Causes, Symptoms, And Treatments Of Fish Toxins

Whether you’re a pescatarian or a lover of all things seafood, you probably have a preference for how you eat fish. You might like sushi, a steaming bowl of seafood ramen, or the good old fish and chips. You might even like all of them equally.

However, news of fish being toxic and causing food poisoning have been doing the rounds lately. Does that mean you’ll have to rethink your favorite food? Not necessarily. There are several types of toxins found in fish, depending on their type, geographical origin, and habitat (freshwater or saltwater). Here are the ones you should watch out for.


1. Ciguatera

Ciguatera accumulates in large fish like barracuda, jacks, and king mackerel

This is a toxin produced by marine algae in subtropical and tropical parts of the world. It accumulates in large fish found in reefs and islands like barracuda, jacks, king mackerel and large groupers and snappers.


Poisoning caused by this toxin is common in Caribbean and Pacific islands, with some outbreaks in southern Florida.1 Poisoning caused by this toxin leads to symptoms like

  • Vomiting
  • Diarrhoea and/or abdominal cramps
  • Headaches, fatigue and fainting
  • Joint and muscle pain
  • Tingling around the mouth, fingers and toes
  • Burning sensation on contact with cold water
  • Extreme itchiness
  • Coma (in severe cases)

Symptoms usually occur after 1–24 hours of eating contaminated fish. Unfortunately, ciguatera is “heat stable,” which means that no amount of cooking can get rid of it. Instead, choose to eat smaller fish and avoid certain parts such as the head, roe, liver and other internal organs where ciguatera toxin is more concentrated.2


2. Scombrotoxin

Scombrotoxin exposure occurs in species of tuna, mackerel, bluefish, marlin, mahi-mahi, jacks, and herring.

This is a toxin that is produced when fish is subjected to high temperatures for extended periods of time after it is caught. This exposure causes species of fish like, tuna, mackerel, bluefish, marlin, mahi-mahi, jacks, and herring to convert large amounts of a naturally occuring amino acid (present in their flesh)  to histamine.


During this process, spoilage bacteria grows rapidly in them. The disease is generally mild and self-resolving, with most symptoms being alleviated with the help of antihistamines.3 However, it’s important to recognize the symptoms of scombrotoxin poisoning, which incudes

  • Tingling and burning around the mouth
  • Facial flushing and sweating
  • Nausea, vomiting and diarrhoea
  • Headaches
  • Palpitations and dizziness
  • Rash.

These symptoms usually go away on their own within 12 hours. To prevent scombrotoxin poisoning ensure that the fish is chilled and refrigerated after it’s been caught, until you cook with it.4


3. Shellfish Toxins

Shellfish toxins accumulate in clams, mussels, scallops, and oysters.

Clams, mussels, scallops, and oysters obtain their food by pumping water through their system and filtering out small organisms. If the water they are in consists of large amounts of toxic algae, then it can accumulate in the shellfish.


An example of one such toxin producing algae bloom is the “red tide”  which leads to  paralytic shellfish poisoning. Like ciguatera, shellfish toxins occur in warmer waters, often during summer months.5 Symptoms caused by shellfish poisoning depend on the type of poisoning itself which include

  • Paralytic shellfish poisoning: This type of poisoning is considered serious and could be fatal. Tingling around the mouth, face, fingers, and toes are common symptoms but in severe cases it could cause an inability to talk, swallow, move, or breathe.
  • Neurotoxic shellfish poisoning: This type of poisoning has symptoms similar to ciguatera poisoning which resolve themselves in a few days.
  • Amnesic shellfish poisoning: This type of poisoning is serious, causing vomiting, abdominal cramps, and diarrhoea. It usually occurs within a day of eating the affected shellfish. Confusion, amnesia, and coma may be caused in severe cases.

The only way to prevent shellfish poisoning is to avoid eating them during algal blooms or when warnings are issued against them by the government.6


4. Mercury

Mercury is found in predatory fish, such as walleye and lake trout as well as shellfish.

Mercury is a popular contaminant in fish. It is present naturally in the environment and enters water bodies through precipitation. Besides this, human activities like gold mining, chemical manufacturing, power generation, waste disposal, and incineration also contribute to an increase in mercury levels in the environment.

The higher the fish species on the food chain, the higher the concentrations of mercury it has.  This includes predatory fish, such as walleye and lake trout as well as shellfish.

Mercury poisoning can cause damage to the nervous system. Although specific symptoms vary, if you do feel sick after consuming fish, do see a medical practitioner. One way to avoid poisoning is to consume fish with low levels of mercury like farm-raised salmon, tilapia, catfish, cod, shrimp, or crab. Pregnant women and young children are advised not to eat shark, swordfish, tilefish, and king mackerel.7

5. Cylindrospermopsin

Species of fish most popular for harboring cyanotoxins include shellfish and tilapia (freshwater fishes)

Cylindrospermopsin is a cyanotoxin produced by cyanobacteria (blue-green algae). This bacteria is found where conditions favor the growth of algal blooms. Species of fish most popular for harboring this toxin include shellfish and tilapia (freshwater fishes). Symptoms of poisoning caused by this toxin include8

  • Headaches
  • Diarrhea
  • Nausea
  • Abdominal pain
  • Dehydration
  • Fatigue
  • Dry eyes
  • Kidney damage

An easy way to get rid of cyanotoxins is to steam your fish for 2 minutes before consuming it. You could boil your fish as well, but recent research shows that steaming does a better job at removing cyanotoxins from fish.9

If you love fish, don’t get bogged down by the toxins found in it. Whenever you can, shop from reliable stores. Store your fish in cold temperatures and opt for smaller fish over larger ones. Lastly, limit your fish consumption to a minimum.