186 



3.2.3 Variations in the sensitti-ity of shrimps to oil 



The response of Crangon to the physically- and 

 chemically-dispersed oil was found to vary between 

 tests. Figure 4 shows the variation in mean mortality 

 of S-hrimps caught at various times in 1976, 1977 and 

 1978 and exposed to physically-dispersed oO, to- 

 gether with the temperature of the estuary water 

 when the animals were caught. No clear seasonal or 

 temperature-related clia.nge in sensitivity is apparent; 

 the increased resistance during the late summer in 

 1976 and 1977 was not obser.'ed in 1978. The short- 

 term fluctuations which occurred during this period 

 may be due to the condition of the animals, the stage 

 of breeding cycle or the extent and method of handl- 

 ing before testing. As the sensitivity of the test ani- 

 mals varies between tests and the toxicity of different 

 batches of the standard oil may var\', it is essentia] to 

 compare simultaneously the toxicity of a dispersant/ 

 oil mixture with that of the physically-dispersed oil 

 using the same stock of test animals. Within each set 

 of rephcate tests the values of the standard error are 

 low enough to allow the detection of differences of 

 toxic effect between dispersants. 



3.2.4 Effect of changes in the degree of agitation 



The speed of the motors operating the stiners used in 

 the standard test was chosen as 1350-1450 rpm be- 

 cause this produced a uniform and reproducible dis- 

 persion without causing the test organisms stress 

 (Section 2). Table 4 shows that when the motor 

 speeds were reduced by half there was a reduction in 

 toxicity of the physically-dispersed oil and a less 

 sicniflcant decrease in the toxicity of the oil and dis- 

 persant rm.\;ure, reflecting the lower mixing energy' 

 in the s>'stem. The relative toxicities of the disper- 

 sions obtained using the four dispersants remained 

 unchanged at the lower speed. Because of the ab- 

 sence of mortality in the control tanks where the o0 

 was poorly dispersed at the lower speeds, however, 

 the toxicity of the chemically-dispersed oil became 

 of greater statistical significance. Thus, where tests 

 are carried out for licensing purposes which involve a 



pass-fail limit (Section I), it is essential to standardise 

 the degree of a&itation by use of a fixed motor speed. 



3.2.5 Effect of changes in the source of oil 



AH the results presented so far have been obtained 

 using fresh Kuwait crude oil, but as part of the valida- 

 tion programme a number of standard sea tests have 

 also been carried out using two fresh Nonh Sea crjde 

 oils. The results of tests using fresh and "weathered' 

 Kuwait, Ekofisk and Auk oils, both alone and with 

 four reference dispersants are shown in Table 5. 

 Differences in toxicity between the oils were reflect- 

 ed in the relative toxicities of the oil and dispersant 

 mixture. Thus, the oil selected for the test did not 

 affect the identity of the dispersants exhibiting the 

 greatest and lowest toxicity. A comparison of the 

 results for fresh and "weathered' oil showed that, 

 although Veathering' reduced the toxicity of the oil 

 and the oil/dispersant mixtures, their relative toxici- 

 ties remained the same. 



The tests which produced the results in Table 5 were 

 carried out over a period of 1 year and the difference 

 in toxicity between the oils could have been affected 

 by the various factors listed in the previous section. 

 To enable a direct comparison of the relative toxici- 

 ties of the oils, a single test was carried out in which 

 a physical dispersion of all three oils, both fresh and 

 "weathered', were tested simultaneously using the 

 same batch of shrimps. The resuhs from this test, 

 which are presented in Table 6, suggest that Auk oil 

 is the most toxic of the three to shrimps. 



Exposure of fresh oil to air for only 24 h results in 

 a significant reduction in its toxicity and it is essen- 

 tial that standard oils be stored in small ainight con- 

 tainers which are discarded after opening. This prac- 

 tice has been adopted with the Kuwait oil used for 

 the standard test. Moreover, the contents of one of 

 these cans has been analysed by gas liquid chroma- 

 tography every month to determine whether any 



Table 4 - Effect of reducing the degree of agitation on results of sea tests using four dispersants 



Motor speeds 

 (rpm) 



1350-1450+ 



Dispersant 



Mean mortality {%) 



BPllOOX 



Synperonic OSD 20 

 Shell Dispersant Cone. 

 Corexit 9527 

 BPllOOX 



Synperonic OSD 20 

 Shell Dispersant Cone. 

 Corexit 9527 



Oil 



ID 

 55 

 30 

 30 



.0 

 





 



Oil and dispersant 



10 

 60 

 55 

 75* 





 10 

 15* 



55* 



•Significajii L-icrease in toxicity (Student's t-tcst;P =<0.05). 

 +AS used in standard test (Section 1.3.1) 



