Linking habitat to density-dependent population regulation: How spawning gravel availability affects abundance of juvenile salmonids (Salmo trutta and Salmo salar) in small streams

Abstract

Rivers with abundant spawning gravel may produce more fry of anadromous brown trout (Salmo trutta) and Atlantic salmon (Salmo salar); however, spawning gravel does not provide suitable shelter to parr, suggesting an optimization problem for spawning habitat in streams. This hypothesis was tested on 40 small spawning streams with anadromous brown trout and Atlantic salmon in western Norway. Juvenile salmonid densities were estimated from samples counted during backpack electrofishing surveys and the available habitat was visually surveyed while walking upstream through the rivers. Consistent with predictions, fry densities were positively related to availability of spawning gravel. However, a second-order polynomial was determined to provide an improved fit (ΔAIC = |8.02|), suggesting a polynomial relationship between the amount of spawning habitat and parr. Our results demonstrate that the abundance of parr strongly depend on gravel avilability and seems to be maximized at 20% to 40% gravel coverage. Beyond ca. 40%, parr mortality becomes density dependent, likely owing to predation and competition for shelter, and below ca. 20% the density-dependent mortality of fry will limit parr numbers. These results should be considered when enhancing or restoring the productivity of spawning streams for anadromous salmonids.