Stehlik: Effects of season on activity rhythms and swimming behavior of Pomatomus saltatrix 
5 
nongliding fish at approximately the same speed and 
along the same path. A fish righted itself by rolling 
smoothly or by flexing the body and turning abruptly. 
After righting itself, a fish returned to its initial level or 
briefly swam horizontally. Gliding occurred only when 
the school was swimming briskly and in an undulating 
path. Gliding nearly always occurred at night in this 
study. In the dark, fish gliding with their bellies facing 
the camera could be seen by the reflection of light off 
Figure 2 
Mean swimming speeds (cm/s), water temperature (°C), and 
hours of light (hr/d) for bluefish ( Pomatomus saltatrix), 10 
Sept 1995-28 June 1996. 
Table 3 
Modes of swimming behavior seen in bluefish ( Pomato - 
mus saltatrix) in 
cycle. 
a laboratory aquarium over an annual 
Mode of 
swimming 
Description 
Powered 
Horizontal swimming in upright 
orientation, propelled by body and tail 
Undulating 
Powered swimming while fish 
continually changed depth 
Burst 
Brief, high-speed swimming 
Gliding 
No propulsion, oriented on side, 
descending or sinking diagonally 
Upswimming 
Powered swimming to ascend from a 
glide to the initial level 
Milling 
Upright orientation, swimming very 
slowly or almost motionless 
Side-swimming 
Powered swimming, oriented on side 
their sides, but fish with their backs to the camera were 
difficult to detect. 
Gliding was sometimes part of a sequence of side- 
oriented swimming behaviors. Side-swimming (Table 
3) occurred during undulatory swimming, while a fish 
was descending, ascending, or moving horizontally, and 
often before or after a glide. Fish ascending from a glide 
sometimes side-swam up all or part of the distance. 
They usually returned smoothly along the undulat- 
ing path of the rest of the school, but at times they 
ascended very steeply and swam against the direction 
of the school. 
Gliding was prevalent throughout the study and oc- 
curred most frequently in fall and winter (Fig. 6). In 
