DISEASES OF MARINE FISHES
41
newly introduced in a susceptible population ; (2) infection pressure
(dosage) or virulence of the pathogen must increase ; or (3) resistance
of the population must be lowered. The role of environmentally
related influences and stresses such as nutrition, temperature and
salinity on disease prevalence must also be considered. Determination
of factors prevailing at the time of an outbreak requires very broadly
based and continuous studies in ecology, immunology and pathology.
Despite the fact that such ideal studies have not often been made in
the past, an appreciable amount of documentation has been assembled
for a few marine fish diseases with histories of epizootic prevalences
and mortalities. Outstanding in this respect is the fungus disease of
herring and other species, referred to earlier, caused by Ichthyophoiius
hoferi. Outbreaks of this pathogen in herring of the western North
Atlantic have been known since 1898 (Cox, 1916; Fish, 1934;
Scattergood, 1948 ; Sindermann, 1956, 1958). The most recent occurred
in the Gulf of Saint Lawrence in 1954-55, when an estimated one-half
of the herring population of that gulf was killed by the disease. The
estimate was based on sampling and field observations during the
outbreak, and the behavior of the fishery since then has supported
the original estimate. Landings declined to about half their previous
level, in the absence of other factors such as change in fishing effort or
market demand. Earlier work (Cox, 1916) indicated that comparable
reductions in population abundance and in the fishery followed the
outbreaks of 1898 and 1914. Reports of outbreaks in the Gulf of Maine
also suggested major dislocations in the fishery (Scattergood, 1948).
It is interesting and perhaps pertinent that the two most recent
outbreaks-the only ones for which we have adequate fishery dataoccurred a t times of herring abundance, as indicated by landing
statistics and general observations.
One of the most remarkable aspects of the fungus disease of herring
in the western North Atlantic has been its apparent periodicity-six
recorded outbreaks 14 to 25 years apart (Fig. 12). The two principal
areas involved, Gulf of Maine and Gulf of Saint Lawrence, each with
discrete populations of herring, have been out of phase during the
most recent outbreaks; an epizootic peak in one gulf coincided with
very low disease prevalence in the other. The comparatively brief
interval between outbreaks suggests a t best only traiisient increase in
resistance of herring populations to the disease. This hypothesis is
supported : by relatively low disease prevalence (an average 27%) at
the epizootic peak, which constitutes low selection pressure ; by the
fact that the most recent outbreak was at least as severe as the first
recorded outbreak in 1898 ; and by determination of mortality rates
0 2
41
newly introduced in a susceptible population ; (2) infection pressure
(dosage) or virulence of the pathogen must increase ; or (3) resistance
of the population must be lowered. The role of environmentally
related influences and stresses such as nutrition, temperature and
salinity on disease prevalence must also be considered. Determination
of factors prevailing at the time of an outbreak requires very broadly
based and continuous studies in ecology, immunology and pathology.
Despite the fact that such ideal studies have not often been made in
the past, an appreciable amount of documentation has been assembled
for a few marine fish diseases with histories of epizootic prevalences
and mortalities. Outstanding in this respect is the fungus disease of
herring and other species, referred to earlier, caused by Ichthyophoiius
hoferi. Outbreaks of this pathogen in herring of the western North
Atlantic have been known since 1898 (Cox, 1916; Fish, 1934;
Scattergood, 1948 ; Sindermann, 1956, 1958). The most recent occurred
in the Gulf of Saint Lawrence in 1954-55, when an estimated one-half
of the herring population of that gulf was killed by the disease. The
estimate was based on sampling and field observations during the
outbreak, and the behavior of the fishery since then has supported
the original estimate. Landings declined to about half their previous
level, in the absence of other factors such as change in fishing effort or
market demand. Earlier work (Cox, 1916) indicated that comparable
reductions in population abundance and in the fishery followed the
outbreaks of 1898 and 1914. Reports of outbreaks in the Gulf of Maine
also suggested major dislocations in the fishery (Scattergood, 1948).
It is interesting and perhaps pertinent that the two most recent
outbreaks-the only ones for which we have adequate fishery dataoccurred a t times of herring abundance, as indicated by landing
statistics and general observations.
One of the most remarkable aspects of the fungus disease of herring
in the western North Atlantic has been its apparent periodicity-six
recorded outbreaks 14 to 25 years apart (Fig. 12). The two principal
areas involved, Gulf of Maine and Gulf of Saint Lawrence, each with
discrete populations of herring, have been out of phase during the
most recent outbreaks; an epizootic peak in one gulf coincided with
very low disease prevalence in the other. The comparatively brief
interval between outbreaks suggests a t best only traiisient increase in
resistance of herring populations to the disease. This hypothesis is
supported : by relatively low disease prevalence (an average 27%) at
the epizootic peak, which constitutes low selection pressure ; by the
fact that the most recent outbreak was at least as severe as the first
recorded outbreak in 1898 ; and by determination of mortality rates
0 2