The physical effects of an acoustic tag on the swimming performance of plaice and cod

Abstract

Drag measurements in a small flume show that the Mitson-West transponding acoustic tag developed at the Fisheries Laboratory, Lowestoft, has a frontal drag coefficient CD0 = 0·6. For fish moving at constant speed the ratio of fish drag to tag drag is given by D/d = 0·82L2CD, where CD is an appropriate drag coefficient for the fish and L its length in cm. Acoustically tagged plaice, Pleuronectes platessa (36–52 cm length), and cod, Gadus morhua (50–70 cm length), tracked by sector-scanning sonar in the southern North Sea have been observed to swim at speed of 1 to 2 L S−1 through the water. Calculations with theoretical and experimental values of CD show that the smallest of these fish could have been slowed down by 7% and the majority by rather less than 5%. The extra ower output required for a tagged fish to maintain the same steady speed as an untagged fish is between 3% and 5% and to maintain the same constant rate of acceleration less than 1%. The results suggest that the swimming performance of plaice and cod observed by sector-scanning sonar is unlikely to have been affected in any significant way by the addition of the acoustic tag.