Oxygen consumption of drift-feeding rainbow trout: the energetic tradeoff between locomotion and feeding in flow

To forage in fast, turbulent flow environments where prey are abundant, predatory fishes must deal with the high associated costs of locomotion. Prevailing theory suggests that many species exploit hydrodynamic refuges to minimize the cost of locomotion while foraging. Here we challenge this theory based on direct oxygen consumption measurements of drift-feeding trout (Oncorhynchus mykiss) foraging in the freestream and from behind a flow refuge at velocities up to 100 cm s-1. We demonstrate that refuging is not energetically beneficial when foraging in fast flows due to a high attack cost and low prey capture success associated with leaving a station-holding refuge to intercept prey. By integrating optimum foraging theory with empirical data from respirometry and video imaging, we develop a mathematical model to predict when drift-feeding fishes should exploit or avoid refuges based on prey density, size and flow velocity. Our foraging and refuging model provides new mechanistic insights into the locomotor costs, habitat use, and prey selection of fishes foraging in current-swept habitats.

Using fine-scale spatial analysis to study behavioural strategies prevalence in wild groups of drift-feeding fish

Key aspects of the social behaviour of groups of drift-feeding fish can be inferred by the way space is shared between group members, because they inhabit a very dynamic and complex environment where spatial positions have a direct impact on fitness-related traits. Therefore, the spatial analysis of such a system can reveal important insights into behavioural ecology of fish, but so far, technical constraints limited this approach to only large salmonids. We used a digital imaging technique to monitor movements and behaviour of free-ranging groups of juvenile galaxiids (Galaxias anomalus) facing two contrasting physical and social contexts. We described the spatial structure of these groups and studied individual space use in relation to their social behaviour. We found that prevalence of territorial behaviour differs greatly between sites, which suggests that groups were displaying different social organisation. This study showed that detailed spatial analysis of space use and behaviour of drift-feeding fish could provide new insights into the social organisation of group-living animals.

Relationship between background invertebrate drift concentration and flow over natural flow recession and prediction with a drift transport model

This study advances understanding of the flow dependency of invertebrate drift in rivers and its relevance to drift-feeding fish. Background drift concentration varied spatially and with flow over natural flow recession (lower mid-range to low flow) in a reach of a New Zealand river, largely consistent with passive entrainment. Seven taxonomic groups (dominated by Leptophlebiidae and Chironomidae) exhibited positive drift concentration–flow relationships, and one (sandy/stony-cased caddisflies (Conoesucidae)) exhibited negative relationships. A mechanistic drift transport model accurately predicted the slope, but not y intercept, of the drift concentration–flow relationship for the total drift community that positively responded to flow but performed more poorly at the taxon or size-class level. Partitioning the relative influence of drift entry and dilution revealed that positive drift concentration–flow relationships arose from entry overwhelming dilution with increasing flow. Drift transport models have potential for predicting relative (%) effects of flow change on concentration and rate of drift-prone invertebrates. This paves the way for drift transport models to inform inputs to net rate of energy intake models for drift-feeding fish.

Mechanics of foraging success and optimal microhabitat selection in Alaskan Arctic grayling (Thymallus arcticus)

Most fishes residing in temperate streams in the Northern Hemisphere are drift-feeders. Despite this fact, little is known about the mechanisms of drift-feeding itself. We used Alaskan Arctic grayling (Thymallus arcticus), an abundant boreal drift-feeder, to examine the effects of water velocity on several aspects of drift-feeding behavior and test predictions of the Grossman et al. (2002) net energy intake model for microhabitat choice. Water velocity had a negative effect on prey capture, a positive effect on holding velocity, and little effect on reactive distance. We also found that dominance was a better predictor of prey capture success than size rank, although neither of these variables influenced holding velocity or reactive distance. The Grossman et al. (2002) model successfully predicted holding velocities of grayling in one Alaskan stream, but not another. Model failure might have occurred due to higher turbulence, increased predation, or interspecific competition with Dolly Varden (Salvelinus malma). These results help inform the study of habitat selection in drift-feeding fishes as well as management and conservation of Arctic grayling.

Habitat-specific production of aquatic and terrestrial invertebrate drift in small forest streams: implications for drift-feeding fish

The influence of stream channel structure on the production of prey for drift-feeding fish is not well understood. We quantified drift production, the entry rate per streambed area, and drift flux, the total export rate per channel unit, in three second-order, forested streams in southwest British Columbia, Canada. We tested whether (1) drift production was higher in riffles than in pools, (2) drift flux increased with riffle length, and (3) prey body size was larger from riffles relative to pools and from terrestrial drop relative to aquatic drift. Total and aquatic-derived drift production (mg·m−2·h−1) was ∼3.5 times higher in riffles relative to pools; however, terrestrial drift did not differ between channel types. Total drift flux (mg·h−1) was positively related to riffle length. Terrestrial invertebrates were approximately three times larger than aquatics, and invertebrates from riffles were approximately three times larger on average than those from pools. These results suggest that channel structure directly affects prey availability and prey quality for drift-feeding fish and that long riffles may be key areas of prey generation.