Horizontal movements of simulated escaped farmed Atlantic salmon (Salmo salar) in a western Norwegian fjord

Transcription

1 1206 Horizontal movements of simulated escaped farmed Atlantic salmon (Salmo salar) in a western Norwegian fjord Ove T. Skilbrei, Jens Christian Holst, Lars Asplin and Stein Mortensen Skilbrei, O. T., Holst, J. C., Asplin, L., and Mortensen, S Horizontal movements of simulated escaped farmed Atlantic salmon (Salmo salar) in a western Norwegian fjord. ICES Journal of Marine Science, 67: The dispersal of simulated escaped farmed Atlantic salmon (Salmo salar) in Hardangerfjord in western Norway was studied by telemetry. Tagged fish were released from fish farms on five different dates in the course of 1 year. Irrespective of the time of year, the fish dispersed rapidly, with a mean displacement from the release site of 5 7 km after 1 d, and 9 12 km after 2 d. Individual rates of movement varied, but were much higher than the rate of displacement from the release site, as fish moved around in the fjord basin in all directions. As a result, the fish were spread over an area of more than 500 km 2 after 1 week. The number of released fish recorded in the fjord gradually declined after each release from 40% remaining in the fjord after 3 weeks to few or none after 7 weeks. In all, 38% moved out of the fjord, and 36 and 2% were reported as captured inside and outside the fjord, respectively. Their rapid dispersal suggests that concentrating efforts to recapture escaped salmon in the proximity of an escape site will probably not be successful, at least from locations of this type. In fact, the recaptures demonstrate that it is both necessary and possible to expand the fishing effort to cover a large area and to sustain it for several weeks to attain high rates of recapture. The wide dispersal of escaped fish potentially acting as vectors within the fjord basin also has implications for the spread of disease and parasites. Keywords: escaped farmed salmon, fish farms, horizontal movements, recapture. Received 19 December 2009; accepted 9 March 2010; advance access publication 13 April O. T. Skilbrei, J. C. Holst, L. Asplin and S. Mortensen: Institute of Marine Research, PO Box 1870 Nordnes, NO-5817 Bergen, Norway. Correspondence to O. T. Skilbrei: tel: ; fax: ; ove.skilbrei@imr.no. Introduction Atlantic salmon (Salmo salar) farming has grown rapidly since the mid-1960s. Production passed t in Norway in 2008 (Directorate of Fisheries; Escapements in Norway and in other countries have been identified as a serious risk associated with fish-farming activities (Ford and Myers, 2008), particularly because escapees enter rivers where they may have a negative impact on the genetic make-up of wild stocks if they spawn successfully (McGinnity et al., 2004; Naylor et al., 2005; Skaala et al., 2006; Ferguson et al., 2007). In addition, if fish that escape from farms are diseased or carriers of potentially harmful pathogens, they represent a potential risk for nearby farming operations and for wild stocks. In Norway, fish farmers are required by law to recapture escaped salmon. At present (Regulation No. 984), this obligation is restricted to an area within a 500-m radius of the production unit, and it is withdrawn when the fish are obviously no longer within that area. If there is a good possibility of recapturing escaped fish, the Directorate of Fisheries and the Department of Environmental Affairs at the County Governor s office may decide to expand the fishing area and fishing period. The last option is usually adopted when a massive escape has taken place. However, attempts to recapture fish have not been successful and are usually not welldocumented. Following two large escape incidents, for instance, fewer than 3% of the escapees were recaptured (Anfinsen, 2005; Buvik, 2005). After examining the catch statistics for a seasonal fishery that targeted escaped farmed salmonids, Skilbrei and Wennevik (2006) concluded that a fjord fishery for escaped salmonids could help to reduce the number of escaped fish, but they were unable to quantify the efficiency of the fishery. One aim of organizing a fishery to capture recently escaped, mostly immature, salmon is to catch them while they are still abundant in the vicinity. After they leave the area and return to the coast later in life as maturing fish, they may be spread over a large area (Skilbrei and Holm, 1998; Hansen, 2006). When the potential ecological consequences of escapes, the size of the fish-farming industry, and the poor outcome of the efforts to capture escaped salmon are taken into account, the need for more-specific knowledge of the behaviour of newly escaped salmon is obvious. Some studies published on post-release behaviour have employed telemetry as a method of tracking the movements of released fish (Furevik et al., 1990; Whoriskey et al., 2006; Lindberg et al., 2009). We studied the post-release behaviour of simulated escaped salmon tagged with acoustic tags in Hardangerfjord, a large fjord in western Norway, where there is a large salmon-farming industry. The vertical distribution in the water column of the simulated escapees has been described already (Skilbrei et al., 2009), demonstrating a tendency for fish to dive on the first day after release, but also that they changed preference and moved very close to the surface during subsequent weeks, especially at night. This behaviour is advantageous to recapture efforts, # 2010 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org

2 Horizontal movements of simulated escaped farmed Atlantic salmon 1207 Figure 1. Location of the acoustic receivers (black stars) and the two cage release sites (boxes). Additional receivers that were present during Release 5 are shown separately (dots). The outermost receivers (see text) are circled. because the fish are easier to target with traditional fishing gear such as gillnets when they are moving in the upper layer. Here, we describe the horizontal distribution obtained from the same study, i.e. the dispersal and movements of the fish tagged acoustically within the fjord, which is the next key question of importance for both the recapture strategy and the spread and control of disease and parasites between fish farms and wild salmonid populations in the fjord basin. It is essential to know whether the fish stay close to the release site, disperse rapidly throughout the fjord, and leave the fjord shortly after release. Material and methods The hydrographic conditions in the middle of Hardangerfjord (Figure 1), in the area around the island of Varaldsøy, are typical of fjords. There are major freshwater sources in the near and inner region of the fjord that tend to produce a distinct upper layer of brackish water at a depth of 5 10 m, with a surface salinity of,15 20 from spring until late autumn. Temperature in the upper layer varies greatly during the year, ranging from 2 to 48C during winter to 158C in summer. Below m, there is less variability in salinity and temperature throughout the year, with typical values ranging between 34 and 35 and 7.5 and 8.58C, respectively. The temperature and salinity profiles during the present experiment are described in Skilbrei et al. (2009). There is a large salmon-farming industry in the fjord. Total salmon production from around 50 sites was close to t in 2003 (Skilbrei and Wennevik, 2006), and it passed t in 2008 (Directorate of Fisheries). There are grounds for concern about the negative effects of escaped farmed salmon on the genetic make-up of several wild salmon stocks in rivers that drain into the fjord (Skaala et al., 2006). Water current and wind observations Recently, currents in Hardangerfjord 2 m deep have been observed from a surface observational buoy positioned across the fjord from the fish release sites (Årvikneset; N E; data.nodc.no/observasjonsboye). The recording current meter is produced by Aanderaa Data Instruments ( using the Doppler Current Sensor technology. Continuous current measurements from the buoy are averaged every 10 min. The observation period was 31 January 26 August The Norwegian Meteorological Institute (met.no) operates an automatic weather station at Kvamsøy farther into the fjord from the Varaldsøy area. Observations of 24-h mean winds were kindly made available by met.no through the service eklima. met.no. Southwest is directionally out of the fjord and northeast is into the fjord. Tagging and releases In all, 132 farmed salmon were tagged with V13P-1L-S256 coded pingers carrying a depth sensor (45 mm long, weight in water 6 g, min/max off time 40/120 s; Vemco Ltd; Shad Bay, NS, Canada) at two commercial salmon farms located close to Varaldsøy in the middle of Hardangerfjord (Figure 1) and released

3 1208 O. T. Skilbrei et al. Table 1. Description of Releases 1 5 (dates of tagging and release of the fish at Release sites 1 and 2, mean size of fish, numbers tagged, percentage and numbers of fish that supposedly moved out of the fjord, and percentage and numbers of captured fish). Description Release 1 Release 2 Release 3 Release 4 Release 5 Date tagged 24 June August December March June 2006 Date released 1 July August December March June 2006 Release site Mean (s.d.) length (cm) 63.7 (3.5) 72.4 (5.6) 54.0 (3.1) 60.5 (3.3) 70.4 (3.9) Mean (s.d.) weight (kg) 2.8 (0.5) 3.0 (0.5) 4.3 (0.8) Number released Percentage (number) moved out 36.8 (7) 29.2 (7) 31.0 (9) 33.3 (10) 56.7 (17) Percentage (number) captured 15.8 (3) 62.5 (15) 37.9 (11) 50.0 (15) 20.0 (6) on five dates in the course of a year in groups of fish (1 July 2005, 28 August 2005, 15 December 2005, 15 March 2006, and 8 June 2006; Table 1). The tagged fish showed no external signs of maturity. The mean fork lengths of the groups of fish released ranged from 54 to 70 cm (Table 1). The fish were domesticated salmon produced at and released from commercial fish farms. All were 1-year-old smolts. Fish from the two first groups to be released were transferred from the smolt hatchery to cages in the sea at the release site in early May 2004, more than a year before the start of the experiments, and the fish used in the subsequent releases were transferred as smolts to the fish farm at the release site in late April or early May Tags were surgically inserted in the abdomen of the fish, as described by Skilbrei et al. (2009), and the fish were kept in a small net-pen for 5 7 d before release. The experiment and the tagging procedure were approved by the Norwegian committee for the use of animals in scientific experiments (FDU). Range-testing trials in the Alta Fjord, performed with tags used in the present study that had been returned by recreational fishers, demonstrated that the tags could be registered by receivers at a distance of m (C. Chittenden, pers. comm.). However, receiver range may vary considerably, and probably did so in the present study. Finstad et al. (2005) demonstrated that the maximum listening distance was.10 the minimum range at the same site. Acoustic receivers In all, 25 VR2 receiver units (Vemco Ltd) were moored at 24 different sites in the fjord in June 2005; the distance between the innermost and the outermost locations in the fjord was 74 km (Figure 1). The distances from the two release sites to the outermost receivers were km, and the innermost was km away. Release site 1 was between, and within the immediate range of, the two receivers moored northeast of Varaldsøy (Figure 1). One receiver was located directly at Release site 2 at the east coast of Varaldsøy, and an additional backup receiver was added to this site in May The receivers were moored to a weight on the seabed, which was also moored to land, and were kept at a depth of m by a float, except for the two that were attached to the floating structure of the cages at Release site 2. From May 2006, before the last release on 8 June 2006, data were also available from a grid of 26 VR2 receivers operated by the Norwegian Institute of Nature Research that covered the inner part of the fjord system and expanded the distance from the release sites to the innermost receivers by 75 km (Figure 1). The area covered by the receivers then increased from 570 to 860 km 2, but note that these added receivers were only used in the analysis of the last release. Data treatment The following definitions and calculations were employed to describe and quantify fish movements. Moved out of the fjord. If the last detection of the fish was made by the outermost acoustic receivers (Figure 1), it was assumed that the fish moved out of the fjord on that date. Disappeared. Fish that were not reported captured and did not move out of the fjord (see above) were classified as disappeared from their last date of detection. However, the possibility exists that fish defined as disappeared might also have left the fjord without being detected by exit receivers or may still have been present within the fjord. Presence in the fjord. A fish was assumed to be present in the fjord from the release day until the date before its last detection or the date of its reported capture in the fjord. Location of fish. To estimate the position of a fish that was not within the range of a receiver at 12:00, it was assumed that it swam at a constant speed from the last receiver that had registered it to the next that did so, taking the shortest possible route. Daily distance to the release site. The distance to the release site was defined as the shortest possible swimming distance to the release site from the location of the fish at 12:00. If a fish left the area covered by the receivers, its last recorded distance was used as a conservative estimate of its distance the following days. Rate of movement between receivers. To estimate the maximum swimming speed, the movement speeds between receivers were calculated. It was assumed that the fastest recorded speeds would reflect the maximum swimming speeds of fish moving rapidly that swam, by chance, directly between two receivers. Only those receivers located more than 8 km apart were used in the analysis, except for day night comparisons when a 5-km minimum distance was used owing to the limited period available for movements between receivers (see below). The reason for not using data from more closely adjacent receivers was to reduce the possible bias under good listening conditions towards an overestimate of the movement speed, because the distance travelled by a fish could then have been considerably shorter than the actual distance between the receivers. Comparison of movement speed between day and night. The diurnal cycle was divided into day and night as follows: day, the period from sunrise to sunset; night, the period from the onset of nautical twilight in the evening to the end of nautical twilight in the morning. The computations of sunrise, sunset, and twilight times were made by the Online Photoperiod Calculator V 1.94L ( The two summer releases (8 June and 1 July) were not used for day night comparisons because of the short duration of the period of darkness at that latitude in summer. Only movements between receivers that started

4 Horizontal movements of simulated escaped farmed Atlantic salmon 1209 Figure 2. Stacked area plots showing the trend in relative proportions of fish present in the fjord, captured, moved out of the fjord, and disappeared for each of the five release groups. and ended within daylight hours of a single day, or during nighttime, were included in the computations. Movement speeds by day and night were compared with two-tailed Student s t-tests. Comparisons between capture rates. A G-test (Sokal and Rohlf, 1981) was utilized to test whether the number of fish reported captured differed between groups. Results Wind and current observations Observations from the buoy from January to August 2009 illustrate typical levels and variability of the current in the Hardangerfjord. We found an average speed of 0.14 m s 21. The direction varied owing to tides and variable winds. We could identify strong wind episodes from periods with current speed.0.5 m s 21, lasting typically for a few hours. For the five release periods, the wind observations at Kvamsøy showed mostly weak winds with speeds,5 ms 21. The exception was 29 August 2006, the day after the second release, when a strong up-fjord wind of 12 m s 21 was recorded as a mean for the whole day (wind direction 2408). Fish abundance and dispersal in the fjord The proportion of the fish detected in the fjord fell steadily with time for all release groups (Figure 2). About 40% of the fish were still recorded 20 d after release, but few fish were observed after.70 d. Some of the fish leaving the observation area and others being caught can explain most of this reduction, and 53 83% of the fish from the various release groups could be accounted for 90 d after release (Figure 2). The fish dispersed rapidly from the release sites. The mean distance of individual fish to the release sites was 5 7 km after 1 d, and 9 12 km 2 d after release (Figure 3). The distance gradually increased in most of the groups during the following 5 d (0 2kmd 21 ; Figure 3). Two days after the release on 1 July 2005, most of the fish were distributed around Varaldsøy, within an area of 150 km 2 (Figure 4a). The exception was one fish that migrated out of the fjord and passed the outermost receiver 52 km away during day 2. One week after release, most of the fish were still in the waters around Varaldsøy, but they were dispersed over an area of.500 km 2 owing to the outward movements of two fish.

5 1210 O. T. Skilbrei et al. Figure 3. Mean distance of fish to release site at 12:00 on days 1 7 post-release. The five release groups are shown separately. Data for Release 3 are not complete owing to the small number of observations on days 2 6. Most of the fish released on 28 August 2005 moved northeastwards towards the inner part of the fjord (Figure 4a) with a mean inward displacement of 0.05 m s 21 during the first 2 d; this is a minimum estimate, because two fish had already passed the innermost receivers. The fish were spread over an area of at least 179 km 2 and a distance of more than 40 km after 2 d. In the course of the next 5 d, many of the salmon continued to enter the inner part of the fjord, and others spread out in other directions, covering an area of at least 374 km 2. The maximum distance between recorded fish was.50 km. On the day after the 15 December release, the fish were dispersed over an area of 122 km 2 over a distance of 30 km, with a concentration around the southern tip of Varaldsøy (Figure 4a). After 1 week, they were dispersed over the whole area covered by the receivers (570 km 2 ) over a distance of 74 km, with few fish remaining in the centre of the fjord. After the fourth release on 15 March, there was again a tendency for the fish to move towards the inner part of the fjord (Figure 4b). They were distributed around Varaldsøy over an area of 200 km 2 within a range of 30 km from the outer- to the innermost fish after 2 d, with most being registered in the north. After a week, several salmon had moved further into the fjord, but others had also reached the outer end of the fjord, an overall distance of.60 km. Rapid dispersal was also documented after the final release on 8 June 2006 (Figure 4b). During the first 2 d, the fish dispersed over 250 km 2 and a distance of 60 km. However, as following the other releases, most remained in the central part of the fjord around Varaldsøy. During the next 5 d, fish moved both up and down the fjord, resulting in a maximum distance between recorded fish of 90 km and an extension of the dispersal area to 710 km 2. On average, the recorded fish were spread over 182 km 2 after 2 d (1 d for the third release) and 556 km 2 after 7 d. However, these are minimum estimates because fish may have moved out of the area covered by the receivers. Speed and pattern of movement in the fjord The speed of movement of the fish was clearly faster than their rates of dispersal. Although their swimming routes between receivers are unknown, and probably longer than the distance between the receivers in many of the cases recorded, the mean movement speed was 0.35 m s 21 (s.d. 0.46), or 30 km d 21, which is 0.5 body lengths s 21. The mean sum of displacements between receivers in the first week was lower, at 6 km d 21. This discrepancy was partly a result of fish visiting the same receiver repeatedly. Maximum movement speed was 1.2 m s 21 (Figure 5), which is equivalent to 104 km d 21 and corresponds to 2 body lengths s 21. The superior individual rate of movement compared with the slower dispersal rate from the release site (Figures 3 and 5) was also a consequence of frequent shifts in swimming direction of the salmon. The trajectories of some of the fish are used to illustrate the behaviour typical of fish residing in the fjord; for example, two salmon from the final release that moved repeatedly up and down the fjord many times before they disappeared after 16 and 18 weeks, respectively (Figure 6). Movements out of the fjord Dispersal or migration out of the fjord was a gradual process that took several months in all release groups (Figure 2). Seven of the 132 fish released moved out of the area before 12:00 of the seventh day (Figure 6; 1, 0, 1, 1, and 4 fish from releases 1 5, respectively). Most fish moved out through the main fjord, but 33% (of 49 fish) that supposedly left the fjord moved out through the two narrow straits in a northwesterly direction (see examples in Figure 6). Two of those with the shortest residence times in the fjord, 4 and 6 d, are shown in Figure 6. Several more from the same release left during the seventh day following the same route (Figure 4b). Day night comparisons The mean speed of movement was significantly slower at night than by day: 0.49 m s 21 (s.d. 0.25) vs m s 21 (s.d. 0.30; p, 0.01). These are the mean speeds of the fish that managed to be registered by two different receivers before the subsequent dusk or dawn. There is reason to believe that these figures underestimate the real differences between day and night movements. The recorded movements between receivers at night only corresponded to 18% of the total number of displacements that could be classified as either day or night movements and to 12% of the distances moved. This clear imbalance between night and day displacements suggests that relatively few fish reached another receiver before dawn because of a slower horizontal night-time movement. Recaptures Almost 38% of the salmon were reported as recaptured (50 out of 132) by recreational fishers, with a range between releases of % (Table 1). The fishing method was reported for 36 of these fish. Most were gillnetted (77.8%), but fish were also caught by bagnet (5.6%), rod from land (13.9%), and by trolling (2.8%). All were caught in the sea or brackish water. The reported catches of the two midsummer releases ( 20%) were lower than those of the three autumn and winter releases (.37%; p, 0.05). The highest recapture rate of 62.5% preceded the release on 28 August. In addition to the captures shown in Figure 2, the total catch of that release group includes the (2)

6 Horizontal movements of simulated escaped farmed Atlantic salmon 1211 Figure 4. Estimated positions of individual fish at 12:00 on day 2 (open red circles) and day 7 (blue stars) preceding the releases on (a) 1 July 2005, 28 August 2005, and 15 December 2005, except that day 1 is shown instead of day 2 following the 15 December release, and (b) 15 March 2006 and 8 June Black arrows indicate fish that have presumably moved out of or towards the inner part of the fjord. The corresponding numbers show the number of fish that had moved before day 2 (red number) or between days 2 and 7 (black numbers). fish that remained in the fjord on day 90 and the capture in other fjords of three of the fish that left Hardangerfjord earlier (two of them are shown in Figure 7). All other catches were reported from Hardangerfjord, mostly in the central area of the fjord (Figure 7). The mean (swimming) distance from recapture to the release site was similar for Release sites 1 and 2: 28.4 km (s.d. 22.9) and 23.3 km (s.d. 18.0; not significant, t-test). Note that one fish caught in a fjord 324 km away was not included in the analysis. On average, the fish were recaptured after 37 d (s.d. 45). Discussion Our study has demonstrated that simulated escaped salmon dispersed rapidly from fish farms in a large fjord. The recorded fish were scattered over an area of several hundred square kilometres after 2 d, and more than 500 km 2 a week after the simulated escapes. Most fish appeared to move at random within the basin and remained in the fjord system for several weeks. The swimming performance of the simulated escaped fish was comparable with the normal swimming activity of farmed salmon in net-pens. Although the actual routes taken by salmon between receivers are unknown and are probably longer than a straight line between the receivers, the mean displacement rate corresponded to speeds of 0.5 body lengths s 21. Normal swimming speeds in net-pens tend to range between 0.4 and 0.6 body lengths s 21 (Dempster et al., 2008, 2009; Korsøen et al., 2009). The clear trends towards lower speed and reduced horizontal activity at night were similar to the diurnal cycle of fish in cages. Korsøen et al. (2009) observed that swimming speeds fell by approximately one-third at night. Except the midsummer releases, the reduced rates of movement by night coincided with a clear tendency of the fish to move closer to the surface than by day (Skilbrei

7 1212 O. T. Skilbrei et al. et al., 2009), implying a diurnal cycle in both horizontal and vertical movements of escaped salmon. It is an open question whether released fish respond to the ambient current. Currents in fjords, as in the ocean, are practically horizontal and with distinct vertical variation. The most variable and energetic parts of the currents are close to the surface, in the upper m, reflecting the forcing mechanisms behind the currents. The total current in a fjord is a combination of various components, each a result of a specific forcing mechanism. Figure 5. Summary of all individual speeds between receivers separated by.8 km. Usually, the main forcing of the fjord water is from freshwater run-off, winds, tides, and various internal waves. We lack detailed information about the current for the release periods, but have some general knowledge that water speed can be comparable with the swimming speed of the fish. Therefore, it is likely that fish might choose to swim with the current (or simply drift) rather than against it. The most obvious pattern was after the 28 August release, when most of the fish released swam up the fjord, coinciding with the period of strong up-fjord wind, so we might expect a relatively strong up-fjord current in the upper few metres. The observed rates of movement are also comparable with the swimming speeds of wild adult salmon in the sea. According to Tanaka et al. (2005), the mean swimming speed of chum salmon (Oncorhynchus keta) is 36.4 km d 21 (0.42 m s 21 ) during oceanic migration, rarely exceeding 1 m s 21. Hansen et al. (1993) observed that few wild adult salmon exceeded 100 km d 21 during migration, and then only when moving with the dominant coastal current. The present estimate of a maximum movement speed of 1.2 m s 21 (2 body lengths s 21 or 104 km d 21 ) displayed by the simulated escapees is somewhat lower than the estimates of sustainable swimming capacity of 3 body lengths s 21 for upstream-migrating adult salmon (Thorstad et al., 1997; Booth, 1998). The observed swimming speeds and the swimming capacity of fish of this size would have taken most of the simulated escapees out of the fjord during their first week at liberty if they had moved either directly towards the mouth of the fjord or with a speed comparable to that of wild salmon during their spawning Figure 6. Simplified movement patterns or trajectories based on visits to receivers of single fish from the final release group. Two fish resident in the fjord are shown from 8 June 2006 until they disappeared on (a) 29 September 2006 and (b) 12 October Trajectories are also shown for (c) two fish that left the fjord rapidly through one of the straits after 4 d (black line) or 7 d (red), and (d) two salmon leaving through the main fjord after 17 d (red line) and 18 d (black).

8 Horizontal movements of simulated escaped farmed Atlantic salmon 1213 Figure 7. Locations in Hardangerfjord and the adjacent fjord for reported catches of simulated escaped salmon following all five releases. migration. Only 7 of the 132 fish released were last recorded at the outermost receivers, located km distant from the release position, within the first week. The speeds of adult wild salmon ranged for the most part from 6 to 33 km d 21 in two other large Norwegian fjords (Hansen et al., 1993) and were close to 40 km d 21 for kelts migrating out through a northern Norwegian fjord (Halttunen et al., 2009). Hansen and Jonsson (1989) observed that adult fish released from net-pens during their second spring gave higher returns as mature salmon than fish released during winter, possibly showing that fish released during spring had a stronger tendency to migrate to the open sea than those released during autumn and winter. If the time of the year affected the migratory behaviour in the present study, its influence must have been minimal because of the slow dispersal out of the fjord over several months of fish in all release groups. Instead, it appeared that the fish were moving at random, with frequent shifts in swimming direction. This behaviour resulted in a prolonged stay in the fjord that would have consequences for the probability of fish entering local rivers, the recapture of escaped fish, and the possible spread of parasites and disease. Most of the escaped farmed salmon in Hardangerfjord (Skilbrei and Wennevik, 2006) and the fish used in the present experiment were assumed to be immature at the time of their escape. However, if some of the escapees remained in the fjord basin for months, the chances are that some would have reached sexual maturity and entered rivers that drain into the fjord system. This may have contributed to the relationship observed between the frequency of farmed salmon in rivers and the local level of fishfarming activity (Fiske et al., 2006). The rate of capture of the simulated escapees, which ranged from 16 to 63%, was higher than expected, because there are no reports of high rates of recapture following escape incidents from commercial farms in Norway. One reason for this may be that most fishing effort focuses on the immediate vicinity of the fish farm. According to the present results, a fishery that targets escapees within 500 m of the fish farm, as required by regulation, will not be effective. At the same time as the fish disperse rapidly into an area of several hundred square kilometres, they also swim deep. The different release groups spent 7 50% of their time below 14 m during the first day after release, and dives to 80 m were frequent (Skilbrei et al., 2009). Because of the vertical and horizontal dispersion of the escaped fish, it may not be realistic in practice to target them effectively at short notice within a deep water column within the 500-m radius. One other release study has also reported that fish disperse rapidly from the release site (Whoriskey et al., 2006). The fish moved much closer to the surface after a couple of days, especially at night (Skilbrei et al., 2009). During the following 4 weeks, they stayed at depths,5 m most of the time. This behaviour is advantageous for recapture efforts, because the fishing effort can focus on a smaller volume where traditional fishing gears such as gillnets and bagnets can be used. The catchability rate reported here might have been relatively high because fish were moving in different directions inside the fjord, because their speed was much higher than their dispersal rate. Hence, a single fish may potentially visit and revisit several fishing-gear sites over a large area. A similar study in a northern fjord (C. Chittenden, pers. comm.) documented escapees tending to travel parallel to the shore and concluded that this movement pattern contributed to the success of bagnets as a recapture method. Our results imply that the fishery for escaped salmon following escape incidents needs to cover a large area, probably at least km from the fish farm, and should last 5 6 weeks while the escapees remain in the fjord. Several types of traditional fishing gear may be used. Gillnets are easy to manage, are widely available, and can be deployed within a few hours. More effort is needed to install and handle bagnets. On the other hand, bagnets and other types of fishing gear developed to keep fish alive may be the only alternative if the fishery targeting escapees conflicts with the need to protect migrating wild salmon, especially if fish escape from sites close to salmon rivers during summer or early autumn. In Alta Fjord, for example, a very high percentage of simulated escapees were captured in the bagnet fishery for wild salmon

9 1214 O. T. Skilbrei et al. migrating through the fjord (C. Chittenden, pers. comm.). In Hardangerfjord, fishing gear that targets salmon is generally not allowed before 1 October, perhaps lowering the potential catch from, and the motivation to report, summer fishing in the present study. Escapes can be a challenge for managing fish health. The observed pattern of movement of escaped salmon implies that they might act as vectors of disease and parasites vis-à-vis both farms and wild populations of salmonids in large fjords with salmon farming. Little is known of the actual transmission mechanisms between wild fish and those inside net-pens. In most fjord systems, distances between farms are short. For example, there are 50 fish farms in Hardangerfjord (Skilbrei and Wennevik, 2006). If both wild fish such as saithe (Pollachius virens; Uglem et al., 2009) and escaped salmon and rainbow trout (Oncorhynchus mykiss) carry pathogens and frequently visit or move around fish farms, such fish should be regarded as a risk factor. Such movements are frequently not included in risk assessments, and the debate on the importance and relevance of horizontal transmission of important fish diseases often focuses on the survival of pathogens in the water column and, hence, on the distance by sea between farms. When modelling the risk of spreading infectious salmon anaemia, Scheel et al. (2007) found a relatively low level of explainability related to distance, shared management, and infrastructure, suggesting transmission from sources other than infected farm sites. A similar study of pancreas disease gave a much higher explainability, suggesting a high degree of horizontal transmission (Kristoffersen et al., 2009). However, those studies focused on the importance of identifying other factors capable of explaining the transmission between farms. Our data on the movements of escaped salmonids within the fjord basin, and presumably also recent data on movement patterns of wild fish such as saithe (Uglem et al., 2009), indicate that fish movements could help to explain some of the unknown factors in the models employed in risk assessment. To conclude, our results have demonstrated that an escape is not a purely local event, but rather a regional one. The salmon disperse rapidly throughout the fjord system, and to recapture them as adults, a grid of fishing gear covering an area of hundreds of square kilometres would appear to be preferable to concentrating fishing effort near the net-pens from which the fish had escaped. 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