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Biotoxins and Disease

Little is known about natural mortality rates of marine mammals in the wild because it is difficult to obtain basic biological information such as age, population size, calving and reproductive rates. In addition to mortality due to predation, some species occasionally suffer from epizootics which have resulted in unusually high mortality events. In 1987, 14 humpback whales washed ashore dead and decomposed along Cape Cod Bay and Nantucket Sound. The cause of this unprecedented stranding of large baleen whales was attributed to a naturally occurring neurotoxin called saxotoxin or STX (Geraci, et al., 1989). STX is produced by a species of dinoflagellate and is more commonly associated with the so-called red tides that may lead to paralytic shellfish poisoning if affected shellfish are consumed. In this particular instance, the humpback whales had been feeding upon mackerel (presumably from the Gulf of St. Lawrence) which had concentrations of STX universally present in their viscera and especially in the liver.

Also attributed to a biotoxin was the massive die-off of bottlenose dolphins (Tursiops truncatus), that occurred in 1987 and again in 1988 along the southern and mid-Atlantic coast from Florida to New Jersey. In total, more than 740 animals washed ashore, with unknown fatalities offshore. Scott, et al. (1988) estimated that 50% or more of the coastal migratory stock between Florida and New Jersey died during this period. Clinical analysis identified brevetoxin, a neurotoxin produced by the dinoflagellate Ptychodiscus brevis, as the proximate cause of poisoning (Geraci, et al. 1989). Since many of these animals also had very high levels of PCBs in their liver, it was speculated that chemical toxicity may have also been a factor in the die-off. In their weakened state, the actual cause of death for many animals was traced to a systemic bacterial infection.

Additional investigations regarding marine mammal diseases suggests that some species of large cetaceans including blue whales (Balaenoptera musculus), fin whales (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae), may suffer significant mortality due to endemic parasitic diseases. It is estimated that 90-95 percent of the fin whales in the North Atlantic are infected by the giant and highly invasive nematode, Crassicauda boopis (Lambertsen, 1986). Transmission of C. boopis from the adult whale to its calf presumably occurs incidentally through ingestion of the larvae shed in the cow's urine during periods of nursing (ibid).

In 1992, New England Aquarium and NMFS conducted serum neutralization studies on live stranded harbor seals and confirmed suspicions of the existence of phocine distemper virus in the Gulf of Maine population. This disease has also been confirmed in harbor seals stranded on Long Island, New York (Duignan, et al., 1993). Further studies led to the conclusion that the disease is endemic in pinnipeds along the east coast and suggested that infection confers long-term immunity (NMFS, 1994). Pinnipeds, particularly grey seals, are also host to three species of nematode worms, collectively called cod worm. The worms live in the stomachs of the seals, where their eggs are shed through the feces, and the larvae are eventually ingested and encyst in the muscle or intestinal mesenteries of cod and other demersal fish (Bonner, 1990).

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