David J. Coughlin, Jason W. Mitchell, Karen A. Stauffer, Widener University, Chester, PA, USA.
During growth from small to large juveniles, salmonids undergo a parr/smolt
transition (PST) associated with the seaward migration in anadromous salmonids.
The hormone thyroxine apparently acts as the signal for PST, leading to
changes in vision, body form, swimming and the regulation of ion balance.
Although rainbow trout are non-anadromous, they do show a number of changes
at PST, including body form and swimming performance. In this study, thyroxine
was used to induce PST in rainbow trout parr while controlling for fish
size. By restricting the diet, the hormone treated fish or "induced
smolts" were the same size as untreated natural parr. After treatment,
steady swimming behavior (maximum steady swimming speed, tailbeat frequency,
tailbeat amplitude) and red muscle kinetics (activation and relaxation
times, Vmax, power production) could be compared between natural
parr, same-sized induced smolts and larger natural smolts. The maximum
steady swimming speed (in BL s-1) of induced smolts was significantly
slower than for natural parr. The swimming of the induced smolts was similar
to natural smolts, with both groups swimming with slower tailbeat frequencies
and greater tailbeat amplitudes than the natural parr. In isometric contractions
of isolated red muscle, the induced and natural smolts have longer activation
and relaxation times than natural parr. As well, the optimal frequency
of oscillation for maximum power production was lower in red muscle of
induced smolts. The molecular basis for the change in red muscle in the
induced and natural smolts is under investigation.