Monitoring Trends in Dolphin Abundance in the Eastern Tropical Pacific Using Research Vessels over a Long Sampling Period: Analysis of 1986 Data, the First Year

The status of five target species of northern offshore spotted dolphins was assessed to determine trends in population size of dolphins incidentally taken by tuna purse seiners. Two vessels, each using line-transect methodology, made observations for 120 d. The data obtained in this first of a five-year study is intended as a baseline. Between the two vessels 1150 schools of marine mammals were detected, of which 749 had dolphins. The abundance estimate was 929,000 animals. Minimum annual declines of at least 17.6% can be detected in five years and 10% in eight years. With additional surveys these detection levels may be reduced.