What Do Fish Smell? The Science of Fishing with Scents

Every time we hit the water, we use baits and lures to deliver a set of sensory inputs designed to elicit strikes from fish. The critical fishy senses that we must appeal to include sight, hearing/vibration, smell and taste. Many of these work much like their parallel senses in land animals, but others have evolved in distinct ways for aquatic organisms.

For example, a fish's senses of hearing and vibration work together, gathering inputs from the bony otoliths in its inner ears, as well as from its unique lateral line system, to detect pressure changes and movements in the surrounding water. Likewise, a fish's unique sense of smell—its ability to detect and respond to chemical "odorants" in the water—plays an important role in how it interacts with its environment and the baits we present.

Fish have two pairs of pores, called nares, on the upper part of their snouts between the jaws and eyes. The nares transport water, and the materials dissolved within it, past a dense network of sensory cells that can detect trace amounts of water-soluble chemical stimulants, collectively called odorants. Nares can be quite prominent on large predatory species but are nonetheless present on every fish, from gamefish to baitfish.

Water flows through the channel connecting the two nares in one direction: in the front pore and out the rear. Many fish have a small flap of tissue that serves to deflect water into the forward of the two nares, while others have evolved a network of cells with pulsing cilia designed to actively transport water through the channel and across the sensory cells. A compact bundle of nerves links the sensory cells to the fish's brain for rapid scent-based decision making.

What Fish Smell

More than 80 years of fisheries studies and meticulous biological research have revealed that fish respond to three general classes of chemical odorants: amino acids, bile acids and pheromones. Each of these may influence fish behavior in a way that is coupled to angling success.

Amino acids are the building blocks of proteins and can act as general feeding stimulants. Nature provides a library of 21 different amino acids, and particular fish are selectively responsive to specific amino acid cocktails.

For example, fish that regularly munch on crabs or crayfish are especially sensitive to the amino acids found in high abundance in their preferred prey, including glycine, alanine, arginine and proline.

Likewise, carnivorous fish respond to a different subset of amino acids than their herbivorous cousins. When a fish detects amino acids, the message is, "food items are nearby."

Fish can also detect bile acids in the water. These are compounds produced as a result of food digestion and are released from fish as part of the normal excretory process. Thus, bile acids serve as a cue that other fish are feeding, and that more food may be close at hand.

Similar to amino acids, bile acids are somewhat non-specific, meaning that a largemouth bass might detect bile acids produced by nearby crappies, and therefore receive a sensory cue that suitable prey items may be available. Bile acids tell fish, "prey is nearby, and other fish are eating it!"

Pheromones reside on the frontier of fish-odorant study. In general terms, pheromones are elaborate compounds produced by a fish—and detected only by the same species—that cause a very specific behavioral response. The first fish pheromone identified, all the way back in the 1930s, is the alarm substance, or "schreckstoff."

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This compound is released into the water when the tissue of a fish is injured, presumably after a predatory attack. Other nearby fish of the same species can detect this pheromone in very minute concentrations, causing them to flee or seek cover.

Other pheromones serve as olfactory signals for migratory fish, especially trout and salmon, to help them identify their native streams and locate suitable spawning partners. Indeed, artificial application of sea lamprey pheromones plays a key role in the management of this destructive, invasive species in the Great Lakes. When a fish detects a pheromone, its behavioral response is likely to be highly specialized: hide, swim in this direction or start looking for a mate.

Smells Like Success

Certain fish are known to be particularly sensitive to the application of scents, including the various catfish, salmon and trout species.

Catfish rely heavily on scent to locate prey items, as the lakes and rivers that frequently hold supersized specimens are often turbid, offering limited subsurface visibility. Common prepared and natural baits, from chicken livers and nightcrawlers to fresh cut bait, exude clouds of readily detected amino acids and other biological stimulants, drawing cats from long distances.

Likewise, trout and salmon rely on their abilities to detect trace odorants in the water during their migratory and reproductive cycles. These same fish are also quite responsive to chemical stimulants applied by anglers to artificial or natural baits, especially as part of the ritualistic salmon-egg curing process.

But what about other popular fish like bass, walleye and panfish?

For these species, the open science is less well-established, although several companies—including, notably, Berkley/Pure Fishing—maintain active research and development efforts to leverage the fish's sense of smell into enhanced catch rates. Since the angling public doesn't have access to the unfiltered scientific data, we must rely on anecdotal evidence to support, or refute, the notion that the right scent can mean more fish.

Some recent offerings, like Berkley PowerBait MaxScent soft baits, have rapidly found their way into tackle bags across the country, fueled by a string of positive results and six-figure checks presented at major bass tournaments. One common thread binding bass and scents together is the frequent observation that highly pressured fish, whether it's sumo smallmouth in gin-clear water or tank largemouth buried beneath grass mats, often respond favorably to the right scent presented at the right time.

My advice? Grab a few baits and try them for yourself, especially when the going gets tough. A scented offering might help convert a slow day into a trip of epic proportions.