The effect of external dummy transmitters on oxygen consumption and performance of swimming Atlantic cod

Telemetry studies have provided unprecedented information about the activity, spatial ecology and energetics of free-swimming fishes (Lucas & Baras, 2000; Cooke et al., 2004). These studies, however, require tagging of fishes with radio or acoustic transmitters. The effects of these transmitters are often assumed to be negligible as far as the transmitter:fish mass ratio (Rtf) is <2% in air (Winter, 1983). This rule of thumb has been challenged by testing survival, growth, healing, health and swimming performance of tagged fishes (Bridger & Booth, 2003). The effect of the transmitter on swimming performance is usually determined by measuring the critical swimming speed (Ucrit) (Mellas & Haynes, 1985; Brown et al., 1999; Cote et al., 1999; Koed & Thorstad, 2001). But information about the possible increase in energy demand during the Ucrit test is absent. This information, though, may be highly relevant, especially if the transmitter is used to estimate swimming costs of fishes. In this case, the direct effect of the transmitter on energy expenditure at different swimming speeds should be evaluated. Atlantic cod Gadus morhua L. is widely used in telemetry studies as information about activity and swimming cost is