Increased Population Density and Suppressed Prey Biomass: Relative Impacts on Juvenile Atlantic Salmon Growth

Abstract

AbstractIndividual growth rates of fish often depend on both population density and resource availability, both of which are subject to anthropogenic impacts and can be manipulated in the interest of restoring or enhancing fish populations. However, direct tests of the relative contribution of these factors to growth variation are lacking for most populations and are critically needed to guide management. We used a large‐scale field experiment to evaluate the relative effects of increased population density and suppressed prey biomass on the growth of juvenile Atlantic salmon Salmo salar. We directly manipulated salmon population density by releasing newly hatched salmon fry from uniform initial conditions at three density treatments in natural streams (18 sites total, repeated over 2 years). We arrayed the density treatments across sites that encompassed a more than 10‐fold range in the biomass of benthic invertebrate prey for salmon, a low prey biomass being associated with heavy shading from riparian forest canopy. This controlled approach clearly demonstrated the enormous growth plasticity of juvenile Atlantic salmon; their mean body mass ranging from 1.2 to 14 g across sites after one growing season. Variation in prey biomass had the strongest effects on fish growth. The fish grew faster at sites with high prey biomass regardless of population density, and prey biomass alone accounted for 71% of the explained variation in their growth. Only after taking the variation explained by prey biomass into account was it apparent that the study fish grew faster at sites with low stocking density and at sites where high losses reduced Atlantic salmon population density. To maintain salmon growth and size‐dependent life histories, restoration and management efforts clearly need to consider anthropogenic factors that suppress prey abundance.