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Summary 0[ Density!dependent factors appeared important for the survival of juvenile Atlantic salmon in the River Imsa whilst density!independent factors were more important for the older _sh at sea[ In fresh water\ density dependence was indicated by a stock−recruitment relationship with increasing loss!rates from eggs to smolts and from eggs to adults as egg density increased[ 62) of the loss!rates were explained by variation in egg density[ At sea\ density independence was indicated by the lack of a signi_cant relationship between loss!rates and smolt densities[ 1[ The relationship between smolt density and initial egg density was best described by an asymptotic {Cushing| type relationship with a plateau at densities higher than approximately 59 999 eggs for the total river areas of 09

Friedland, K. D., MacLean, J. C., Hansen, L. P., Peyronnet, A. J., Karlsson, L., Reddin, D. G., Ó Maoiléidigh, N., and McCarthy, J. L. 2009. The recruitment of Atlantic salmon in Europe. – ICES Journal of Marine Science, 66: 289–304. The stock complex of Atlantic salmon, Salmo salar, in Europe has experienced a multidecadal decline in recruitment, resulting in the lowest stock abundances observed since 1970. Here, physical forcing, biological interactions, and the resultant growth response of post-smolt salmon are examined with a view to understanding the mechanism controlling recruitment. Sea surface temperature (SST) has increased in the Northeast Atlantic, with the pattern and seasonal change in SST negatively correlated with post-smolt survival during summer and in a region that spatially matches the post-smolt nursery. Constituents of the pelagic foodweb, including potential post-smolt food and plankton that may affect post-smolt forage, have changed on a decadal scale and correlate with salmon survival. Retrospective growth analyses of eight stock/sea age components show that post-smolt growth during summer is positively correlated with salmon survival and recruitment. The Atlantic Multidecadal Oscillation appears to be a more closely aligned climate forcing index than the North Atlantic Oscillation with respect to salmon recruitment. European Atlantic salmon recruitment appears to be governed by factors that affect the growth of post-smolts during their first summer at sea, including SST and forage abundances; growth appears to mediate survival by the functional relationship between post-smolts and their predators.

Mean estimated straying rate for Atlantic salmon Salmo salar L. leaving the River Imsa as smolts during 1976–1999 was 15% for hatchery fish and 6% for wild conspecifics. Hatchery Atlantic salmon selected for production traits during four or more generations strayed >50%. The straying rate was higher for Atlantic salmon staying 2 rather than 1 year at sea before attaining maturity. For spawning, 96% of the strays entered streams within 420 km from the River Imsa, and c. 80% entered streams within 60 km of the mouth of the River Imsa, whether the fish were wild or hatchery released. Within the 60 km zone, the number of strays caught in a river increased with the Atlantic salmon catch in that river, but there was no significant relationship between straying rate and water discharge or distance from the river to the River Imsa. The observed straying rate of hatchery Atlantic salmon decreased with increasing number of fish entering the River Imsa. Sexual maturation as parr did not influence the tendency to stray. The results suggest that the establishment of temporary zones, free of fish farms, outside important Atlantic salmon rivers by the fisheries authorities in Norway should be large, whole fjords, to be effective.

Summary 1.Catches of Atlantic salmon Salmo salar L. decreased in the 1980s and 1990s over its entire area in the North Atlantic and smolts were often released for stock enhancement. However, there are questions about their survival and performance relative to fully wild fish. This paper reports on the survival and sea growth of River Imsa salmon released from 1981 to 1999 as 1-and 2-year-old hatchery and wild smolts. 2. Survival was significantly higher for wild than hatchery fish. Hatchery salmon released as 2-year-old smolts had lower survival, were captured more in coastal than freshwaters, grew more slowly and attained maturity younger than corresponding 1-year-old smolts. 3. The survival rate of hatchery fish released as 2-year-old smolts, but not 1-year-olds and wild smolts, decreased during the 1980s and 1990s. Growth rates at sea, adult size and the proportion of multi-sea-winter fish of all three groups also decreased over time. 4. Catches in coastal relative to freshwaters were higher for two-than one-sea-winter fish. Salmon captured in coastal water were greater in length than those captured in rivers. Mean specific growth rate at sea was similar for wild and hatchery salmon released as 1-year-old smolts, and higher than in hatchery fish released as 2-year-olds. 5. The proportion of two-sea-winter salmon correlated positively with the specific growth rate in the first year at sea. Total capture of wild adult salmon in rivers and Norwegian home waters each year correlated positively with the specific growth rate in the first year at sea. The same correlation held for hatchery fish released as 2-but not 1-yearold smolts. 6. Synthesis and applications. The coastal fishery was size-selective in reducing the size and age of salmon. Releases of 1-year-old smolts were financially more profitable than those of 2-year-olds. Decreasing production of River Imsa salmon since 1981 was chiefly caused by reduced sea-age at maturity and growth rate at sea of both hatchery and wild fish. A counteracting measure would be to reduce the size selectivity of the salmon fisheries.

Energetic expenditure during spawning of male and female 1 -sea-winter Atlantic salmon, Sulmo sular L., was measured. Before spawning, the somatic energy content per unit weight did not differ between the sexes. The testescontent offat wasO.24, ofcarbohydrate8.89, ofwater 1.21 and of ash 1.61 times as high as that of the ovaries. Just prior to spawning, mean gonadosomatic index (GSI; wet weight, YO) in males was 4.36 and in females 20.26, and expressed as energy ratios (kJ, YO) 4.47 and 27.75, respectively. During spawning the energy loss of male soma was higher (35.57%) than that of females (2540%). This was a result of higher loss of fat in males than in females. Total energy cost of spawning in males and females was on average 514 and 51.4%, respectively.