FISHERY BLXLETIN: VOL. 69, NO. 2 



tank (a plastic swimming pool 15 ft in diameter) 

 was maintained by a temperature regulation 

 system at about 18.5° C. The mean test temper- 

 ature was 18.48 ± 0.03° C. The fish were cap- 

 tured near Santa Catalina Island, Calif., on 12 

 September 1969. Tests began 2 weeks later and 

 ended on 21 November 1969. Fish were fed an 

 abundant ration of chopped squid, anchovies, 

 and frozen brine shrimp. Probit analysis, a sta- 

 tistical technique first applied to sustained speed 

 data by Brett (1967), was used to estimate sus- 

 tained speed threshholds. 



Variability in length posed a problem in the 

 analysis. Although all fish were from the same 

 school, differences in length existed; also the 

 fish grew in the course of the study. These dif- 

 ferences were insufficient, however, to determine 

 the form of the relationship between length and 

 sustained speed. In general, the relationship 

 between length and sustained speed for other 

 species (Bainbridge, 1962; Brett, 1965), theo- 

 retical considerations (Gray, 1968; Fry and 

 Cox, 1970), and relationships between length 

 and other swimming capabilities (Magnuson, 

 1970), indicate that speed is proportional to a 

 fractional power of length equal to about U"^ - "'. 

 In addition, the minimum swimming speed 

 of Trachurus was proportional to L"^ when esti- 

 mated from Magnuson's equation (Hunter and 

 Zweifel, 1971). In light of the above evidence 

 it seemed preferable to use 0.6 as the coefficient 

 of length, although unity has been commonly 

 employed in cases where length coeflficients were 

 unknown. As an alternative to this procedure 

 I also estimated the percent fatigued at different 

 speeds in centimeters per second and in body 

 lengths per second for a narrow length range 

 (10.0 to 11.9 cm total length) where the effect 

 of diflFerences in length would be negligible. 



RESULTS 



Within a few minutes after Trachurus were 

 placed in the swimming compartment they be- 

 came quiescent, swam steadily, and remained in 

 about the same position in the compartment 

 throughout the test or until they became fa- 

 tigued and fell against the rear screen. This 



was in contrast to some other species which did 

 not swim steadily, but swerved and oscillated 

 from side to side. 



The relationship between water speed and 

 percent fatigue had the normal sigmoid form 

 of a dosage response curve (Finney, 1952). 

 Probit estimate of the applied water speed at 

 which 50^; fatigue occurred in 360 min of swim- 

 ming and the 95 5r confidence limits were 94.40 

 ± 5.15 cm/sec for Trachurus 10.0 to 11.9 cm 

 total length, N = 127 (Figure 1, Table 1). Thus, 



Table 1. — Swimming endurance of Trachurus sym- 

 metricus in cm/sec and in L^Vsec. 



' Totol speed range divided into 20 equal intervals; speeds listed are 

 midpoints of those intervals. 



50 ''r of Trachurus in this length range could 

 be expected to sustain a speed of about 8.4 L/sec 

 or 22.1 L" Vsec for 360 min. For all Trachurus 

 (N = 294) the water speed at which 50 ':r fa- 

 tigue occurred after 360 min of continuous 

 swimming and the 95 ''r confidence intervals 

 were 22.4 ± 1.2 L" Vsec. The first estimate, 

 based on a narrow length range, and the second 

 one, based on all data, were reasonably close. 

 On the other hand, when all data were in the 

 form V L'" the 50^r threshold was 9.34 L sec 

 which is higher than the preceding estimates. 

 Inspection of these data, however, showed that 

 the coefficient for length clearly was less than 

 one and that use of unity biased the estimate. 



268 



