The survival of stocked Atlantic salmon smolts during sea run and the timing of migration in the river Simojoki, northern Finland

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1 Aquaculture 219 (2003) The survival of stocked Atlantic salmon smolts during sea run and the timing of migration in the river Simojoki, northern Finland Erkki Jokikokko a, *, Samu Mäntyniemi b,1 a Finnish Game and Fisheries Research Institute, Bothnian Bay Fisheries Research Station, Jenssintie 2, FIN Simo, Finland b Finnish Game and Fisheries Research Institute, Oulu Game and Fisheries Research Station, Tutkijantie 2A, FIN Oulu, Finland Received 17 December 2001; received in revised form 24 September 2002; accepted 16 October 2002 Abstract The migration of Atlantic salmon smolts in a river phase was studied in in the Simojoki, a river flowing into the northernmost part of the Gulf of Bothnia, northern Finland. The influence of migration distance on the recapture rate of stocked smolts was evaluated and the timing of their migration compared with that of wild smolts. We assumed that the recapture rate would indicate the relative survival of stocked smolts during their migration from different parts of the river. A log-linear model was used to describe the number of fish recaptured with a smolt trap at the river mouth. The recapture rate of marked smolts stocked in different rapids did not depend on the migration distance ( km) but varied randomly. The stocked smolts migrated at the same time as the wild smolts in but not in The correlation between the daily catch of wild and released smolts ranged from 67% to 95% in stocking groups released from 30 to 100 km above the smolt trap. Generally, no correlation was found with smolts released only 11 km above the trap. We concluded that the migration distance of stocked smolts did not have a significant effect on the survival of sea-running smolts in the Simojoki and that hatchery-rearing had not altered the migration ability of stocked smolts, which migrated simultaneously with wild smolts. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Salmo salar; Smolt migration; Survival; Timing of migration; Recapture rate * Corresponding author. Tel.: ; fax: address: erkki.jokikokko@rktl.fi (E. Jokikokko). 1 Current address: Rolf Nevanlinna Institute, University of Helsinki, P.O. Box 4, FIN Helsinki, Finland /03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S (02)

2 432 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) Introduction During their migration from nursery areas to the sea, the smolts of salmonids are vulnerable to a number of fish and avian predators, and losses of smolts may be very high (Lindroth, 1955; Larsson and Larsson, 1975; Larsson, 1985; Jepsen et al., 1998; Aarestrup et al., 1999). The losses have been evaluated by mark-recapture methods, in which smolts have been marked with Carlin tags (Larsson, 1985; Kennedy et al., 1991), Panjet tattoos (Rasmussen et al., 1996), fin clips (Jepsen et al., 1998), PIT tags (Skalski et al., 1998; Muir et al., 2001) or radio tags (Rasmussen et al., 1996; Jepsen et al., 1998; Aarestrup et al., 1999). Statistical models have been developed to derive reliable survival estimates from the data of release recapture studies of migrant smolts (Skalski et al., 1998), and mortality has been estimated by assessing the abundance of predators (Wood, 1987). Another important factor affecting the success of smolt migration is the timing of migration, as this may be adapted through natural selection to the environmental conditions prevailing in each area. There is thought to be an ecological and physiological window for smolt migration that gradually closes when the time is over (Hansen and Jonsson, 1989; McCormick et al., 1998; Whalen et al., 1999) and it is recommended that smolt releases should coincide with the migration of wild smolts (Kennedy et al., 1984). This practice has also been observed to decrease predation (Hansen and Jonsson, 1989). However, because the behaviour of reared smolts differs in many respects from that of their wild counterparts (Jonsson et al., 1991), it is important to ensure that stocked smolts are able to utilize this migration window and descend simultaneously with wild smolts. The Atlantic salmon, Salmo salar L., stocks in the Baltic Sea area have declined after the second World War, at first due to the damming of the major salmon rivers and after that increasingly largely as a result of heavy sea fishing (Eriksson and Eriksson, 1993; Anon., 2000). In 1990s, also the M74, a reproduction disorder of salmon, which appears as the death of yolk-sac fry in the wild, is assumed to be a serious threat for wild stocks (Keinänen et al., 2000). Large number of salmon smolts is released annually to compensate the loss of natural reproduction in dammed rivers. These releases are carried out only to maintain the salmon fishing in the sea area, i.e. it is pure sea-ranching. In the Gulf of Bothnia area, where salmon still have possibilities for spawning, smolts and parr have been released during recent decades in many rivers also for enhancement purposes. In Finland, e.g. most of these releases are nowadays based on a special enhancement program adopted by the International Sea Fishery Commission (IBSFC) in The goal of this Salmon Action Plan (SAP) is to restore wild salmon populations in present salmon rivers or establish them in potential rivers in the Baltic region. In the river Simojoki, northern Finland, enhancement stocking has been carried out already since 1984 by the Finnish Game and Fisheries Research Institute (Jokikokko and Jutila, 1998). In this river, where salmon reproduce naturally, a considerable number of reared salmon parr and smolts have been released in the rapids every year to strengthen the natural stock. Due to a heavy fishing during the feeding migration in the sea, the number of returning salmon has been low (Jokikokko and Jutila, 1998). Thus, despite good intentions to enhance the natural reproduction, in reality the program is sea-ranching to maintain fisheries. As long as sea fishing continues and the wild stocks are as low as they

3 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) have been in the Simojoki River, supporting releases are needed and extra spawners are valuable in safeguarding the existence of the salmon stock. Any decisions concerning the sea fishing in the Baltic Sea, which has a dominating effect on the success of the enhancement especially in the case of the river Simojoki, are made on the political level, internationally by IBSFC and nationally by fishing authorities. In practice, almost the only way for fisheries biologists and fish farmers to influence the yield of the releases is to use the highest possible quality and knowledge in all phases of production from the eggs to the stocking of smolts. Thus, we considered it important to study the correlation between the migration distance and the recapture rate of stocked smolts in This had not been studied during earlier stocking efforts, which had already been underway for over 10 years in the Simojoki River. Releases are made along the whole 110-km-long stocking area in order to ensure the imprinting. If there is a correlation between migration distance and survival to sea entry, then stocking strategies might be profitably modified. In the Simojoki River, all the stocked fish have been fin-clipped before release and thus can be distinguished from the wild fish. Since most of the rapids are located in the lower part of the river, the main stocking area for salmon parr is about 100 km upwards from the river mouth. No fish have been stocked in the upper part of the river except in some experiments with Carlin-tagged smolts. However, these gave very poor results, the recapture rate being < 1% compared with the 5 10% or even higher achieved in groups stocked in the lower part of the river. There are long pool sections and several lakes in the upper reaches of the river that are considered to be detrimental for migrating smolts (Rasmussen et al., 1996). We used a smolt trap at the river mouth to catch the migrating smolts, and assumed that the recapture rate would reflect the relative losses of migrating smolts. We could not evaluate total migration losses because the mortality due to stocking and handling, although it may have been high (Schreck et al., 1989), was not known and because the smolt trap caught only a portion of the descending smolts. The marked salmon smolts were stocked at different sites in the river. Our null hypothesis was that the recapture rate decreases when the migration distance increases. We also examined the migration pattern of wild and reared smolts, and analysed the relationship between the catch of marked smolt groups and wild smolts. Our assumption was that the higher the correlation between the daily catch of wild and stocked smolts, the better the stocked smolts could utilize the migration window and thus survive in a natural environment. 2. Materials and methods 2.1. Study area The Simojoki River (65j38VN, 25j00VE) flows into the northern part of the Gulf of Bothnia (Fig. 1). The river is 175 km long and its mean discharge rate is 38 m 3 s 1. The entire river is accessible to salmon, but the results of annual electrofishing surveys show that they spawn only in the lowest 100 km. This behaviour is attributed to several factors such as the location of suitable spawning areas, the number of ascending spawners and the

4 434 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) Fig. 1. Location of the river Simojoki in northern Finland, and the sites of the smolt trap and of the rapids where smolt groups were released (1 = Isopetäjä, 2 = Alaniemi, 3 = Tainikoski, 4 = Hosio, 5 = Raiskio, 6 = Portimo, 7 = Kaitavirta). stocking practice (Jokikokko, 2002). The smolts used here were of Simojoki origin, because it is the only stock used for enhancement purposes in this river Stocking and trapping During the 4 years from 1996 to 1999, 2-year-old salmon smolts were marked and released at various sites along the river (Fig. 1). The migration distance from the release site to the river mouth varied from 11 to 174 km. The migrating smolts were counted with the aid of a smolt trap placed in the same place in each year at the river mouth. Smolt trapping started after the spring flood as soon as the water level was low enough for the trap to be put in the river. In most years, this was in late May (Table 1), when the water temperature was < 10 jc (Fig. 2). Wild smolts normally migrate from late May to late June. Smolt trapping ended after the smolt run was over, by which time the water temperature had risen to jc. The trap closed about one-third of the river, which was m wide at this point. The breadth of the river minimised the possible effects of the water level and current changes during the smolt trapping but caused a quite low catchability. However, it was not possible to increase the catchability due to a high number of smolts during a peak run, when several thousand smolts per day could be a very typical

5 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) Table 1 The smolt trapping period and its duration in the river Simojoki in Year Trapping period Trapping days Stocking of marked smolts Date of last recapture May 27 June 34 3 June 12 June May 22 June June 15 June May 21 June May 19 June May 28 June May 17 June The stocking date and the date for the last recapture of marked smolts are also given. catch. With all the handling, counting and sampling too many fish would have had detrimental effects for smolts. The mesh size of the codend was 8 mm (from knot to knot). Each marked smolt group in the study contained 1000 reared fish. In 1996, four groups were marked by hot branding and one by Panjet inoculation (Hart and Pitcher, 1969). Since Panjet marking seemed to be easier and faster than branding, all groups in 1997, 1998 and 1999, five per year, were marked with Panjet tattoos. An alcian blue was used, and the colour dot was injected near the base of the pelvic or pectoral fins, a unique mark for each release site. The mean weight of the smolts was 62, 52, 52 and 39 g in 1996, 1997, 1998 and 1999, respectively. The smolts stocked in 1996 and 1997 were raised at the Kainuu Fisheries Research and Aquaculture station of the Finnish Game and Fisheries Research Institute. They were brought to the Institute s Simojoki fish farm, which is located on the Simojoki, some weeks before stocking. The smolts stocked in 1998 and 1999 were raised at the Simojoki fish farm. In each year, the fish were marked at the Simojoki fish farm 1 or 2 weeks before stocking. After marking each group was kept in a separate plastic rearing pond, where the fish could recover from handling. The smolts were Fig. 2. Fitted curves of model 1. The log-likelihood of the model is

6 436 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) transported to the release site in a plastic tank with oxygen addition, and were released at the same time as unmarked smolts in accordance with routine stocking practice. The smolt groups were not released before the smolt trap was in operation, and the trap was not removed until the wild smolt run was over (Table 1). In 1996, 1997 and 1998, smolts were released at five sites (Isopetäjä, Alaniemi, Tainikoski, Hosio and Raiskio), which were 11, 30, 47, 73 and 98 km upstream from the trap, respectively (Fig. 1). In 1999, two new places (Portimo and Kaitavirta), 117 and 174 km from the trap, respectively, were tested and two sites used in previous years were rejected (Alaniemi, Hosio). The new sites were in the upper part of the river, thus providing more variation to the migration distance. It was assumed that all the smolt groups and their treatments were similar in each year, so the differences in the recapture rates between groups were caused only by the differences in migration losses. The years could not be compared with each other, because the catchability of the trap varied from year to year, depending, for instance, on the length of wings used in the trap and the water discharge rate during the migration. It was assumed, however, that catchability did not vary during the trapping period Statistical methods To study the influence of the migration distance on recapture rate of stocked smolts, the following log-linear model was formulated to describe the number of recaptured fish. Loglinear models with Poisson or negative binomial distribution are widely used with counttype response variables (McCullagh and Nelder, 1989). logðsþ ¼ logðnþ þa þ year þ ðb þ yearþlogðmþ; ð1þ where s = the expected number of recaptured fish from each stocking group; n = the size of the stocking group; a = the log (recapture rate), when the migration distance is 1 km in the year 1999, other years are contrasted to this year by adding the year effect; b = the slope of the line for the year 1999, other years are contrasted to this year by adding the year effect; year = an addition to the log(recapture rate) or to the slope for each year; and m = the distance between the stocking site and smolt trap. Because it is possible that smolts move in shoals rather than independently, we assumed that the number of recaptured fish from each stocking group followed a negative binomial distribution (Seber, 1982) with mean s. If it appears that the model can be simplified so that each year has its own level of the recapture rate (a + year), but all years have a common slope (b), the model becomes logðsþ ¼logðnÞþa þ year þ b logðmþ ð2þ Thus, if the common slope is negative, the recapture rate declines when the distance increases but if the slope is positive the converse is true. The likelihood ratio was used to compare models 1 and 2. We assumed that the number of recaptured fish from each stocking group followed a negative binomial distribution with mean s. On the basis of the results, the smolts released in the lowest rapid (Isopetäjä) were omitted from the log-linear analysis, because they

7 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) Table 2 The recapture rate of marked smolts (%) stocked in different rapids in the river Simojoki in and the distance (km) between stocking site and smolt trap Distance (km) Isopetäjä Alaniemi Tainikoski Hosio Raiskio Portimo Kaitavirta clearly had not had enough time to recover from stocking before being caught with the smolt trap. The timing of the run of tagged smolts was compared with that of wild smolts. To this end, a correlation, if any, was sought between the number of tagged smolts and wild smolts in the smolt trap. The null hypothesis (H 0 ) was that there is no correlation between tagged and wild smolts. 3. Results The recapture rate of smolts did not depend on distance from the trap, because this varied almost randomly from one stocking site to another in all 4 years (Table 2). The Fig. 3. Fitted curves of model 2. The log-likelihood of the model is The absolute difference between log-likelihoods of models 1 and 2 multiplied by two is , which, when compared with the v 2 (3) distribution, gives a P-value of 0.18.

8 438 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) Table 3 Details of the recapture of marked smolt groups in the river Simojoki in A B C A B C A B C A B C Wild smolts 8 June June June May 5 38 Isopetäjä 4 June June June May 1 6 Alaniemi 7 June June June 15 4 Tainikoski 7 June June June May 6 14 Hosio 7 June June June 17 9 Raiskio 7 June June June May 6 22 Portimo 31 May 7 10 Kaitavirta 16 June 23 2 A: median date of descent of marked and wild smolts in according to catch statistics; B: days elapsing from the beginning of smolt trapping to the median date (wild smolts) and from stocking to the median date (marked smolts); C: duration of smolt run (in days) from the first to the last caught fish. likelihood ratio test between models 1 and 2 suggested that if equal gradients were assumed across the years, the frequency probability of obtaining more extreme data than those observed was not especially low ( P c 0.18); model 2 was therefore considered acceptable (Figs. 2 and 3). The common slope in formula 2 was , implying that the recapture rate of smolts stocked in the upper part of the river was slightly lower than that of smolts stocked in the lower reaches. The dispersion of the slope was, however, so big that the slope could also be positive within the 95% confidence interval. According to the estimation, the variation observed in recapture rates is more likely to be caused by differences in the efficiency of the smolt trap between the years than by the distance between the stocking site and the trap. In 1996 and 1997, the marked smolts started to migrate as soon as they were released. The median descent date was 1 day after release for the lowest stocking group and 4 5 days for the four upper smolt groups (Table 3). In 1997, 50% of the smolts released in the Fig. 4. Water temperature in the river Simojoki during the smolt trapping period in Arrows point to dates when marked smolt groups were released in different years.

9 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) Table 4 Correlation between the daily catch rate of wild and released smolts in Isopetäjä Alaniemi Tainikoski Hosio Raiskio Portimo Kaitavirta j 0.766*** 0.707*** 0.790*** 0.798*** NS 0.968*** 0.974*** 0.961*** 0.962*** * 0.931*** 0.894*** 0.960*** 0.955*** *** NS NS NS NS ***: P < 0.001; *: P < 0.05; j: P < 0.1; NS: P>0.1. Alaniemi and Tainikoski rapids migrated to the trap in 5 days, whereas the smolts released 1 day later in the two uppermost rapids, Hosio and Raiskio, reached the trap in only 4 days. The peak run was thus on the same day in all these stocking groups despite the difference in distance and release date. The water was clearly cooler in 1998 than in other years at the time of stocking (Fig. 4), and more than 2 weeks elapsed before the temperature had risen high enough for the smolt run to start. In 1999, the migration pattern was similar to that in 1996 and 1997, and the smolt groups stocked in the Tainikoski, Raiskio and Portimo rapids had the same median trap capture date, which was several days later than that of smolts stocked in the Isopetäjä rapid. Over 3 weeks elapsed, before the smolts (only two) from the uppermost stocking site, Kaitavirta rapid, were Fig. 5. The cumulative catch of smolts released in different rapids in The first date on the x-axis indicates the date of release, and the last date that when the last marked smolt was caught. For 1996 and 1997, see the legend for 1998.

10 440 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) caught. In each year, the median date was the same for almost all stocking groups, and the duration of the smolt runs, once started, was similar irrespective of stocking site and variation in migration distance. The migration of the stocked smolt groups correlated fairly well with the main run of the wild smolts in 1996, 1997 and 1998 (Table 4). The median capture dates were about the same for both the wild and the stocked smolts although the total number of wild smolts caught with the trap net was quite low. The smolts stocked in the Isopetäjä rapid, the lowest rapid in this study, tended to migrate earlier than other stocking groups or wild smolts (Fig. 5). In 1998, the median descent day was the same for the lowest stocking group and the wild smolts, whereas it was a couple of days later for the upper groups. Despite having the same median date, the wild smolts and the lowest stocking group did not have a similar migration pattern and thus the correlation was low (Table 4). The daily migration rate of the four upper groups correlated significantly with that of the wild smolts. In 1999, the recapture rate was very low for all but the lowest stocking group, and, unlike other groups, its migration pattern correlated well with that of the wild smolts. 4. Discussion Judging by the recapture rate of smolts found in this study, smolt mortality does not depend on migration distance in a river where conditions are equal to those in the Simojoki. According to the study of Hvidsten and Ugedal (1991), the recapture rate of stocked smolts was often higher among fish released in the upper than in the lower section of the river. Romakkaniemi et al. (2000) observed that the recapture rate of reared 2-yearold smolts remained between 7.6% and 9.4% even though the migration distance of different stocking groups varied from 105 to 450 km. In some studies, the proportion of non-recaptures was 16.4% when the migration distance was only 700 m (Kennedy et al., 1991) and % when smolts were released 1 km above a smolt trap (Hansen and Jonsson, 1985). In the light of these figures, we would have expected a greater decrease in recapture rate in the present study. However, Muir et al. (2001) did not observe a marked decline in the survival of migrating chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) in their study until after about km of migration. The assumption of steady catchability of the trap net throughout the migration period may be invalid for the river Simojoki, because catchability may vary daily, depending on environmental conditions such as water level and temperature (Schwarz and Dempson, 1994; Romakkaniemi et al., 2000). It is possible that smolts released at different distances behave differently due to differences in their adaptation time and experience in the river and to the variation in smolt size. The possible variation in catchability deserves a specific study seeking to improve the reliability of smolt run estimates. However, the variation in catchability may be negligible due to the short duration of the migration period of the tagged smolts. Smolts released km from the smolt trap were caught almost simultaneously at the trap. One reason for this may have been that the trap was emptied only once a day. This sampling interval was too long for us to establish whether any differences in migration

11 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) time existed between the smolts released at different sites. Furthermore, there are no lakes in that part of the Simojoki to slow down migration and to cause differences in migration time (see Hansen et al., 1984). The slowing effect of the long quiet sections and lakes in the upper part of the river was clearly visible in 1999, when it took smolts at least three times as long to cover the distance (174 km) from the uppermost rapid (Kaitavirta) as that from the three lower stocking sites. Clearly, the long migration time combined with the pike population in the lakes increased the risk of predation and caused the observed low recapture rate of stocked smolts. High mortality of migrating smolts has been reported by Jepsen et al. (1998). In one Danish river, the total mortality of released salmon smolt groups passing through a 12-km-long reservoir was 86.4% for untagged smolts and 87.5% for radio-tagged smolts. The main predator was pike, causing over 50% of the recorded mortality. Our results confirm the opinion of Rasmussen et al. (1996) that if lakes are present in a river, efforts to re-establish an anadromous salmonid population in that river will face very serious problems. It would be difficult to create a naturally reproducing population of salmon in the upper part of the Simojoki, because releases of parr and smolts would not be profitable. However, when smolts were stocked in the middle or lower reaches of the river, differences in recapture rate and, thus, in relative migration losses were minimal. The simultaneous migration and almost equal duration of the smolt run despite the different migration distances from all but the uppermost stocking site may be the reason for the almost equal recapture rates. The length of time smolts are exposed to predators is very important (Wood, 1987; Jepsen et al., 1998), and in this respect, the risk of smolts being eaten in the Simojoki is similar to that of smolts stocked in rapids up to 100 km away. It is possible that smolts released in the lower part of a river stay there until the descending wild smolt schools stimulate their migration (see also Kennedy et al., 1984; Hvidsten et al., 1995). Such a stimulus caused by wild smolts in the Simojoki may explain the fairly rapid and simultaneous descent of the hatchery smolts. A similar correlation in the timing of the migration of wild and stocked smolts was observed by Kennedy et al. (1984) and Haikonen (1996). This is a very important factor in attempts to avoid or reduce predation, because it gives predators, e.g. mergansers (Mergus merganser), the shortest possible time in which to gorge (Wood, 1987). Another reason for minimizing delays in the downstream passage is that the period between the onset of migration and the loss of a smolt s physiological characteristics may be brief (Whalen et al., 1999). In the Simojoki, the high correlations between the wild and the stocked smolts in most years show that, in general, the timing of releases has been successful. According to the results for 1998, a year with a long period between release and descent, reared smolts are able to select a suitable migration time and the smolt run is an active process (Solomon, 1978; Hansen and Jonsson, 1985; Aarestrup et al., 1999; Hembre et al., 2001). Contrary to the suggestion of Cresswell (1981), smolts do not undertake only post-stocking movements. As reported by Kennedy et al. (1984), positively directed downstream swimming may not necessarily occur, but the behavioural mechanism for either holding station or drifting must be subject to control. Migration does not start before conditions, e.g. water temperature, are suitable for triggering the migration. The ability of wild and reared smolts to respond to such stimuli in similar ways seems to be a hereditary character, not one affected by the juvenile experiences of the reared smolts. It has often

12 442 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) been observed that the downstream migration of Atlantic salmon was initiated when the temperature reached 10 jc (Österdahl, 1969; Fried et al., 1978; Jonsson and Ruud- Hansen, 1984; Erkinaro et al., 1998). Clearly, it was the falling water temperature in 1998 that delayed the start of migration, not the trauma of tagging and handling observed by Kennedy et al. (1991), because in other years no such delay occurred. Some stocked smolts may move passively, as shown by those caught 1 day after release in the Isopetäjä rapid near the trap in Obviously they had not recovered sufficiently from transport and stocking stress to resist the current before they entered the trap (see Schreck et al., 1989). Probably some of the smolts released in the upper rapids drifted downstream, but because of the longer distance they did not reach the smolt trap until they had recovered from stocking. Such downstream movement of smolts would be evidence of passive displacement with the current as reported by Thorpe et al. (1981). Pirhonen et al. (1998) likewise hypothesized that smolt migration is a passive process. Our results from the river Simojoki show, however, that stocked smolts (like wild smolts) migrate when the time is optimal and that, once started, the migration is soon over. Releases at the wrong time should be avoided, because prolonged migration or a long waiting period before descent, as in 1998, may cause unnecessary predation (see Wood, 1987; Jepsen et al., 1998). Hansen and Jonsson (1989) also observed that the survival of smolts in the sea was highest for fish released at the same time as natural smolts leave the river. These findings emphasize the importance of timing releases to coincide with the wild smolt migration when possible, as suggested by Kennedy et al. (1984). Acknowledgements The study was financed by the Finnish Game and Fisheries Research Institute. The Bothnian Bay Fisheries Research Station was responsible for the smolt marking and trapping. The Institute s Kainuu Fisheries Research and Aquaculture station and the Simojoki fish farm produced the smolts, put the facilities for smolt marking at our disposal and were responsible for stocking the smolts in the rapids. Dr. Jaakko Erkinaro, Mr. Eero Jutila and Mr. Markku Julkunen gave useful comments on the manuscript in its various phases. The authors wish to thank all of the above and especially the personnel of the Research Station and of the Aquaculture Unit for smooth cooperation. References Aarestrup, N., Jepsen, N., Rasmussen, G., Økland, F., Movements of two strains of radio tagged Atlantic salmon, Salmo salar L., smolts through a reservoir. Fisheries Management and Ecology 6, Anon., Report of the Baltic Salmon and Trout Assessment Working Group. ICES CM 2000/ACFM: pp. Cresswell, R.C., Post-stocking movements and recapture of hatchery-reared trout released into flowing waters a review. Journal of Fish Biology 18, Eriksson, T., Eriksson, L.-O., The status of wild and hatchery propagated Swedish salmon stocks after 40 years of hatchery releases in the Baltic rivers. Fisheries Research 18, Erkinaro, J., Julkunen, M., Niemelä, E., Migration of juvenile Atlantic salmon in small tributaries of the River Teno, northern Finland. Aquaculture 168,

13 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) Fried, S.M., McCleave, J.D., LaBar, G.W., Seaward migration of hatchery-reared Atlantic salmon, Salmo salar, smolts in the Penobscot river estuary, Maine: riverine movements. Journal of the Fisheries Research Board of Canada 35, Haikonen, A., Lohen, Salmo salar, villien ja istutettujen smolttien vaellusnopeus ja vaellusnopeuteen vaikuttavat tekijät Tornionjoessa vuosina (The migration speed of wild and stocked salmon, Salmo salar, smolts and factors affecting it in the river Tornionjoki in ). Parainen. Graduate study at the Finnish Fisheries School. 48 pp. + 5 app. (in Finnish). Hansen, L.P., Jonsson, B., Downstream migration of hatchery-reared smolts of Atlantic salmon (Salmo salar L.) in the River Imsa, Norway. Aquaculture 45, Hansen, L.P., Jonsson, B., Salmon ranching experiments in the river Imsa: effect of timing of Atlantic salmon (Salmo salar) smolt migration on survival to adults. Aquaculture 82, Hansen, L.P., Jonsson, B., Døving, K.B., Migration of wild and hatchery-reared smolts of Atlantic salmon, Salmo salar L., through lakes. Journal of Fish Biology 25, Hart, P.J.B., Pitcher, T.J., Field trials of fish marking using a jet inoculator. Journal of Fish Biology 1, Hembre, B., Arnekleiv, J.V., L Abée-Lund, J.H., Effects of water discharge and temperature on the seaward migration of anadromous brown trout, Salmo trutta, smolts. Ecology of Freshwater Fish 10, Hvidsten, N.A., Ugedal, O., Increased densities of Atlantic salmon smolts in the river Orkla, Norway, after regulation for hydropower production. American Fisheries Society Symposium 10, Hvidsten, N.A., Jensen, A.J., Vivås, H., Bakke, O., Heggberget, T.G., Downstream migration of Atlantic salmon smolts in relation to water flow, water temperature, moon phase and social interaction. Nordic Journal of Freshwater Research 70, Jepsen, N., Aarestrup, K., Økland, F., Rasmussen, G., Survival of radio-tagged Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.) smolts passing a reservoir during seaward migration. Hydrobiologia 371/ 372, Jokikokko, E., Migration of wild and reared Atlantic salmon (Salmo salar L.) in the river Simojoki, northern Finland. Fisheries Research 58, Jokikokko, E., Jutila, E., The effects of stocking with salmon parr, Salmo salar, on the smolt production in the River Simojoki, Northern Finland. In: Cowx, I.G. (Ed.), Stocking and Introduction of Fish. Fishing News Books, Blackwell, Oxford, pp Jonsson, B., Ruud-Hansen, J., Water temperature as the primary influence on timing of seaward migrations of Atlantic salmon (Salmo salar) smolts. Canadian Journal of Fisheries and Aquatic Sciences 42, Jonsson, B., Jonsson, N., Hansen, L.P., Differences in life history and migratory behaviour between wild and hatchery-reared Atlantic salmon in nature. Aquaculture 98, Keinänen, M., Tolonen, T., Ikonen, E., Parmanne, R., Tigerstedt, C., Rytilahti, J., Soivio, A., Vuorinen, P.J., Itämeren lohen lisääntymishäiriö-m74. Kalatutkimuksia-Fiskundersökningar, vol Finnish Game and Fisheries Research Institute, Helsinki. 38 pp., in Finnish with English abstract. Kennedy, G.J.A., Strange, C.D., Andersen, R.J.D., Johnston, P.M., Experiments on the descent and feeding of hatchery-reared salmon smolts (Salmo salar L.) in the River Bush. Fisheries Management 15, Kennedy, G.J.A., Strange, C.D., Johnston, P.M., Evaluation of Carlin tagging as a mark-recapture technique for estimating total river runs of salmon smolts, Salmo salar L. Aquaculture and Fisheries Management 22, Larsson, P.-O., Predation on migrating smolt as a regulating factor in Baltic salmon, Salmo salar L., populations. Journal of Fish Biology 26, Larsson, H.-O., Larsson, P.-O., Predation på nyutsatt odlad smolt i Luleälven (English Summary: Predation on Hatchery Reared Smolts After Release in River Lule 1974). Report of the Swedish Salmon Research Institute, vol pp. Lindroth, A., Mergansers as salmon and trout predators in the river Indalsälven. Report - Institute of Freshwater Research, Drottningholm, vol. 36, pp McCormick, S., Hansen, L.P., Quinn, T.P., Saunders, R.L., Movement, migration and smolting of Atlantic salmon (Salmo salar). Canadian Journal of Fisheries and Aquatic Sciences 55 (Suppl. 1), McCullagh, P., Nelder, J.A., Generalized Linear Models, 2nd ed. Chapman & Hall, London. Muir, W.D., Smith, S.G., Williams, J.G., Hockersmith, E.E., Skalski, J.R., Survival estimates for migrant

14 444 E. Jokikokko, S. Mäntyniemi / Aquaculture 219 (2003) yearling chinook salmon and steelhead tagged with passive integrated transbonders in the lower Snake and lower Columbia Rivers, North American Journal of Fisheries Management 21, Österdahl, L., The smolt run of a small Swedish river. In: Northcote, T.G. (Ed.), Salmon and Trout in Streams. H.R. MacMillan Lectures in Fisheries. University of British Columbia, Vancouver, B.C., Canada, pp Pirhonen, J., Forsman, L., Soivio, A., Thorpe, J., Movements of hatchery reared Salmo trutta during the smolting period, under experimental conditions. Aquaculture 168, Rasmussen, G., Aarestrup, K., Jepsen, N., Mortality of sea trout (Salmo trutta L.) and Atlantic salmon (Salmo salar L.) smolts during seaward migration through rivers and lakes in Denmark. ICES C.M. 1996/T:9. Romakkaniemi, A., Haikonen, A., Mäntyniemi, S., Lohi-ja meritaimenkantojen seuranta Tornionjoessa (Monitoring of the Salmon and Trout Stocks in the River Tornionjoki in 1999). Kala-ja riistaraportteja, vol Finnish Game and Fisheries Research Institute, Simo. 66 pp., in Finnish with English figure and table texts and summary. Schreck, C.B., Solazzi, M.F., Johnson, S.L., Nickelson, T.E., Transportation stress affects performance of coho salmon, Oncorhynchus kisutch. Aquaculture 82, Schwarz, C.J., Dempson, B.D., Mark-recapture estimation of a salmon smolt population. Biometrics 50, Seber, G.A.F., The Estimation of Animal Abundance and Related Parameters, 2nd ed. Macmillan, New York, NY, USA. 654 pp. Skalski, J.R., Smith, S.G., Iwamoto, R.N., Williams, J.G., Hoffman, A., Use of passive integrated transponder tags to estimate survival of migrant juvenile salmonids in the Snake and Columbia rivers. Canadian Journal of Fisheries and Aquatic Sciences 55, Solomon, D.J., Some observations on salmon smolt migration in a chalkstream. Journal of Fish Biology 12, Thorpe, J.E., Ross, L.G., Struthers, G., Watts, W., Tracking Atlantic salmon smolts, Salmo salar L., through Loch Voil, Scotland. Journal of Fish Biology 19, Whalen, K.G., Parrish, D.L., McCormick, S.D., Migration timing of Atlantic salmon smolts relative to environmental and physiological factors. Transactions of the American Fisheries Society 128, Wood, C.C., Predation of juvenile Pacific salmon by the common merganser (Mergus merganser) on eastern Vancouver Island: I. Predation during the seaward migration. Canadian Journal of Fisheries and Aquatic Sciences 44,