Salmon escape from fish farms at all life stages, to both fresh and salt water. They are caught in ocean fisheries, and should they mature will move to freshwater to spawn (e.g. Hansen et al. 1987; Gausen & Moen 1991; Webb and Youngson, 1992;
Youngson et al. 1997; Crozier 1998; Carr et al. 1998; Whoriskey & Carr 2001).
Farmed salmon are taken in large numbers in Norwegian coastal commercial salmon fisheries (about 24% of total nominal catch in 2002). Their proportion is lower in fjord and freshwater catches, but increases in spawning populations. Tagging experiments have shown that farmed salmon from Norway are caught in the Faroes’ fisheries (Hansen et al. 1987). The abundance of farmed salmon in oceanic areas at Faroes is high (Hansen et al. 1999). Farmed fish have been captured at much lower frequencies in fisheries in Scotland, Ireland and Northern Ireland, despite the presence of extensive salmon farm production in these regions (ICES CM 2001/ACFM:15). This may be due to differences compared to Norway in the siting of salmon farms in relation to the salmon rivers and fisheries, or it may be due to different dispersal patterns of the farmed fish after they escape.
Wild salmon smolts leave their home rivers in the spring and move quickly into oceanic areas. In the north east Atlantic zone, smolt tagging experiments and post-smolt surveys have strongly indicated that ocean currents are the vectors that force the fish northwards (Holm et al. 2000). Salmon smolts imprint, or learn cues sequentially on their way from the river to the sea, and use that information for homing on the return migration. The homeward migration may be divided in two phases, an oceanic phase with fast movement from the ocean to coastal areas, and a slower migration from coastal areas to the natal river (Hansen et al. 1993). Migration patterns of hatchery-reared salmon released as smolts in freshwater are similar to those of wild salmon. Hatchery smolts released on the coast also tend to return to the area where they were released, but
areas from where they escaped, and the fact that Norway as the most significant producer of farmed salmon in the Atlantic should contribute many of the escaped farmed salmon observed in that area.
Results from an experiment that released large salmon from two farms on the south and mid- Norwegian coast showed that salmon escaping in the autumn had lower survival rates than fish released in the winter/early spring ((ICES CM 2001/ACFM:15; Hansen 2002). The released fish were recaptured in the sea, as well as in freshwater north of their experimental “escape” point. Some of the fish from the southern farm moved to the southeast and entered freshwater in this area. The movements could be explained by the direction and strength of ocean currents. Assuming that fish entering freshwater had made their final decision on where to spawn, it could be concluded that these farmed salmon were not imprinted to any particular river or marine site, and could therefore be regarded as "homeless". This contradicts Hansen and Jacobsen’s (2000) speculation that farmed homed to the area from which they escaped.
Ocean movements of the farmed salmon could be controlled by prevailing currents ((ICES CM 2001/ACFM:15; Hansen 2002). This may explain why so few of the fish released in the autumn in the previously described experiment were ever recovered. These fish could have been transported with the currents so far north that when they attained sexual maturity, they either were too far off route to find a river for spawning, or were simply lost in the cold Artic water. Fish that escape later in the year (closer to maturation) could have a higher probability of entering freshwater to spawn than early escapees, but the low recovery rates (less than 6%) of experimentally late released fish (Hansen 2002) suggest that significant numbers of them are also lost.
Based on the above, the following hypothesis is proposed: Farmed salmon escaping from cages in different countries are displaced with the currents, and any fish that become sexually mature when they are relatively close to the coast enter local fisheries and rivers. The signification of this is that escaped farmed salmon may spread into fisheries and rivers far away from where they escaped.
2.6.2 Methodology to improve knowledge on the distribution and movements of escaped farmed salmon Farmed salmon that have escaped from sea cages can easily be identified in fisheries and stocks, but it is more difficult to detect fish that escaped as parr or smolt. Sampling and examination of salmon in marine areas at different times of the year, especially in areas that have not been sampled before, would improve the general knowledge of the spatial and temporal distribution of farmed salmon.
At present it is difficult to determine from which country or area farmed fish caught in the ocean originated from. To approach this problem, it would be feasible to tag farmed fish, conduct experimental ”escapes”, and determine the ultimate fate of the fish. Recoveries could come from existing fisheries, and planned scientific sampling programmes. A number of different tags and tagging procedures could be used, including:
1. External tags (Carlin, Lea, Floy, etc.)
2. Visible implant tags (including visual implant elastomers) 3. Coded wire tags (CWT)
4. Passive Integrated Transponder (PIT) tags 5. Sonic tags
6. Data storage tags (DST) 7. Genetic tags
8. Physiological tags (otholith marking, trace elements in bones and otoliths, fatty acids, etc.)
External tags can be reliably detected in fisheries and scientific sampling programmes. Visible implant tags can be recovered in sampling programmes, but may be difficult to detect for fishermen.
Data storage tags are new technologies, and are still expensive. However, information on the behaviour (postion, environmental conditions, movements) of the recovered fish will be significant. Tagged fish can be recovered in sampling programmes or by fishermen.
Genetic and physiological tagging are new methods that can be used for mass marking. However, ”tagged” individuals have to be recovered in sampling programmes, and the marks are expensive to identify.
2.6.3 Experimental tagging programme for investigating the behaviour of escaped farmed salmon
To test the hypotheses that salmon escaping from fish farms in the Northeast Atlantic are homeless, transported with the currents, enter fisheries and rivers in other countries than the one they escaped from, or are lost in the Arctic, several tagging programmes using different tag types could be developed. Below a simple programme using individually numbered external tags that can be recovered both from fishermen and in sampling programmes is outlined, including a pilot project to be expanded to a main project. The programme is expected to give information on migration, distribution, survival and growth of escaped farmed salmon.
1. Pilot project
This should be carried out to compare migration and distribution of one single group (500-1000) of farmed salmon released in each of the countries producing farmed salmon (i.e. Ireland, Scotland, Faroes, Iceland and Norway). To maximise the probability for recaptures ((ICES CM 2001/ACFM:15; Hansen 2002) the farmed salmon to be released should be expected to be sexually mature the following autumn and should preferably be released in March/April.
External tags of the same origin and type should be used, and the releases should be co-ordinated in time. The recovery information should be used for developing a detailed design of the main project.
2. Main project
Groups of externally tagged farmed salmon should be released sequentially over the year (e.g. monthly, bimonthly etc), or over periods when escapes from salmon farms are known to occur, usually during the winter. The fish should be released in the same countries as suggested above, and the numbers of tagged fish in each group should be optimised based on results from the pilot project. The releases should be coordinated and the same types of tags should be applied.
This exercise is expected to give information on variation in migration, distribution, survival and growth of salmon escaping from fish farms at different times of the year.
Given the large numbers of farmed salmon escaping from cages in the Northeast Atlantic, the number of farmed salmon released for the purpose of this experiment will only be a small fraction of the total number of escaping salmon.
2.6.4 Sonic tracking of escapees in Maine (USA)
An experimental release of farmed salmon fitted with acoustic tags is planned to start in the Cobscook Bay region of Maine in autumn, 2003. This region produces the majority of the USA’s east coast farmed Atlantic salmon, and adjoins Canada’s Bay of Fundy region where the Canadian east coast industry is concentrated. The goals of the study are to:
• Document the residency time of “escaped” fish in the vicinity of the cages following the release.
• Track the directions and rates of any movements that the fish exhibit, and correlate them with tidal currents and other environmental cues.
• Based on histories of detection of the tagged fish on the receiver grid, attempt to determine their survival time at sea.
• Maintain a cross border detection grid in order to document the degree to which escapees stray between US and Canadian waters.
• Determine if the fish tend to move to particular rivers in the region at spawning time, presuming they survive for this long.
The project will provide short to medium term information about rates of dispersal of farmed fish, post-escape. Results should help with the development of recapture strategies, or if the program shows that the fish in this region are not likely to be recaptured, it will refocus efforts and scarce resources on ensuring containment.
2.7 Compilation of Tag Releases and Finclip Data by ICES Member Countries in 2002