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Between June 1987 and May 1988, the bodies of at least 740 [#permalink]
03 Apr 2009, 14:52
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. Since some of the dead animals never washed ashore, the overall disaster was presumably worse; perhaps 50 percent of the population died. A dolphin die-off of this character and magnitude had never before been observed; furthermore, the dolphins exhibited a startling range of symptoms. The research team that examined the die-off noted the presence of both skin lesions and internal lesions in the liver, lung, pancreas and heart, which suggested a massive opportunistic bacterial infection of already weakened animals.
Tissues from the stricken dolphins were analyzed for a variety of toxins. Brevetoxin, a toxin produced by the blooming of the alga Ptychodiscus brevis, was present in eight out of seventeen dolphins tested. Tests for synthetic pollutants revealed that polychlorinated biphenyls (PCBs) were present in almost all animals tested.
The research team concluded that brevetoxin poisoning was the most likely cause of the illnesses that killed the dolphins. Although P. brevis is ordinarily not found along the Atlantic coast, an unusual bloom of this organism—such blooms are called “red tides” because of the reddish color imparted by the blooming algae—did occur in the middle of the affected coastline in October 1987. These researchers believe the toxin accumulated in the tissue of fish and then was ingested by dolphins that preyed on them. The emaciated appearance of many dolphins indicated that they were metabolizing their blubber reserves, thereby reducing their buoyancy and insulation (and adding to overall stress) as well as releasing stores of previously accumulated synthetic pollutants, such as PCBs, which further exacerbated their condition. The combined impact made the dolphins vulnerable to opportunistic bacterial infection, the ultimate cause of death.
For several reasons, however, this explanation is not entirely plausible. First, bottlenose dolphins and P. brevis red tides are both common in the Gulf of Mexico, yet no dolphin die-off of a similar magnitude has been noted there. Second, dolphins began dying in June, hundreds of miles north of and some months earlier than the October red tide bloom. Finally, the specific effects of brevetoxin on dolphins are unknown, whereas PCB poisoning is known to impair functioning of the immune system and liver and to cause skin lesions; all of these problems are observed in the diseased animals. An alternative hypothesis, which accounts for these facts, is that a sudden influx of pollutants, perhaps from offshore dumping, triggered a cascade of disorders in animals whose systems were already heavily laden with pollutants. Although brevetoxin may have been a contributing factor, the event that actually precipitated the die-off was a sharp increase in the dolphins’ exposure to synthetic pollutants.
14. The passage is primarily concerned with assessing (A) the effects of a devastating bacterial infection in Atlantic coast bottlenose dolphins (B) the progress by which illnesses in Atlantic coast bottlenose dolphins were correctly diagnosed (C) the weaknesses in the research methodology used to explore the dolphin die-off (D) possible alternative explanations for the massive dolphin die-off (E) relative effects of various marine pollutants on dolphin mortality
15. Which one of the following is mentioned in the passage as evidence for the explanation of the dolphin die-off offered in the final paragraph? (A) the release of stored brevetoxins from the dolphins’ blubber reserves (B) the date on which offshore dumping was known to have occurred nearby (C) the presence of dumping sites for PCBs in the area (D) the synthetic pollutants that were present in the fish eaten by the dolphins (E) the effects of PCBs on liver function in dolphins
16. Which one of the following is most analogous to the approach taken by author of the passage with regard to the research described in the third paragraph? (A) A physics teacher accepts the data from a student’s experiment but questions the student’s conclusions. (B) An astronomer provides additional observations to support another astronomer’s theory. (C) A cook revises a traditional recipe by substituting modern ingredients for those used in the original. (D) A doctor prescribes medication for a patient whose illness was misdiagnosed by another doctor. (E) A microbiologist sets out to replicate the experiment that yielded a classic theory of cell structure.
17. Which one of the following most accurately describes the organization of the last paragraph? (A) One explanation is criticized and different explanation is proposed. (B) An argument is advanced and then refuted by means of an opposing argument. (C) Objections against a hypothesis are advanced, the hypothesis is explained more fully, and then the objections are rejected. (D) New evidence in favor of a theory is described, and then the theory is reaffirmed. (E) Discrepancies between two explanations are noted, and a third explanation is proposed.
18. It can be inferred from the passage that the author would most probably agree with which one of the following statements about brevetoxin? (A) It may have been responsible for the dolphins’ skin lesions but could not have contributed to the bacterial infection. (B) It forms more easily when both P. brevis and synthetic pollutants are present in the environment simultaneously. (C) It damages liver function and immune system responses in bottlenose dolphins but may not have triggered this particular dolphin die-off. (D) It is unlikely to be among the factors that contributed to the dolphin die-off. (E) It is unlikely to have caused the die-off because it was not present in the dolphins’ environment when the die-off began.
19. The explanation for the dolphin die-off given by the research team most strongly supports which one of the following? (A) The biological mechanism by which brevetoxin affects dolphins is probably different from that by which it affects other marine animals. (B) When P. brevis blooms in an area where it does not usually exist, it is more toxic than it is in its usual habitat. (C) Opportunistic bacterial infection is usually associated with brevetoxin poisoning in bottlenose dolphins. (D) The dolphins’ emaciated state was probably a symptom of PCB poisoning rather than of brevetoxin poisoning. (E) When a dolphin metabolizes its blubber, the PCBs released may be more dangerous to the dolphin than they were when stored in the blubber.
20. The author refers to dolphins in the Gulf of Mexico in the last paragraph in order to (A) refute the assertion that dolphins tend not to inhabit areas where P. brevis is common (B) compare the effects of synthetic pollutants on these dolphins and on Atlantic coast dolphins (C) cast doubt on the belief that P. brevis contributes substantially to dolphin die-offs (D) illustrate the fact that dolphins in relatively pollution-free waters are healthier than dolphins in polluted waters (E) provide evidence for the argument that P. brevis was probably responsible for the dolphins’ deaths
21. Which one of the following factors is explicitly cited as contributing to the dolphins’ deaths in both theories discussed in the passage? (A) the dolphins’ diet (B) the presence of P. brevis in the Gulf of Mexico (C) the wide variety of toxins released by the red tide bloom of October 1987 (D) the presence of synthetic pollutants in the dolphins’ bodies (E) the bacterial infection caused by a generalized failure of the dolphins’ immune systems