Measuring the bioenergetic cost of fish activity in situ using a globally dispersed radiotracer (Cs-137)

The energetic cost of activity is an important component of the bioenergetic budget of fish, yet this parameter has rarely been quantified for wild populations. Using a Cs-137 mass balance approach, we estimated the annual bioenergetic budgets for individual age-classes of 19 species of North American freshwater fish. Immature fish have low activity-related metabolic costs that agree with estimates based on swimming speed or integer multipliers. Mature fish have 2- to 4-fold higher activity than immature fish and 2- to 4-fold higher activity than estimates based on swimming speed or integer multipliers. The higher activity in mature fish may be due to reproductive efforts. Underestimation of activity in conventional bioenergetics models leads to underestimation of consumption rates. Thus, our in situ and age-specific estimates of activity costs provide a means to improve bioenergetic predictions. Although our analysis was done on an annual basis, it is possible to use the Cs-137 technique over shorter intervals (weeks). The Cs-137 method has general applicability to aquatic systems because Cs-137 is globally dispersed and can be accurately measured in all aquatic organisms using gamma spectrometry.