Effect of origin, sex and sea age of Atlantic salmon on their recapture rate after river ascent

Transcription

1 J. Appl. Ichthyol. 22 (2006), Ó 2006 Blackwell Verlag, Berlin ISSN Received: June 16, 2005 Accepted: September 22, 2005 doi: /j x Effect of origin, sex and sea age of Atlantic salmon on their recapture rate after river ascent By E. Jokikokko 1, I. Kallio-Nyberg 2, E. Jutila 3 and I. Saloniemi 4 1 Finnish Game and Fisheries Research Institute, Bothnian Bay Fisheries Research Station, Simo; 2 Finnish Game and Fisheries Research Institute, Quark Fisheries Research Station, Vaasa; 3 Finnish Game and Fisheries Research Institute, Helsinki; 4 Department of Biology, Section of Ecology, University of Turku, Turku, Finland Summary 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. Introduction Reduced natural populations of salmon are commonly supported or enhanced by stocking programmes in rivers flowing into the Pacific and Atlantic (Ritter, 1997; Hilborn and Eggers, 2000), in order to alleviate problems caused by overfishing and other human impact such as the damming of spawning rivers. In the Baltic Sea, for example, in recent decades about 90% of the annual smolt production of Atlantic salmon (Salmo salar L.) was of reared origin, even though the proportion of wild smolts has begun to increase considerably (Anon., 2004). Due to high fishing pressure the spawners may be selected according to age and size (Olsen et al., 2004). A prevailing approach has been to evaluate the management and success of salmon stocks mainly through the catch yield, but the survival of spawned salmon (kelts) in the river or later in the sea has not been considered to such an extent. In the Baltic Sea, a considerable proportion of male Atlantic salmon mature as one-sea-winter (1SW) grilse (Salminen, 1997; Romakkaniemi et al., 2003), but their ability to survive as kelts for a new sea migration and repeat spawning is not completely known. Repeat spawning, the ability of salmon to spawn a second or even third time within a 1 or more years interval, was earlier more common among males and females in the absence of human impact (Alm, 1934; Ja rvi, 1938, 1948). This has diversified the age structure of the spawning population with different river and sea age groups. In recent years the proportion of repeat spawners has increased as a result of fishing restrictions (Romakkaniemi et al., 2003). The intensified regulation of sea fishing since 1996 has resulted in the enhancement of salmon in many northern rivers flowing into the Baltic Sea (Romakkaniemi et al., 2003). This recovery of the salmon stocks has consequently increased fishing pressure in the rivers (e.g. Jutila et al., 2003a). This may have considerable effects on the spawning stock, because the recapture rate of ascended salmon may be high in rivers where sport fishing is popular (Erkinaro et al., 1999). Therefore, it is uncertain whether more efficient restrictions should also be implemented not only at sea but also in rivers. Ascending salmon that have reached their natal rivers are reproductively the most valuable part of the salmon stock and thus should be exploited with caution. Few studies have been concerned with the tagging of adult Atlantic salmon in order to follow their survival after spawning (Jonsson et al., 1991a). Ascending salmon have mainly been tagged with radio transmitters to study their migration and recapture rate in the river phase (e.g. Erkinaro et al., 1999; Jokikokko, 2002), or tagged to follow their sea migration before spawning (Karlsson et al., 1999). In summer 1997 and 1998, the consequences of the newly strengthened sea fishing regulations were examined by catching and Carlintagging ascending adult salmon spawners in the Simojoki River. The number of salmon in this river was minimal in earlier years, but after implementing the new regulations considerably more salmon ascended the river (Jutila et al., 2003a; Romakkaniemi et al., 2003). This allowed the comparison of recapture rate and the spatial and temporal distribution of wild and reared spawners after river ascent. These data are needed both for fishery regulation purposes and for modelling the life cycle of Atlantic salmon in the Baltic Sea area (Anon., 2004). The present study especially describes the effects of fishing, which still is the most important factor affecting the survival of adult salmon despite relieved fishing pressure in recent years (Jutila et al., 2003a). The early ascending salmon, which are often predominantly wild multi-sea-winter (MSW) females (e.g. Karlsson et al., 1999; Jokikokko et al., 2004) and thus valuable for reproduction, are exposed to a longer period of river fishing than are later migrants. Therefore, we also aimed to determine the probability of salmon capture in the river before spawning in relation to their ascent timing, to enable the consideration of possible reorganization in the river fishery. U.S. Copyright Clearance Centre Code Statement: /2006/ $15.00/0

2 490 E. Jokikokko et al. Material and methods Study area The ascending adult salmon were caught in the boreal Simojoki River (65 38 N, E), which drains into the northern part of the Gulf of Bothnia in northern Finland (Fig. 1). The river is 175 km in length with a mean discharge of 38 m 3 s )1 and a drop of 176 m. The entire river is accessible to salmon, which reproduce naturally in the river, but most spawning has been noted to take place at a distance of up to 110 km from the sea (Jokikokko and Jutila, 1998). Due to extensive salmon fishing in the Baltic Sea, the natural Atlantic salmon stock of the river declined rapidly in the 1970s; in the mid-1980s hatchery-reared salmon parr and smolts were annually stocked in the rapids to support the natural stock. Only offspring of the river s own salmon stock were released. They had been fin-clipped by removing the adipose fin, a pelvic fin, or both, depending on the age of fish at stocking, in order to distinguish them from wild fish when recaptured later in the river or at sea. In the present study, all stocked salmon were combined and analysed together due to the low number of some stocked groups among the ascending fish. Trapping and tagging The ascending salmon were trapped and tagged in 1997 and In 1997, a trap net was set in a pool about 4 km upstream from the river mouth (Fig. 1). Mesh size of the codend was 30 mm (from knot to knot), and the wings 43 and 60 mm. Because the trap net caused some disturbances and negative reaction among the fishermen (Jokikokko, 2002), in 1998 the net was installed 4 km upstream from the previous site. In both years the entire river was closed in an attempt to capture all ascending salmon, although this goal could not be completely attained (Jokikokko, 2002). In 1998, the trapping was delayed due to river flooding. Tagging was carried out in a boat attached along the codend of the trap net. The fish were taken from the codend using a dip net and placed in a container filled with fresh water. All captured salmon were tagged, excluding some fish injured in the trap. Before tagging, the fin clippings were checked, scales sampled and the length of the fish recorded. A Carlin tag was attached externally below the dorsal fin. Fish were not anaesthetized during tagging, as there was the possibility that fishermen would catch the fish soon after their release; any potential harmful effects of anaesthetic traces in fish used for human consumption were to be avoided. Despite the absence of anaesthesia the fish were calm during the tagging as well as when they were released into the river upstream of the trap. Between the 6 June and 1 August 1997, 585 Atlantic salmon, of which 561 could be sexed and their origin determined (60 wild and 501 reared salmon), were Carlin-tagged at the trap. The determination of sex was based on the external characteristics of the salmon (e.g. the length and form of the jaw and snout) as described by Jokikokko et al. (2004). In addition, 11 captured salmon (three males and eight females) had already earlier been tagged as smolts; after checking their tags they were released into the river and considered in the analysis like the other fish. Thus, 596 tagged salmon, of which 572 were of known origin and sex, were released above the trap to continue their upstream migration in 1997 (Table 1). In 1997, a part of the sexed and tagged salmon was aged by scale reading; most of them were MSW fish (Table 2). In 1998, 229 salmon, of which 10 wild and 218 reared salmon could be sexed, were tagged (Table 1). Because the trapping and tagging were carried out from 15 July to 2 August, which was late in the ascending season, the salmon were mostly 1SW fish, with only seven MSW salmon tagged (Table 2). Salmon lengths in both years were also measured. Six of the aged salmon in 1997 and one in 1998 had spawned earlier, which corresponds to 2.1% and 0.4% of the aged fish respectively. Tag returns in the river phase came from recreational fishermen and later, in the sea phase, from commercial fishing. No additional effort was put into recapturing the tagged fish. Data on tag recaptures of salmon caught in the sea and river Table 1 Total number of tagged ascending salmon in the Simojoki River in 1997 and 1998 Year Total number tagged Origin Sex Wild (n) Reared (n) Wild (%) Reared (%) Male Female Male Female Total The number of fish whose origin and sex could be determined and the proportion of wild and reared salmon are also given. Eleven reared salmon previously tagged as smolts are included in the figures for Table 2 Number of tagged salmon in sea age classes during their ascent of the Simojoki River in 1997 and 1998 Year Sex Sea age, n (length) 1SW MSW Total (n) 1997 Male 39 (53.3 ± 5.92) 100 (88.8 ± 13.52) Female 2 (52.0 ± 1.41) 148 (85.9 ± 12.44) Male 219 (57.2 ± 6.21) 6 (83.7 ± 12.02) Female 2 (60.0 ± 14.14) 1 (85.0) 3 Total 262 (56.6 ± 6.34) 255 (87.0 ± 12.88) 517 Fig. 1. Map of Simojoki River and location of trap net and tagging site in 1997 (a) and 1998 (b) Sexes are shown separately. Only fish with information on both sea age and sex are included. The mean length (cm) ± SD of fish in the each group is given. 1SW, one-sea-winter; MSW, multi-sea-winter.

3 Recapture rate of S. salar after river ascent 491 fisheries were compiled by the Tagging Office of the Finnish Game and Fisheries Research Institute. Statistical methods The recapture rate (%), which is the proportion of the tagged fish recaptured, was evaluated for sex and origin (wild or hatchery-reared). The recapture rate of salmon ascending at different times during the season was studied in 1997 by dividing the trapping season into three periods so that the number of salmon caught was approximately similar in each period, and then comparing the number of recaptures from the river before spawning. In 1998, no such division was carried out due to the short trapping season. As the river recaptures were also examined in relation to the spawning, which was not possible to check in the field, its timing was based on the stripping dates in the nearby Lautiosaari hatchery. Spawning was assumed to have finished by mid-october; fish recaptured after this time were treated as spawned salmon. Merely comparing, for example, mean lengths between the recaptured and non-recaptured fish does not acceptably describe the effect of length on the recapture probability (Saloniemi et al., 2004). Therefore, the recapture rate was also analysed by applying generalized linear models (McCullagh and Nelder, 1989) for the salmon tagged in 1997 to determine which of the recorded factors and interactions were the most important predictors of salmon recapture after their ascent of the river. We used the length of salmon together with origin, sex and sea age (1SW, MSW) at tagging as potential explanatory variables. Salmon tagged in 1998 were not included in the analysis due to biased data (mainly male grilse). Data analysis were carried out with the SAS statistical package (SAS 8.2) using the GENMOD procedure with a binomial distribution (SAS Institute, 1999). Other SAS procedures for the regression analysis and chi-squared test were also used. The significance of the statistical tests is expressed in the following with symbols: ns P > 0.1, P < 0.1, *P < 0.05, **P < 0.01 and ***P < Results River recaptures before their return to the sea The mean recapture rate of all tagged salmon was 9.8%, being 8.1% in 1997 and 14.4% in 1998 (Table 3). The proportion of salmon recaptured in the river before the spawning season, i.e. mid-october, was 55% of all recaptures among fish tagged in 1997 and 46% among fish tagged in When the tag returns after the spawning season in winter and spring were included (about 20% of river recaptures), over half (65%) of total recaptures were realized in the river before salmon descended to the sea to begin a new feeding migration. The proportion of recaptured and non-recaptured fish in the river was similar for the salmon ascending the river in three different periods in 1997 (in June, and in the early or latter part of July; v 2 ¼ ns ; Table 3). Two salmon tagged in 1997 had descended to the sea and had been recaptured there in the same autumn before the spawning season. They were included in the total recaptures and considered as sea recaptures in Table 3, but they were not utilized in the analysis due to unknown sex and origin. Recaptures by origin, sea age, sex and size of fish The total recapture rate of wild and reared salmon was similar (8.5% and 9.5%, respectively; v 2 ¼ ns ), when 1997 and 1998 were examined together. The total recapture rate of male and female salmon was also similar (9.3% and 11.6%, respectively; v 2 ¼ ns ). The recapture rate of MSW fish was higher for females (12.8%) than males (5.7%), but statistically only indicatively (v 2 ¼ 3.5 ; Table 4). All MSW males were recaptured from the river, whereas one-third of the male grilse were recaptured after their descent to the sea. One half of MSW females were recaptured in the river and the other half thereafter (Table 4). No tags were returned from the 11 salmon tagged as smolts. In 1997, the recapture rate was somewhat higher for females (10.9%) than for males (6.5%; v 2 ¼ 3.4 ). In 1997, females Table 3 The number of tagged salmon in different periods in 1997 and the number and proportion (%) of recaptures from the river before and after spawning, from the sea during the feeding migration and from the river when the salmon had returned to spawn the second time (II) Tagging time Tagged Returns, n (%) River before spawning River after spawning Sea River II Total 6 30 June (3.7) 3 (1.4) 6 (2.8) 1 (0.5) 18 (8.3) 1 15 July (4.2) 1 (0.5) 5 (2.3) 1 (0.5) 16 (7.4) 16 July to (5.5) 2 (1.2) 2 (1.2) 0 13 (7.9) 1 August 1997 Total (4.4) 6 (1.0) 13 (2.2) 2 (0.3) 47 (7.9) Total (6.6) 5 (2.2) 12 (5.2) 1 (0.4) 33 (14.4) The corresponding figures from 1998 are also given. Table 4 Number of tagged 1SW and MSW male and female salmon in 1997 and 1998 and the distribution of recaptures 1SW male MSW male 1SW female MSW female Number tagged River recaptures before descent 23 (8.9) 6 (5.7) 1 (25.0) 9 (6.0) Sea recaptures after descent 10 (3.9) (6.0) Second recapture from the river 1 (0.4) (0.7) Total recaptures, n (%) 34 (13.2) 6 (5.7) 1 (25.0) 19 (12.8) The second recapture from the river means that salmon had survived after returning to the sea and again ascended the river. The proportion (%) of recaptures of tagged fish in each group is given in parentheses. 1SW, one-sea-winter; MSW, multi-sea-winter.

4 492 E. Jokikokko et al. Table 5 Spatial and temporal distribution of recaptured salmon (tagging years and sexes are shown separately) Spatial distribution Temporal distribution Year Sex River GB MB Total Total 1997 Male Female Male Female Total, n (%) 54 (71) 12 (16) 10 (13) 76 (100) 44 (56) 20 (26) 14 (18) 78 (100) River, Simojoki River; GB, Gulf of Bothnia; MB, main basin of the Baltic Sea. Temporal distribution: 1 ¼ tagging year, 2 ¼ next year after tagging, 3 ¼ second year after tagging. Total numbers of recaptures differ slightly between spatial and temporal distribution due to imperfect data, as fishermen did not always mention the catch area or the date. Table 6 Effect of sex, origin (wild, all reared), sea age (1SW, MSW) and length of tagged salmon and interactions on the recapture rate of tagged fish after river ascent were more often (59% of female recaptures) caught after the tagging year than were males (13%; v 2 ¼ 10.8**; Table 5). Females were also recaptured more often (45% of female recaptures) at sea (the Gulf of Bothnia and the Baltic main basin) than were males (5%; v 2 ¼ 8.2*). In 1998, most of the salmon were male grilse; their recapture pattern was very similar to the MSW females in 1997 (Table 5). According to the generalized linear model the only significant factor predicting the recapture rate of tagged salmon was the interaction between sex and length (Table 6). As Fig. 2 illustrates, this interaction reflects the fact that the recapture rate of females increased with length, but the situation was the opposite in males. The probability of recapture as a function of the length of wild and reared salmon was slightly different within sexes, but it was not significant according to the model in Table 6. As the length of adults tagged in 1997 increased with sea age, the proportion of recaptured females increased and males decreased with age (linear regression; females: R 2 ¼ 0.518***, n ¼ 149; males: R 2 ¼ 0.800***, n ¼ 139; Fig. 3). Discussion Sex length Variables Effect of origin on recapture rate Sex Origin Sea age Length Deviance v * d.f Likelihood ratio test statistics for each variable are given (data based on 1997 taggings). 1SW, one-sea-winter; MSW, multi-sea-winter. This study recorded a similar recapture rate of wild and reared Atlantic salmon during and after river ascent. However, the survival of reared salmon, measured as the recapture rate of tagged smolts, has often been reported to be much lower than that of wild fish (Jonsson et al., 1991c, 2003; Saloniemi et al., 2004) and the wild-reared ratio in the smolt phase is known to be much lower than that among spawners (Jutila et al., 2003a). At the time of release, reared smolts have no experience of living in the wild. Their response to predators differs from that Probability of recapture Hatchery females Hatchery males Wild females Wild males Length of fish (mm) Fig. 2. Smoothed curves for probability of recapture as a function of length of fish in different salmon groups according to the model presented in Table 6 (salmon tagged in 1997) Recapture rate % Males Females 1SW 2SW 3SW 5SW Fig. 3. Proportion of recaptured female and male salmon in sea age groups tagged in The numbers of tagged fish in these age groups were 39, 74, 25 and 1 for one-sea-winter (1SW), 2SW, 3SW and 5SW males, respectively, and 2, 104, 40 and 3 for females. No 4SW salmon were tagged of wild fish (Olla et al., 1998; Johnsson et al., 2001). Reared fish must also adjust their feeding habits from hatcherysupplied food to living prey, and the stress of handling and transportation may cause deficiencies in behaviour (Olla et al., 1998). Only the fittest specimens therefore survive to be tagged as adult fish after the sea migration. Such fish have survived intense natural selection in early life (Fleming et al., 2000), including the mortality after sea entry reported by Willette et al. (1999). These observations could explain the similar recapture rate of the mature wild and reared salmon in the present study.

5 Recapture rate of S. salar after river ascent 493 The recapture rate of adult wild and reared salmon may, however, be variable due to spawning and fishing. It is known that spawning behaviour differs between reared and wild salmon, causing differences in their survival after spawning (Fleming et al., 1996; Fleming and Petersson, 2001), and also that the energetic costs of spawning result in increased mortality (Jonsson et al., 1991b). Jutila et al. (2003b) and Kallio-Nyberg et al. (2004) showed that wild Simojoki salmon migrate more often to the main basin of the Baltic Sea than reared salmon, a larger proportion of which may stay in the Gulf of Bothnia. Therefore, differences between the sea fisheries in these areas (Anon., 2004) may affect the tag recapture rate of both origin groups. Effect of sex, sea age and size on recapture rate The relationship between the sex and length or age of fish was the most important factor affecting the recapture rate of ascended Atlantic salmon. The recapture rate of females increased with size and age, the opposite relationship being recorded in males. Proportionally more tagged females were recaptured after spawning than males (see also N. Jonsson et al., 1990a, 1991a). The better survival of MSW females is important for the salmon stock, because the overall number of offspring is dependent on the eggs produced by these females. Spawning is believed to be more strenuous for males than for females, because breeding competition is more intense among males (Fleming and Petersson, 2001). Males lose a greater proportion of their somatic energy reserves (Jonsson et al., 1991b) and they sustain more injuries in spawning competition (B. Jonsson et al., 1990b, 1997). The higher total recapture rate of male grilse compared with MSW male salmon in the present study agrees with the observations of Jonsson et al. (1991a) that mortality is higher for larger than for smaller kelts, which is probably due to the cost of reproduction and the older age of the former group. Thus, grilse are better able to remain alive, as are also MSW females, and both salmon groups therefore produce relatively more recaptures in later life phases after leaving the river Recaptures in the river The recapture rate of Carlin-tagged salmon in the Simojoki River was lower than that in a previous study in 1996, when ascending salmon were tagged with radio transmitters (Jokikokko, 2002). Five of 38 tagged salmon (13%) were caught by fishermen from the river some time after tagging, but the different tagging method used then may have affected the reporting activity. In the Burrishoole River, Ireland, similar exploitation rates have also been reported (Mills, 1989). These are, however, low figures when compared to 47% for the River Wye between 1965 and 1974 (Mills, 1989) and to 40 70% for the radio-tagged salmon in the Tenojoki River (Erkinaro et al., 1999). Due to non-reporting, the true exploitation rate may, of course, be higher than that observed here. Non-reporting may clearly be a problem in relation to illegal gill net fishing, but it is not the case with rod fishing where fishermen are generally eager to obtain more information on their prey fish. Despite the higher proportion of recaptures originating from the river than from the sea after spawning, Jutila et al. (2003a) showed that an increase in angling effort did not prevent the recovery of Simojoki salmon, whereas small changes in sea fishing had considerable effects on the salmon population in this river. This is because sea fishing exploits and removes most of the salmon during their feeding and spawning migration in the sea before they reach the river for the first time. Only a fraction of the original smolt population is left to ascend the river, and the total number of salmon thus caught is very small compared to the sea exploitation. Effect of ascent timing on the catches The early ascending Atlantic salmon were not caught in proportionally greater numbers in the river than salmon ascending later in the season, despite their different periods of exposure to fishing. This is probably partly due to the decreased movements of salmon after settling on their spawning area quite soon after the river ascent (Jokikokko, 2002). This decreases their probability of capture with gill nets and may also reduce their reaction to lures and flies. The observed results lessen the need for a special seasonal fishing closure in the Simojoki River in addition to the ordinary fishing closure during the spawning season, especially because the fishing methods are relatively constant from year to year. However, because a considerable proportion of salmon remain in the river over the winter after spawning (see also Jokikokko, 2002), being thus potential repeat spawners, caution is also well deserved for the river phase. About 20% of the fishing mortality in the river occurred after the spawning season from late-autumn to the following spring, and almost all of these salmon were caught in connection with the gill net fishery targeted at other species. Therefore, the fishing of MSW females and male grilse, the groups having survival potential after ascending the river, needs to be controlled. The survival difference in the post-smolt phase between the wild and reared salmon, for example, in the Simojoki River (Jutila et al., 2003a; Saloniemi et al., 2004), seems to disappear in the mature phase. This finding allows fishery managers to better evaluate the applicability of stocking as a management method in salmon rivers. Furthermore, the similar survival of wild and reared salmon and the higher survival of females compared to males could be applied in modelling to achieve a more precise picture of the life cycle of Atlantic salmon. Acknowledgements The staff of the Bothnian Bay Fisheries Research Station carried out the tagging and trapping of the ascending salmon. The tagging office of the Finnish Game and Fisheries Research Institute organized the collection of tag return data. Mrs Irmeli Torvi aged the captured salmon. Dr Roy Siddall corrected the English in the manuscript. Our warmest thanks are due to all those who participated in the study for their invaluable help. References Alm, G., 1934: Salmon in the Baltic Precincts. Rapp. P V. Réun. Cons. Int. Explor. Mer 92, Anon., 2004: Report of the Baltic Salmon and Trout Assessment Working Group. ICES CM 2004/ACFM:23. Available at: (accessed 5 February 2005). Erkinaro, J.; Økland, F.; Moen, K.; Niemelä, E., 1999: Return migration of the Atlantic salmon in the Tana River: distribution and exploitation of radiotagged multi-sea-winter salmon. Boreal Environ. Res. 4, Fleming, I. A.; Petersson, E., 2001: The ability of released, hatchery salmonids to breed and contribute to the natural productivity of wild populations. Nord. J. Freshw. Res. 75,

6 494 E. Jokikokko et al. Fleming, I. A.; Jonsson, B.; Gross, M. R.; Lamberg, A., 1996: An experimental study of the reproductive behaviour and success of farmed and wild Atlantic salmon. J. Appl. Ecol. 33, Fleming, I. A.; Hindar, K.; Mjolnerad, B.; Jonsson, B.; Balstad, T.; Lamberg, A., 2000: Lifetime success and interactions of farm salmon invading a native population. Proc. R. Soc. Lond., B 267, Hilborn, R.; Eggers, D., 2000: A review of the hatchery programs for pink salmon in Prince William Sound and Kodiak Island, Alaska. Trans. Am. Fish. Soc. 129, Järvi, T. H., 1938: Fluctuations in the Baltic stock of salmon ( ). Rapports et Procès-Verbaux des Réunions du Conseil International pour l Exploration de la Mer CVI, 114. Järvi, T. H., 1948: On the periodicity of salmon reproduction in the northern Baltic area and its causes. Rapports et Procès-Verbaux des Réunions du Conseil International pour l Exploration de la Mer CXIX, 131. Johnsson, J. I.; Ho jesjo, J.; Fleming, I. A., 2001: Behavioural and heart rate responses to predation risk in wild and domesticated Atlantic salmon. Can. J. Fish. Aquat. Sci. 58, Jokikokko, E., 2002: Migration of wild and reared Atlantic salmon (Salmo salar L.) in the River Simojoki, northern Finland. Fish. Res. 58, Jokikokko, E.; Jutila, E., 1998: The effects of stocking with salmon parr, Salmo salar, on the smolt production in the River Simojoki, northern Finland. In: Stocking and introduction of fish. I. G. Cowx (Ed.). Blackwell Scientific Publications, Oxford, UK, pp Jokikokko, E.; Kallio-Nyberg, I.; Jutila, E., 2004: The timing, sex and age composition of the wild and reared Atlantic salmon ascending the Simojoki River, northern Finland. J. Appl. Ichthyol. 20, Jonsson, B.; Jonsson, N.; Hansen, L. P., 1990a: Does juvenile experience affect migration and spawning of adult Atlantic salmon? Behav. Ecol. Sociobiol. 26, Jonsson, N.; Jonsson, B.; Hansen, L. P., 1990b: Partial segregation in the timing of migration of Atlantic salmon of different ages. Anim. Behav. 40, Jonsson, N.; Hansen, L. P.; Jonsson, B., 1991a: Variation in age, size and repeat spawning of adult Atlantic salmon in relation of river discharge. J. Anim. Ecol. 60, Jonsson, N.; Hansen, L. P.; Jonsson, B., 1991b: Energetic cost of spawning in male and female Atlantic salmon (Salmo salar L.). J. Fish Biol. 39, Jonsson, B.; Jonsson, N.; Hansen, L. P., 1991c: Differences in life history and migratory behaviour between wild and hatcheryreared Atlantic salmon in nature. Aquaculture 98, Jonsson, N.; Jonsson, B.; Hansen, L. P., 1997: Changes in proximate composition and estimates of energetic costs during upstream migration and spawning in Atlantic salmon Salmo salar. J. Anim. Ecol. 66, Jonsson, N.; Jonsson, B.; Hansen, L. P., 2003: The marine survival and growth of wild and hatchery-reared Atlantic salmon. J. Appl. Ecol. 40, Jutila, E.; Jokikokko, E.; Julkunen, M., 2003a: Management of Atlantic salmon in the Simojoki River, northern Gulf of Bothnia: effects of stocking and fishing regulation. Fish. Res. 64, Jutila, E.; Jokikokko, E.; Kallio-Nyberg, I.; Saloniemi, I.; Pasanen, P., 2003b: Differences in sea migration between wild and reared Atlantic salmon (Salmo salar L.) in the Baltic Sea. Fish. Res. 60, Kallio-Nyberg, I.; Jutila, E.; Saloniemi, I.; Jokikokko, E., 2004: Association between environmental factors, smolt size and the survival of wild and reared Atlantic salmon from the Simojoki River in the Baltic Sea. J. Fish Biol. 65, Karlsson, L.; Ikonen, E.; Westerberg, H.; Sturlaugsson, J., 1999: Data storage tag study of salmon (Salmo salar) migration in the Baltic: The spawning migration of wild and hatchery-reared fish and a comparison of tagging methods. ICES CM 1999/AA:05. Available at: (accessed 5 February 2005). McCullagh, P.; Nelder, J., 1989: Generalized Linear Models, 2nd edn. Chapman and Hall, London, UK, 511 p. Mills, D. H., 1989: Ecology and management of Atlantic salmon. Chapman and Hall, London, UK, 351 p. Olla, B. L.; Davis, M. W.; Ryer, C. H., 1998: Understanding how the hatchery environment represses or promotes the development of behavioural survival skills. Bull. Mar. Sci. 62, Olsen, E. M.; Heino, M.; Lilly, G. R.; Morgan, M. J.; Brattey, J.; Ernande, B.; Dieckmann, U., 2004: Maturation trends indicative of rapid evolution preceded the collapse of northern cod. Nature 428, Ritter, J. A., 1997: The contribution of Atlantic salmon (Salmo salar L.) enhancement to a sustainable resource. ICES J. Mar. Sci. 54, Romakkaniemi, A.; Perä, I.; Karlsson, L.; Jutila, E.; Carlsson, U.; Pakarinen, T., 2003: Development of wild Atlantic salmon stocks in the rivers of the northern Baltic Sea in response to management measures. ICES J. Mar. Sci. 60, Salminen, M., 1997: Relationships between smolt size, postsmolt growth and sea age at maturity in Atlantic salmon ranched in the Baltic Sea. J. Appl. Ichthyol. 13, Saloniemi, I.; Jokikokko, E.; Kallio-Nyberg, I.; Jutila, E.; Pasanen, P., 2004: Survival of reared and wild Atlantic salmon smolts: size matters more in bad years. ICES J. Mar. Sci. 61, SAS Institute, 1999: SAS/STAT user s guide, Version 8. SAS Institute Inc., Cary, NC, USA. Willette, T. M.; Cooney, R. T.; Hyer, K., 1999: Predator foraging mode shifts affecting mortality of juvenile fishes during the subarctic spring bloom. Can. J. Fish. Aquat. Sci. 56, Author s address: Erkki Jokikokko, Finnish Game and Fisheries Research Institute, Bothnian Bay Fisheries Research Station, Jenssintie 2, FI Simo, Finland. erkki.jokikokko@rktl.fi