8 
Fishery Bulletin 107(1 ) 
Figure 5 
Line drawings from video images of bluefish (Pomcitomus salta- 
trix). (A) Vertically-oriented 250-mm bluefish, all fins extended, 
( B ) vertically oriented fish swimming toward the observer, pecto- 
ral and pelvic fins extended, (C) fish gliding toward the observer, 
rolled onto its left side, body curved downward and tail fin lobes 
curved up, (D) fish gliding toward the observer on its right side, 
dorsal, anal, and pelvic fins curved up, (E) side view of fish 
gliding with belly toward the observer, sculling with pectorals, 
tail fin lobes curved up, and (F) fish gliding with back toward 
the observer. 
Daily and seasonal rhythms 
In this and earlier studies (Olla and Stud- 
holme, 1972), bluefish in the laboratory were 
active during the day and schooled more 
cohesively during day than at night. Gliding 
and milling occurred at night. This timing of 
passive behaviors may alternate with active 
behaviors such as feeding during the day. Field 
studies corroborate these laboratory-observed 
rhythms. Stomach fullness in bluefish was 
greatest in the afternoon (Marks and Conover, 
1993; Juanes and Conover, 1994). Bluefish 
were more vulnerable to otter trawls during 
daylight hours than at night (Munch, 1977; 
Wiedenmann and Essington, 2006). The latter 
authors hypothesized that bluefish descend 
to near-bottom during the day to feed upon 
schools of anchovies, Anchoa spp., and then 
ascend at night to where they are less acces- 
sible to otter trawls. 
The present study is the first ever published 
on the behavior of bluefish over a yearly cycle 
in real time in a research aquarium. Blue- 
fish changed activity patterns by season, as 
exemplified in the fall, when their swimming 
speeds and the day-night differences in speeds 
gradually decreased as temperature decreased. 
No period of restlessness or accelerated swim- 
ming was seen during the fall, as had been 
seen in an earlier study (Olla and Studholme, 
1971). In that study, a school of adult bluefish 
was acclimated to 19.5°C, under a winter pho- 
toperiod, and were swimming at a mean speed 
of 20-30 cm/s. When the water was chilled to 
11.5°C over 29 days (0.25°C/d), the fish, in- 
stead of swimming more slowly as predicted by 
the bioenergetic response, swam dramatically faster at 
60-100 cm/s. Increased activity in reaction to chilling 
was also seen in tautog ( Tautoga onitis) and sablefish 
( Anoplopoma fimbria) (Olla et ah, 1980; Sogard and 
Olla, 1998). The fish in these latter studies may have 
been undergoing short-term stress responses associ- 
ated with the urge to migrate or escape. In the present 
study, temperature was lowered half as fast (0.105°C/d 
in 57 days) as in the earlier study (Olla and Studhol- 
me, 1971), and the fish could adjust to the temperature 
change without a stress response. 
In spring, the bluefish in the present study acceler- 
ated their swimming speed in response to the cue of 
increased daylength, without any cue of increased tem- 
perature. Similarly, bluefish on their wintering grounds 
respond to changes in day length and begin migration 
in time to arrive at their summer grounds when toler- 
able temperatures are available. 
Modes of swimming 
Bluefish exhibit speed and swimming endurance 
although they do not have the body and caudal-fin shapes 
optimally adapted for sustained swimming performance, 
as are found in pelagic taxa such as scombrids (Webb, 
1978). The bodies of scombrids are cylindrical, elliptical 
in cross-section, and highly streamlined to minimize 
frictional drag. The optimal caudal fin for sustained 
swimming is stiff, has a high aspect ratio, and a lunate 
shape that reduces drag produced by the wake of the fish 
(Nursall, 1958; Webb, 1978). In the scombriform mode of 
propulsion, thrust is generated by the caudal peduncle 
and tail only (Blake, 2002). In contrast, bluefish use the 
anguilliform swimming mode, in which thrust is gener- 
ated by muscular contractions along the body and tail 
(Webb, 1978). The tail of a bluefish produces 65% of the 
total thrust (Ogilvy and DuBois, 1982). Like scombrids, 
bluefish are streamlined and have elliptical body cross 
sections, but their tails are flexible and semilunate, 
suited for burst swimming as well. 
To glide, an aquatic animal must be negatively buoy- 
ant, have sufficient forward momentum, and possess a 
foil shape that produces lift to counteract sinking. A 
foil has a downward-facing arch and an asymmetrical 
camber ratio that produces less pressure below it than 
above it. At sufficient speed, the horizontal body plan 
