Determinants of parr size variations within a population of brown trout Salmo trutta L.

The size of 2-month old trout Salmo trutta parr differed between sites and between years along the River Esva catchment (Asturias, northwestern Spain) . Such variation was in a direction opposite to the variation observed in p arental size. Parr were smaller in sites where parents grew faster, whereas larger parr occurred in sites where parents grew less. Parr size of six co horts (1990-1996, except 1995) at 14 sites along the River Esva was inverse ly related to the growth rate and length of parents and positively related to canopy, egg size, and water temperature. The latter acted similarly on a ll parr independently of egg size and the site where the egg originated. Co variation patterns among parr size, parental traits, and canopy suggest tha t a canopy-regulated, growth-determined trade-off between egg size and numb er, previously described for the Esva trout, also extends to alevin size. I n forested, shaded sites, adult trout grew less and spawned fewer larger eg gs that resulted in larger alevins, whereas in fully insolated, production- rich sites, trout grew faster and spawned higher numbers of smaller eggs th at produced smaller parr. I hypothesize that the phenotypic plasticity illu strated by the environmentally induced trade-off between egg size and numbe r further extended to alevin size may be evolutionarily advantageous becaus e it relates the size of trout alevins to food availability, as predicted b y the growth previously experienced by parents.