Body condition metrics explain fish movement in experimental streams

Body condition refers to an array of physiological or nutritious states in animals, but body condition is typically measured using a single metric in animal behavior studies. In this paper, we conducted a three-day laboratory study to evaluate whether metrics of body condition affected movement of wild-caught creek chub (Semotilus atromaculatus) at 16 and 22°C. Movement was counted between two circular tanks connected by a straight corridor, which was equipped with a pair of passive integrated transponder antennas. Fish movement was most frequent during periods of darkness in a simulated photoperiod, and more mobile individuals consistently moved more frequently across the 3 days. In general, individuals in better body condition were more mobile than those in poorer condition, and different metrics of body condition affected movement to different degrees. Specifically, per cent weight loss and dry matter content (ratio of wet versus dehydrated weight) were better predictors of movement than weight-at-length, which is most typically used as a surrogate of body condition in animals. In addition, body condition and length interacted to affect fish movement, where body condition affected movement of smaller individuals more significantly than that of larger individuals. Body condition effects on activity were similar at 16 and 22°C. This study demonstrates that body condition metrics may not always affect certain behavior in a similar way. Body condition metrics need to be carefully selected when used in the context of animal behavior, and more than a single metric should be tested when feasible.