Bone deformities occur regularly in fish farms around the world. The operculum is one of the earliest craniofacial bones to form embryologically and is subject to a range of developmental and acquired abnormalities. Opercular shortening is one of the most prevalent diseases in larval and juvenile salmonids (and other species), sometimes affecting up to 80% of fish in a population.
In those species that rely on the operculum to help move water over the gills, loss of efficiency in this part of the pumping mechanism puts an increased burden on respiration and excretion. Although salmonids have a respiratory reserve capacity, opercular shortening will inevitably compromise this, and result in increased blood flow (hyperaemia) in order to compensate.
Shortening can be bilateral or unilateral, and can be the result of erosion and remodelling, or simply failure to develop properly (hypoplasia). Bone development (osteogenesis) requires high levels of oxygen, so anything that interferes with a well-oxygenated blood supply to the developing tissue will lead to abnormalities.
In addition to loss of efficient respiration, opercular shortening exposes the underlying gill tissue which predisposes it to mechanical damage and infection. In the population overall, such changes can lead to a decrease in food consumption and variable mortality, and in addition can affect the presentation of the final product to the consumer.
The predisposing factors are varied but include environmental ones such as temperature (high temperature during incubation), pH, low dissolved oxygen, supersaturation (leading to gas bubble disease), heavy metals, salinity, light exposure times and high levels of bacteria.
Genetic alterations can lead to a high prevalence of opercular alterations; this has been seen in triploid Atlantic salmon. Nutritionally, shortening can occur with inadequate levels of fatty acids, proteins, ascorbic acid (vitamin C) and retinoic acid (vitamin A).
Other factors include teratogens such as malachite green and high biomass, possibly resulting in high bacterial loading, and elevated proteolytic enzymes that can erode the trailing edge of the operculum.
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