Fish Sensory Systems

This endangered pallid sturgeon lives on the bottom in the deep, dark waters of the Mississippi River. Taste and smell are important for finding food.

The cavernous mouth of this flathead catfish is lined with sensory cells for tasting food.

The chemical senses: Taste and smell

Part 1 of this three-part series explored vision. Light travels rapidly through water and, as such, provides real-time information about a fish’s environment. If the fish sees it, it’s there. If it moves, the fish sees it move. As a source of information, chemicals are at the opposite end of the communication spectrum. Chemicals travel slowly and, at best, provide imprecise information about the location of the source. Nevertheless, detection of chemical cues can be very important to feeding.

Taste and smell

Fish detect chemicals in the water by their senses of taste and smell. Taste receptors are largely in the mouth but are also on the barbells of catfish and other fish bearing these organs. Smell receptors are in the nostrils (pits) on the top of the head. The taste and smell organs send information to different parts of the brain, but they work essentially the same. The main functional difference between the two sensory systems is largely in the chemicals detected. Fish smell chemicals at much lower concentrations than they can taste them.

By human standards, fish have highly acute abilities to detect chemicals in the water — acuity on the order of a couple drops of a chemical in a swimming pool. Detection thresholds vary among chemicals and among fish, but Pacific salmon are a good example about both the chemical acuity of fish and its importance to survival. Pacific salmon spend their adult life in the ocean but spawn in streams. They return to the stream where they hatched and spent their juvenile years by detecting the chemical signature of the water. They start their one-way journey to spawn in a river as large as the Columbia and end up in a small tributary that contributes only a tiny fraction of the water flowing down the Columbia.

Chemical triggers

Taste is probably easier to understand than smell. A fish takes a chunk of something into its mouth. Sensory cells lining the mouth detect the chemical signature of that chunk, and the brain interprets the signals as acceptable or unacceptable for consumption. The effective chemicals are tightly associated with the ingested object. Given the quickness of fish responses, particularly reflex responses, chemicals that mimic acceptable taste — or chemicals that mask an unacceptable taste — would be a desirable addition to something an angler is trying to pass off for food.

The functionality of smell is easy to understand in terms of orienting to a place, as spawning Pacific salmon do, or maybe escaping a place because of a noxious pollutant or the chemical signature of a predator. But the effectiveness of the sense of smell to obtain information about the location of prey is less apparent. To be truly effective for communicating information, the chemical needs to disperse rapidly and create a perceptible chemical gradient — a scent trail. A cut shad fished on a river rig in flowing water may produce an odor trail that will lead a big blue cat to the bait, but it is hard to imagine that a soft plastic bait retrieved even at a very slow pace disperses enough chemical information to attract a largemouth bass from more than a few inches away.

Minute quantities of alarm chemicals, called Schreckstoff by scientists who study animal behavior, released by an injured fish alert other fish in the area of danger. From my own experience, releasing or losing a bass can turn off a good bite when the fish are in a tight bunch, such as on a brush pile or the corner of a rock reef.

Scientists also know that pheromones — chemicals released into the environment to communicate among individuals of the same species — affect fish behavior. Many so-called “fish attractants” rely on this principle. One thing to keep in mind as you search for fish attractants: chemicals most readily detected by fish are water-soluble compounds.

Scents or nonsense

Much remains to be learned about exploiting the sense of smell to attract fish or make them bite. Which odors attract, which repel? Scents may function more as deal makers-deal breakers. A fish is attracted to prey, or a lure, by visual or acoustic stimuli. At very close range, the chemical odor allows the attack to continue or causes it to abort. While this sequence may be hard for us slow-reacting, sensory-limited humans to accept, we are dealing with an organism that can detect chemicals at concentrations of a part per billion and slurp and reject a crankbait before we can even blink.