Challenging conditions for Bluff oyster harvesters have seen some operators pull up anchor early for the season.

While the industry is notoriously cyclical, this season, which usually runs until the end of August, has been especially bad, operators say.

The weather, a bad year for an endemic virus and other environmental factors have hit the fishery hard, Graeme Wright of Barnes Oysters and operations manager of the Bluff Oyster Management Company said.

Out of the four months Barnes has been running boats, it has only fished 42 percent of those days because of the weather, Wright told RNZ's Nine to Noon.

"We've had some extended periods of up to 10 days in a row when the boats haven't been able to leave the wharf. So, the weather has been a major player."

The parasite Bonamia exitiosa, endemic in the fishery, has caused an increased level of mortality this season, he said.

"We're talking up to 20 percent mortality rates. So, that and the weather and for whatever reason overall the quality is not as good as we would normally expect to see."

Other fisheries in the area are also seeing a decline in quality, he said.

"Things like mutton birds have had really poor quality, blue cod is battling a bit, snapper hasn't been good quality. So, there's some environmental stuff that's happening out there we're just having to live with."

The prevalence of Bonamia had doubled this year, Wright said.

While a lack of nutrition of nutrition has impacted the wider fishery in the Foveaux Strait, Oysters are particularly vulnerable, he said.

"Fish can swim away if they don't like the environment, they can move somewhere else, oysters can't obviously move themselves."

And bad weather can also make it difficult for oysters to feed.

"They've got to have the conditions, no big storm events continually covering them over and making it difficult for them to feed."

The season ends 31 August, and Barnes intends to fish through, he said.

"Certainly, with the major delays with the weather all being equal we will stick around until the end of August this year."

Reproduced with permission