to reach 2 ppm and 29%n, respectively. After reach- 

 ing these levels (about 200 min), test water was 

 shut down and normal seawater restored. The fol- 

 lowing morning fish were removed, weighed, and 

 measured, and survivors were returned to holding 

 tanks. Oxygen and salinity levels were monitored 

 by oxygen meter and salinograph; samples were 

 taken periodically for laboratory analysis to verify 

 the instruments. 



Results and Discussion 



Behavioral responses to decreasing levels of sa- 

 linity were unremarkable; Table 1 summarizes 

 results from three skipjack tuna and one yellowfin 

 tuna. No consistent swimming speed changes 

 were observed during periods when salinity de- 

 creased from about 34%n to 29%o. Although sample 

 size is small, these tunas did not make any dra- 

 matic response to salinity changes of magnitudes 

 expected within their normal habitat. 



Figure 1 illustrates typical results from tunas 

 encountering slowly changing oxygen concentra- 

 tion. At or about 4 ppm, skipjack tuna (Figure la) 

 demonstrated an abrupt increase in swimming 

 speed. In most fish tested, speed increased to over 2 

 lengths/s. Yellowfin tuna, in contrast, showed no 

 alteration in swimming speed as oxygen levels 

 decreased (Figure lb). 



Figure 2 summarizes the oxygen experiment 

 observations from eight skipjack tuna and three 

 yellowfin tuna. Individual points plotted are me- 

 dian swimming speeds for the eight skipjack tuna 

 grouped by: 1) before treatment and 2) 1-ppm dis- 

 solved oxygen intervals both decreasing and in- 

 creasing, i.e., 6-5, 5-4, 4-3, 3-2, and 2-3, 3-4, 4-5, 

 5-6. Number of swimming speeds sampled ranged 

 from under 5 to over 100 depending on the number 

 of laps swum during each interval. Heavy line 

 connects the grand median of each interval. Simi- 



TABLE 1. — Effect of decreasing salinity on mean swimming 

 speed in tunas. 



DISSOLVED OXYGEN 



C5 

 Z 

 UJ 



SKIPJACK TUNA 



Id 



a. 



in 



1.0 



L 



YELLOWFIN TUNA 



UJ 



o 

 en 

 (/> 



o 



50 100 150 200 250 300 350 400 



ELAPSED TIME (MINUTES) 



FIGURE 1. — Effect of dissolved oxygen level on swim speed in 

 two species of tunas. Swim speeds are median values sampled for 

 10-min periods. 



lar data for each yellowfin tuna (decreasing oxy- 

 gen intervals only) are included for comparison. 



Not all of the skipjack tuna survived the treat- 

 ment; three of the eight died when oxygen levels 

 dropped below about 2.5 ppm. Survival times for 

 skipjack tuna under conditions of low oxygen are 

 as follows: in excess of 240 min at 4 ppm, 59 min at 

 3 ppm, and 18 min at 2 ppm ( Gooding and Neill see 

 footnote 2). My data are consistent with this 

 resistance-time distribution, and both studies 

 support the Barkley et al. (see footnote 1) 

 hypothesis that there does exist a low oxygen level 

 that limits the observed oceanic distribution of 

 skipjack tuna. 



Yellowfin tuna are not apparently stressed dur- 

 ing the exposures to the low oxygen water 

 employed. In separate tests done after the expo- 

 sures to decreasing oxygen, two additional yel- 

 lowfin tuna survived and made no overt locomo- 

 tory changes when introduced directly into water 



651 



