486 



FISHERY BULLETIN OF THE FPSH AND WILDLIFE SERVICE 



Table 9. — Experiment 9a: Effects on Mugil cephalus of certain test materials initially used in experiment 9 

 [4 small flsh tested in each container; see table 8 for container contents) 



Container 



Number (in millions) of Oymnodinium 

 brevis per Uter 



Remarks 



No. 9.. 

 No. 10 

 No. 11. 



No. 12 



No. 14 



No. 15. 



0.9 (sample taken 30 minutes after 4 

 fish were added). 



2.4 (sample taken 25 minutes after 4 

 fish were added). 



1.3 (sample taken 15 minutes after 4 

 fish were added). 



1.4 {sample taken 15 minutes after 4 

 fish were added). 



Materia] aerated continuously during previous 24-hour period, aerated vigor- 

 ously for last 12 hours and during experimental period. Material not cloudy. 



Material neither aerated during previous 21-hour period nor during experi- 

 mental period. Material not cloudy. 



Material not aerated during previous 22-hour period. Vigorous aeration began 

 18 minutes before flsh were added and continued throughout experiment. 

 Material not cloudy. 



Material neither aerated during previous 23-hour period not during experi- 

 mental period. Material not cloudy. 



Material neither aerated during previous 10-hour period nor during experi- 

 mental period. Material was cloudy when fish were added. Two fish 

 developed heavy microbial growth on caudal fin; they died between 30 and 

 48 hours after study was begun. Two fish aUve after 5 days. Material 

 relatively clear at this time. 



Material neither aerated during previous 9-hour period nor during experimental 

 period. Material cloudy when fish were added, but relatively clear 5 days 

 later. Three fish aUve after 5 days. 



' Time (hr.:min.) required for fish to show first signs of imbalance. 



' Time (hr.:min.) of cessation of opercular movement. 



' Distress or death did not occur durmg the 24-hour test period. 



Although the experiment was discontinued after 

 24 hours, containers 14 and 15 were set aside and 

 observed for 4 more daj^s. Two of the fish in the 

 culture medium died between 30 and 48 hours, 

 and the other two were alive after 5 days. The 

 three fish remaining in the sea water also lived 

 through 5 days. There was some indication that 

 nonaerated G. brevis cultures were more toxic than 

 the aerated. 



RESULTS OF EXPERIMENTS WITH BACTERIA- 

 FREE CULTURES 



The results of the experiments with bacteria-free 

 G. brevis confirm that this organism produces a 

 fish-killing substance (s) as indicated by tests 

 with unialgal cultures. Bacteria-free G. brevis 

 was toxic to the two species of fish tested 

 (Cyprinodon variegatus and Mugil cephalus). The 

 minimal death time for C. variegatus was about 

 3% hours. The mullet were more sensitive, with 

 a minimum death time as lov/ as 15 minutes. 

 Furthermore, small mullet appear to be less 

 sensitive to the substance than the large ones, 

 since the three small fish in each container of G. 

 brevis culture outlived the large one. The con- 

 centration of G. brevis in these bacteria-free 

 cultures varied from 2.3 to 4.8 million organisms 

 per liter. Such concentrations are considerably 

 less than the 10 to 60 million per liter sometimes 

 encountered in areas where dead or dying fish 

 occur. 



There was good agreement between the results 

 of experiments 8 and 9 with regard to the toxicity 

 of bacteria-free G. brevis cultures to small M. 

 cephalus, which was the only species used in both 

 experiments. Nevertheless, the small mullet sur- 

 vived much better in the control materials in 

 experiment 8 than in experiment 9. In the latter 

 experiment, the small mullet died rapidly in one 

 particular batch of sterile culture medium (con- 

 tainers 13 and 14). Large M. cephalus in this 

 batch of medium, however, survived much better 

 than the small mullet. 



We attribute the early death of the small midlet 

 in the culture medium placed in containers 13 and 

 14 of experiment 9 to the abnormally low pH of 

 this particular batch of medium. Moreover, the 

 relatively poor survival of these fish in some of the 

 other control containers of experiment 9 was pos- 

 sibly due to their being damaged by the vigorous 

 thrashing about of the large mullet. The length 

 of some of the large test fish slightly exceeded the 

 inside diameter of the experimental containers. 



The low pH of the material in container 14 sug- 

 gests that the batch of medium placed in container 

 14 was abnormal before the fish were added. Ap- 

 proximately 14 hours after experiment 9 began 

 the pH of the medium in container 14 was 5.9 and 

 the bacterial count was between 25 and 50 million 

 per ml. Since the pH of this material increased 

 from 5.9 to 6.4 during the next 11 hours in spite of 



