Does recruitment trigger negative density-dependent feedback loops in stream-dwelling salmonids?

Abstract

I explored the hypothesis that recruitment may stabilize the numerical dynamics of stream-dwelling salmonids by triggering density-dependent feedback loops through the operation of recruitment dependence on individual growth, mortality, life span and maximum size and their effects on fecundity. I examined 98 cohorts of two Salmo trutta populations of northern Spain and a population of Jutland (Denmark) located 2400 km apart. Recruitment, growth, mortality, life span and maximum size were inter-related, were recruitment-dependent and described negative power trajectories. In the Spanish populations, faster-growing individuals of weakly recruited cohorts with lower mortality attain longer life span and larger size. Hence, larger females spawning more abundant, larger eggs that, in turn, induce stronger cohorts of higher spawners’ abundance, recruitment and mortality. The mortality patterns match the self-thinning patterns, an ultimate expression of competition. Significant relationships among self-thinning slopes and mortalities rates with increasing recruitment demonstrate that the rate at which density-driven mortality is higher, the stronger the intensity of intraspecific competition. Space-limited habitat and size-dependent resource availability underpinning site-specific carrying capacities suggest that interference competition is the primary mechanism underpinning population regulation.