DISEASES OF MARINE FISHES
43
Europe, particularly Scandinavia, provides other evidence of widespread epizootics in which great numbers of eels were killed (Bruun and
Heiberg, 1932, 1935). An outbreak in eels of the Baltic coast of
Germany in 1959, reported by Mattheis (1960), followed a period of
several decades of low prevalence. Mattheis also observed that an
epizootic in eels preceded by one year an outbreak in pike.
The very recent epizootic of bacterial etiology in Chesapeake Bay
white perch seriously reduced the population, as indicated by field
observations of shoals of dead fish, and numbers washed up on shore,
as well as by a sharp decline in production. The fishery in 1964, the
first post-epizootic year, was markedly reduced in comparison with
previous years. Landings of white perch in 1964 were only 622000
pounds, as compared with an average of 1500 000 pounds for the
three preceding years ; no indication existed that other major variables
had changed. Statements by sportsmen and local residents indicated
that white perch were extraordinarily abundant in 1962, the year
immediately preceding the outbreak.
Mass mortalities appear t o be, and often are, catastrophic events
when viewed from close proximity. Their long-term effects may be
severe, even resulting in extinctions in part of the host range, or they
may be slight, causing only minor depressions in population size. If
reduction is severe enough and the ecological niche ” vacated long
enough, the species may be replaced by another with similar ecological
requirements. The original species may then persist in low abundance
for a long period. This is probably the extreme consequence of disease.
The more probable course of events, in view of the high reproductive
potential of most teleosts, is a gradual return to former population
size and only temporary disturbances of many parts of the ecosystem
in which the species is enmeshed. An extensive and still emerging
documentation of the degree of this dislocation can be found in studies
of the fisheries in the Gulf of Saint Lawrence during and after the
recent epizootic of fungus disease (Kohler, 1961 ; Sindermann, 1963 ;
Tibbo and Graham, 1963). Herring were most seriously involved, and
landings during the post-epizootic years were less than half their
previous level (Fig. 13). Mean ages in landings decreased, growth
rates increased, fewer age group8 were represented in the fishery, and
relative abundance of herring larvae decreased sharply. Alewives,
which have been described as less susceptible to the pathogen (Sindermann and Scattergood, 1954), acquired infections and were killed in
sufficient numbers to be observed. Mackerel were also heavily infected ;
mortalities occurred ; and landings decreased during the post-epizootic
years. Cod did not become infected but fed on disabled herring to
43
Europe, particularly Scandinavia, provides other evidence of widespread epizootics in which great numbers of eels were killed (Bruun and
Heiberg, 1932, 1935). An outbreak in eels of the Baltic coast of
Germany in 1959, reported by Mattheis (1960), followed a period of
several decades of low prevalence. Mattheis also observed that an
epizootic in eels preceded by one year an outbreak in pike.
The very recent epizootic of bacterial etiology in Chesapeake Bay
white perch seriously reduced the population, as indicated by field
observations of shoals of dead fish, and numbers washed up on shore,
as well as by a sharp decline in production. The fishery in 1964, the
first post-epizootic year, was markedly reduced in comparison with
previous years. Landings of white perch in 1964 were only 622000
pounds, as compared with an average of 1500 000 pounds for the
three preceding years ; no indication existed that other major variables
had changed. Statements by sportsmen and local residents indicated
that white perch were extraordinarily abundant in 1962, the year
immediately preceding the outbreak.
Mass mortalities appear t o be, and often are, catastrophic events
when viewed from close proximity. Their long-term effects may be
severe, even resulting in extinctions in part of the host range, or they
may be slight, causing only minor depressions in population size. If
reduction is severe enough and the ecological niche ” vacated long
enough, the species may be replaced by another with similar ecological
requirements. The original species may then persist in low abundance
for a long period. This is probably the extreme consequence of disease.
The more probable course of events, in view of the high reproductive
potential of most teleosts, is a gradual return to former population
size and only temporary disturbances of many parts of the ecosystem
in which the species is enmeshed. An extensive and still emerging
documentation of the degree of this dislocation can be found in studies
of the fisheries in the Gulf of Saint Lawrence during and after the
recent epizootic of fungus disease (Kohler, 1961 ; Sindermann, 1963 ;
Tibbo and Graham, 1963). Herring were most seriously involved, and
landings during the post-epizootic years were less than half their
previous level (Fig. 13). Mean ages in landings decreased, growth
rates increased, fewer age group8 were represented in the fishery, and
relative abundance of herring larvae decreased sharply. Alewives,
which have been described as less susceptible to the pathogen (Sindermann and Scattergood, 1954), acquired infections and were killed in
sufficient numbers to be observed. Mackerel were also heavily infected ;
mortalities occurred ; and landings decreased during the post-epizootic
years. Cod did not become infected but fed on disabled herring to