Abstract
Migrating wild salmon smolts on the R. Bush, Northern Ireland, are tagged with coded wire tags (CWTs), as part of a study of salmon marine survival. We report the results of a comprehensive comparison of the effects of tagging on wild smolt survival, made possible by the total trapping facilities on the river, which allows simultaneous assessment of returns to the river of untagged wild fish. Analysis of one sea winter (grilse) returns to fresh water of tagged and untagged wild salmon over a 13-year period indicated that the tagged groups displayed significantly poorer return rates in each year of the study and overall ( P<0.001). The difference between tagged and untagged fish varied from year to year, ranging from 0.33:1 to 0.84:1, tagged fish on average surviving at a rate of 43.6%, relative to untagged fish. While procedures were kept constant as far as possible during the study, evidence of avian predation of tagged fish released during daylight prompted a change to night-time release. This significantly improved survival of the tagged fish, but did not explain the source of the mortality differential, which remained significant throughout. Other factors possibly contributing to mortality in the tagged fish are outlined and discussed with reference to literature examples. The importance of being able to correct for mortality when using data from tagged groups of wild salmon smolts in studies of marine survival is emphasised.
Published Version
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