Since the 1950s, more than 6 million dolphins have died as by-catch in the American yellowfin tuna fishery. These deaths were not caused by accidental incidents between fishermen and dolphins but resulted from a method of fishing that purposefully targeted these animals in order to catch yellowfin tuna. Referred to as “fishing on porpoise,” this technique remained an industry secret for decades. By the early 1970s, however, dolphin by-catch had become a major environmental issue in the United States, thanks to the work of William F. Perrin. In the following years, politicians, scientists, environmentalists, and members of the tuna industry struggled with how best to resolve the problem. While the debates that arose from the “tuna-porpoise controversy” helped to dramatically reduce dolphin by-catch, these solutions did not come easily. This thesis looks to bring this forgotten moment in American environmental history to the historical forefront by exploring the origins and early years of the tuna-porpoise controversy. By examining this period, this thesis will show why fishermen first used dolphins to catch tuna in the 1950s, how and why dolphin by-catch became such a major environmental issue in the 1970s, and what various groups and individuals did to ameliorate the problem during the period. / Graduate