Ontogenetic divergence of growth among rainbow smelt morphotypes

Multiple, sympatric morphotypes of rainbow smelt (Osmerus mordax) are known in Lake Utopia, New Brunswick. The largest, ‘giant’ form is predominantly a piscivore, the smaller, ‘dwarf’ form are predominantly planktivores, and there is an intermediate body-sized form. The forms exhibit some genetic variability, but it is body size that best defines morphotypes, trophic status, and spawning behaviour. We compared egg size, spawning date, incubation time, size at hatching, and daily and annual growth to determine when divergence in body size occurs among morphotypes. Giant form larvae hatched earlier and grew faster during their first year. Dwarf and intermediate form larvae displayed inter-annual variability in degree of overlap and divergence in growth which occurred in their first growing season or at age 1+ or 2+. We conclude that earlier hatching, early growth trajectories, and later niche shifting are linked to the persistence of morphotypes, i.e., the process is controlled by the environment and sustained to some degree by spawning segregation.