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Author: Dr. Panos Varvarigos
Freelance Veterinarian - Fish Pathologist, Athens, Greece.





(Identification, description)

Unknown in most cases. Several environmental and nutritional parameters have been associated with particular anatomic disorders affecting the skeletal, fin or swim bladder development. Among these, the disturbance of fertilised eggs during the first 10 hours of embryonic development, water flow/currents in the larval tanks, water pH, levels of dissolved CO2 and heavy metal ions, intensity of light, vitamin deficiency (mainly ascorbic acid), or hyper-vitaminosis (vitamin A) are considered important. However, in the majority of cases a clear link between specific factors with particular deformations has not been established. As a result, in a single hatchery, there may be different predominant types and levels of deformities in fry batches originating from the same brood-fish and under apparently unchanged management and feeding practices.

Economic Implications:


Frequency of occurrence:

Frequent (All fry batches contain a percentage of deformed fish)

Farmed fish species affected:

All artificially propagated, farmed species are subject to anatomic disorders.

Age/size of fish mostly susceptible:

Anatomic disorders are either hereditary or nutrition and environment related and usually develop early during the larval stages of the fish in the hatchery tanks. As the fish grow in the cages some types tend to become less obvious to the inexperienced (e.g. mild spinal disorders), while other types are emphasised (e.g. gill cover inward folding) and render the fish unmarketable.

Seasonal occurrence:


Regional pertinence:


Predisposing factors:

Poor health and nutrition of brood-fish. Shocks (alterations in water quality, temperature, light, vibrations) of fertilised ova during incubation, especially during the first hours of embryo development. Poor larval nutrition, inferior water quality, improper intensity of light and relatively strong water movement in the larval tanks.

Main deformities (categories):

The common anatomic problems may be classified into deformities of the:

-          Spine (lordosis, cyphosis, scoliosis),

-          Fins (caudal, dorsal) dysplasia or aplasia,

-          Head (face, jaws, opercula) dysplasiae.

The lack of a developed swim bladder used to be a very serious problem of the past. The disorder was caused by a thin film of lipids forming on the water surface in the larval tanks. This film obviates the larvae from gulping air in order to inflate their developing swim bladders. It consists of lipids that escape from the lipid rich diet necessary for larval nutrition. Surface skimmers have been introduced radically reducing the proportion of larvae lacking a swim bladder. Later, at the nursery stage, fry are tested for swim bladder development by floating or "epipleusis". Anaesthetised fry are placed for a short time in hyper-saline (briny) sea-water. Those without swim bladder sink to the bottom and are discarded.

Diagnosis (field, laboratory):

Fish inspection. Several samples of larvae must be inspected under a stereoscope. Random samples of fry should be inspected on an ichthyoscope and X-ray mammograms performed. Experienced staff should grade nursery fry subsequent to light anaesthesia. Handling has to be performed quickly on a wet, smooth surface, with few fish at a time to minimise stress and injury. An opaque glass surface with light from underneath (grading table) is recommended.

(mortality, growth reduction, extra labour):

Fry with a high percentage of deformed fish (in excess of 3%) comprise a very significant cost element, since the majority of such fish survive in the cages and utilise farm resources. At harvest they have to be discarded as substandard product.

Example: A rather small cage farm producing 300 metric tonnes of fish a year (860,000 fish at 350g average weight) requires to introduce about 1 million fry in order to account for overall annual losses of 7% (disease, accidents, escapes). If the percentage of deformed fry is 3% (realistic average), 30,000 unmarketable or degraded fish would be raised, representing considerable opportunity cost (potential sale of additional 10.5 tonnes of fish) and feed waste (about 16 tonnes of feed). In addition, extra labour to inspect and reject the "crooked" fish at the packing plant is necessary.

Hence, the pressure is on the hatcheries to produce quality fry. There is a very significant labour and time cost element to carry out regular sampling and repeated fish grading in the nurseries. Such handling comprises an important stress factor for the young fish and occasionally sparks disease. Besides, hatcheries are forced to give credit or donate additional fry when the percentage of deformed fry in a delivered batch is unreasonably high.



Management advice (prevention):

If the proportion of deformed larvae exceeds 30% the whole batch might be rejected as uneconomic. Management measures, such as those listed below, will minimise the intensity of anatomic anomalies and reduce the percentage of deformed fry in any batch. Deliveries of proper quality fry to the on-growers must be ensured by:

-    Careful selection, nutrition and conditioning of brood-stock.

-    Proper collection and incubation of fertilised eggs in view to minimising stress or shocks to the developing embryos.

-    Meticulous control and recording of water parameters and larval management.

-    Routine sampling of larvae and nursery fry for inspection.

-    Repeated (if necessary) hand grading of nursery fry under light anaesthesia.

-    X-ray mammograms of statistically significant random samples of every batch of fry prior to delivery.

-    Training and education of staff.

Environmental issues:

Deformed escapees into the sea are seriously handicapped hence, considered incapable of surviving and spawning in the competitive natural environment.





Author: Dr. Panos Varvarigos
Freelance Veterinarian - Fish Pathologist, Athens, Greece.


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Every effort has been made to ensure that the information is accurate until the date of last editing. It is based upon the accumulated personal experience of applied veterinary work. The author cannot take responsibility for incorrect interpretation or any resulting consequences. The contents may be used as an educational guide and are definitely not meant to become a stand-alone diagnostic tool or operations manual.


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