Last week, I came across my fishing neighbor and I stopped to have a chat with him. During the conversation, as he threw out his bait, he asked me: ‘How do fish know our bait is there? Can fish smell? Do they have noses?’.
This is a question I’ve never thought about myself but I just couldn’t seem to get it out of my mind over the past few days. I have been fishing myself for years, almost always use bait and was therefore surprised that I did not know this. Over the past week I have been researching, and in this article I will try to answer this question as clearly as possible.
Can Fish Smell?
Yes, fish are able to smell. Compared to humans, in fish, smell and breathing are completely separated from each other. A fish smells through the two openings that sit on its head that form an independent organ and do not pass through to the lungs as with us. Along these openings, the water can flow inwards towards the underlying part of the smell organ, this is called the rosette because of its shape. In the scent rosette, the smell nerves of the fish end and this makes it possible for the fish to smell. The movement of the fish ensures that water flows in through these nostrils in the same way as with their gills. When the incoming water contains odorants, they will be detected by the nerve endings and the fish’s brain will be given a signal that food is nearby, for example. Then the water flows out through the rear openings on the head. In the base, the smell organ of all fish works in much the same way. However, fish species adapt to their environment so depending on the species and habitat, a fish will make more or less use of the sense of smell. A catfish species in Africa that is usually found in muddy murky water will be more focused on the sense of smell than a pike in shallow and clear water. The pike has more possibilities to hunt for sight and, unlike the catfish, needs the nose less.
Important fact!
The smell ability of fish can decrease as more CO2 enters the ocean. The influence of CO2 in the water on the behaviour of fish has been investigated. This has emerged from a new study, done by the University of Exeter. The lead researcher, Dr. Cosima Porteus indicated that in water where more CO2 was present, the fish were less active and more slowly recognizing danger.
What do fish use their sense of smell and what do they smell?
A fish uses the sense of smell for multiple things that are important for survival. This allows them to detect food, warn them against predators and identify species or habitats. These signals are sent to the brain when fragrances are recognized. But what fragrances does a fish recognize? The main substances are amino acids, bile acids and sex hormones. What the recognition of these fragrances is good for will be explained below.
Recognizing amino acids is one of the most important functions of a fish’s sense of smell and contributes, for example, to finding food. Because the fish recognizes the amino acids in the food, this is an immediate indication that it is a nutritious snack! This is also a major factor that is often experimented with different scents for bait. Think of boilies and pellets for carp fishermen, both of which can be processed with fragrances such as amino acids to make them more attractive to the fish.
In addition, the sense of smell of fish is very sensitive to bile acids. These are substances that are secreted after food digestion and could, for example, demonstrate the quality of the food. The biological application is not fully known, but it is thought this ability can also be utilised to migrate or forage. To find a partner to spawn with, many species of fish secrete sex hormones through, for example, urine. In this way, a male knows that the female is sexually mature and this is the signal to mate.
Structure of the rosette organ
In general, the smell organ of fish sits on the back of the head and runs to about the eyes. When the fish swims, water flows along the two nostrils on the head. The water flows into the nose and then reaches the scent rosette. The rosette can be seen as a collection of filters that recognize fragrances in the water and then send a signal to the fish’s brain. The bigger the rosette, the better the smell ability of the fish! For example, when the fish detects an amino acid in the water, this may be a signal that food is present in the environment.
When the water has passed the scent rosette, it leaves the fish along the rear nostrils. This is the general structure of the smell organ of the fish. Nevertheless, the organ can adapt to the habitat and way of life of the different fish species. Think of adjustments in its size and place on the body. For example, a stingray has the nostrils at the bottom of the head and the white shark has them on the top of the head.
At what distance can fish smell bait/prey?
There is not one clear distance at which fish can observe bait or prey. As a fisherman you base your bait on the fish, and the fish does not base his preference on you! Think of catfish, for example. Because these fish often have dark habitats, they do not hunt for sight and are therefore more dependent on smell. A strong smelling bait, for example a dead fish, will have a better chance of success than a piece of lure. Therefore, think about the kind of fish you want to catch before you choose your bait. What is the habitat, food and behaviour of the fish? Answering these questions can give you a good idea of the best bait to use. In many angling shops nowadays there are many decoys and substances for sale that can be used to excite the fish. Also keep in mind that what you smell is not necessarily what the fish smells! Eventually, practice makes perfect!
Are there any fish species that can’t smell?
Every species of fish has an odour organ, but it is not fully developed at all and sometimes this is not even used. As mentioned, this depends on the species and habitat of the fish. Barracuda, for example, is known to hunt mainly in the open water and around reefs. Because the water often provides good visibility for the fish and there are enough prey animals present in such places, the smell organ is less important. This makes the eye more developed. It is known that there are fish species that can distinguish colors and even see ultraviolet!
Conclusion
Yes, fish are able to smell odours but this is not always the case. Through two nostrils on the head, water flows in, the rosette filters it for fragrances and the water leaves the fish through the rear nostrils. The smell organ develops more or less depending on the species of fish.
The bigger the scent rosette, the better the sense of smell. Often you will see that fish with a small rosette are more focused on sight and vice versa. When you go fishing there are a number of questions that you can answer in advance to choose suitable bait, based on the characteristics of the fish. Questions such as: ‘What is the habitat, the food and behaviour of the fish?’ can provide good indications.
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