Fishy smells
  |  First Published: December 2013

The sense of smell is a well-developed and important one for most fish species, and plays a significant role when it comes to fooling and catching these critters.

We know that dogs have a far superior sense of smell to humans, but it would seem that many fish leave dogs floundering in their wake (pardon the pun!) when it comes to scent detection. In fact, some scientists believe that a fish’s sense of smell is, on average, up to a thousand times keener than a dog’s, and more than a million times better than a human’s… Perhaps customs officers should be using ‘sniffer mullet’ at out airports instead of their trusty canine companions!

The low concentrations at which some marine creatures can detect substances is the stuff of legend, and it’s true that certain sharks can effectively detect a few drops of blood in an Olympic swimming pool full of seawater!

Fish species with good eyesight that hunt near the surface in clear water generally rely less on smell than those grubbing around the bottom in murky conditions. For example, we can reasonably assume that a marlin’s sense of smell is probably less refined than, say, a catfish’s. Species like bream, trout, Murray cod and mulloway will rely on a combination of sight and smell when hunting. But the fact is that most of the species we pursue as anglers do rely to some extent on their generally excellent sense of smell when it comes to finding food, avoiding being eaten and successfully reproducing (the three key motivators of all fish behaviour).

Scientists have also demonstrated that many fish can determine differences between groups of chemicals, and even between separate compounds within those chemical groups, especially when amino acids are involved (amino acids are essential building-block compounds found within the cells of living organisms). Incredibly, it seems that fish have the ability to recognise the smell of specific amino acids, and that they will respond differently to each of these scents.

One amino acid called L-Serine, found in natural oils on the skin of humans, has been identified as a significant repellent to some fish species. If transferred to lures, flies or baits when handling and rigging these items of tackle, L-Serine could definitely turn fish off striking. Interestingly, it has also been shown that some people have much higher concentrations of L-Serine on their hands than others. This fact alone could help to explain why some individuals seem to be consistently ‘luckier’ anglers than others!

To backtrack for a moment, we need to understand that fish ‘smell’ things in a slightly different way to humans and other air-breathing creatures. The senses of smell and taste are closely related in most animals, and this link is especially important beneath the surface of the water, where ‘odours’ are effectively tiny bits of stuff tasted by fish. This involves minute concentrations or traces of materials being carried by the water before coming into contact with receptors in the fish’s nostrils, mouth, any whiskers or barbles they might have, and even via special sensory pores on the skin.

To aid in this task, the nostrils (nares) found in most fish are quite different to those of humans and other mammals. We use our nostrils as part of our respiratory (breathing) system, whereas fish use theirs exclusively for detecting scents.

A fish’s nostrils are typically positioned between its mouth and eyes and are connected to an olfactory organ that contains millions of odour-sensing cells. Water must be constantly passed through the nostrils and over these olfactory cells for scents or particles of material to be detected. Many of the fish species we pursue have U-shaped nostril canals, with water being forced in one end and out the other as the fish swims, or simply holds position facing into any current flow.

The sense of smell is important in other ways, too. Fish species that aggregate in schools are known to communicate with each other using chemical secretions called pheromones that are detected via the senses of smell and taste. These pheromones are not only sensed by fellow school members, but also by potential predators. Furthermore, some species produce distinct fear, feeding and reproductive pheromones, depending on prevailing circumstances. Many experienced anglers believe that this is one of the reasons why freely-striking fish may suddenly go off the bite if one of their school mates is hooked and (especially) if it’s then released in the vicinity of the school. The existence of pheromones may even help to explain how entire populations of fish can effectively ‘wise up’ to fishing pressure and become much better at avoiding hooks or nets overtime… It’s a fascinating subject and one we’ll be returning to in this series.

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