Understanding and comparing early mortality of European salmon populations at sea.

SMOLTrack I was initiated in January 2017 and ended in December 2018.

Wild Atlantic salmon were tagged with acoustic transmitters and their subsequent migration followed via acoustic listening stations.

Wild salmon smolts were caught and carefully tagged with acoustic transmitters and then released. The tagged smolts were detected as they moved past Automatic Listening Station (ALS) arrays on the way from their natal river to the open sea. This project setup was carried out in five areas spanning almost the entire North – South distribution area in the EU (Denmark, England, Ireland, Northern Ireland and Spain).

Table 1. Fate of the released smolts for each study area. The percentage of smolts successfully crossing the study area is shown in brackets in the last column.

CountryRiverSmolts releasedSmolts lostSuccessful smolts
DenmarkSkjern865531 (36%)
EnglandTamar1003664 (64%)
IrelandErriff402812 (30%)
Northern IrelandBush996138 (38%)
SpainMinho502327 (54%)
SpainUlla100955 (5%)
∑ = 475∑ = 298∑ = 177 (37%)

The SMOLTRACK I project revealed variable, but generally low survival rates in the lower freshwater and transition environments through study areas across the European area of distribution of the Atlantic salmon. The methodology applied worked well and it was possible to acquire highly valuable data sets about loss of salmon smolts in lower rivers / estuaries, comparable over several countries. The results directed attention towards the conditions under which the smolts must move from river to sea as well as the presence of multiple predators. The results provide a solid, comparable one-year estimate on the survival of salmon smolts through the lower river and estuary / fjord from 6 river systems in 5 countries. The results show that potential bottlenecks for Atlantic salmon exists already in the initial migratory phase, which may have a large impact on the overall return rate of adult salmon.

In addition to the scientific aims, the project brought together a group of experts to provide advice on best practice and to produce a Standard Operating Procedure for this type of study.