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Bacteria A. sobria could cause red discolouration in channel catfish fillet: U.S. study

The probable cause for red discolouration in channel catfish (Ictalurus punctatus) fillet has been suggested by a U.S. study, bringing the industry a step closer to fighting economic losses. 

More than one per cent of the catfish fillets from each harvest is rejected by processing plants because of diffuse red discolouration in the fillet. Per 2019 data, the estimated annual loss was US$683,000 for catfish processors.

In the study by Shay S. Keretz et al., “Effects of Aeromonas sobria on physiology and fillet quality of market-sized channel catfish (Ictalurus punctatus),” fillets from channel catfish inoculated with bacteria A. sobria demonstrated redness values similar to that of the catfish fillets rejected by processors. 

Fillet redness was analyzed on days three, six, and nine post-inoculation. Treated fillets were redder than control on day three but statistically similar on days six and nine. Fillet redness in control fish was lower on days three and nine, and higher on day six.

In the control group, which was sham-injected with phosphate-buffered saline, the overall average redness of fish fillets was similar to that of acceptable processed catfish fillets.

The study was published in the Journal of World Aquaculture Society.

Physiological responses of inoculated animal “included decreases in hemoglobin and hematocrit presumably because of hemolytic anemia caused by hemolytic toxins produced by A. sobria and increased histological lesion grades for multiple organs, because of inflammation and necrosis.”

A. sobria was cultured and juvenile catfish was initially used in the experiment to determine the effective bacterial dose subsequent to the inoculation of market-sized animal. 

A. sobria is a pathogenic bacteria prevalent in acceptable catfish fillets at channel catfish plants in the Mississippi Delta. Some aeromonad strains have been identified as highly virulent and are known to lead to hemorrhaging and, it was explained, could either cause or contribute to the production of red fillets.

Aside from discolouration, processors reject catfish fillet on the basis of texture being too soft or hard.

Some cited that the small, singular red discolouration marks could be caused by the fish locking their pectoral pins in response to predator stimulation or high stress levels. “When there are high stocking densities of catfish, locked pectoral fins may cause puncture damage to adjacent catfish, which result in red spots on the fillets,” it was explained.

Previous studies had also factored in the impact of water quality. Although poor water conditions are a stressor, it was proven that it does not induce the formation of red fillets.