Application of acoustic tagging, satellite tracking and genetics to assess the mixed stock nature of coastal net fisheries

Scottish Marine and Freshwater Science Vol 9 No 5
An array of 78 acoustic receivers was distributed among 47 rivers predominantly on the east and north coasts of Scotland. A further 8 receivers were moored on the north and east coasts near to shore. A total of 81 salmon captured in the Armadale coastal net fishery between 7 July and 25 August 2017 were tagged externally with small acoustic transmitters and released. Genetic samples were taken from each fish. Of the tagged fish, 44 (54%) were detected in rivers, one on the east coast (River Spey) and the others on the north coast (Rivers Naver, Borgie, Strathy, Halladale, Kinloch and Polla). The results are consistent with the main impact of the fishery being in the order of 100km, with occasional salmon captured from further away. Genetic analysis (in the absence of confirmation by tracking) would have suggested that the fishery was substantially more mixed stock with 30% of the catch being from the east coast and Hebrides. In fact, many of the fish assigned by genetics to east coast stocks appeared to return to north coast rivers. Possible explanations for this discrepancy are discussed. A range of patterns of movement of salmon among different rivers was observed. Tagged fish were detected near to shore on the north coast in the marine receiver array.

Results of these 2017 experiments were compared with data collected in 2013 and 2014 using satellite tagging and associated genetics. In that case there was evidence of greater interception of east coast salmon at Armadale and good correspondence between predictions of home river from genetics and observations from satellite tagging, albeit based of a small sample size.

Each salmon sampled in 2017 was assessed for presence of red vent syndrome (RVS) whereby the vent area of the fish was swollen and reddened. The incidence of RVS was 82%, higher than recorded previously. No difference could be detected in the subsequent survival of fish with and without RVS, but the power to detect any effect was very low because there were so few fish without the syndrome.