Density-dependent growth in stream-living Brown Trout Salmo trutta L.

Summary

• 1
  Several studies have offered evidence for the occurrence of density-dependent growth in stream-living Brown Trout. However, such evidence has been gleaned for low-density populations, whereas studies on persistently high-density populations have claimed that growth is density-independent. Such a paradoxical observation is shared with other salmonids and has been assumed by several authors to suggest that stream salmonid populations may be regulated by two different mechanisms: density-dependent growth at low densities and density-dependent mortality, in the absence of density-dependent growth, at high densities.
• 2
  This comparative long-term study explored the occurrence of density-dependent growth by examining growth during the lifetime across cohorts in three stream-living Brown Trout populations representing the opposite extremes of growth and density documented throughout the species’ distributional range.
• 3
  This comparison highlighted identical growth–recruitment patterns in a high-density population with low potential for growth, in a low-density population with high potential for growth and in a population with intermediate traits. In the three populations, growth declined with increased recruitment describing negative power trajectories. These observations are consistent with there being a single, negative power relationship between growth and density where the effects of density dependence are stronger at low densities and become negligibly low at high densities.
• 4
  Stream-living Brown Trout populations may be regulated by the continuous operation of density dependence on growth and mortality. In poorly recruited cohorts density dependence may operate on growth but not on mortality during a time period after which density dependence operates on both growth and mortality. In highly recruited cohorts, density dependence operates simultaneously on growth and mortality from the youngest life stages.