Fishy-tasting eggs almost always come down to one of three things: what the hen ate, a genetic quirk in certain breeds, or how the eggs were stored. The most common culprit is a compound called trimethylamine (TMA), which has a distinctly fishy smell and can accumulate in egg yolks when specific conditions line up in the hen’s diet or biology.
Trimethylamine: The Compound Behind the Taste
Trimethylamine is the same molecule responsible for the characteristic smell of old fish. In chickens, TMA is produced when gut bacteria break down certain nutrients, particularly choline and a plant compound called sinapine. Normally, an enzyme in the hen’s liver converts TMA into an odorless form before it can reach the eggs. When that system works properly, you never notice a thing.
Problems start when TMA production outpaces the liver’s ability to neutralize it. Excess TMA circulates through the hen’s body and deposits directly into the egg yolk, giving it that unmistakable fishy flavor. This can happen because of what the hen is eating, because of her genetics, or both at the same time.
Feed Ingredients That Cause Fishy Eggs
Rapeseed (canola) meal is one of the best-documented triggers. It contains sinapine, a compound that gut bacteria readily convert into trimethylamine. Hens fed diets high in rapeseed meal produce eggs with noticeably higher TMA levels, and the taint is strong enough that even casual eaters can detect it.
Fishmeal and fish oil in poultry feed are another suspect people often point to, but the research is more nuanced than you might expect. Studies testing diets supplemented with fish oil and whole flaxseed found no significant effect on egg flavor, yolk appearance, or general acceptability in fresh eggs. Even at relatively high inclusion levels (9% whole flaxseed, for instance), trained taste panels reported no unpleasant fishy flavor in hard-boiled eggs. That said, professional panelists can detect subtle differences at very high levels of flaxseed meal, so there’s likely a threshold. For most backyard and commercial flocks, standard amounts of fishmeal in feed are unlikely to be the sole cause of a strong fishy taste.
Choline-rich diets are also worth considering. Choline is an essential nutrient for laying hens, and it’s often added to feed as choline chloride. Gut bacteria from several bacterial families use a specific enzyme system to cleave choline into TMA. In one study, hens fed a diet supplemented with choline chloride for 60 days produced yolks with TMA levels around 1.06 mg/kg, enough to be detectable.
The Genetic Factor: FMO3 Mutations
Some hens are genetically predisposed to producing fishy eggs regardless of diet. The key is a gene called FMO3, which codes for the liver enzyme that converts smelly TMA into its odorless form. A specific mutation in this gene (a single amino acid change called T329S) disrupts that enzyme’s function. Hens carrying this mutation can’t efficiently neutralize TMA, so it builds up and ends up in their eggs.
This is the chicken equivalent of a rare human condition called trimethylaminuria, sometimes known as “fish odor syndrome,” where people excrete TMA in their urine, sweat, and breath due to the same type of enzyme deficiency. In chickens, the mutation has been mapped to chromosome 8 and identified across several different breeding lines, meaning it’s not limited to one breed. Importantly, the issue isn’t that these hens produce less of the enzyme. They produce normal amounts, but the enzyme itself doesn’t work as well because of the structural change.
Brown egg layers, particularly Rhode Island Reds and their crosses, tend to carry this mutation more frequently than white egg breeds. If you keep backyard chickens and notice that only certain hens produce fishy-tasting eggs, genetics is the most likely explanation.
Odor Absorption During Storage
Eggshells are porous. Thousands of tiny pores cover the surface, allowing air and moisture to pass through. This also means eggs can absorb strong odors from their surroundings. If you store eggs near fish, onions, or other pungent foods in the refrigerator without a sealed container, the eggs can take on those flavors over time. This is a simpler and more fixable cause than feed or genetics, but it’s real and surprisingly common.
Bacterial contamination can also play a role in stored eggs. Pseudomonas bacteria, which thrive in cold, moist environments, are known to produce fishy odors on eggs, meat, poultry, and dairy products held in cold storage. This type of spoilage is different from the classic rotten-egg sulfur smell. If your eggs smell fishy and also seem off in texture or appearance, spoilage bacteria may be the issue rather than anything that happened before the egg was laid.
How to Fix the Problem
Your approach depends on the cause. If you buy eggs from a store, the most likely explanation is odor absorption. Store eggs in their original carton or a sealed container, away from strong-smelling foods. This alone solves the problem for many people.
If you raise your own chickens, start by looking at the feed. Switch away from diets heavy in rapeseed or canola meal and check whether your feed contains unusually high levels of fishmeal. Reducing choline supplementation can also lower TMA production in the gut, though choline is important for egg production, so drastic cuts aren’t ideal.
For hens with the FMO3 genetic mutation, dietary changes help but may not eliminate the issue entirely. Research has shown that supplementing feed with riboflavin (vitamin B2) can boost the activity of the TMA-neutralizing enzyme, resulting in lower TMA levels in egg yolks compared to hens on a standard diet. This is one of the more practical interventions for backyard flock owners dealing with a persistent fishy taint from specific birds.
If the problem is isolated to eggs from one or two hens in your flock, those birds likely carry the FMO3 mutation. You can identify them by hard-boiling eggs from individual hens and doing a smell test, since cooking concentrates TMA and makes it easier to detect. From there, you can choose to keep those hens for purposes other than egg production or simply accept the trait and manage it through diet adjustments.