FISHERY BULLETIN: VOL. 69, NO. 2 



the tip of the tail measured perpendicular to the 

 axis of progression plus the mirror image of 

 that measurement on the other side of the axis 

 of progression. 



The film sequences selected for analysis were 

 usually ones in which the fish held a constant 

 position in the current and swam steadily. Oc- 

 casionally at higher speeds it was not possible 

 to obtain such a sequence because the fish did 

 not maintain a constant position but rather ac- 

 celerated and decelerated. In this case a se- 

 quence was chosen in which no net movement 

 existed between the beginning and end of the 

 sequence although the fish moved slightly for- 

 ward and backward within the sequence. Usu- 

 ally 5 complete tail beats were analyzed per 

 speed level but occasionally as few as 2.5 and 

 as many as 11 were analyzed. 



Sixteen speed levels were used in the exper- 

 iments; seven of the levels, 15 to 60 cm/sec, 

 were graduated at intervals of 25''r of the pre- 

 ceding level and nine of the levels, 69 to 212 

 cm/sec were graduated at intervals of 15%. A 

 speed level interval greater than 10% was used 

 because of the possible 10 "^f variability in flow 

 within the swimming compartment. 



A grand total of 176 speed tests was analyzed 

 for 14 jack mackerel, varying in total length 

 from 4.5 to 27.7 cm. Owing to the diflTerences 

 in length, no fish was able to swim at all levels. 

 All 14 fish swam at five levels, 24, 30, 38, 48, 

 and 60 cm sec, and all but the two smallest fish 

 swam at the next five higher levels, 79, 91, 105, 

 121, and 139 cm^sec. Only fish of 16 cm or 

 larger were tested at speeds above 139 cm sec 

 and only those less than 16 cm were tested at 

 15 cm/sec. 



Other species were tested for comparative 

 ])urposes but fewer observations were made. 

 Seventy-four swimming sequences of five Scom- 

 ber, 26.3 to 32.2 cm total length, were analyzed, 

 nine sequences of five Sai-dinops, average length 

 13.6 cm, and seven sequences of one Triakia, 

 23.6 cm. 



All fish were tested singly except for Sardi- 

 nops, which was tested in a group of five. Fish 

 were held in the swimming compartment at a 

 low speed for about 30 min before an experi- 



ment began. Seawater temperature ranged 

 from 17.0° to 19.5° C among experiments but did 

 not vai-y over a degree within an experiment. 



RESULTS 



The tail beat amplitude of Trachurus did not 

 change with speed but was constant at all speed 

 levels and was directly related to length (Fig- 

 ures 2 and 3) . Tail beat frequency, on the other 



VELOCITY (cm /sec) 



Figure 2. — Tail beat amplitude at various speeds for six 

 Trachurus, 4.5 to 27.7 cm total length. 



LENGTH (cm) 



FiGl'RE 3. — Relationship between tail beat amplitude and 

 total length for 14 TracliKnis, 4.5 to 27.7 cm total length. 

 {A = 0.23177L, s„^ = 5.068, and N = 176.) 



hand, changed with speed and, therefore, was 

 the only speed modulator measured in these ex- 

 periments. In all species studied the relation- 

 ship between tail beat frequency and velocity 



256 



