DISEASES OF MARINE FISHES
61
has emerged from studies of several outbreaks includes a long enzootic
phase of very low prevalence (< 0-1%), several years of rapidly
increasing incidences reaching an epizootic peak when about 25 yo of
individuals are infected and acute infections are common, then several
years of rapidly declining incidences and return to the enzootic phase.
Annual sampling since the most recent outbreak in Gulf of Saint
Lawrence herring has provided some data to support these general
observations (Fig. 15). The consistency in this sequence of events, as
deduced from fragmentary observations during the years immediately
preceding and following the epizootics of 1931, 1947 and 1954 suggests
that future outbreaks of Ichthyophonus disease may be predicted
several years in advance, if sampling of herring populations is
continuous.
Other diseases of herring have not exhibited the periodicity characteristic of the fungus disease. Continuous sampling for myxosporidean
(Kudoa) infections has disclosed only minor variations in prevalence
in those areas where the parasite occurs. It is of course possible, and
even likely, that longer-term variations in abundance might be expected
for this and other parasites. Similar stability in parasite abundance
has been found in a 5-year study of larval nematodes (Anisakinae)
and larval cestodes (Trypenorhyncha) in herring from the North
American east coast.
Other examples of dramatic increases in abundance of pathogens
and parasites can be found in the literature. Dogie1 and Lutta (1937)
studied such an outbreak in sturgeon caused by monogenetic trematodes
introduced into the Aral Sea with their normal hosts. Increases of
this kind are often seen in marine aquaria, and are considered in a
later section.
C. Age effects
The effect of host age on the parasite fauna has been examined
repeatedly. Recent summarizations of Russian work (Liaiman, 1956 ;
Polyanski and Shulman, 1966 ; Polyanski, 1968) indicate that significant variations with age can result from segregation into schools of
similar age that often occupy different habitahs at different ages, and
from changes in diet of fish with increasing age. Increase in occurrence
and intensity with age is often characteristic of worm infestations, but
is not necessarily true with microbial agents, many of which may be
more common in young fish than in old. Polyanski and Shulman
(or). cit.) have described the changes with age in the parasite fauna of
cod and Reshetnikova (1955) of mullet in the Black Sea. The authors
made the point that " juvenile " parasites, characteristic of very
61
has emerged from studies of several outbreaks includes a long enzootic
phase of very low prevalence (< 0-1%), several years of rapidly
increasing incidences reaching an epizootic peak when about 25 yo of
individuals are infected and acute infections are common, then several
years of rapidly declining incidences and return to the enzootic phase.
Annual sampling since the most recent outbreak in Gulf of Saint
Lawrence herring has provided some data to support these general
observations (Fig. 15). The consistency in this sequence of events, as
deduced from fragmentary observations during the years immediately
preceding and following the epizootics of 1931, 1947 and 1954 suggests
that future outbreaks of Ichthyophonus disease may be predicted
several years in advance, if sampling of herring populations is
continuous.
Other diseases of herring have not exhibited the periodicity characteristic of the fungus disease. Continuous sampling for myxosporidean
(Kudoa) infections has disclosed only minor variations in prevalence
in those areas where the parasite occurs. It is of course possible, and
even likely, that longer-term variations in abundance might be expected
for this and other parasites. Similar stability in parasite abundance
has been found in a 5-year study of larval nematodes (Anisakinae)
and larval cestodes (Trypenorhyncha) in herring from the North
American east coast.
Other examples of dramatic increases in abundance of pathogens
and parasites can be found in the literature. Dogie1 and Lutta (1937)
studied such an outbreak in sturgeon caused by monogenetic trematodes
introduced into the Aral Sea with their normal hosts. Increases of
this kind are often seen in marine aquaria, and are considered in a
later section.
C. Age effects
The effect of host age on the parasite fauna has been examined
repeatedly. Recent summarizations of Russian work (Liaiman, 1956 ;
Polyanski and Shulman, 1966 ; Polyanski, 1968) indicate that significant variations with age can result from segregation into schools of
similar age that often occupy different habitahs at different ages, and
from changes in diet of fish with increasing age. Increase in occurrence
and intensity with age is often characteristic of worm infestations, but
is not necessarily true with microbial agents, many of which may be
more common in young fish than in old. Polyanski and Shulman
(or). cit.) have described the changes with age in the parasite fauna of
cod and Reshetnikova (1955) of mullet in the Black Sea. The authors
made the point that " juvenile " parasites, characteristic of very