Have you ever caught a bass with random (but sometimes seemingly not random) black blotches on its body? Until recently, scientists didn’t really know what caused this. The black blotches were obviously linked to melanin production (the same process by which human skin color is regulated) and were mostly irregular, suggesting some kind of genetic mutation or anomaly. For a long time it was just known as “blotchy bass syndrome” and assumed to be harmless.

However, scientists started to notice some patterns with the syndrome, particularly related to density. It seemed that the prevalence of the syndrome was not random, which is what we would expect if it were a genetic anomaly. Recently, a scientist named Vicki Blazer with USGS did a study where she assayed the melanistic lesions on fish with the syndrome and discovered a virus!

The lesions are called Hyperpigmented Melanistic Skin Lesions or (HPML). Blazer performed a genetic analysis on the lesions and found genes belonging to a virus in a newly described family called Adomaviridae.

It is still unknown whether the virus has any direct negative effects on fish. It could indirectly affect them by messing with their ability to camouflage and hide from predators. However, it seemed to only affect fish larger than 200mm, which may be less vulnerable to predation.

Researchers on the Susquehanna River in Pennsylvania have been sampling smallmouth bass to record data on the prevalence of HPMLs in the population. They have found up to 13% of populations can show symptoms of the virus, and also that it is distributed in clusters, suggesting that it is communicable.

I’ve personally observed it in largemouth bass, alabama bass, and especially in shoal bass, where prevalence seems to be linked to seasonal patterns in behavior where fish aggregate in shoals during spawning season.

More research is needed to fully describe “blotchy bass syndrome”, but for now, hopefully this satisfies your curiosity.

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