Why Do Fish Smell So 'Fishy'?

Having a fishy odour relates to something being ‘not quite right’ as in ‘something smelling fishy’ — an adjective associated with doubt or suspicion. The saying probably comes from the fact that when dead fish are old, they start to smell funny or unpleasant. But why do fish smell fishy and why do they sometimes smell of other things like caramels or peaches?

A researcher at Friedrich-Alexander Universtät (FAU) in Germany is studying fishy odours using chromatography — to find out why fish sometimes smell fishy and sometimes smell of other things. Ultimately, he wants to find out how the unpleasant fishy smells can be reduced to improve the taste of fish. Let’s check out how he is getting on.

Farmed or wild — can it affect the smell?

Governments and food agencies encourage people to eat a healthy, balanced diet — alongside exercise, the prime driver to a healthier and less obese society. This includes eating less meat and sugary products and more fruit, vegetables and fish. Generally, eating several portions of fish each week is healthy as it is relatively low in saturated fat and high in protein — and some fish also contains long-chain fatty acids which are also thought to be beneficial.

But as more fish is consumed, demand increases and this places a burden on wild fish stocks. This impacts both the environment as the food chain is disturbed and our long-term food security. Enter fish farms. Unfortunately, farmed fish can have unwanted odours and tastes when compared with wild fish — something consumers notice. Professor BÏ‹ttner — the Chair of Food Chemistry at the FAU — explains in a press release:

‘Microorganisms can form in aquaculture ponds and other substances can successively break down in the water, creating highly potent aromas which in turn cause deficiencies in the fish.’

Earthy, manure smell?

To try and understand what causes the odours Mohamed Mahmoud, a researcher at FAU, has been studying rainbow trout from aquaculture farming in Germany. Mahmoud sampled and analysed both the water the fish are farmed in and the rainbow trout themselves. He used a combination of gas chromatography linked with olfactometry and mass spectrometry to separate and identify the odour compounds in both the water and the fish. GC is a commonly used technique when working with volatiles as discussed in the article, Sample Preparation Options for Aroma Analysis.

The musty smell associated with fish is typically from the compound geosmin which has a mouldy smell, whilst the compound 2-methylisoborneol has a rotten fish smell. But Mahmood detected ten other compounds in fish which have a musty, earthy odour — including one that smells of manure. Mahmoud thinks that the manure smell probably comes from run-off from the surrounding land. Substances run off the land into the water and get into the fish.

Mahmoud is still investigating the problem, trying to identify the sources and causes of odours that are present in farmed fish but not in their wild cousins.