COUGHLIN, D.J. Widener University, Chester, PA, USA.
Rainbow trout undergo a transition from small (< 15 cm) to large
(>20 cm) juveniles. This parr/smolt transition (PST) is associated with
seaward migration in anadromous salmonids. Although rainbow trout do not
migrate to the sea, they do show changes in body form and swimming at PST.
For instance, trout parr have deeper bodies and swim with less body curvature
than the larger smolts. I examined the contraction kinetics of red muscle
from different longitudinal positions in trout parr and smolts, predicting
that the larger smolts would show slower muscle kinetics. In isometric
contractions, the anterior muscle activates and relaxes more quickly than
posterior muscle for both life history stages. In addition, parr red muscle
activates and relaxes faster than that of smolts. Red muscle kinetics correlate
with steady swimming kinematics. For example, the relationship of tailbeat
frequency to swimming speed (BL s-1) differs between parr and
smolts. Since normal PST occurs over a period of growth, the effects of
growth alone are difficult to separate from the effects of PST. Therefore,
I treated trout with thyroxine to artificially induce PST. The induced
smolts superficially resemble larger natural smolts but were the same size
as natural parr. However the induced smolts were similar to natural smolts
in muscle kinetics and swimming kinematics.