The phenotypic divergence between natural salmon stocks and the corresponding hatchery-reared sto... more The phenotypic divergence between natural salmon stocks and the corresponding hatchery-reared stocks was investigated in two Atlantic salmon (Salmo salar) populations from the Tornionjoki and Simojoki Rivers, where hatchery reared smolts of local origin were released during the second half of the last century. The reared salmon were larger as smolts and also heavier at sea than wild salmon in both stocks. The recapture rate was highest for wild Tornionjoki salmon (mean 2.8%) and lowest for reared Simojoki salmon (mean 1.2%). During 1999-2008, survival was higher in wild than reared fish, while the overall survival of tagged smolts declined. Survival was close to similar in Tornionjoki and Simojoki salmon when the year, background (wild or reared) and smolt length were taken into account. Migration distances differed between wild and reared stocks. In the second winter at sea, the majority of wild salmon (94-96%) were captured in the southern part of the Baltic Sea, while the reared salmon, especially those of the Simojoki stock, remained closer to their native river in the northern parts of the Baltic. Rearing also decreased the duration of the sea migration before the first maturation in Simojoki salmon, but not in Tornionjoki salmon. Of the wild Simojoki salmon, 64% returned to their home river as multisea-winter (MSW) fish compared to 40% of the reared fish. The population and rearing background, but not smolt length, explained the probability of maturing as one-sea-winter instead of MSW fish. Rearing produced 1.60 times more grilse than occurred among wild salmon. The reared salmon showed different life-history patterns from wild fish, and especially in the Simojoki River, the salmon stock as a whole presently deviates from previously documented wild rivers. Even though population sizes appear to be recovering, it may take time before the stocks regain their adaptive potential following the period of collapse and supportive releases.
The recapture rate and survival of hatchery-reared Atlantic salmon Salmo salar stocked as 1 year-... more The recapture rate and survival of hatchery-reared Atlantic salmon Salmo salar stocked as 1 year-old parr (semi-wild) with that of hatchery-reared Atlantic salmon stocked as 2 year-old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986-1988 and 1992. The recapture rate of the semi-wild groups varied from 1Á0 to 13Á1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1Á7-17Á0%). The recapture rate of the semi-wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery-reared groups stocked as smolts (1Á3-6Á3%). The survival of semi-wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery-reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery-reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.
The survival of Atlantic salmon Salmo salar in the Baltic Sea was examined in relation to smolt t... more The survival of Atlantic salmon Salmo salar in the Baltic Sea was examined in relation to smolt traits (length and origin) and annual environmental factors [sea surface temperature (SST) and seasonal North Atlantic Oscillation (NAO) index], and prey fish abundance (herring Clupea harengus and sprat Sprattus sprattus) in the main basin and the southern Gulf of Bothnia. The study was based on recapture data for Carlin-tagged hatchery-reared and wild smolts from the Simojoki, a river flowing into the northern Gulf of Bothnia. The survival of the wild and reared groups was analysed using an ANOVA model and a stepwise regression model, with the arcsintransformed proportion of recaptured fish as the response variable. The results demonstrated a combined influence of smolt traits and environmental factors on survival. For the reared Atlantic salmon released in 1986-1998 (28 groups), the increasing annual mean SST in July in the southern Gulf of Bothnia and increasing mean smolt size improved survival. If the SST in July was excluded from the model, the NAO index in May to July also had a positive effect on survival (P < 0Á10). The log 10 -transformed abundance of 0þ year herring in the southern Gulf of Bothnia entered the model (P < 0Á15) if the SST and NAO index were excluded. For the wild Atlantic salmon released in 1972-1993 (21 groups), only the increasing SST in July showed a significant association with improved survival (P ¼ 0Á004). Prey fish abundance in the main basin of the Baltic Sea had no influence on the survival of reared or wild smolt groups. The interaction between smolt size and the SST in July was not significant. The origin was a better, but not a significant, predictor of marine survival compared to the smolt size or the SST in July. The mean recapture rate of the wild groups was twice that of the reared groups in the whole data. The results suggest that cold summers in the Gulf of Bothnia reduce the survival of young Atlantic salmon in both wild and reared groups. The larger smolt size of the reared groups compared with the wild groups to some extent compensated for their lower ability to live in the wild. # 2004 The Fisheries Society of the British Isles
The migration pattern of Atlantic salmon (Salmo salar L.) post-smolts in the Baltic Sea was inves... more The migration pattern of Atlantic salmon (Salmo salar L.) post-smolts in the Baltic Sea was investigated based on tag recoveries of Carlin-tagged wild and hatchery-reared smolts released in the Simojoki river in 1972-2005. Exact date of sea entry was known only for the wild smolts. Tag recoveries of wild salmon in the estuary within 10 km from the river mouth were received on average 3.5 days (±2.0 SD) after release. Time required for emigration from the estuary was dependent on the sea surface temperature (SST) off the river (R 2 = 0.625, P = 0.004), being shorter in years with warmer than colder sea temperatures. Outside the estuary, the wild and hatcheryreared post-smolts migrated southwards along the eastern coast of the northern Gulf of Bothnia, the tag recoveries coinciding with the warm thermal zone in the SST occurring along the coastal area. After arriving in the southern Gulf of Bothnia in late summer the post-smolts mostly migrated near the western coast, reaching the Baltic main basin in late autumn. The relationships between the marine conditions and migration patterns are discussed.
The recapture rate of Atlantic salmon (Salmo salar L.) after river ascent was examined by the tra... more The recapture rate of Atlantic salmon (Salmo salar L.) after river ascent was examined by the trapping and tagging of ascending spawners in the lower reaches of the Simojoki River, which flows into the northern Baltic Sea. In 1997 and 1998, altogether 825 Carlin-tagged salmon were released to continue their upstream migration. Of these, 800 could be sexed and categorized as reared (91%) or wild (9%) salmon. In 1997, most of the ascending salmon were multi-sea-winter (MSW) fish, whereas in 1998 almost all were one-sea-winter (1SW) male grilse due to the late trapping season. About 10% of all tagged fish were recaptured, two-thirds of which were caught in the river before their descent to the sea. There was no difference in the recapture rate between salmon of wild (8.5%) or reared (9.5%) origin, or between females (11.6%) and males (9.3%). Generalized linear models for data from 1997 showed that the recapture rate increased with length and age of females, but that the opposite was true for males. River fishing did not seem to remove proportionally more early ascending salmon than fish that ascended later.
... Abstract. The effect of origin, smolt size and year of release on the sea migration pattern o... more ... Abstract. The effect of origin, smolt size and year of release on the sea migration pattern of Atlantic salmon (Salmo salar L.) in the Baltic Sea was examined by tagging experiments conducted in 19911993 on wild and reared smolts of the Simojoki river salmon stock. ...
The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta)... more The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) was examined in relation to marine conditions during post-smolt migration and in relation to stock traits. In 1970-2001, Carlin-tagged smolts were released in the Iijoki and Oulujoki rivers, the northern Baltic Sea. When both species were analysed together, the abundance of the three prey fish, herring (Clupea harengus), smelt (Osmerus eperlanus) and vendace (Coregonus albula) correlated positively with the survival of salmonids. In addition, the increase in smolt size appeared to improve the survival rate. Sea surface temperature (SST) may have affected indirectly through the abundance of prey fish during the post-smolt migration of salmon and sea trout. The smelt and vendace showed a statistical effect on survival only when the temperature effects were not included in the models. In sea trout, an increasing smolt length was not significantly correlated with the survival in good herring recruitment years, but in poor years survival increased very rapidly with increasing smolt size. The recapture rates of the salmonids tended to decrease between the years 1970 and 2001. During the same time period, the June SST slightly decreased. The positive correlation between the annual summer SST and recapture rate of salmon may partly explain the decreasing trend in recapture rates. An increase in smolt size did not compensate for the decline in the recapture rate of either species.
The endangered Baltic salmon (Salmo salar) stock in the Simojoki river has mainly been managed by... more The endangered Baltic salmon (Salmo salar) stock in the Simojoki river has mainly been managed by stocking hatchery-reared smolts, but parr were also used between the years 1986 and 2007 together with tagged wild smolts. The parr were semi-wild compared to hatchery-reared smolts, as they experienced the river conditions for at least a year before descent to the sea. We studied year-and environment-related changes in the life-history traits of these three groups differing in their source. The salmon originating from reared smolts had a larger smolt size compared to semi-wild or wild salmon. Wild salmon caught during the second winter were smaller than fish in the other source groups, but at greater sea ages the three groups did not differ in size. The annual survival of the wild salmon was higher than that of the reared source groups. Wild salmon were captured more often in the Main Basin, far from their home river, as opposed to semi-wild or reared salmon that remained near the home river in the Bothnian Sea. A higher proportion of wild salmon were multi-sea-winter fish on the return migration (57%) compared to reared and semi-wild salmon (32%). Low survival was associated with a low recruitment of young herring (Clupea harengus membras) and the mean catch weight of salmon in all source groups, but the reasons for the long-term decreasing trend in survival remain unclear.
In the Simojoki River in the northern Gulf of Bothnia, reared salmon stocked as smolts produced c... more In the Simojoki River in the northern Gulf of Bothnia, reared salmon stocked as smolts produced considerable numbers of ascending one-sea-winter (1 SW) males, whereas the proportion of male 1 SW salmon was low among spawning migrants of wild or reared parr origin. The sex ratio among ascending wild fish and reared salmon stocked as parr was similar, with females predominating, while reared salmon stocked as smolts were mainly males. The multi-sea-winter (MSW) salmon entered the river annually within a fairly short time period from the beginning of the migration season, independent of their sex or origin. 1 SW males migrated into the river significantly later in the season than MSW males. The results indicate that the delayed opening of the fishing season in the Gulf of Bothnia is effective in reducing the harvest of MSW salmon at sea. However, as the timing of the ascent may vary by several weeks from year to year, the effect of this regulation bound to certain calendar days may also vary considerably from year to year.
The phenotypic divergence between natural salmon stocks and the corresponding hatchery-reared sto... more The phenotypic divergence between natural salmon stocks and the corresponding hatchery-reared stocks was investigated in two Atlantic salmon (Salmo salar) populations from the Tornionjoki and Simojoki Rivers, where hatchery reared smolts of local origin were released during the second half of the last century. The reared salmon were larger as smolts and also heavier at sea than wild salmon in both stocks. The recapture rate was highest for wild Tornionjoki salmon (mean 2.8%) and lowest for reared Simojoki salmon (mean 1.2%). During 1999-2008, survival was higher in wild than reared fish, while the overall survival of tagged smolts declined. Survival was close to similar in Tornionjoki and Simojoki salmon when the year, background (wild or reared) and smolt length were taken into account. Migration distances differed between wild and reared stocks. In the second winter at sea, the majority of wild salmon (94-96%) were captured in the southern part of the Baltic Sea, while the reared salmon, especially those of the Simojoki stock, remained closer to their native river in the northern parts of the Baltic. Rearing also decreased the duration of the sea migration before the first maturation in Simojoki salmon, but not in Tornionjoki salmon. Of the wild Simojoki salmon, 64% returned to their home river as multisea-winter (MSW) fish compared to 40% of the reared fish. The population and rearing background, but not smolt length, explained the probability of maturing as one-sea-winter instead of MSW fish. Rearing produced 1.60 times more grilse than occurred among wild salmon. The reared salmon showed different life-history patterns from wild fish, and especially in the Simojoki River, the salmon stock as a whole presently deviates from previously documented wild rivers. Even though population sizes appear to be recovering, it may take time before the stocks regain their adaptive potential following the period of collapse and supportive releases.
The recapture rate and survival of hatchery-reared Atlantic salmon Salmo salar stocked as 1 year-... more The recapture rate and survival of hatchery-reared Atlantic salmon Salmo salar stocked as 1 year-old parr (semi-wild) with that of hatchery-reared Atlantic salmon stocked as 2 year-old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986-1988 and 1992. The recapture rate of the semi-wild groups varied from 1Á0 to 13Á1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1Á7-17Á0%). The recapture rate of the semi-wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery-reared groups stocked as smolts (1Á3-6Á3%). The survival of semi-wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery-reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery-reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.
The survival of Atlantic salmon Salmo salar in the Baltic Sea was examined in relation to smolt t... more The survival of Atlantic salmon Salmo salar in the Baltic Sea was examined in relation to smolt traits (length and origin) and annual environmental factors [sea surface temperature (SST) and seasonal North Atlantic Oscillation (NAO) index], and prey fish abundance (herring Clupea harengus and sprat Sprattus sprattus) in the main basin and the southern Gulf of Bothnia. The study was based on recapture data for Carlin-tagged hatchery-reared and wild smolts from the Simojoki, a river flowing into the northern Gulf of Bothnia. The survival of the wild and reared groups was analysed using an ANOVA model and a stepwise regression model, with the arcsintransformed proportion of recaptured fish as the response variable. The results demonstrated a combined influence of smolt traits and environmental factors on survival. For the reared Atlantic salmon released in 1986-1998 (28 groups), the increasing annual mean SST in July in the southern Gulf of Bothnia and increasing mean smolt size improved survival. If the SST in July was excluded from the model, the NAO index in May to July also had a positive effect on survival (P < 0Á10). The log 10 -transformed abundance of 0þ year herring in the southern Gulf of Bothnia entered the model (P < 0Á15) if the SST and NAO index were excluded. For the wild Atlantic salmon released in 1972-1993 (21 groups), only the increasing SST in July showed a significant association with improved survival (P ¼ 0Á004). Prey fish abundance in the main basin of the Baltic Sea had no influence on the survival of reared or wild smolt groups. The interaction between smolt size and the SST in July was not significant. The origin was a better, but not a significant, predictor of marine survival compared to the smolt size or the SST in July. The mean recapture rate of the wild groups was twice that of the reared groups in the whole data. The results suggest that cold summers in the Gulf of Bothnia reduce the survival of young Atlantic salmon in both wild and reared groups. The larger smolt size of the reared groups compared with the wild groups to some extent compensated for their lower ability to live in the wild. # 2004 The Fisheries Society of the British Isles
The migration pattern of Atlantic salmon (Salmo salar L.) post-smolts in the Baltic Sea was inves... more The migration pattern of Atlantic salmon (Salmo salar L.) post-smolts in the Baltic Sea was investigated based on tag recoveries of Carlin-tagged wild and hatchery-reared smolts released in the Simojoki river in 1972-2005. Exact date of sea entry was known only for the wild smolts. Tag recoveries of wild salmon in the estuary within 10 km from the river mouth were received on average 3.5 days (±2.0 SD) after release. Time required for emigration from the estuary was dependent on the sea surface temperature (SST) off the river (R 2 = 0.625, P = 0.004), being shorter in years with warmer than colder sea temperatures. Outside the estuary, the wild and hatcheryreared post-smolts migrated southwards along the eastern coast of the northern Gulf of Bothnia, the tag recoveries coinciding with the warm thermal zone in the SST occurring along the coastal area. After arriving in the southern Gulf of Bothnia in late summer the post-smolts mostly migrated near the western coast, reaching the Baltic main basin in late autumn. The relationships between the marine conditions and migration patterns are discussed.
The recapture rate of Atlantic salmon (Salmo salar L.) after river ascent was examined by the tra... more The recapture rate of Atlantic salmon (Salmo salar L.) after river ascent was examined by the trapping and tagging of ascending spawners in the lower reaches of the Simojoki River, which flows into the northern Baltic Sea. In 1997 and 1998, altogether 825 Carlin-tagged salmon were released to continue their upstream migration. Of these, 800 could be sexed and categorized as reared (91%) or wild (9%) salmon. In 1997, most of the ascending salmon were multi-sea-winter (MSW) fish, whereas in 1998 almost all were one-sea-winter (1SW) male grilse due to the late trapping season. About 10% of all tagged fish were recaptured, two-thirds of which were caught in the river before their descent to the sea. There was no difference in the recapture rate between salmon of wild (8.5%) or reared (9.5%) origin, or between females (11.6%) and males (9.3%). Generalized linear models for data from 1997 showed that the recapture rate increased with length and age of females, but that the opposite was true for males. River fishing did not seem to remove proportionally more early ascending salmon than fish that ascended later.
... Abstract. The effect of origin, smolt size and year of release on the sea migration pattern o... more ... Abstract. The effect of origin, smolt size and year of release on the sea migration pattern of Atlantic salmon (Salmo salar L.) in the Baltic Sea was examined by tagging experiments conducted in 19911993 on wild and reared smolts of the Simojoki river salmon stock. ...
The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta)... more The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) was examined in relation to marine conditions during post-smolt migration and in relation to stock traits. In 1970-2001, Carlin-tagged smolts were released in the Iijoki and Oulujoki rivers, the northern Baltic Sea. When both species were analysed together, the abundance of the three prey fish, herring (Clupea harengus), smelt (Osmerus eperlanus) and vendace (Coregonus albula) correlated positively with the survival of salmonids. In addition, the increase in smolt size appeared to improve the survival rate. Sea surface temperature (SST) may have affected indirectly through the abundance of prey fish during the post-smolt migration of salmon and sea trout. The smelt and vendace showed a statistical effect on survival only when the temperature effects were not included in the models. In sea trout, an increasing smolt length was not significantly correlated with the survival in good herring recruitment years, but in poor years survival increased very rapidly with increasing smolt size. The recapture rates of the salmonids tended to decrease between the years 1970 and 2001. During the same time period, the June SST slightly decreased. The positive correlation between the annual summer SST and recapture rate of salmon may partly explain the decreasing trend in recapture rates. An increase in smolt size did not compensate for the decline in the recapture rate of either species.
The endangered Baltic salmon (Salmo salar) stock in the Simojoki river has mainly been managed by... more The endangered Baltic salmon (Salmo salar) stock in the Simojoki river has mainly been managed by stocking hatchery-reared smolts, but parr were also used between the years 1986 and 2007 together with tagged wild smolts. The parr were semi-wild compared to hatchery-reared smolts, as they experienced the river conditions for at least a year before descent to the sea. We studied year-and environment-related changes in the life-history traits of these three groups differing in their source. The salmon originating from reared smolts had a larger smolt size compared to semi-wild or wild salmon. Wild salmon caught during the second winter were smaller than fish in the other source groups, but at greater sea ages the three groups did not differ in size. The annual survival of the wild salmon was higher than that of the reared source groups. Wild salmon were captured more often in the Main Basin, far from their home river, as opposed to semi-wild or reared salmon that remained near the home river in the Bothnian Sea. A higher proportion of wild salmon were multi-sea-winter fish on the return migration (57%) compared to reared and semi-wild salmon (32%). Low survival was associated with a low recruitment of young herring (Clupea harengus membras) and the mean catch weight of salmon in all source groups, but the reasons for the long-term decreasing trend in survival remain unclear.
In the Simojoki River in the northern Gulf of Bothnia, reared salmon stocked as smolts produced c... more In the Simojoki River in the northern Gulf of Bothnia, reared salmon stocked as smolts produced considerable numbers of ascending one-sea-winter (1 SW) males, whereas the proportion of male 1 SW salmon was low among spawning migrants of wild or reared parr origin. The sex ratio among ascending wild fish and reared salmon stocked as parr was similar, with females predominating, while reared salmon stocked as smolts were mainly males. The multi-sea-winter (MSW) salmon entered the river annually within a fairly short time period from the beginning of the migration season, independent of their sex or origin. 1 SW males migrated into the river significantly later in the season than MSW males. The results indicate that the delayed opening of the fishing season in the Gulf of Bothnia is effective in reducing the harvest of MSW salmon at sea. However, as the timing of the ascent may vary by several weeks from year to year, the effect of this regulation bound to certain calendar days may also vary considerably from year to year.
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