The Effect of Drifting Debris on Drift-Feeding Fish and Foraging Models

Most models of drift-feeding behavior implicitly assume that all prey capture attempts are successful, leading to the ingestion of food that offsets the energetic and opportunity costs of each capture maneuver.  The same assumption underlies the common use of prey capture attempts as a proxy for energy intake and fitness.  This study examines the validity of that assumption for juvenile stream-type Chinook salmon in the Chena River, a clearwater river in central Alaska. 

We used close-up, high-definition video to record drift-feeding behavior as the fish grew through the 30-80 mm size range.  We discovered that the fish spit out most of the potential prey items they captured, and spent considerable time visually inspecting items they ultimately declined to capture.  The only other similar studies we found were performed on brook trout in still water, and by comparison we found that fish spent far more foraging attempts pursuing and rejecting inedible debris.  We attribute this difference to the fact that flowing rivers contain more suspended debris than still waters, and they give the fish less time to visually distinguish prey from debris.  The ubiquity of these apparent mechanistic explanations suggests that our results may be more typical for small salmonids feeding in moving water.

We examined the implications of this effect for a modern foraging model by incorporating the opportunity cost of handling debris into the model.  Under several realistic parameter scenarios, the time spent pursuing debris dramatically reduces the model’s predictions of the fish’s net energy intake.  The effect is particularly strong for juvenile salmonids, whose small prey competes for attention with minute detritus that may be abundant even in clear streams.