Between June 1987 and May 1988, the bodies of at least 740 bottlenose dolphins out of a total coastal population of 3,000 to 5,000 washed ashore on the Atlantic coast of the United States.
Topic
The cause of a massive die-off of bottlenose dolphins on the Atlantic coast in 1987–1988.
Framework
Answer / Explain
Main Point
The passage examines why a severe die-off of bottlenose dolphins occurred, presenting a commonly accepted explanation (brevetoxin poisoning from an algal bloom) but ultimately arguing that a sudden increase in synthetic pollutants, such as PCBs, was likely the main precipitating factor, with brevetoxin possibly playing a secondary role. (The “most valuable sentence” is at the end of the final paragraph: )
P1: The Scale and Features of the Dolphin Die-Off
Between 1987–1988, about half the coastal bottlenose dolphins died in an unprecedented event, with affected dolphins showing a wide variety of symptoms, likely due to a massive, opportunistic bacterial infection.
P2: Possible Toxins Identified
Researchers tested dolphin tissues for natural and synthetic toxins, finding brevetoxin from an algal bloom in some, and PCBs—a manmade pollutant—in almost all dolphins.
P3: Brevetoxin Hypothesis
The research team initially concluded the red tide algal toxin (brevetoxin) was the main culprit, as there had been a bloom of this alga in the area. The dolphins’ stressed, emaciated state also likely released more pollutants from their fat, making them even more vulnerable to infection.