DISEASES OF MARINE FISHES
45
their ability to escape predators and to survive variations in the
physical environment ; by blinding fish ; by making infected fish more
conspicuous; or by altering behavior in ways that render fish more
vulnerable to predation. A disoriented, erratic circular swimming
movement at or near the surface has been observed as a generalized
symptom of a number of diseases. It was first described by Plehn and
Mulsow (191 1) as a symptom of Ichthyophonus infection in European
salmonids, and has since been seen during mass mortalities of Pacific
and Atlantic herring. Exophthalmia, with subsequent destruction of
the eye, is another generalized symptom of several diseases, including
bacterial infection and larval trematode invasion. Cataracts, usually
of both eyes, were noted by Raney (1952) in striped bass. Spinal
curvatures, evidently of neuro-muscular origin, have also been described
as generalized signs of bacterial and fungal infections. In studies
of herring diseases it has been noted that individuals with bacterial
infections exhibited characteristic and easily visible whitish patches
near the tail. Field observations of immature herring schools disclosed
that individuals so affected were easily seen even in turbid water and
during twilight. Other studies brought out the fact that diseased
herring aggregated differentially in deeper water, and that samples of
immature members of this pelagic species taken in bottom trawls
contained higher frequencies of abnormal fish than did samples taken
with conventional surface gear (purse seines and gill nets) in the same
areas. Bruun and Heiberg (1932) and others have noted that, eels with
" red disease " were lethargic and often motionless on the bottom
where they could be caught easily by hand. Locomotion became
disoriented and swimming became a series of stiff wriggling movements.
Examinations of fish with extensive intramuscular myxosporidean
cysts and necrosis, with heavy larval nematode infestation of muscles,
or with many copepods imbedded in their flesh, lead inevitably to the
conclusion that such a parasite burden, although it may not be the
primary cause of death, must seriously reduce statistical chances for
survival in an environment in which early and sudden death is the
rule. Disease may also reduce the reproductive capacity of marine fish
populations. Castration has been reported (Pinto, 1956) in European
sardines as a result of invasion of the testes by Eirneria sardinae.
Later studies disclosed that over 50% of the individuals sampled were
parasitized. Mechanical interference with the discharge of sex products
can result from massive visceral infections of smelt by Glugea hertwigi.
Cysts of the parasite occupy much of the body cavity, occlude the
vent and prevent normal spawning.
45
their ability to escape predators and to survive variations in the
physical environment ; by blinding fish ; by making infected fish more
conspicuous; or by altering behavior in ways that render fish more
vulnerable to predation. A disoriented, erratic circular swimming
movement at or near the surface has been observed as a generalized
symptom of a number of diseases. It was first described by Plehn and
Mulsow (191 1) as a symptom of Ichthyophonus infection in European
salmonids, and has since been seen during mass mortalities of Pacific
and Atlantic herring. Exophthalmia, with subsequent destruction of
the eye, is another generalized symptom of several diseases, including
bacterial infection and larval trematode invasion. Cataracts, usually
of both eyes, were noted by Raney (1952) in striped bass. Spinal
curvatures, evidently of neuro-muscular origin, have also been described
as generalized signs of bacterial and fungal infections. In studies
of herring diseases it has been noted that individuals with bacterial
infections exhibited characteristic and easily visible whitish patches
near the tail. Field observations of immature herring schools disclosed
that individuals so affected were easily seen even in turbid water and
during twilight. Other studies brought out the fact that diseased
herring aggregated differentially in deeper water, and that samples of
immature members of this pelagic species taken in bottom trawls
contained higher frequencies of abnormal fish than did samples taken
with conventional surface gear (purse seines and gill nets) in the same
areas. Bruun and Heiberg (1932) and others have noted that, eels with
" red disease " were lethargic and often motionless on the bottom
where they could be caught easily by hand. Locomotion became
disoriented and swimming became a series of stiff wriggling movements.
Examinations of fish with extensive intramuscular myxosporidean
cysts and necrosis, with heavy larval nematode infestation of muscles,
or with many copepods imbedded in their flesh, lead inevitably to the
conclusion that such a parasite burden, although it may not be the
primary cause of death, must seriously reduce statistical chances for
survival in an environment in which early and sudden death is the
rule. Disease may also reduce the reproductive capacity of marine fish
populations. Castration has been reported (Pinto, 1956) in European
sardines as a result of invasion of the testes by Eirneria sardinae.
Later studies disclosed that over 50% of the individuals sampled were
parasitized. Mechanical interference with the discharge of sex products
can result from massive visceral infections of smelt by Glugea hertwigi.
Cysts of the parasite occupy much of the body cavity, occlude the
vent and prevent normal spawning.