Stehlik: Effects of season on activity rhythms and swimming behavior of Pomatomus saltatrix 
11 
endure more cold than adults and may not winter as far 
south as other age classes (Slater et al., 2007). 
When fish cannot escape unfavorable temperatures, 
they resort to behavioral thermoregulation (Olla et 
al., 1985; Sogard and Olla, 1998). Starved walleye 
pollock ( Theragra chalcogramma) reduce swimming 
speed and spend most of their time in colder waters 
below a thermocline, thus decreasing their metabolic 
cost (Sogard and Olla, 1996). Summer-spawned age-0 
bluefish may use the same strategy to suppress meta- 
bolic rates by wintering in colder waters off North 
Carolina just south of Cape Hatteras instead of farther 
south (Morley et al., 2007; Slater et al., 2007). Milling 
is certainly another way in which bluefish reduce their 
activity in winter. 
The 121-kL research aquarium is a large experimen- 
tal space, but confinement may potentially alter fish 
behavior. Despite qualifications, the author believes 
gliding behavior in bluefish is authentic natural be- 
havior. This behavior began in the first few weeks of 
captivity and persisted throughout the experiment. 
The researchers who initiated this line of investigation 
observed gliding in the aforementioned unpublished 
experiment in 1984-85, and the author witnessed it in 
a seven-month experiment in 2006-07. The behavior 
each time was composed of similar elements of body 
curvature, fin extension, glide distance, and glide angle. 
It is unknown, however, how far bluefish would glide 
given unlimited space. 
Bluefish may be unique among laterally compressed 
teleosts in gliding on their sides, thus radically chang- 
ing their hydrodynamic profiles. Although gliding has 
been witnessed in more than one group of age-0 bluefish 
in the laboratory, it has not been studied in very small 
age-0 fish or older age classes. Energy benefits may be 
different according to the age of a fish, because the bod- 
ies of adult bluefish are relatively more cylindrical and 
less flexible than age-0 fish, and perhaps cannot attain 
as efficient a foil shape. 
Much remains to be discovered about daily behav- 
ior routines, migration routes, and overwintering in 
various cohorts and age classes of bluefish. Considerably 
less energy is spent during the glide and upswim mode, 
possibly comparable to energy savings by migrating 
raptors, and should be quantified or modeled. Internal 
archival tags that record depth would be valuable for 
field studies of gliding, in places where recovery is pos- 
sible, such as a migration corridor. Bluefish with acous- 
tic tags could be detected at strings of fixed or roving 
receivers at ocean observatories. Information from such 
tags may locate concentrations of milling bluefish in 
areas offshore that would be accessible to fishing. Daily 
vertical migrations of bluefish on the continental shelf 
should be investigated to determine their relation to 
feeding and long-distance travel. It was surprising that 
bluefish in the laboratory continued to glide briskly dur- 
ing midwinter when they would have reached wintering 
grounds in the wild. Perhaps the modes of swimming, 
speeds, and routes during migration and wintering in 
bluefish are more variable than we suspect. 
Acknowledgments 
This article is dedicated to A. Studholme and A. Bejda, 
who designed this experiment and ran a preliminary ver- 
sion of it in 1984. I acknowledge B. Olla, who established 
protocols and methods for studying behavior in fishes 
at the James J. Howard Marine Sciences Laboratory. 
D. Roe calculated energy savings for gliding bluefish. 
I acknowledge J. Buckel, B. Phelan, J. Rosendale, F. 
Scharf, and reviewers and editors for assistance. 
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