The Angler’s Guide To Fish Anatomy

The Angler's Guide to Fish Anatomy

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Sport fishing is a growing activity throughout the world, but especially in the United States. From grouper fishing in Florida to trout fishing in Idaho, locals and tourists alike travel to participate in bustling fishing scenes all year round. 

Would-be anglers must brush up on their fish-related information before heading onto the open water. Understanding different fish parts and their functions is essential because it helps hobbyists identify different specimens and learn more about the environment around them. Keep reading to learn more about fish anatomy.

Fish Fossil
Photo by Jacqueline Martinez on Unsplash

External Anatomy Of A Fish


A fish’s fins are its primary tool for navigating and moving through water. These external appendages are often located near a fish’s centerline for optimal balance and steering. 

However, not all fins are the same, and most fish have several types on their body – each fulfilling a different purpose:

    • Dorsal fin: The dorsal fin is located along the fish’s back, like a horse’s mane. While it helps with steering a fish’s body, it often serves as protection. Some fish have spiny or sharp dorsal fins that can easily slice through flesh.
    • Pectoral fin: Pectoral fins are usually located right beside a fish’s gills. These small, arm-like appendages help fish steer and turn in the water. They also control depth.
    • Pelvic fin: Otherwise known as a ventral fin, the pelvic fin sits at the bottom of a fish’s body. This fin is essential for most complex movements, functioning as both a brake and propulsor. Additionally, pelvic fins can be used to “ground walk” and hover over surfaces.
    • Anal fin: Anal fins are usually located halfway between the pelvic and tail fins. These are essential for stabilizing a fish’s body, similar to a keel at the bottom of a boat. Narrow or laterally-compressed fishes generally have longer anal fins to keep them upright.
  • Tail fin: The tail fin (or caudal fin) is a fish’s primary tool for locomotion, like how oars propel a boat forward. These fins vary quite a bit depending on how a fish moves – larger fins are more commonly found on slow-moving fish while forked tails help smaller fish escape predators.
Mackerel in hands
Photo by OMG Snap on AdobeStock


A fish’s gills work very similarly to human lungs, except they filter oxygen from water instead of “breathing”. These branching organs are located on each side of a fish’s head and contain many blood vessels for transporting oxygen to the rest of the fish’s body. 

As a fish opens its mouth to “inhale”, water is filtered through the gills to extract its oxygen content. Then, capillaries located in the gills transport the oxygen into the rest of the fish’s body.

These sensitive organs are protected by a bony covering called an operculum, the visible “flaps” or openings located on a fish’s head. 

Lateral Lines

A fish’s lateral line is a small system of tiny holes or pores along a fish’s body. While relatively unnoticeable, it is a powerful sensory system that allows a fish to understand and interact with the world around it. Specifically, they enable a fish to detect movements, vibrations, and changes in the pressure around them.

Furthermore, these tiny pores give a fish the necessary spatial awareness to stick to its school, defend itself from predators, or chase prey down. Even blind fish can perform these functions without much of an issue. This is achieved through a complex system of “hair” cells or receptors that send signals to the nerves running along a fish’s body.

Grouper Fish Face
Photo by David Clode on Unsplash


Nares are small openings at the front of a fish’s head, resembling nostrils in appearance and function. However, unlike human nostrils, a fish’s nares come in two pairs, one on each side. These small connected openings form an important sensory structure that allows fish to “smell” the surrounding area.

A fish “inhales” water through its foremost nares. This runs the water over sensory cells that pick up chemicals, signals, and other vital information to help them avoid predators and locate mates. The water is then expelled from the second set of nares once a fish has successfully “sniffed” the area around them.

Peacock Bass
Photo by Kwanza on AdobeStock

Scales And Slime

A fish’s scales are a built-in layer of armor that protects against bacteria and general life underwater. That said, these scales don’t work alone. A layer of slime or secretions sits atop a fish’s scales, sealing them off from potential parasites while covering open wounds. 

Some fish (like parrotfish) have slimy layers laced with poisonous chemicals to deter predators, while others use this added slipperiness to get away from larger animals. 

Note: Anglers should be careful not to scrape off this layer of slimy secretions when handling a fish.


A fish’s mouth says a lot about its diet and eating habits based on shape, size, location, and orientation. For example, fish with teeth (like sharks and barracudas) are carnivores that like to “hit and run” or attack prey in short, quick bursts. Meanwhile, fish with protruding mouths are “suckers”, upturned mouths hint at surface prey, and downturned mouths indicate bottom feeding.


Internal Anatomy Of A Fish


A fish’s heart functions very similarly to a human one. Specifically, it pushes blood and oxygen throughout the body while pumping waste through the kidneys and liver. But despite functional similarities, a fish’s heart only has two chambers because it has a single-circuit circulatory system.

Unlike other land-based animals and mammals, fish require less oxygen overall. That means a fish’s blood does not have to return to the heart (as it does in four-chambered living beings) to be repressurized and sent to the rest of the body. Instead, the blood is reoxygenated at the gills before it makes another round.


Fish livers are in charge of waste management, storage, and digestion. This is achieved through two primary functions: 

  • The production of bile that helps break down stored fat inside a fish’s intestines 
  • The maintenance of proper blood chemistry through carbohydrate storage and waste excretion

Pyloric Caeca

Little is known about the pyloric caeca’s actual functions, and researchers have noted that not all fish have them – a variance that is sometimes present even between members of the same species. Some experts theorize that this finger-like organ may help with digestion and secretion because of its proximity to the stomach and intestines. 


Gonads are the collective term for a fish’s sex organs. This refers to the testes used for sperm production in male fish and the ovaries for female fish. Fish may lay eggs to be fertilized externally, lay pre-fertilized eggs, or birth live fry similar to mammals.

Swim Bladder

A swim bladder is a gas-filled sac located in a fish’s dorsal or upper side. Because fishes can be pretty heavy, they need an inner structure that helps them maintain buoyancy despite their body weight. The air in a fish’s swim bladder lets them “float”.

Here are the two types of swim bladders found in fish:

  • Physostomous: The physostomous swim bladder is often found in fish living in shallower water. This flexible sac expels gas via bubbles as the fish swims closer to the surface, but it’s also reinflated just as quickly. A fish will come up for air and inhale gas to push into the swim bladder on the way down.
  • Physoclistous: Typically found in deepwater fish, the physoclistous swim bladder is completely cut off from a fish’s mouth. Instead, gas is expelled through the thin membranes of the organ’s walls and diffused via blood. Likewise, oxygen is reintroduced to the swim bladder through oxygenated blood.


The stomach is where most digestion occurs, and its general size and shape vary based on a fish’s feeding habits. For example, the intestines found in the internal anatomy of a tilapia fish (a herbivore) would be longer than carnivorous fish because plant matter is generally tougher to break down into usable parts.


A fish’s kidneys filter liquid waste from the blood. This is particularly helpful for maintaining and regulating saltwater concentration within a fish’s body. In some cases (like with salmon), a fish’s kidneys may allow them to exist in both freshwater and saltwater habitats.

Final Thoughts

Fishing might be fun, but that doesn’t mean you can simply show up in a boat and cast your line. You’ll need a fishing license and the knowledge to help you while you’re out on the water. 

Familiarizing yourself with different fish body parts and functions can help even the most inexperienced anglers understand their quarry better. The result? An entertaining, productive, and educational fishing session.


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