Area use and movement patterns of wild and escaped farmed Atlantic salmon before and during spawning in a large Norwegian river

Abstract

We compared the within-river movements and distribution of wild and escaped farmed Atlantic salmon Salmo salar before and during spawning in the Namsen river system of Central Norway. A total of 74 wild and 43 escaped farmed salmon were captured at sea, tagged with radio transmitters and released. Based on our examinations, most, if not all salmon (farmed and wild) entering the River Namsen were sexually mature. Farmed salmon entering the river system had a higher probability than wild individuals of reaching the migration barrier in the upper part of the river, 70 km from the sea. During the pre-spawning and spawning periods, farmed salmon were located mainly in the upper parts (50 to 70 km from the sea), whereas wild salmon were evenly distributed along the entire river during both periods. Consequently, the probability of farmed × wild inter-breeding varied among river sections. Our finding that the distribution of escaped farmed salmon may differ from that of wild salmon and among river sections in the prespawning and spawning periods-and that it may also vary over time-must be taken into consideration when (1) designing monitoring programs aimed at estimating the proportion of escaped farmed salmon in rivers and (2) when interpreting monitoring results. Furthermore, targeted fishing in the river aimed at reducing the number of farmed salmon prior to spawning may be more effective in upper rivers sections, and below major migration barriers.

Half a century of genetic interaction between farmed and wild Atlantic salmon: Status of knowledge and unanswered questions

“…Data from early surveys conducted in Norway revealed unweighted annual average maturation of escapes captured in rivers as 91.9% (range 77%–100% over the 12 years) and 86.8% (range 64%–100%) for males and females, respectively (Fiske et al., ). Also, in a recent study conducted in the River Namsen, middle Norway, most of the escapees entering the river were mature or maturing (Moe et al., ). In contrast, all of 29 small (0.4 kg) escapees captured in the River Steinsdalselva in western Norway in 2012 were immature (Madhun et al., ), and observations of large numbers of immature adults have been reported in rivers in Canada (Carr, Lacroix et al., ; Lacroix et al., ).…”

“…While farmed escapees may successfully spawn in the same areas of rivers as wild fish (Butler et al., ), studies have shown that adult farmed escapees do not necessarily use the same regions of a river during the spawning season as wild fish (Moe et al., ; Okland et al., ; Thorstad et al., ). Furthermore, in the absence of significant migration barriers such as large waterfalls, farmed escapees have a tendency to migrate to the upper reaches of rivers (Moe et al., ; Thorstad et al., ). In addition to area use differences, the timing of farmed salmon spawning may not be synchronized with the native population (Fleming et al., ; Moe et al., ; Saegrov et al., ; Webb et al., ).…”

“…Furthermore, in the absence of significant migration barriers such as large waterfalls, farmed escapees have a tendency to migrate to the upper reaches of rivers (Moe et al., ; Thorstad et al., ). In addition to area use differences, the timing of farmed salmon spawning may not be synchronized with the native population (Fleming et al., ; Moe et al., ; Saegrov et al., ; Webb et al., ). Variations in “time and space,” in addition to the documented competitive inferiority of farmed escapees under spawning, may contribute to a partial or total miss‐match of spawning relative to wild salmon under certain conditions and thereafter influence patterns of introgression and offspring survival.…”

Behaviour and survival of wild Atlantic salmon Salmo salar captured and released while surveillance angling for escaped farmed salmon

“…Whitney et al 2016) of the recovery time of salmon after exercise. In general, longer buffer periods will be best but this must be considered against the run timing of farmed fish to ensure a representative sample (Moe et al 2016, Svenning et al 2017. Farmed salmon also tend to have a different distribution within the river than wild fish, and have been observed migrating farther upriver than wild fish (Moe et al 2016); therefore, efforts to refine surveillance fishing methods may be necessary to further ensure representative sampling.…”

“…However, farmed Atlantic salmon are known to enter rivers later in the season than wild fish (e.g. Moe et al 2016, Svenning et al 2017, and often after the recreational fishery has closed. Therefore, scale samples obtained during the summer underestimate the extent of farmed salmon in the population.…”