Potential methods for measuring the activity patterns and energy use of Atlantic cod (Gadus morhua)

Abstract

As there are no commercially available acoustic telemetry devices for quantifying the swimming activity and activity-related metabolic expenditures of a wide range of marine species, we (i) examined the suitability of three methods (electromyography; sonomicrometry; and tail differential pressure tags (DPT)) for measuring the swimming speed of Atlantic cod ( Gadus morhua ), and indirectly, metabolic rate (MO2) and (ii) measured the activity pattern of free-swimming cod carrying the DPT. All three methods yielded significant relationships with swim speed during a critical swimming speed (Ucrit) test. However, only the DPT was able to discern between swimming speed differences of 0.1 body lengths (BL)·s–1and provide a strong relationship with MO2. Further, we found that free-swimming cod fitted with the DPT swam at an average speed of 0.33 BL·s–1, the speed previously reported to result in minimal cost of transport for this species. While the DPT has considerable potential for assessing the bioenergetics of marine fishes, the swimming economy of tagged Atlantic cod was lower above 0.4 BL·s–1as compared with untagged fish, and Ucritwas reduced by 25%. These latter effects are likely related to the tag’s present size (39 g) and design.