Task: Question Answering
Input (English): Passage: The northern gannet is not heavily predated. The only known habitual natural predators of adults are bald eagles and white-tailed eagles. Predators of eggs and nestlings include the great black-backed gull and American herring gull, common ravens, ermine, and red fox. Attacks at sea are insignificant though large sharks and seals may rarely snatch a gannet out at sea.Kleptoparasitism by skuas, particularly the great skua, occurs at breeding sites. The skua chases its victim until it disgorges its stomach contents, providing a meal for the attacker. Skuas may catch the tip of the gannet's wing, causing it to fall into the sea, or seize the tail to tip its victim into the water. The gannet is only released when it has regurgitated its catch.External parasites include feather lice, although there are relatively few species and none are found on the head. As with grebes and divers it may be that the short head feathers provide insufficient cover for the parasite. In one species, Michaelichus bassani, immature lice are found in the membranes lining the subcutaneous air-cells. Ixodes mites include the widespread I. uriae.The spiny-headed worm Corynosoma tunitae appears to occur only in gannets and closely related seabird families such as the cormorants. The tapeworm Tetrabothrius bassani adsorbs toxic heavy metals at a higher concentration than the gannet's own tissues, with an average 12 times as much cadmium as the gannet's pectoral muscles and 7–10 times the lead level of the bird's kidney and liver. Since levels of these toxic metals are detectable in the parasite earlier than in the host, the tapeworm might be used as an early indicator of marine pollution. 
Question: What can levels of toxic metals in its parasites tell us might be present in the environment of the northern gannet?
Output (English): marine pollution.