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The Bass Blotch Mystery

Bass Blotching
BFHP: Ralph Manns / June 9, 2009
The Bass Blotch Mystery
In 1989, Douglas Carlson of the New York Department of Environmental Conservation
reported* that largemouth bass with unusual dark blotches were present in the Hudson River
Estuary. The markings were a result of an increased number of pigment cells in the skin and
occurred on various body surfaces. Bass collected by biologists using electro-shocking
equipment were more likely to have these blotches (23 percent) than those caught by bass
anglers (11 percent), that there was no noticeable change in the frequency of these sightings
between 1985 and 1988, and that biologists in other states contacted in 1986 had also seen
similarly marked bass. At that time, most of the sightings were in eastern coastal areas and a few
impoundments in the central and southern states.
In 1994, Kathleen Skinner and other biologists at Russell Sage College* * reported
several investigations into these markings. They noted that in 1985, New York biologist John
Schachte found about 33 percent of Hudson River bass over 12-inches long had blotches, but that
50 percent of bass over 14 inches had them. Other studies made in that year found parasites
were probably not the cause and that the bass did not show any other specific cellular pathology.
Previous studies of unusual markings on other fish species related abnormal blotches to
environmental contaminants, viruses, heredity, the inflammatory responses of the skinís immune
system, or changes in hormones that regulated skin pigment responses. The team noted that
another study ruled out a viral cause. Unmarked bass failed to become blotched when inoculated
with sera from affected bass, there were no associated skin lesions, and other fish species were
not affected.
Skinnerís team checked to see if the Hudson River pollutant, PCB, was a cause. They
found PCBs were high in Hudson River bass, concentrations were different in male and female
fish, and blotched bass contained more PCBs than unblotched bass from the same areas. But,
they noted the blotches occurred in samples from areas without high PCB concentrations and in
bass with the same concentrations as the unmarked control bass. They concluded that PCBs
ďmay not be the cause of LMB black blotch syndrome.Ē
Other as yet unstudied chemicals might be the cause. Increased pigmentation in animal
tissue is a natural defense mechanism against heavy metals, aromatic hydrocarbons, acids, and
other environmental contaminants.
Clearly, more study is needed to explain what is going on. Before Carsonís report was
published, Iíd seen a few bass with the dark blotching. Since then, the number of blotched fish
has seemed to steadily increase and sightings are now common on most Texas waters I fish. I
suspect there is a tie-in with the increase in catch-and-release bass fishing since the early 1980s.
Carsonís observation that electrofishing produced more blotched bass than angling could be a
result of learned, short-term, lure-avoidance responses by recently handled, and thus blotched,
bass. The increased percentage of blotched bass in larger sizes could also be associated with
Bass Blotching
catch-and-release, particularly the forced releases in waters with slot limits.
I moved near a lightly-fished pond 10 years ago. At first, the fish I caught had no
blotches. Since then, Iíve caught and released about 700 bass a year in this pond, many more
than once. Iíve noticed that about a third now show blotching, often around the mouth. Iíve
identified re-caught individuals by their marks. Iíve also caught fish with distinct and
identifiable blotches at Lake Fork.
Lakes with slot and special limits like Fork that force the release of many large bass seem
to hold more blotched fish. Although contaminants may be associated with the blotches, I
suspect that handling during catch and release and the hooking and capture process itself are at
least related factors. Marks near mouths are easily attributed to lipping techniques. Those on
other parts of the body are less frequent on my pond bass than on bass at Fork, but seem to be
located at places where bass may have touched a boat, the ground, or have banged into cover
while hooked. The lower tail and anal fins seem to be particularly vulnerable. One slot-fish at
Lake Fork appeared to have the imprint of a hand across its back.
The marks seem to eventually disappear, suggesting they may be part of a skin or slime
repairing process. I was fish the pond less in December and early January. The fish I catch in
late January seldom have blotches, but the number of blotches increases over the remainder of a
year. Iíve also noted that they seem to disappear faster from smaller adult bass. The largest fish
seem to hold them for a year or more.
It is not that C&R causes blotching. I suspect the blotching is an increasingly present
genetic mutation that makes skin blotch in response to injury. A mutant factor(s) is/are possibly
spread by bass fishermen, like the LMBV to explain the great increase in sightings of blotched
bass in recent years. Although diseases are not considered part of the cause, a genetic mutation
might be. C&R plays a part by increasing the sightings and handling of blotch-able fish.
Blotching might be in response to any source of skin damage or irritation, but C&R fisherman
are a well-known factor, as evidenced poor handling procedures periodically used by proanglers
in bass fishing videos.
If we wish to understand blotching we need to statistically investigate the relationship of
blotching to the introduction of Florida strain largemouth bass into northern strain
stocks. But this possibility explains nothing about the recent increase in SMB blotching. but an
easily mutable pigment gene might react to a mutagen routinely carried and spread by anglers.
Dr. Van Gelder, aan occasional contributor to the BFHPs, has instigated a TPWD
investigation of any possible association of spawning with blotching. However, if my genetic
postulate has any validity, test validity would depend upon the genetics of the bass stock tested.
In any case, these blotches donít seem to harm bass in any way, and might even prove
useful in the identification of individual fish or analysis of capture and release rates if anglers are
a cause.
Bass Blotching
* Carlson, D. M. 1989. Unusual pigmentation on largemouth bass. Presentation to the 45th
Annual NE Fish and Wildl. Conf., Ellenville, NY, (Mimeo) 8p.
** Skinner, K. M., L. Pagels, and K. A. Peregrim. 1994. Black blotch largemouth bass in the
Hudson River, New York. Presentation given at the New York natural History Conference III,
April 13-16, 1994. ( Mimeo) 29 p.
this item is under copyright rules and can only be reprinted or reused with the authorís

by Ralph Manns


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