Mystery of the Black Blotched Bass, by Don Orth

Is there a Piscine Dermatologist anywhere?   If so, I need your expert assistance.  People post anomalous fish photos on Flickr, Facebook, Instagram, Pinterest, or Twitter all the time and ask “what is wrong with this fish?”   One mystery that has been around for a long time, and not yet solved is the mystery of the black blotched bass.     Too often all we have is the photos, which are insufficient for any diagnosis.

Smallmouth Bass with Melanosis, from Susquehanna River.  Photo by Bill Yingling

These bass have irregularly shaped black blotches on their bodies.    Largemouth Bass and Smallmouth Bass with these black blotches have been reported by anglers and by fisheries biologists alike.   These black blotches have been reported as early as 1980 from rivers, such as the Hudson and Susquehanna, as well as lakes, reservoirs, rivers and ponds in eastern, southern, and Gulf states.

What is it?    The black blotches are referred to as ‘melanosis’ or hyperpigmented melanosis.    Darkening is hyperpigmentation and lightening is hypopigmentation.    The normal coloration pattern that we observe is the result of genetics and the antagonistic reaction between the melanocyte-stimulating and melanin-concentrating hormones (Rakers et al. 2010).  

Fish skin, just like human skin, has an overlying epidermal layer and deeper dermal layer.  Pigment-holding cells are chromatophores and are named melanophores (melanin), iridophores (those that reflect light), or xanthophores (red, orange, and yellow).    Human skin gets its color primarily from melanin, a pigment produced by melanophores in the skin.  Melanophores in fish are equivalent in human skin.     Pigment patterns have a genetic basis and may also change rapidly in response to stress of a variety of sources.    This leads to short-term responses of patterns due to hormone levels -- for example some fish darken in response to social stresses due to interacting with other fishes in close quarters.     For a visual depiction of how rapidly coloration may change, click here to view the video of fish melanophores responding to adrenaline (Courtesy of Richard Wheeler RichardWheeler.net)

Melanosis is rare but has been reported in many types of fishes.     It is a stable pattern of pigmentation.   In humans a hyperpigmentation condition called melasma is sometimes referred to as "the mask of pregnancy," because it commonly appears on the faces of pregnant women, a response to hormone imbalances. 

Early papers described the condition from trout, salmon, and lungfish in large public aquaria (Nigrelli 1934; Smith and Coates 1936).  The lungfish melanosis resulted from “more or less prolonged or permanent expansion of melanophores, or from an actual increase in the melanophores of the skin,” whereas the trout and salmon was from paralysis of nerves controlling melanophores in the tail region.    Gordon (1931) developed a breeding protocol that resulted in melanosis in hybrid Platy x Swordtail fish.  This model system has been used in numerous early studies to investigate factors that may induce melanoma (Patton et al. 2010) and subsequent research on the Zebrafish (Danio rerio) model demonstrated that many of the genes that are involved in melanocyte development are the same genes that are mis-regulated in melanoma development.

Photo of hybrid female Swordtail crossed with a male Platy (Xiphophorus).  The male parent contributed the factor for macromelanophores and the female parent provided modifying factors that disturb normal development of pigment cells (Gordon 1931).  Photo: Texas State University.

What causes it?   Most of the observations by anglers and fisheries biologists have been bass 12 inches and larger, but this represents the size range that are typically encountered.   It is unclear whether black blotches are produced at birth at some later point in life in the fish caught by anglers. Fish have generalized responses to stresses and the normal skin melanization factors may be altered from pathogens, wounds, UV radiation, nutrition, stress, and environmental contaminants. 

Hyperpigmentation has been observed (though usually not the endpoint) in controlled toxicant exposure studies.     Exposures to dioxins resulted in hyperpigmentation in Zebrafish, Carp, and Largemouth Bass.     Because the black blotches are caused by multiple factors, we cannot derive a cause from symptom alone.   We could bring the fish to Dr. Don’s Piscine Dermatology Associates, if only it existed, to biopsy and examine affected tissues.    A thorough diagnosis would require a microscopic examination of the tissues to determine if there are cellular or tissue anomalies, pathogens, parasites, or simply higher levels of melanophores.

In seeking to understand, scientists hypothesize the sequence of conditions that might lead to black blotches and attempt to create black blotches in unaffected fish.   Perhaps an exposure to some unusual substance in water or diet leads to abnormal development and hormonal responses.  Or is it handling stresses during catch and release?    William L. Yingling, M.D., writing a comment to a blog post related to the Susquehanna River wrote “The increased frequency of these affected fish is not from better reporting.  Never saw this in the upper mainstem where I fish until 2010 and I've fished the same water since 1976. It is a new occurrence in these waters and it comes right on the heels of intersexing and bacterial infections that are killing the fish.”   Many black blotched bass reports emerge from the Susquehanna River where the fishery is in decline, yet “there is currently no voluntary or mandatory action plan to identify the causes and sources of the problem and find solutions (Arway and Smith  2013).”     In the New River, Virginia, where fisheries are thriving, fisheries biologists and anglers rarely encounter and report Smallmouth Bass with black blotches, and when they do it is only a few small blotches. 

Should the black blotched bass be a cause of our concern?        Should we be concerned about eating black blotched bass?   Most who have captured the bass with black blotches report that they appear to be actively feeding or otherwise in good condition.       The hyperpigmentation pattern represents a strange catch, but should not affect quality of flesh for human consumption.

One concern that remains is the changing environment.   Melanoma has been studied in laboratory investigations of Zebrafish and Platy x Swordtail hybrids and UV-B will induce melanoma in susceptible fish (Mitchell et al. 2010).       More recently, melanoma with patterns similar to UV-induced melanomas has been confirmed in Coral Trout Plectropomus leopardus at two sites in the Great Barrier Reef Marine Park (Sweet et al.  2012).     Increased UV affects wild animal populations in addition to humans!

Relevant and meaningful questions appear to be “Do these black blotched bass ever develop melanoma?”  “Does melanosis develop in response to acute handling stress and/or UV exposure during catch and release by anglers?” or “Are these melanotic bass produced at birth?”   or  “Is there something in the water?”  As bass fisheries have been effectively managed to permit longer-lived, trophy sized fish and more bass are caught and released, we certainly have older bass in our waters.    Humans, as we age, develop more dark spots for reasons that are not clear to us.  Are the black blotched bass associated with presence of many more old bass in our waters?      I am curious.

References

Arway, J.A., and G. Smith.  2013. The Susquehanna River-A fishery in decline. Fisheries 38(5):235-236.

Gordon, M.  1931.  Hereditary basis of melanosis in hybrid fishes.  American Journal of Cancer 15:1495-1523.

Kleeman, J.M, J.R. Olson, and R.E. Peterson.  1988.  Species differences in 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity and biotransformation in fish.  Fundamental and Applied Toxicology 10(2):206-213.

Mitchell, D. L. A. A. Fernandez, R. S. Nairn, R. Garcia, L. Paniker, D. Trono, H. D. Thames, I. Gimenez-Conti.  2010.  Ultraviolet A does not induce melanomas in a Xiphophorus hybrid fish model. Proceedings of the National Academy of Sciences,   DOI: 10.1073/pnas.1000324107

Nigrelli, R.F.  1934. Pseudo-melanosis in the tail of trout and salmon.  Copeia 1934(2):61-66.

Patton, E.E., D. L. Mitchell, and R.L. Nairn.  2010.  Genetic and environmental melanoma models in fish.  Pigment Cell Melanoma Research 23:314-337.

Rakers, S., M. Gebert, S. Uppalapati, W. Meyer, P. Maderson, A.F.Sell, C. Kruse, and R. Paus.  2010.  ‘Fish matters’: the relevance of fish skin biology to investigative dermatology.    Experimental Dermatology 19:313-324.

Smith, G.M., and C.W. Coates.  1936.  Cutaneous melanosis in lungfishes (Lepidosirenidae).  Biological Bulletin 71(2):282-285.

Sweet, M., N. Kirkham, M. Bendall, L. Currey, J. Bythell, and M. Heupel.  2012.  Evidence of melanoma in wild marine fish populations.  PLOS One 7(8): e41989. doi:10.1371/journal.pone.0041989

Zodrow, J.M., and R.L. Tanguay.  2003.  2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits Zebrafish    caudal fin regeneration.    Toxicological Sciences 76(1):151-161.