Effects of stock-specific and environmental factors on the feeding migration of Atlantic salmon (Salmo salar) in the Baltic Sea

Abstract

Atlantic salmon (Salmo salar L.) stocks in the Baltic Sea are mainly exploited during their sea migration. The offshore fishery in the feeding grounds of these salmon permitted us to analyse the migrations of certain stocks on the basis of tag recovery data. Four salmon stocks from rivers draining into Bothnian Bay (25°E, 65.5°N) were selected for study. During 1984-1991, about 135 000 2-year-old hatchery-reared smolts were tagged and released. We applied logit models, with the site of recovery as a multicategory response variable, to analyse the distribution of tag recoveries in the Baltic Sea feeding grounds. The results showed the combined influence of stock, prey abundance, and smolt size on the spatial marine distribution of the salmon. Although stock-specific sea migration patterns were apparent, annual environmental factors had a stronger influence on the sea migration route of salmon than did the stock factor. The salmon released as small smolts (total length [Formula: see text] 17.0 cm) were more frequently caught farther from the release site than were those released as larger smolts (>17 cm). The abundance of suitable prey (age 0+ herring) in the year of smolt release was found to be a key factor influencing the migration distance. The salmon released in the years of strong herring recruitment in the Gulf of Bothnia had a higher probability to stay in the Bothnian Sea for feeding and not to migrate farther south.

Spawning-Age Differences and their Temporal Trends in Wild and Sea-Ranched Atlantic Salmon Stocks, from Stock Mixture Data

“…Factors common to all stocks during the feeding migration include changes in the sea environment, such as the sea surface temperature, conditions in the feeding area, such as the availability of food, and the offshore and coastal fishery. This reasoning has proved correct in several previous studies related to salmon abundance, growth, and migration distance, in which synchronous trends have been related to common environmental factors [6,13,62,63]. The decrease in the proportion of old males has also been observed previous studies , as Järvi [64] reported approximately 11% of the spawners to be 4-6 SW salmon in the 1930s and 1940s, but an analysis sixty years later reported only 0.5-2% of salmon of that age [15].…”

“…The home rivers of all these stocks drain into the Bothnian Bay, the northernmost part of the Baltic Sea, and the stocks have very similar migration patterns. The fish from all these stocks undertake their feeding migration to the Baltic Main Basin, and return as spawners to their home rivers early in the summer, mainly during a few weeks in June [6,25].…”

“…The spawning environment and its selective factors differ between the wild and the sea-ranched hatchery stocks, but the conditions and feeding areas in the sea are common to all fish, so the observed differences between wild and searanched groups are assumed to be a result of either direct differences in the reproduction environment or the genetic selection it causes [6].…”

“…The homing of salmonids to their natal river has led to adaptation to local environmental conditions [2,3]. Spawners of a large river are known to be older and larger than those of a small river, as fish size is a response to the discharge of the river, and to the distance to the feeding areas in the sea [4][5][6][7].…”

The role of coastal ocean variation in spatial and temporal patterns in survival and size of coho salmon (<i>Oncorhynchus kisutch</i>)

“…In a tagging study over a period of 7 years, Kallio-Nyberg et al (1999) noted that environmental factors had a strong influence on migratory route in young Atlantic salmon and that prey abundance influenced migratory distances. Although our cluster results suggest that animals within clusters are influenced by coherent patterns of environmental variability, coho salmon may respond similarly and demonstrate interannual variability in spatial and temporal patterns of ocean utilization.…”