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11-08-2010, 02:52 PM #1
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The status of wild and hatchery propagated Swedish salmon stocks after 40 years of ha
By T. Eriksson, L.-O. Eriksson
Fisheries Research, Volume 18, Issues 1-2, Biological Interactions of Natural and Enhanced Stocks of Salmon, October 1993, Pages 147-159, ISSN 0165-7836, DOI: 10.1016/0165-7836(93)90045-9.
The Baltic stocks of Atlantic salmon have been subjected to human exploitation for many centuries. During the last century, however, these salmon populations have been subjected to especially dramatic changes in spawning stream access, fishing patterns, and fishing pressure. At the end of the nineteenth century, salmon spawned in 60-70 rivers in the Baltic proper. From 1940 onwards many of the large salmon rivers were dammed, most of them below the lowest spawning rapids for salmon. Methods for rearing salmon up to the smolt stage were developed, and the first smolt releases started around 1950. The number of smolts released in Swedish rivers gradually increased to about 2-2.5 million by the middle of the 1980s. During the last 10 years other countries have started salmon release programs, resulting in an annual release of about 3 million smolts. Smolt releases have been successful, showing high survival rates of stocked fish. Today, only about 20 suitable rivers, most of them in Sweden, are available for natural spawning runs. The runs are very weak.
Before the Second World War the Baltic salmon fishery mainly consisted of coastal and river catches of spawning migrators ascending the rivers. However, since the end of the war an offshore drift gillnet fishery has been developed, and nowadays makes up the major part of the total catch. As a consequence, the spawning runs of wild salmon have decreased dramatically. One hundred years ago the natural production of salmon from the Baltic salmon rivers was around 7-8 million smolts annually. In the early 1970s, the wild smolt run to the Baltic included about 2 million fish. At present, only about 0.5 million wild smolts are recruited to the whole Baltic proper, which is only about 20% of the smolt production capacity still remaining in the rivers.
The offshore fishery on mixed populations in the central areas of the Baltic proper leads to an extremely high fishing pressure on the Baltic salmon stocks. Wild stocks are unable to cope with the present exploitation rate. As a result, the number of wild spawners in natural streams has gradually been diminishing. In the absence of a compensatory program, a reduction in stock-size of the magnitude shown by wild salmon in the Baltic would decrease catch-per-unit-effort to such a degree that there would be no economic incentive for a commercial salmon fishery. At present, however, about 90% of the salmon smolts leaving Baltic rivers are of hatchery origin. Owing to their extremely high egg-to-smolt survival rates the reared stocks can withstand this high exploitation. Thus the success of hatchery enhancement programs in combination with a lack of effective regulations, allowing a high-catch-per unit effort in the offshore fishery, is largely responsible for the sad fate of remaining wild salmon stocks in the Baltic.
Any biologically sound management program for the Baltic salmon would have to include regulations prohibiting or severely restricting the offshore fishery on mixed stocks. Salmon stocks should be exploited in relation to the capacity of individual populations, by a more terminal fishery at the coast or in rivers. However, an established offshore fishery in the main Baltic, including (at least) seven nations, has so far made any kind of regulation difficult.