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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