Fish develop cancer across a wide range of species, organ systems, and environments. Tumors have been documented in everything from bottom-dwelling flatfish in polluted harbors to pet goldfish in home aquariums and zebrafish bred in pristine laboratory conditions. The disease follows many of the same biological rules as cancer in mammals, which is precisely why fish have become one of the most important animal models in human cancer research. But beyond the laboratory, tumors in wild fish also serve as a kind of early-warning system for environmental contamination, and in recent years, researchers have even discovered a form of melanoma in catfish that appears to spread from fish to fish like an infection.
What Kinds of Cancer Do Fish Get
The variety of tumors found in fish is surprisingly broad. In a retrospective study of pet fish examined by veterinary pathologists, the most common diagnosis across all organ systems was soft tissue sarcoma, accounting for about a quarter of cases. The eye was the most frequently reported site for a primary tumor, and koi had the highest rate of neoplasia among species examined, followed by goldfish.1PubMed Central. Retrospective Study of the Prevalence, Histopathology, Therapy, and Survival Time of Neoplastic Disease in Fish These findings come from captive fish brought to veterinary clinics, so the picture is shaped by which species people keep as pets and which problems prompt an owner to seek help. Still, the range of tumor types is striking.
Laboratory zebrafish develop an even wider catalog of tumors, including lymphosarcoma and other blood-cell cancers, soft tissue sarcomas, optic pathway tumors, melanomas, liver tumors, kidney tumors, and thyroid tumors.2ScienceDirect. Nonexperimentally Induced Neoplastic and Proliferative Lesions in Laboratory Zebrafish Some of these arise spontaneously with age, much the way cancer does in aging humans.
In wild fish, liver tumors are especially well documented. English sole collected from Puget Sound, Washington, showed a full spectrum of liver lesions, from early preneoplastic changes to full hepatocellular and biliary cancers, all of which closely resembled the lesions produced by known cancer-causing chemicals in lab experiments with both fish and rodents.3JNCI: Journal of the National Cancer Institute. Pathologic Anatomy and Patterns of Occurrence of Hepatic Neoplasms, Putative Preneoplastic Lesions, and Other Idiopathic Hepatic Conditions in English Sole (Parophrys vetulus) From Puget Sound, Washington Wild pejerrey fish in South America have also been found with hepatocellular carcinoma, classified using the same system applied to human liver cancer. Some of those tumors were the conventional type, while a smaller number showed a more aggressive sarcomatoid pattern.4Bulletin of the European Association of Fish Pathologists. Occurrence of hepatocellular carcinoma in wild pejerrey Odontesthes bonariensis Teleostei Atherinidae
Outside the liver, wild fish in the Bering Sea have been found with epidermal papillomas, lymphocystis (a viral skin disease that produces tumor-like nodules), and pseudobranchial tumors in Pacific cod, with the cod tumors appearing on both sides of the head in every affected individual.5Journal of Fish Biology. The frequency, distribution, and pathology of three diseases of demersal fishes in the Bering Sea
Why Fish Develop Cancer
The causes mirror what drives cancer in other animals: a combination of environmental exposures, genetic predisposition, viral infection, and sometimes just the accumulated damage of aging. But because fish live submerged in their environment, they tend to absorb waterborne chemicals through their gills and skin continuously, which makes them especially sensitive to pollutants.
Chemical contamination is the best-studied driver. Winter flounder caught in Boston Harbor, an area heavily contaminated with industrial chemicals, showed a high percentage of visible liver cancer. Flounder taken from cleaner waters outside the estuary had no liver lesions at all.6PubMed Central. Carcinogenesis studies in rodents for evaluating risks associated with chemical carcinogens in aquatic food animals That kind of stark geographic contrast, tumors in polluted areas and none in clean areas, has been replicated across multiple species and waterways. Bottom-dwelling fish like sole and flounder are at greatest risk because they live in constant contact with contaminated sediment. Researchers have also demonstrated the pollution link experimentally: medaka fish exposed to the known carcinogen diethylnitrosamine in the lab developed liver tumors at high rates, with every single hepatocellular tumor in adult-exposed fish classified as malignant.7Carcinogenesis. Progression of hepatic neoplasia in medaka (Oryzias latipes) exposed to diethylnitrosamine
Ultraviolet radiation is another clear cause, at least for skin cancers. Platyfish-swordtail hybrids (crosses of two Xiphophorus species) are genetically susceptible to melanoma, and UV exposure dramatically increases their tumor rate. Multiple UV exposures given to young fish resulted in melanoma in roughly 20 to 40 percent of animals within four months, compared to a background rate of just 2 to 12 percent in unexposed fish, depending on the strain.8PubMed Central. Animal model for ultraviolet radiation-induced melanoma: platyfish-swordtail hybrid Later work showed that adult fish exposed to acute UVB developed melanomas at rates equivalent to those irradiated as newborns, and that chronic solar-spectrum exposure over months also triggered tumors.9PubMed Central. Recent advances in sunlight-induced carcinogenesis using the Xiphophorus melanoma model These Xiphophorus fish remain one of the most important animal models for understanding how sunlight causes melanoma.
Genetics also plays a significant role, independent of environmental exposure. Laboratory zebrafish carrying mutations in the TP53 gene, the single most commonly mutated gene in human cancers, developed tumors by about 14 months of age. Roughly 28 percent of the fish homozygous for one particular TP53 mutation grew malignant peripheral nerve sheath tumors, along with one case of melanoma.10Molecular Cancer Research. Zebrafish as a Cancer Model The fact that a single genetic change can drive tumors in fish just as it does in humans underscores how deeply conserved the biology of cancer is across vertebrates.
The Catfish Melanoma That Spreads Like an Infection
One of the most unexpected discoveries in fish oncology came from a lake straddling the Vermont-Quebec border. Starting around 2012, brown bullhead catfish in Lake Memphremagog began showing unusually high rates of melanoma. Researchers initially suspected a contaminant in the lake or possibly a viral cause. The answer turned out to be stranger: the melanoma itself appears to be transmissible, passing directly from fish to fish as living cancer cells. This makes it only the fourth known naturally occurring transmissible cancer in animals, alongside a venereal tumor in dogs, the facial tumor disease devastating Tasmanian devils, and a contagious leukemia found in several species of bivalves (clams and mussels).11Nature. Brown bullhead catfish melanoma represents a novel transmissible cancer
Transmissible cancers are bizarre by any standard. In most animals, cancer dies with the individual because a recipient’s immune system would normally destroy foreign cells. The handful of known exceptions involve cancers that have evolved ways to evade immune recognition, often in populations with reduced genetic diversity. The Lake Memphremagog catfish discovery adds fish to a very short and peculiar list, and raises questions about whether other unrecognized transmissible cancers might be circulating in aquatic species that have never been screened for them.
Fish as Stand-Ins for Human Cancer Research
If fish getting cancer sounds like bad news, there is a flip side: their tumors have become enormously useful for understanding and fighting cancer in people. The zebrafish in particular has emerged as one of the workhorses of cancer biology. The evolutionary conservation of cancer-related genetic programs between zebrafish and humans is strong enough that findings in fish can be meaningfully translated back to human medicine.12PubMed Central. Zebrafish Models of Cancer-New Insights on Modeling Human Cancer in a Non-Mammalian Vertebrate
Zebrafish bring practical advantages that mice simply cannot match. They reproduce quickly and in large numbers, they are cheap to house, and their larvae are transparent, meaning researchers can watch tumor cells grow, migrate, and respond to drugs in a living animal under a microscope. That last feature is especially powerful for studying metastasis, the process by which cancer spreads from one site to another, which is what ultimately kills most human cancer patients.13PubMed Central. Zebrafish In Vivo Models of Cancer and Metastasis
Recent work has pushed this further. Human cancer cells can be transplanted into zebrafish, creating what researchers call xenograft models, and the fish’s response to those cells can be tracked in real time. This has opened the door to screening potential drugs on a large scale at reasonable cost. It has also raised the possibility of personalized medicine applications: take tumor cells from an individual cancer patient, implant them in dozens of zebrafish, test different drug combinations, and see which ones work before committing the patient to a particular treatment.12PubMed Central. Zebrafish Models of Cancer-New Insights on Modeling Human Cancer in a Non-Mammalian Vertebrate
Some of the mechanistic insights have been genuinely novel. Using zebrafish imaging, one group showed that when a cell acquires an oncogenic Ras mutation (one of the most common mutations in human cancers), the cell initially becomes senescent, essentially shutting itself down, and is cleared from the tissue. But when a TP53 mutation is added on top, the cell survives, stays senescent, and begins secreting inflammatory molecules that convert neighboring normal cells into senescent cells too. The result is a mass of dysfunctional cells that resembles a tumor, built not from one mutant cell dividing uncontrollably but from a mutant cell corrupting its neighbors.14Nature Communications. Zebrafish imaging reveals TP53 mutation switching oncogene-induced senescence from suppressor to driver in primary tumorigenesis That finding, which would have been extremely difficult to observe in a mouse, challenges some standard assumptions about how tumors form.
Fish Tumors as Environmental Alarm Bells
Wild fish populations with elevated cancer rates have become one of the more reliable indicators that something is wrong with a waterway. The logic is straightforward: if bottom-dwelling fish in a harbor are developing liver tumors and the same species in a nearby clean area are not, the environment is probably the variable. Researchers have argued that certain types of tumors in vulnerable species, particularly liver cancers in bottom-dwelling fish and skin lesions in several species groups, can serve as useful monitors of ecosystem health, at least for contamination by chemicals that damage DNA or promote tumor growth.15Journal of Aquatic Ecosystem Health. The use of tumors in wild populations of fish to assess ecosystem health
Not all fish tumors carry this signal, though. Tumors caused by purely genetic mechanisms or by viruses would not indicate pollution, so the species being examined and the type of lesion both matter. A lymphocystis outbreak (caused by a virus) tells you something very different from a cluster of hepatocellular carcinomas in sole living over contaminated sediment.15Journal of Aquatic Ecosystem Health. The use of tumors in wild populations of fish to assess ecosystem health The distinction is important because environmental monitoring programs that use fish tumor surveys need to pick the right species and the right lesion types, or risk missing real contamination or sounding false alarms.
There is also a human health dimension. Fish and shellfish caught in polluted waters can contain carcinogenic chemicals at levels high enough to raise concerns about human consumption. The Boston Harbor flounder example prompted researchers to propose long-term feeding studies in lab rodents, using fish from contaminated waters as the diet, to test whether the chemicals concentrated in those fish could cause cancer in mammals. The reasoning was that if rodents fed contaminated fish developed tumors, it would strengthen the case that eating large amounts of chemically contaminated fish poses a real risk to people.6PubMed Central. Carcinogenesis studies in rodents for evaluating risks associated with chemical carcinogens in aquatic food animals To be clear, you cannot catch cancer from eating a fish that has a tumor. The concern is about the chemical contaminants the fish has accumulated in its flesh, not the tumor cells themselves.
Can Pet Fish Be Treated for Tumors
For aquarium hobbyists who notice a growth on their koi or goldfish, the question quickly shifts from “do fish get cancer” to “can anything be done about it.” The answer is yes, at least in some cases. Veterinary surgery on fish is an established, if niche, practice. Surgical excision of tumors in individual fish has been documented and can be a viable treatment option, particularly for external masses that are accessible and have not spread.16Europe PMC Central. Surgical excision of a tumor in a fish. The fish is typically anesthetized using a dissolved agent in the water, the procedure is performed with the fish partially out of the water or in a shallow anesthetic bath, and the animal is returned to clean water to recover.
Prognosis depends heavily on the tumor type and location. A benign external mass that can be cleanly removed may never recur. An internal malignancy or a soft tissue sarcoma, the most common diagnosis in pet fish, carries a worse outlook. The retrospective study of pet fish neoplasia found that koi and goldfish were the species most frequently affected, which makes sense given their popularity, their long lifespans (koi can live for decades), and the close attention owners pay to their appearance.1PubMed Central. Retrospective Study of the Prevalence, Histopathology, Therapy, and Survival Time of Neoplastic Disease in Fish If you notice a lump, discoloration, or unusual growth on a pet fish, an aquatic veterinarian (yes, they exist, and their numbers are growing) can perform a biopsy and discuss options.
Why Some Fish Seem to Resist Cancer
Not all fish are equally susceptible, and the differences are interesting. Some species appear to develop tumors rarely, while others, particularly long-lived species or those with certain genetic backgrounds, seem prone to them. The Xiphophorus hybrid crosses mentioned earlier develop melanoma because hybridization disrupts the normal regulation of pigment-cell growth genes, essentially removing a genetic brake that the parental species each maintain in different ways. This is a reminder that cancer susceptibility in fish, as in mammals, is not just about exposure to carcinogens but about the genetic context in which those exposures occur.
Sharks are often cited in popular media as animals that “don’t get cancer,” but this is a myth. Sharks do develop tumors, including benign and malignant types. The misconception likely persists because sharks are less studied than bony fish, not because they possess some special cancer immunity. The broader pattern in fish biology is that cancer appears to be a possibility for virtually any species with enough cell division and enough time, which is to say, all of them.
How Exposure Timing Shapes the Disease
One pattern that emerges clearly from experimental work is that when a fish is exposed to a carcinogen matters as much as what it is exposed to. In medaka exposed to the chemical carcinogen diethylnitrosamine, the tumors that developed differed dramatically depending on whether the fish were exposed as larvae or as adults. Adult-exposed fish developed overwhelmingly malignant hepatocellular carcinomas, while larval-exposed fish developed a more mixed picture, with about half of their liver tumors being benign and a much larger proportion of biliary (bile duct) tumors. Adult-exposed fish also showed a unique lesion called “nodular proliferation,” small clusters of abnormal cells with completely disrupted tissue architecture that may represent microcarcinomas, essentially cancers caught at their earliest stage.7Carcinogenesis. Progression of hepatic neoplasia in medaka (Oryzias latipes) exposed to diethylnitrosamine
This age-dependent vulnerability is not unique to fish. In human medicine, childhood exposures to radiation or certain chemicals produce different cancer profiles than adult exposures. The medaka data reinforces that the developmental stage of the tissue at the time of exposure determines which cell types are most vulnerable and which kind of tumor ultimately results. For aquaculture and environmental monitoring, the implication is that early-life exposures in fish, even if the animals appear healthy as juveniles, can quietly set the stage for tumors that manifest much later.
The Immune System Connection
Fish possess both innate and adaptive immune systems, and both interact with tumors in ways that parallel what happens in humans. Zebrafish have become a key model for studying what is called the tumor microenvironment, the complex ecosystem of immune cells, blood vessels, and signaling molecules that surrounds a growing tumor and can either fight it or be co-opted to support it.17Frontiers in Cell and Developmental Biology. Tipping the Scales With Zebrafish to Understand Adaptive Tumor Immunity Because zebrafish immune development can be tracked in detail, researchers can watch how immune cells infiltrate a tumor, how the tumor evades immune attack, and how experimental drugs alter that balance, all in real time in a living animal.
This work has practical consequences for human medicine. Immunotherapy, the strategy of training or unleashing a patient’s own immune system to attack cancer, has become one of the most important advances in oncology over the past decade. Understanding how tumors suppress immune responses in zebrafish feeds directly into the development of better immunotherapies for people. The fish are not just getting cancer for their own unfortunate reasons; they are helping researchers figure out how cancer hides from the immune system and how to strip away that camouflage.