Effects of a benthivorous and a drift-feeding fish on a benthic stream assemblage

Area of Science:

• Freshwater Ecology
• Aquatic Ecosystem Dynamics
• Trophic Interactions

Background:

• Fish predation is a significant factor influencing benthic macroinvertebrate communities.
• The feeding strategy of fish (benthic vs. drift-feeding) can differentially impact prey availability and community structure.
• Understanding these interactions is crucial for managing freshwater ecosystems.

Purpose of the Study:

• To assess the impact of benthic-feeding bullheads (Cottus gobio) and drift-feeding brown trout (Salmo trutta) on a benthic invertebrate assemblage.
• To investigate the role of direct predation versus behavioral avoidance in structuring invertebrate densities.
• To determine the effects of these fish on periphyton biomass.

Main Methods:

• A 1-month field experiment using enclosures with cobble/gravel substrata.
• Four factorial treatments: control, bullheads only, brown trout only, and both species.
• Dietary analysis of fish and measurement of invertebrate densities, immigration/emigration rates, and periphyton biomass.

Main Results:

• Bullheads significantly reduced densities of seven invertebrate taxa, while brown trout only affected one (Baetis rhodani).
• Brown trout primarily consumed terrestrial prey (>80%), explaining their weaker impact on benthic invertebrates.
• Bullheads increased periphyton biomass, likely through consumption of grazers, whereas brown trout had no effect.

Conclusions:

• Benthic-feeding fish exert stronger top-down control on benthic invertebrates compared to drift-feeding fish with alternative terrestrial prey.
• Fish feeding strategy and diet composition are critical determinants of their impact on aquatic food webs.
• Alternative prey availability for drift-feeding fish can alter fish-benthic-periphyton interactions, potentially explaining variable impacts reported in literature.