Foraging Fish as Zoogeomorphic Agents: An Assessment of Fish Impacts at Patch, Barform, and Reach Scales

Abstract

Flume studies have demonstrated that foraging by fish can modify the structure and topography of gravel substrates, thereby increasing particle entrainment probabilities and the amount of sediment mobilized during subsequent experimental high flows. However, the zoogeomorphic impact of benthic foraging has not previously been investigated in the field. This paper reports field experiments that examined the nature and extent of disturbance of riverbed gravels by foraging fish, predominately Cyprinids, at patch, riffle, and reach scales and complementary ex situ experiments of the impacts on bed stability. At patch scale, benthic feeding fish displaced particle sizes ≤90 mm in diameter, increased bed surface microtopography and grain protrusion, and loosened surface structures. Although enhanced mobility was expected from these structural changes, foraging also caused localized coarsening of sediments, and the ex situ experiments recorded significantly reduced grain entrainment, bedload flux, and total transported mass from foraged patches. Foraging disturbed bed materials at all 12 riffles in the study reach and, on average, disturbed 26.1% of riffle area per 24 h feeding period. These findings demonstrate for the first time that foraging fish, which are widespread and feed perennially, can act as zoogeomorphic agents in rivers, affecting grain‐size distributions and bed material structure, with potential implications for bed stability and bedload transport at reach and river scales. Whether fish increase or reduce bed mobility is probably dependent on a host of factors, including the net effects of both structural disturbance and biogenic particle sorting, as these affect entrainment stresses under subsequent competent flows.

Zoogeomorphological behaviours in fish and the potential impact of benthic feeding on bed material mobility in fluvial landscapes

“…Simple devices like washer disturbance indicators would be valuable for gathering such information. In addition, field experiments ( sensu Pledger et al, ) could investigate the extent of substrate conditioning by wild fish communities. Estimate the cumulative impact of benthic feeding for river‐scale sediment fluxes. With a fuller understanding of which species and fish communities disrupt bed materials and how abiotic and biotic factors mediate their effects, a feasible goal becomes the development of a generic model for predicting the impact of foraging on sediment flux.…”

“…During the in situ experiment, Pledger et al . () detected significant changes in the size distribution of experimental tray substrates, with foraging resulting in better sorted and coarser sediments, as indicated by increases in all grain‐size percentiles. Benthic foragers preferentially displaced finer particle sizes from experimental trays which caused a statistically significant, 2.8 kg (33%) decrease in the total mass of sediment remaining in foraged trays.…”

“…There are two sites in the database on the River Idle that fall within the study area used by Pledger et al . (). These two sites have ΣBIS site = 0.138 and 0.253, respectively, equivalent to the 3rd and 10th percentiles of all site scores; that is, between 90 and 97% of sites have higher ΣBIS site scores.…”

The zoogeomorphology of case‐building caddisfly: Quantifying sediment use

“…Gottesfeld et al ., ; Hassan et al ., ; Buxton et al ., ; Fremier et al ., ). In addition, foraging by benthic‐feeding fish can increase sediment mixing and fine sediment suspension (Pledger et al ., ; Huser et al ., ), affecting both gravel and sand substrates (Statzner et al ., ; Pledger et al ., ).…”

Predator, prey, and substrate interactions: the role of faunal activity and substrate characteristics

“…However, the classic view of fine sediment dynamics at the water-substrate interface has been conceived entirely around geophysical principles in which bed shear and fluid turbulence drive entrainment, transport, and deposition of fine sediment (Beschta and Jackson 1979, Diplas and Parker 1992, Kuhnle et al 2016. There is a growing body of literature demonstrating that animals including fish and macroinvertebrates can also alter the accumulation and distribution of fine sediment (Statzner et al 1996, Zanetell and Peckarsky 1996, Nogaro et al 2006, Pledger et al 2017) via the expenditure of biotic energy . For example, macroinvertebrate prey may winnow fine sediment from interstitial spaces and thereby maintain and/or re-establish vertical connectivity and migration pathways within the river bed (Visoni and Moulton 2003, Mermillod-Blondin et al 2004, Mermillod-Blondin and Rosenberg 2006, Nogaro et al 2006, Stumpp and Hose 2017.…”