Sanford's green mussels at its Havelock processing plant in the Marlborough Sounds. The company is set to close its Christchurch plant with the loss of more than 200 jobs.

Seafood processor Sanford has linked warmer sea to the likely closure of its mussel plant employing 232 people in Christchurch.

Chief executive Volker Kuntzsch said higher ocean temperatures had affected the growth of its farmed mussels in the Marlborough Sounds and its supply of wild, young mussel spats that it harvests from other places for future stock.

All fishing companies operating along the northern South Island coastline were finding and growing fewer mussels and it seemed to be linked to different weather, he said.

Recent El Nino and La Nina weather cycles had warmed the ocean and made the loss of mussels "much more pronounced than usual".

Normally weather-related problems did not get "worse and worse" like this, Kuntzch said.The long-term outlook for consistently high spat-growth in the wild was " a little grim", he said.

Sanford staff shucking mussels at the threatened Matipo St, Riccarton plant in Christchurch had been on unpredictable shifts of two to five days a week for "a long time", he said.

The listed company could no longer be sure whether it would be "open every single week for the next six months" and was no longer prepared to ride out this much uncertainty.

The seafood industry was seeing the effect of warmer seas, from Kingfish being found around Stewart Island to an unusual number of salmon dying at a New Zealand King Salmon farm in the Marlborough Sounds.

Sanford was asking itself whether a deeper pattern of climate change was setting in. "I don't think we have an answer to that."

If the warmer water was part of a more regular pattern it may last seven to 15 years, Kuntzsch said.

Sanford had a spread of mussel farms, including one in Bluff, at Havelock in the Marlborough Sounds and in the Coromandel area.

The company's market share in mussels was 40 per cent. Its production had fallen because of the upper South Island problems but Kuntzch expected it to soon be producing more than it had been, using less-affected bases. "What is bad here, may no be as bad in Coromandel or Bluff."

Kuntzsch expected other companies with less geographical buffer could lose more of their production, creating an "impact on supply" of 10 to 20 per cent.

The shortfall was likely to lift the export value of New Zealand mussels "but I wouldn't think it has an impact on the (retail) local price", he said.

A representative for the Service and Food Workers Union, Chas Muir, said Sanford's Havelock staff were told last week their night-shift was about to shut down because of the mussel shortage.

He said the signs were for Sanford's Christchurch facility were "ominous" once the firm's top people confirmed a site visit for early Thursday afternoon.