The Pacific oyster was discovered in large numbers along the west coast of Sweden in 2007. The mortality rate in some places during the past two winters has been 100%, but researchers at the University of Gothenburg who have studied the Pacific oyster can now say that the species copes with cold winters and is here to stay.
The Pacific oyster has proved to be tolerant of low temperatures. Large populations of the oyster remained after the harsh winter of 2009/2010. In 18 locations studied along the Bohuslän coast, the mortality rate ranged between 32% and 100%, and averaged 84%. The survival rate for the oysters increased with increasing depth. Some oyster sites consequently still have large populations of living oysters.
"Pacific oysters that were exposed to low air temperatures by low water levels or were frozen in ice died during the first winter (this could refer to the winter 2007, change to the first of the two latest winters, or something similar?), while those that were beneath the ice coped well," says the researcher Åsa Strand of the Department of Marine Ecology at the University of Gothenburg.
This year's winter (December to February) was warmer than last year, and the water level was generally higher. No major mortality can therefore be expected this winter, as those oysters that would be in the risk zone already died last winter. However, Pacific oysters born in 2010 are at great risk of dying during the winter if they have settled in shallow water. Strand and her colleagues will therefore study morality after this year's winter at the end of May and beginning of June. The fact that the Pacific oyster survived the winter of 2009/2010 means that it is, in all probability, here to stay.
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