Carlin tag recoveries as an indicator of predation on salmon smolts by goosanders and red‐breasted mergansers

• Published in: Journal of Fish Biology
• Main Authors: Feltham, M. J., MacLean, J. C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1996
• Subjects: Sawbill; Salmo salar
• Online Access: http://dx.doi.org/10.1111/j.1095-8649.1996.tb01118.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1095-8649.1996.tb01118.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1095-8649.1996.tb01118.x

Between 1984 and 1990 a total 221 Carlin tags used to mark salmon Salmo salar smolts in the River North Esk, NE Scotland, were recovered from the stomachs of goosanders Mergus merganser and red‐breasted mergansers M. serrator . Both Carlin‐tagging and adipose‐clipping affected the predation of salmon smolts by sawbill ducks. The mean (± S.D.) sizes of tagged smolts taken by both species were similar (117 ± 3 mm) and significantly smaller than the mean sizes of smolts in the river, possibly due to a reduction in the swimming performance of small smolts bearing tags. Large adipose‐clipped smolts (±mean smolt size) were predated significantly more than unclipped smolts, but no such difference was observed for small smolts (<mean smolt size). Data suggested that some ducks cued into smolt release sites where they took a disproportionately high number of tagged smolts on or near the day of release and continued to do so for several days. Although tag recoveries are commonly used to estimate the impact of piscivores on fish stocks, our study suggests that some predators respond to the presence of tagged fish in ways that directly reflect the tagging procedure and that caution should be exercised when extrapolating conclusions based on such data to untagged fish.