(Stuff).- The long, hot summer may have benefited fruit growers around the Nelson region but it has been bad news for another key primary producer with the warm conditions reducing production in many of the marine farms in the top of the South Island.

Mussel spat catching sites in Golden Bay have been hit hardest with one of the biggest company's experiencing a 90 percent drop in catch rates so far this season. Mussel and salmon growth rates are also down with Sealord aquaculture and New Zealand King Salmon both reporting a decline in production.

Managing director of the Golden Bay-based Waitapu Fishing Company, Winnie Rountree said Tuesday morning that its mussel spat catch was down by 90 percent and it had been the worst season in "at least the last decade". Waitapu owns and manages seven marine farms in Golden Bay and Tasman Bay including key spat catching sites at the Ring Road, offshore from Port Tarakohe, and at Wainui Bay.

Mr Rountree attributed the decrease to the warmer water temperatures that were unsuitable for mussels. The warm water affected both mussels' spat production and growth rates, he said.

"The only difference has been the water temperature - it's like bath water. It's been up to 22 degrees at the height of summer and mussels die at 26 degrees. Ideally it should be around 19 or 20 degrees."

Mr Rountree said the company's Ring Road spat catching site had closed for the season. Although Waitapu had a good supply of spat left from last season, it was now relying on the Wainui site to supply sufficient spat for next season's farming operations.

New Zealand Marine Farming Association chief executive Graeme Coates said there would still be enough spat for farmers to seed their lines with 75 percent of the spat for the country's mussel farms coming from Kaitaia Beach. Only about 20 percent came from Golden Bay and Tasman Bay and 5 percent from the Marlborough Sounds.

Mussel growth rates had also been affected with Sealord aquaculture manager Dorje Strang saying the company had seen a significant drop in mussel yield compared with the last four years.

"These yields have been two to three percent lower than normal," he said.

Mr Strang said mussel farmers were like any other farmers in that seasonal variation impacted year to year.

"It's about dealing with it when it happens in a way that mitigates the effect and then learning and planning ahead so that farm production is not knocked for a six in the next period of climatic adversity."

Salmon farms have also been affected with New Zealand King Salmon sea farms manager Mark Preece saying warmer water temperatures reduced the growth rates of the salmon and increased their stress levels. However, the company protected itself from such seasons by planning for the worst possible summer and ensuring it had enough 4kg fish for harvest, he said.

National Institute of Water and Atmospheric Research scientist John Zeldis confirmed the warm La Nina summer and low winter rainfall caused poor growth rates in mussels. Dr Zeldis said La Nina caused more easterlies and El Nino more westerlies. The westerlies created higher nutrient water by promoting up-welling at Kahurangi and the western strait, and driving the enriched water through Cook Strait.

Source: http://www.stuff.co.nz