In water supersaturated with gas, gas bubble disease
(GBD) involving the eyes, may occur (see Fig. 10.3). Eye
lesions include gas bubbles in the anterior chamber, cataract,
synechia and panophthalmitis.
Physical damage with perforation of the cornea or endogenous spread of bacteria to the ocular structures may result in
endopthalmitis or panopthalmitis, which may be accompanied
by haemorrhage and subsequent puncture of the eye. Damage
to the cornea may be superficial or deep to penetrating
(perforating). Superficial damage will heal by increased
mitotic activity and migration of epithelial cells from the
periphery of the lesions. During the healing process, the eye
and the open socket may completely fill with scar tissue.
Another source of eye damage is some parasitic
infections as for example infections by metacercaria of
Diplostomum spathaceum in wild fish or those reared in
earth ponds. The parasite affects the lens or in the anterior
eye chamber (see Fig. 9.36). Similar eye changes have been
seen in fish infected with the trematode Tylodelphys clavata.
10.5 Developmental Disorders
Deformities, skeletal disorders or pigment abnormalities are
an unwelcome but inherent aspect in salmonid farming, fortunately, with low prevalence. The similarity of malformations
across fish species and culture systems implies that there is a
general causal effect within the environment. Particular groups
or batches of salmon fry may show a range of deformities
which are evident within the egg, or are apparent shortly after
hatching. Such deformities include a general discolouration or
pigment abnormality of the egg, ‘Siamese twin’ fry, kyphosis,
lordosis and others associated with the cranium or jaw. Many
of these conditions are attributed to genetic factors, nutritional
imbalance, hypoxia, poor husbandry, or contact with parasites
such as Myxobolus cerebralis. Furthermore, fish can be the
recipients of numerous injuries that are potentially deleterious
to aquaculture production performance and welfare. For many
deformities the proposed causes in natural populations remain
speculative.
10.5.1 Deformities Among Eggs and Fry
High mortality during early life is common and may be
caused by numerous aetiologies, but often developmental
(Fig. 10.31). All salmonids can be affected but most reports
are from widely farmed species, i.e. salmon and rainbow
trout. Hereditary and environmental factors are the most
important causes. Moderate abnormalities may become evident as the fish grows, and provided these do not interfere
significantly with normal function, such fish can generally
survive. However, as a farmed animal they will not perform
well, they will be removed during grading and consequently
rejected at market.
10.5.2 Yolk Sac Constrictions
Yolk sac constrictions affect the posterior part of the yolksac which is separated and becomes unavailable as an energy
source for developing fry. The cause(s) are not known, but
may be related to density and presence of inadequate
substrate. Insufficient water flow and/or accumulation of
waste metabolites are also possible causes, alone or in
combination.
10.5.3 Soft Egg Disease
Soft egg disease is a condition where the eggs become soft
and flaccid and collapse, probably as a result of altered
water-flow through the membranes of the egg. Possible
causes include high concentration of ammonia and amoeba
infestation.
10.5.4 Blue Sac Disease
Blue sac disease or dropsy (hydrocoele embryonalis) is
characterized by abnormal accumulation of fluid with a
bluish tint between the yolk-sac and the outer membranes
of the fry (Fig. 10.32). This can be observed shortly after
hatching and become more apparent within a few days.
Affected individuals often display humpback, a wide open
mouth and focal haemorrhage. Affected fry are lethargic
with reduced respiration and heart rate. Fry may recover
if the condition has not had a long duration or if the lesions
are moderate. The causes are believed to be accumulation
of toxic waste products, unfavourable environment, rough
handling and transport. Xenobiotic chemicals may act
directly or following enzymatic catalysis to intermediate
products, becoming more or less toxic to the organism. The
condition has been reported to be induced by dioxininducing the syndrome characterized by increased mortality at early life stages and spinal defects, oedema and
haemorrhage.
10.5.5 White Spot Disease
White spot disease can occur in eggs in late stages of incubation and the yolk-sac of newly hatched fry. The white or
greyish spots in the yolk are coagulated proteins following a
leak in the foetal membrane with consequent mortality
(Fig. 10.33). Unfavourable environmental factors including
164
10 Production Diseases and Other Disorders
(GBD) involving the eyes, may occur (see Fig. 10.3). Eye
lesions include gas bubbles in the anterior chamber, cataract,
synechia and panophthalmitis.
Physical damage with perforation of the cornea or endogenous spread of bacteria to the ocular structures may result in
endopthalmitis or panopthalmitis, which may be accompanied
by haemorrhage and subsequent puncture of the eye. Damage
to the cornea may be superficial or deep to penetrating
(perforating). Superficial damage will heal by increased
mitotic activity and migration of epithelial cells from the
periphery of the lesions. During the healing process, the eye
and the open socket may completely fill with scar tissue.
Another source of eye damage is some parasitic
infections as for example infections by metacercaria of
Diplostomum spathaceum in wild fish or those reared in
earth ponds. The parasite affects the lens or in the anterior
eye chamber (see Fig. 9.36). Similar eye changes have been
seen in fish infected with the trematode Tylodelphys clavata.
10.5 Developmental Disorders
Deformities, skeletal disorders or pigment abnormalities are
an unwelcome but inherent aspect in salmonid farming, fortunately, with low prevalence. The similarity of malformations
across fish species and culture systems implies that there is a
general causal effect within the environment. Particular groups
or batches of salmon fry may show a range of deformities
which are evident within the egg, or are apparent shortly after
hatching. Such deformities include a general discolouration or
pigment abnormality of the egg, ‘Siamese twin’ fry, kyphosis,
lordosis and others associated with the cranium or jaw. Many
of these conditions are attributed to genetic factors, nutritional
imbalance, hypoxia, poor husbandry, or contact with parasites
such as Myxobolus cerebralis. Furthermore, fish can be the
recipients of numerous injuries that are potentially deleterious
to aquaculture production performance and welfare. For many
deformities the proposed causes in natural populations remain
speculative.
10.5.1 Deformities Among Eggs and Fry
High mortality during early life is common and may be
caused by numerous aetiologies, but often developmental
(Fig. 10.31). All salmonids can be affected but most reports
are from widely farmed species, i.e. salmon and rainbow
trout. Hereditary and environmental factors are the most
important causes. Moderate abnormalities may become evident as the fish grows, and provided these do not interfere
significantly with normal function, such fish can generally
survive. However, as a farmed animal they will not perform
well, they will be removed during grading and consequently
rejected at market.
10.5.2 Yolk Sac Constrictions
Yolk sac constrictions affect the posterior part of the yolksac which is separated and becomes unavailable as an energy
source for developing fry. The cause(s) are not known, but
may be related to density and presence of inadequate
substrate. Insufficient water flow and/or accumulation of
waste metabolites are also possible causes, alone or in
combination.
10.5.3 Soft Egg Disease
Soft egg disease is a condition where the eggs become soft
and flaccid and collapse, probably as a result of altered
water-flow through the membranes of the egg. Possible
causes include high concentration of ammonia and amoeba
infestation.
10.5.4 Blue Sac Disease
Blue sac disease or dropsy (hydrocoele embryonalis) is
characterized by abnormal accumulation of fluid with a
bluish tint between the yolk-sac and the outer membranes
of the fry (Fig. 10.32). This can be observed shortly after
hatching and become more apparent within a few days.
Affected individuals often display humpback, a wide open
mouth and focal haemorrhage. Affected fry are lethargic
with reduced respiration and heart rate. Fry may recover
if the condition has not had a long duration or if the lesions
are moderate. The causes are believed to be accumulation
of toxic waste products, unfavourable environment, rough
handling and transport. Xenobiotic chemicals may act
directly or following enzymatic catalysis to intermediate
products, becoming more or less toxic to the organism. The
condition has been reported to be induced by dioxininducing the syndrome characterized by increased mortality at early life stages and spinal defects, oedema and
haemorrhage.
10.5.5 White Spot Disease
White spot disease can occur in eggs in late stages of incubation and the yolk-sac of newly hatched fry. The white or
greyish spots in the yolk are coagulated proteins following a
leak in the foetal membrane with consequent mortality
(Fig. 10.33). Unfavourable environmental factors including
164
10 Production Diseases and Other Disorders