题目选项分析

Lockeport's commercial fishing boats use gill nets, which kill many of the netted fish, including some fish of endangered species. The fishing commission has proposed requiring the use of tent nets, which do not kill fish: boat crews would then throw back fish of endangered species. Profitable commercial fishing boats in similar areas have already switched over to tent nets. The proposal can therefore be implemented without economic harm to Lockeport's commercial fishing boat operators. Which of the following, if true, casts the most serious doubt on the argument made for the proposal?

【选项】The endangered species of fish in Lockeport's commercial fishing area did not become endangered as a result of the use of gill nets by fishing fleets.