Climate change futures: health, ecological and economic dimensions

CASE STUDIES 84 | NATURAL AND MANAGED SYSTEMS
Dermo disease caused extensive oyster mortalities in
the Gulf of Mexico in the late 1940s. Later, it caused
chronic and occasionally massive mortalities in the
Chesapeake Bay. Since 1990, Dermo has been
detected in Delaware Bay, Long Island Sound,
Massachusetts, Rhode Island and Maine.
MSX (multi-nucleated spore unknown) is now known to
be caused by the parasite Haplospordium nelsoni.
MSX caused massive oyster mortalities in Delaware
Bay in 1957 and two years later in Chesapeake Bay.
The parasite has been found from Florida to Maine,
but has not been associated with mortalities in all
areas. MSX was found in oysters from Connecticut
waters 30 years ago. MSX disease is suppressed by
low salinities and low temperatures. As described by
the Connecticut Department of Agriculture, there is low
oyster mortality during the winter months and the
prevalence and intensity of the disease decreases. A
second mortality period occurs in late winter and early
spring (Ford and Tripp 1996).
THE ROLE OF CLIMATE
As with human diseases, the sequential occurrence of
events, such as a warm winter followed by a warm,
dry summer, can result in disease prevalence and
intensities greater than normal (Hofmann et al. 2001).
The intensification of Dermo disease coincided with a
period of sustained drought, diminished freshwater
inflow to the Chesapeake Bay, and warmer winters.
The drought and warmer temperatures, which cause
increased evaporation, combined with the reduced
freshwater input, resulted in increased salinity of the
Bay waters. The increased salinity allowed the disease-causing
parasite to increase in prevalence and
intensity. The milder winters allowed the parasite to survive
and remain at high levels in the oyster population.
Reduction and/or cessation in harvesting pressure
have not resulted in significant recovery of the stocks.
The mitigation strategies were too late and did not
anticipate the increase and spread of Dermo disease
that has occurred as the climate has warmed.
NORTHWARD MOVEMENT
From the late 1940s when Dermo disease was first
identified (Mackin et al. 1950) until the 1990s,
Dermo disease was found primarily from Chesapeake
Bay south along the Atlantic coast of the United States
and into the Gulf of Mexico. In 1990 and 1991 the
parasite causing this disease was found in locations
from Delaware Bay, NJ, to Cape Cod, MA. It is now
found in oyster populations in Maine and southern
Canada, where it has caused epizootics (epidemics
among animals — in this case, shellfish) that have devastated
oyster populations and the oyster fishery.
Several hypotheses have been suggested for the
observed expansion in range of this disease. The one
that is most consistent with the available evidence is
that this parasite was introduced into various northern
locations where it remained at low levels until the
recent warming climate allowed it to proliferate (Ford
1996). In particular, above-average winter temperatures
during the 1990s along the eastern United States
and Canada (Easterling et al. 1997) have allowed
the parasite that causes Dermo disease to become
established (Ford 1996; Cook et al. 1998). Also, the
interannual variation in prevalence and intensity of
Dermo disease in oysters along the Gulf of Mexico
has been shown to be related to shifts in the ENSO
cycle (Kim and Powell 1998), also an indication that
climate is a strong contributor. This relationship arises
through changes in temperature and salinity that result
from the ENSO cycle, which directly affect Perkinsus
marinus growth and development. The disease is much
more intense and prevalent throughout the Gulf of
Mexico during La Niña events, which produce warmer
and drier conditions and often drought (Kim and
Powell 1998).
HEALTH AND ECOLOGICAL IMPACTS
Oysters and other bivalves (clams and mussels), in
addition to serving as food for humans and shorebirds,
are filter-feeders. Each oyster can pull in many gallons
of water a day, filtering out the nutrients and plankton
for its own nourishment. In this way, they provide a critical
ecological service: controlling the nutrient and
algae level in bays and estuaries. Without bivalves,
these coastal waters would turn murky, and contaminated,
and algal mats would create hypoxic or anoxic
conditions. Such waters become less productive for
fish, shellfish and sea grasses that support the fish, and
shellfish can die off and the few fish left tend to