A pest is something that causes problems for people, usually because of unnaturally large numbers (ecologists also include plants and animals that are a problem for other plants and animals). Most pest fishes are found in areas that have been disturbed, because numbers seldom get to the pest level in natural communities where predators and disease usually keep populations in check.
Some fishes could be considered annoyances if not pests, even at low numbers. Many of us have had the unpleasant experience of wading in a pond and having sunfishes such as Bluegill Sunfish (Lepomis macrochirus, Centrarchidae) picking at hairs, scabs, or moles on our body. This unsettling behavior is really just an extension of the natural feeding activities of the Bluegill. Bluegill are generalist feeders, and one of their roles in lakes is to serve as cleanerfish, removing parasites from the skin of other fishes. The Bluegill is really doing you a favor, not realizing that your leg hairs are not an attached parasitic copepod.
Bluegill and other sunfishes may reach pest levels as a result of a process called “stunting.” A stunted population consists of many small fish. It results largely from a lack of predators, usually due to overfishing of sport fishes. When most of the large predators, such as basses, pikes, Walleye, and sturgeons have been removed from a lake, the sunfishes continue to reproduce. Their numbers grow but because of competition for food their food supplies become reduced. As a result, the fish do not grow to large size but still keep on breeding, leading to a lake chock full of small Bluegill or Redbreast Sunfish. Bullhead catfishes and Yellow Perch are other North American native fishes that produce stunted populations.
More commonly, pest fishes are introduced species that do not serve the purpose for which they were introduced. Again, stunting is a common complaint. Goldfish, Common Carp, and tilapia are notorious for stunting. These fishes are introduced to provide food for people but due to stunting wind up being too small to be desirable. Other introduced pest species cause problems because they are predators on native fishes or invertebrates. Walking catfishes, lionfishes, Asian Swamp Eels, Chinese carps, Guppies, Mosquitofish, Ruffe, Round Goby, and snakeheads reach pest status in many places in the United States. Two sci-fi/horror movies were produced in 2004, capitalizing on the discovery that snakeheads had been introduced and were breeding in a Maryland pond. Frankenfish portrayed genetically engineered snakeheads. Snakehead Terror had snakeheads transforming from pests to predators when human growth hormones were dumped into a local lake.
In other countries, as well as in the United States, introduced sport fishes (Brown and Rainbow trout, Smallmouth and Largemouth bass, Northern Pike, sunfishes, Peacock Bass) are predators that wipe out their prey base and also feed on native sport fishes. The lesson from this list is that introducing any fish that does not occur naturally in an area should only be done after careful study, if at all.
Stunted populations, discussed above, are an example of too many fish in one place. Most anglers complain about too few rather than too many fish, except where anglers compete with predatory fishes for sport fishes. For example, anglers in Florida complain about too many gars (Lepisosteus, Lepisosteidae) and Bowfin (Amia calva, Amiidae) eating the Largemouth Bass that the anglers would rather catch.
From a nuisance standpoint, the physiology of some fishes results in unpleasant conditions for humans. Alewives (Alosa pseudoharengus, Clupeidae) entered the Great Lakes through man-made canals in the 1940s. They fed on zooplankton and replaced the native whitefishes and their numbers exploded. Alewives are naturally saltwater fish but can survive in fresh water. However, their physiology is better adapted to salt water. This makes them physiologically stressed and therefore sensitive to sudden environmental changes, such as rapidly reduced water temperature. When Alewives move inshore to spawn in the spring, they are often exposed to reduced temperatures from inflowing streams and cold water upwellings. The result is that they die by the millions and wash up on beaches, where they rot. In Lake Michigan, beaches have to be closed and bulldozed clear of stinking fish. Again, problem fish are more likely to be fish that are not native to an area.
Brown Trout (Salmo trutta, Salmonidae) are native to Europe and western Asia but have been introduced throughout the world. They provide good fishing but have wiped out native fishes in North America, Australia, New Zealand, and South Africa, where large predatory fishes did not occur naturally.
Most adult ducks are safe from fish predation, but ducklings are undoubtedly eaten by a variety of large predatory fish such as Largemouth Bass and pike. Muskellunge and large Northern Pike are perfectly capable of eating all sizes of ducks. Goosefish (Lophius americanus, Lophiidae), admittedly a marine species, get their name from their presumed goose-eating habits, if that counts. But given that many ducks including mergansers, sawbills, scaup, mottled ducks, goldeneyes, and long-tailed ducks eat fishes, a little turnabout is fair play.
Very few fish attack people or pets (we will ignore sharks for the time being). Some injuries occur when people accidentally step on or place their hands on spiny bottom fishes that are well camouflaged and do not move (scorpionfishes, stonefishes, weeverfishes, stingrays). But most injuries occur when people handle fishes carelessly, and the fish tries to defend itself with its spines or teeth. Gars, Bowfin, barracudas, Bluefish, halibut, piranhas, tigerfish, and pufferfishes are among fish that will bite and should be handled with care and respect.
Almost all fishes have sharp spines, some that contain venom that produces a painful sting. People are commonly hurt while handling catfishes, pinfishes, rabbitfishes, and even sunfishes. A few fishes can produce a strong electric shock that can stun and even knock a person out (Electric Eel, torpedo rays, electric catfishes), but again the shock is almost always a defensive tactic on the part of a frightened or handled fish.
There’s a right way and a wrong way to pick up a stingray, as a student on an ichthyology field trip who missed the demonstration on how to pick up a stingray found out the hard way. Treatment traditionally employs compression to stop the bleeding, heat to destroy the toxin, then ice to reduce the swelling.
Contrary to what scary movies might make you think, piranhas (Serrasalmus, Characidae) seldom if ever attack live humans (see “Do fishes bite?” in chapter 4). Most reports turn out to have involved drowning victims, after they drowned. The exceptions involve a recent upsurge in attacks in new reservoirs along the Amazon River. But the fish are not feeding, just protecting their young. Australian tandan or eel-tailed catfish (Tandanus, Plotosidae) also have a reputation for attacking waders and swimmers who come near their nests.
A good subject for a factual scary movie would be the behavior of a small pencil catfish, the Candiru (Vandellia cirrhosa, Trichomycteridae) of the Amazon River basin. Pencil, or parasitic, catfishes normally eat mucus and scales from other fishes or pierce the skin or gill cavities of other fishes and feed on blood. But the Candiru occasionally makes a mistake that can be an excruciating experience for a swimmer and fatal for the catfish. Candiru are attracted to the small currents that come out of fish gills. They swim “upstream,” lodge themselves in the gills with spines that project backward from their gill covers, and feed on gill tissue. A human bather, usually a male, who pees while in the water can attract a Candiru. Documented cases (with photographs) exist of a Candiru stuck in someone’s penis, requiring surgery to be removed. Young boys in the Amazon are taught not to stand up and pee over the side of a canoe for fear that one of these fishes will swim up the stream. Being nibbled on by a Bluegill pales in comparison.
Small but scary, pencil catfish, or Candiru (Vandellia cirrhosa, Trichomycteridae) are less than 15 centimeters (6 inches) long, but there are verified accounts of them swimming up a man’s penis and lodging themselves there, requiring surgical removal. Photo by P. Henderson, PISCES Conservation Ltd.
Few fish diseases can be passed on to humans. However, fishes are intermediate hosts for some parasites that can be transmitted to people and cause severe abdominal pain, vomiting, and diarrhea. These parasites include intestinal tapeworms, liver flukes, and parasitic nematodes, all from freshwater fishes. Almost all can be killed by proper cooking.
A number of harmful bacteria live on fish skin and spines, so when people get stabbed while cleaning fish, the wounds can become infected. At least 12 bacterial species found in fishes can cause infections, including Salmonella, Vibrio, Mycobacterium, and Streptococcus. People stabbed while cleaning St. Peter’s Fish (Sarotherodon galilaeus, Cichlidae) have become infected by Vibrio and had to have limbs amputated.
Sad to say, many fish are unsafe to eat. As discussed above, fish that are caught in contaminated water and eaten without thorough cooking can pass disease-causing protozoa, bacteria, and viruses to people. Even worse are fish contaminated with compounds that are not destroyed by cooking. For too many years, we have treated lakes, rivers, and the ocean as convenient places to dump our wastes. As a result, many fish have incorporated these toxins into their bodies. These fish contain toxins that are poisonous to them and us.
Because of our polluting habits, nearly 40% of U.S. lakes and rivers have fish consumption advisories, meaning it is not safe to eat the fish that live there. Fishes in these waters have unsafe levels of mercury, pesticides (DDT, chlordane, diazinon, atrazine, malathion, mirex, toxaphene), herbicides (atrazine), dioxins, polychlorinated biphenyls (PCBs), heavy metals (aluminum, arsenic, cadmium, chromium, lead, selenium), endocrine disrupters, creosote, and many, many others. Before eating freshwater fish, it is a good idea to check with a local health department to see if a fish consumption advisory has been issued for the place where you are going fishing.
A fish consumption advisory sign in coastal Delaware. Fishes here are contaminated primarily with PCBs.
Marine fishes are often not as heavily contaminated as freshwater fishes, although many species that live in coastal lagoons and other nearshore environments are not safe to eat. Again, checking with state and local health departments is a good idea.
Not all problems from eating fish are caused by human pollution. A toxin found in tropical reef fishes around the world is ciguatera. People who eat ciguatoxic fish suffer stomach cramps, heart rate changes, reversal of sensations (i.e., ice cream feeling hot), and possibly death from respiratory failure. The most common culprits are large predators such as moray eels, groupers, snappers, and Great Barracuda, and the toxin is not destroyed by freezing or cooking.
Ciguatera fish poisoning on coral reefs. A one-celled alga called Gambierdiscus toxicus produces a potent toxin that passes up the food chain, becoming more concentrated at each step. When people eat contaminated fish, they receive a heavy dose of the toxin. Modified from Helfman (2007); algae photo courtesy of the Florida Marine Research Institute
Ciguatera is a good example of an accumulated, magnified toxin, one that is passed along the food chain and thus concentrated in top predators. The toxin is first produced in certain kinds of reef algae, which are eaten by herbivores, which are eaten by small carnivores, which are eaten by larger predators. People are at the top of this food web and receive a huge dose of toxin by eating a single, large fish. The algae that produce the toxin grow best on recently disturbed surfaces, such as what happens during dredging, dynamiting, and ship anchoring.
Ironically, because the toxin does not break down from freezing, persons far from coral reefs have suffered ciguatera poisoning. Twenty people at a dinner in Calgary, Canada, suffered ciguatera poisoning from eating thawed-and-cooked reef fishes imported from Fiji.
Bites and stab wounds from spines should be treated as any deep cuts by stopping bleeding, cleansing, antibiotics to prevent infection, and so on. Some highly venomous stings, such as those caused by stonefishes, lion-fishes, eel-tailed catfishes, and stingrays, can be excruciatingly painful. Professional medical help may be needed because some people are particularly sensitive and can go into anaphylactic shock and stop breathing. For stingray and stonefish stings, first aid manuals suggest applying hot compresses (but not so hot as to burn) to the wound to destroy the proteins in the venom. Call 911 or a poison control center.
