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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