designated commercial fishery in 1999.
Annual catch limit: Set at 2,021 tonnes in 2007-08.
Recorded catch: Reported landings of 790 tonnes in 2006 in Foveaux Strait and 170.9 tonnes in Tasman Bay and Golden Bay in 2005-06. In Foveaux Strait current catches are less than 10 percent of catches in the 1980s.
Population size: In 2006 the recruited stock size in Foveaux Strait was estimated at about 21% Bo which is up from 10% in 1992. The biomass in Tasman Bay and Golden Bay has declined by 60 percent since 1998.
Stock trends:Uncertain in Foveaux Strait with biomass is projected to be stable or increase, while decreaseed in Tasman and Golden bays.
Fishing method
Method: Bottom dredging with a heavy metal dredger.
Habitat damage: Dredging in effect bulldozes the sea floor with the loss of bryozoan reefs from large areas of Foveaux Strait.
Bycatch: In an experiment, bottom dredging killed 19-36% of small oysters (Cranfield et al 1999). Bryozoan reefs are also important settlement areas for oysters.
Ecological effects: A disease (Bonamia) has repeatedly infected oysters in the Foveaux Strait fishery in recent years causing large-scale mortality. This is likely to have been exacerbated by the impacts of dredging, which has highly modified the bottom community and stresses oysters. Blue cod population and dredge oyster numbers increased in areas where dredging stopped and benthic habitat regenerated (Cranfield et al 2001).

Management
Stock assessment: Quantitative stock assessment based on 2007 survey in Foveaux Strait and 2005 in Nelson-Marlborough.
Management plan: No.
Quota Management Species: Yes, since 1996 for Nelson/Marlborough and 1998 for Foveaux Strait.

Biology
Distribution: Widespread around New Zealand with commercial fisheries in Foveaux Strait and Tasman Bay. They are also caught as bycatch in the Chatham Islands scallop fishery.
Maximum age (years): 33+
Age at sexual maturity: 3-4
Growth rate: Moderate.
Reproductive output: High to very high.
Age exploited: 4-8.

References:Cranfield H J, Michael K P and Doonan I J, 1999 Changes in the distribution of epifaunal reefs and oysters during 130 years of dredging for oysters in Foveaux Strait, southern New Zealand. Aquatic Conservation: Marine and Freshwater Ecosystems 9, 461-483. Report from the Fishery Assessment Plenary, May 2007: stock assessments and yield estimates. Part 1: Albacore to Moonfish, Sullivan et al (comp), Science Group, Ministry of Fisheries; Guidebook to New Zealand Commercial Fish Species, Revised Edition 1990, NZ Fishing Industry Board, Ministry of Fisheries 2004. H J Cranfield, G Carbines, K P Michael, A Dunn, D R Stotter, D J Smith (2001) Promising signs of regeneration of blue cod and oyster habitat changed by dredging in Foveaux Strait, southern New Zealand. NZ J of Marine and Freshwater Research, 2001, Vol. 35.

This page was updated on 10 November, 2007