(A) Many of the species of fish caught by Snare Island fishermen and Chinuan fishermen are closely related.
This might actually help the plan, as similar species might have similar taste or characteristics, making it easier for Snare residents to accept. Doesn't cast doubt.
(B) Consumers in Snare Island eat fish in restaurants about as often as do consumers in Chinua.
This doesn't directly relate to the plan. It just says frequency of eating fish is similar, but doesn't address preference for local taste or the low-fat marketing. Not particularly doubting.
(C) At times in the past, Snare Island fishermen have sold much of their catch to buyers in Chinua.
This might indicate that Chinuan people like Snare fish, but it doesn't say anything about Snare people liking Chinuan fish. Doesn't cast doubt on the plan to sell to Snare.
(D) The warmer climate in Chinua allows Chinuan fishermen to harvest fish more months of the year than can Snare Island fishermen.
This could be an advantage for Chinuan fishermen, as they might have a more consistent supply. It doesn't cast doubt; it might even support the plan by making lower prices possible.
(E) Most Snare Island residents are not concerned about their diet and believe that locally-caught fish are low-fat.
This directly undermines the marketing strategy. The plan relies on marketing Chinuan fish as a low-fat alternative. If Snare residents aren't concerned about diet, they won't care about the low-fat claim. Moreover, they already believe local fish are low-fat, so there's no perceived benefit to switching. This makes the plan much less viable.
Therefore, option (E) casts the most doubt on the plan.
Answer: (E)