50
CARL J. STNDERMANN
As examples, the myxosporidean Kudoa clupeidae has been reported
from herring of the western Atlantic but not the eastern, the larval
nematode Eustoma rotundaturn from the eastern but not the western
Atlantic, while parasites such as the coccidia mentioned above and the
trematode Brachyphallus crenatua (Rud.) occur in varying frequencies
on both sides of the Atlantic.
~
FIG. 15. Prevalence of Ichthyophonus in Atlantic herring of the Gulf of
Saint Lawrence and the Gulf of Maine, 1965-63.
B. Temporal variations
Temporal variations in prevalence of disease are most apparent in
studies of epizootics. Unfortunately, long-term studies of prevalences
have rarely been carried out, so that the annual increases in incidence
building to epizootic levels, or the subsequent annual declines in
disease abundance following epizootics, have rarely been documented
for marine fish populations. Referring again to Ichthyophonus disease
of western North Atlantic herring, the general sequence of events that