Looks a bit like bacterial infection. Maybe an infection of a lesion caused by predators (e.g. herons)? But I have no specific idea.

It looks like a leech got to it, but I could be mistaken.

In order to be able to respond more objectively, please give us some information about: type of water (freshwater, seawater), water temperature and salinity. With this information we can give some ideas or approximations.

The very circular lesion suggests a sucking attachment by a predator (like a lamprey, but not a lamprey because of geography), maybe some other aquatic predator or ectoparasite- the damage looks physical but it could be bacterial as well, or a combination.

I previously saw lesions like that on fish in an overcrowded environment. The fish attacked/bit each other. In this instance the fish were trying to migrate upstream and large numbers got trapped got trapped in small pools below a dam wall.

It may be worth your while to make a smear to rule out infections.

Thank you for the answers so far. Here is some additional information: the fish were caught in a floodplain lake adjacent to the Oguemoué River approximately 50 km upstream of Lambarene, Gabon. It is tropical freshwater.

appears to be an infection that has been walled off, suggesting to me that there was either trauma, foreign object or a parasite.  is this in just one fish or multiple fishes?

Really interesting one this. The wound screams lamprey to me but as mentioned by Anindo, unlikely to be the case in Gabon (to my knowledge there aren't any lamprey species recorded from Gabon). The wound looks to be infected so it could be a bacterial infection cause or a wound caused by something else that has since got infected. How many fish have been seen with these lesions and is it just one fish species affected? Are different sized fish affected?

Following up to see how many fish affected. Since it has been reported in two different sites, it may be an infection, or just a coincidence. Thanks!

The toothed funnel-like sucking mouth of the lamprey is largely responsible for similar very circular lesion at the level of attachment. However, in the second picture with whole fish, a small circular lesion is visible on the posterior dorsal part of the fish and it may reflects an infectious origin (viral, bacterial or both).

Nice! Looks exactly like signes after lamprey's bites. I've found them on flounders. If the skin is cut, with sharp edges, it means that this fish was attacked by other fish or something with round mouth and teeths around.  Secondary infection is probably bacterial.

Beside it, everything depends on the number of fish. Single event means that this lesion is not the effect of infectious disease, more - it is worth of observation. 

Hi James

Have you had the lesions sampled and histology done? Ulcerative lesions like this can be caused by many things but it is almost impossible to identify the cause without careful examination. Some more sinister problems can be identified promptly using histology, particularly EUS, which has a history in Africa and you may want to rule this particular disease out because it has significant importance, but also because EUS actually presents with very similar lesions.

it don't like lesion caused by bites of other fish or animals, but like ulcerative lesions by some pathological microroganisms or viruses,

Hi James

I agree with David on this one.  I think it is important to get some diagnostic information to at least potentially rule out an infectious cause.  Numerous infectious agents can cause ulcerative lesions as presented in your images such as parasites viruses, bacteria and fungi or fungus like organisms.  Epizootic Ulcerative Syndrome is of particular concern as it is prevelent on the African continent and seems to be spreading.  First records originated from the Zambesi, Chobe River system but since then I am aware of outbreaks in South Africa, Zambia, Zimbabwe, Botswana, DRC.

Aeromonads, crustaceans, lampreys and predators can give tihis kind of lesions, but it is important how many fish are affected.

It's ulcerative hemoragic lesion (with necrotic tissus). Many agents can cause this syndrome : a parasites (lamprey for exemple), bacteria and many environmentals toxic organic (HAP, PCBs...) . Analyticals laboratory is necessary

Classic bite lesion either from a lamprey or a cookie cutter shark. In this case it would be the lamprey. Key signs are the perfect circular outline, the lack of inflammation around intact skin outside the lesion and the healing of the floor of the lesion. Think about the alternative - an infection which results in an open wound will be much larger than the wound, and will interfere with the healing process.

The fish usually present lesions in different parts of the body, and the causes are numerous, eg bites where later develops a crust, product of parasites, blows and even commensalist species where they usually settle and live on their host, you can review numerous bibliography In this regard

In reply to John Brian Jones - I would agree but the geography makes it HIGHLY UNLIKELY to be lamprey as there are no records of lamprey from that region of the world - unless I've missed something!

There are certainly no lampreys known from Gabon, and cookiecutter sharks would not occur in freshwater habitats. Therefore we can rule out those as causing the lesions.

My mistake about the distribution of lamprey, but look at what the lesion is telling you. The cause (whatever) has resulted in the loss of a circular piece of skin down to the musculature without the attendant underlying inflammation that would indicate a sub-dermal infection as the aetiology. The only other explanation I can think of is a blister which has burst.

"No lampreys" does not mean that there's no animal able to bite in a way like lampreys or cookie sharks :). Is in Gabon any animal biting like lamprey or not? Are you able to exclude this possibility? :)

I'd like to contribute with literature:

http://aquaticpath.phhp.ufl.edu/lesionguide/lesionguide.pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4235013/pdf/1751-0147-50-S1-S7.pdf

It is very difficult determine what was the cause of this lesion. if it was a bite caused by lamprey or other fish, there is some time passed from that, what could be hard to observe the borderlines and agressor-mouth characteristics for the clinical signs and diagnoses. Several parasites can cause lesions on the fish skin and permits that other pathogens acts on the fish. For example, Isopoda Cymothoidae can causes punctual lesions that can increases in area (circular area) due to the bacterial infection ou fungal infection. Other Crustacea such as Copepoda Caligidae (Caligus spp.) can causes deep and broad skin lesions, too. 

A reference that may be of interest in understanding the structures involved. It was a webinar for fish pathologists in Australasia in 2014.

My first guess was lamprey too but then I noticed that lampreys are only known from cooler climates and not from Africa. I agree that it must have been some fish with sharp edges on the mouth obviously. It is interesting that there is a second much older leasion of nearly the same size that is healed just in front of the fresh leasion. So the fish must have been attacked two times or there was prolonged parasitism of a single parasite that changed the place.

Having looked at the photos again and originally suggesting leeches...I still want to say that it must have been an animal causing damage to the fish resulting in secondary infections. However, having seen other posts regarding EUS I am now leaning towards a microbial pathogen. Largely because you had earlier mentioned that there were multiple fish (on the first page of suggested answers) from two separate areas showing similar markings.

this is physically injured, of course, will lead to bacterial infection. 

My question would be was this fish caught alive with the lesion like this or caught dead? 

Answer to this would help. Thanks

Thank you again for your suggestions-samples have now been taken by the Gabonese fisheries agency. As soon as I find out anything, I will post an update. 

Thanks Amina and James. 

look like physically damage. of course to lead to bacterial infection.

How many fish have this kind of lesion?

I think some physical damage.This of course will lead to bacterial infection. 

It looks like some kind of bite with secondary bacterial infection. However, the diagnosis will depend also on the number of affected fish.

Regards

SM Najim

yes, it looks bite  or physical damage

Thanks

I need to ask some questions, was this lesion in the fresh fish or dead?. Please elaborate the history behind this lesions more to come up with most probable reason