"Fish counters, catches ... All the reports point in the same direction," said Jaakko Erkinaro, a professor at the Finnish Game and Fisheries Research Institute.

"The figure of (a) 50 percent (drop between 2005 and 2006) is likely. Whatever the exact figure is, it's a marked decline," he told AFP, attributing the decrease to several hypotheses: a fall in salinity, warming of the waters or a rise in seal stocks.

"But it remains a big question mark," he said.

Wild salmon quotas were drastically reduced in the 1990s by the International Baltic Sea Fishery Commission, which over the years reduced them from 700,000 fish to 300,000 in 2005.

The European Union decided in October to reduce by an additional five percent the quotas for 2007.

The Baltic Sea is a shallow and virtually closed sea and therefore very polluted.

Pregnant women are, for example, advised to avoid eating wild salmon because of levels of PCBs (polychlorobiphenyls) and PCTs (polychloroterphenyls) five times higher than farmed salmon, according to the European food safety authority.

Erkinaro said salmon was not the only fish species in the Baltic Sea that was threatened.

"There are problems with other species. Cod, for example, in the southern Baltic, or river trout. The causes are varied," he said.

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