logic here is that, since, CPUE is constant therfore the pop is same
Using C we say that CPUE is not the only cause, perhaps there can be yet another cause : FISHING NETS
Reasoning : Since 1980's technology is so improved that people can catch more number of sharks so precisely in the stipulated time than that of in 1973.
Here E gives an alternate reason (technology) there by weakening the conclusion that CPUE would be same as that of 1973.