188 



5% to 100%. About one third of the dispersants tested were 

 rigniTicantly more toxic than oil, with little difference 

 between conventional dispersants and concentrates. 



4.2 Tests under non-standard conditions 



As with the sea test, the sensitivity of the test to variations 

 in test conditions has been investigated and the influence of 

 several variables is described below (Sections 4.2.1-4.2.4). 



4.2.1 Variations in the sensitivity of limpets to oil 



Some indication of the variation of sensitivity of 

 limpets to oil was obtained by examination of the 

 mean mortality (%) of limpets exposed to oil during 

 beach tests carried out in 1977 and 1978 (Figure 5). 

 It is apparent that limpets are slightly less sensitive to 

 the toxic effects of oil during the summer months 

 and that the short-term fluctuations in mortality are 

 low. The standard deviations within each set of 

 replicate tests are low enough to allow detection of 

 differences, between the toxic effects of oil and those 

 of dispersants, with some confidence. 



4.2.2 Effect of changes in the source of oil 



Approval of a dispersant for licensing purposes is 

 based on its toxicity relative to that of a standard 

 oil; thus variations in toxicity between oils could lead 

 to different resulls. The sensitivity of limpets to 



three oils is shown in Table 11 where the toxicities 

 of Auk and Ekofisk oils, both fresh and "weathered', 

 are compared with that of the standard Kuwait oil 

 using two batches of limpets. There appeared to be 

 no significant diff.-once in toxicity between the 

 different types ol oil, or between fresh and 

 Veathered' oil. These results suggested that a coating 

 of oil will result in similar mortalities regardless of the 

 exact concentration or composition of the oil, and 

 imply that the limpets were killed by being physi- 

 cally coated with oil, rather than because of chemical 

 toxicity. The detailed mechanism by which the oils 

 used in these tests exerted their effects on limpets is 

 not clear. 



Table 11 - Relative toxicities of three oils, both fresh and 

 Veathered' to two populations of limpets in the 

 beach test 



"S 



(_ 

 o 

 E 



OJ 



en 

 a 



lOOr 



60- 

 20- 



1977 \ \ \\ 



J 



M J J 



N D 



"^lOOr 



a. 



60 



20- 



1978 



M J J 



N D 



Figure 5. Seasonal varutions in the mortality of Patella exposed to fresh Kuwait crude oil under standard beach test 

 conditions. 



Mean % mortality and standard deviation. 



