Stehlik: Effects of season on activity rhythms and swimming behavior of Pomatomus saltatrix 
3 
Table 1 
Number of bluefish (Pomatomus saltatrix) measured, average total lengths (TL, mm), and weights (g), with ranges, at intervals 
during the activity rhythm experiment, 1995-96, 
Date 
Number 
of fish 
measured 
Average 
total length 
(TL, mm) 2 
Length range 
(mm) 
Average weight 
(g) 
Weight range 
(g) 
10 August 10 1995 
110 
161.5 
125-192 
34.8 
16.6-60.5 
12 October 1995 
24 
229.4 
185-261 
139.5 
68.1-205.3 
15 November 1995 
26 
251.8 
210-278 
196.7 
100.7-259.5 
17 January 1996 
25 
265.7 
223-290 
219.9 
126.0-283.9 
22 May 1996 
27 
302.6 
268-334 
285.2 
186.7-389.7 
28 June 1996 
64 
325.2 
250-357 
369.0 
243.7-470.5 
1 Fork length = 0.8647 (TL) + 7.0656 for TL range 125 to 388 mm. 
utes of each hour. During two weeks in January, counts 
were made to determine the proportion of fish that were 
gliding. All fish that passed in each 5 -min observation 
period were counted, usually 300-600 observations. 
Then the videos were replayed and only the gliding fish 
were counted. Ascending fish were difficult to see and 
could not be counted. 
Glide angles (a), upswimming (returning to initial 
depth) angles i/3), and glide and upswim distances (a 1 , 
a 2 ) (Weihs, 1973; Magnuson, 1978) were determined for 
individual fish from videotapes from the first week of 
January 1996. Angles were determined from successive 
still images and reported as the angle deviating from a 
horizontal path. Speeds of individuals over ground were 
also obtained from successive images. 
An additional study was conducted from November 
2006 through May 2007, under the same experimental 
conditions, to focus on details of gliding behavior. Age-0 
summer-spawned bluefish (n=71, length range 135-216 
mm TL) were used. Glide angles were measured as be- 
fore, and body roll angles were measured from images 
of these fish gliding straight toward a video camera. 
Results 
Feeding and growth 
At the start of the experiment, the mean length of the 
bluefish was 162 mm total length (TL) (147 mm fork 
length) and the mean weight was 35 g (Table 1). Their 
condition factor (Fulton’s K) averaged 1.08. The day 
after placement into the aquarium the fish fed read- 
ily. In early September, at 20°C, they consumed 13% 
body weight/d (BW/d) (Table 2). Consumption rates 
decreased thereafter, falling to 2. 0-3. 8% BW/d while 
the water temperature was 15°C. After the temperature 
was raised once again to 20°C, food consumption rose to 
5.9% BW/d. Fish grew throughout the experiment, but 
the rate of increase in length and weight slowed as fish 
size increased. At the end of the experiment on 28 June 
1996, the mean length of the bluefish was 325 mm TL 
and the mean weight was 369 g (Table 2). At that time 
67 bluefish survived in excellent health and their condi- 
tion factor averaged 1.55. Growth rate over the entire 
experiment was 0.96 g/d. 
From about 8 April to 4 May 1996, the fish were in- 
fested with the parasitic protozoa Brooklynella hastili. 
Swimming speed decreased, pale patches appeared on 
the skin, some scales were lost, and food consumption 
was unusually low (0.9% BW/d on week of 28 April to 4 
May). Two fish died. The parasites were killed by lower- 
ing the salinity to 13 psu and then raising it to ambient 
psu from 1 May through 4 May. The following week the 
appetites and swimming speeds of the fish returned to 
levels similar to those before the infestation. 
Daily and seasonal rhythms 
After one week the bluefish settled into a pattern of 
swimming in a counterclockwise circle and individuals 
were spaced evenly around the aquarium perimeter. 
They occupied all depths of the aquarium. Individuals 
occasionally shot ahead or out of the circular path, but 
usually the fish remained apace with one another. In all 
months the bluefish swam faster during daylight than 
at night, accelerated each morning after light increased 
and usually attained maximum speed around noon. A 
representative diel rhythm during fall is shown for the 
week beginning 29 October 1995 (Fig. 1A). During the 
week of 31 December (Fig. IB), the peaks of activity near 
noon were brief. Speeds in cm/s were highest when the 
fish had grown, as shown on the week of 26 May 1996 
(Fig. 1C). 
Activity varied seasonally in relation to water tem- 
perature and light in the aquarium. From early Octo- 
ber through early December, mean swimming speeds 
decreased gradually from 38.3 to 31.2 cm/s (Fig. 2, 
Table 2). Mean speeds were irregular in winter, and at 
a minimum in early March (Fig. 2). Speeds increased 
