The effect of ultrasonic tags on the growth rates of Atlantic salmon, Salmo salar L., parr of varying size just prior to smolting

Abstract

The attachment of dummy, ultrasonic, acoustic tags, of the Stirling Mk. VI design, to pre‐smolt Atlantic salmon parr significantly affected fish growth rates. Fish less than 160 mm in length lost weight and showed no change in length. Fish over 160 mm in length put on weight, though at a rate significantly lower than that of untagged controls. For fish between 160 and 180mm in length, the increase in length was significantly smaller in tagged fish than in untagged controls. Over 180 mm in length, no difference could be detected in the length gains of tagged and untagged fish.Smaller tagged fish gave up maintaining station and showed signs of distress at an earlier stage in the experiment than larger tagged fish. Tag‐related mortality was inversely related to fish size. All but two of the control fish survived the experiment and continued to maintain station throughout.The results suggest that the minimum length of Atlantic salmon parr on which this design of acoustic tag could be employed is 160 mm, and that caution is needed in the interpretation of track lengths longer than l0 daysof parr in the size range 160–180 mm.