Osterhaus emphasizes that both kinds of fish were taken from catches destined for human consumption. For 2 years, the researchers sampled blood from the seals every six to nine weeks and made various measurements of immune system function. Almost immediately after the experiment began, vitamin A levels dropped 20 to 40 percent in the blood of seals fed fish from the Baltic and remained low throughout the 2-year experiment. The Baltic-fed seals showed other changes in their blood. The concentration of granulocytes was consistently elevated 10 to 15 percent, compared to the Atlantic-fed seals.
Granulocytes are white blood cells that fight bacterial infections.
Osterhaus speculates that the Baltic-fed seals may have suffered from higher levels of chronic infection. Other key components of the immune system were compromised in the Baltic-fed seals. The Osterhaus experiments are not the final word on what's been killing seals for the last 10 years, but they are highly suggestive that pollution harms the immune system of marine mammals. Because they have large stores of blubber, because organochlorine compounds accumulate in fat, and because they eat high on the food chain, marine mammals tend to collect organochlorine compounds in their bodies. Last December a group of wildlife specialists, representing diverse disciplines, met to discuss the global situation.