The Secret Lives of Fish: Essential Facts from NOAA Fisheries

The world's oceans, rivers, and lakes teem with an estimated 20,000 to 40,000 species of fish, each exhibiting unique adaptations to their environment. Scientists at NOAA Fisheries study these diverse populations to better understand marine ecosystems. This compilation addresses some of the most frequently asked questions regarding the biology, physiology, and behavior of these aquatic vertebrates.

Dimensions and Diversity

What is the world’s largest fish?

The title belongs to the whale shark (Rhincodon typus), which can exceed 50 feet in length and weigh several tons. Following distantly is the basking shark, reaching lengths of 35 to 40 feet.

What is the world’s smallest fish?

The smallest known species is a type of goby native to the Philippines. Rarely exceeding half an inch in adulthood, its population density is so high that it supports a commercial fishery despite its diminutive size.

What is the most common fish in the ocean?

The most abundant fish are various species of deep-water bristle mouths (family Stomiidae). Roughly the size of a minnow, these fish inhabit depths of 500 meters or more across all the world's oceans.

What are the oldest fish lineages?

Primitive, jawless fish such as lampreys and hagfish represent the oldest surviving evolutionary lines. Their unique anatomical structures, particularly their sucking mouths, provide zoologists with critical insights into the transition from simple to complex vertebrate life forms.

Biology and Physiology

How do fish breathe?

Fish do not breathe "air"; they respire by extracting dissolved oxygen from water. This process occurs via the gills, which contain dense networks of capillaries that diffuse oxygen into the bloodstream.

Do fish sleep?

While most fish lack eyelids (sharks being an exception), they do enter states of rest. During these periods, metabolic rates slow, and the fish may float in place, wedge themselves into coral or mud, or occupy nests. They remain alert to danger but are functionally asleep.

How do fish swim?

Locomotion is primarily achieved through the sequential contraction of lateral muscle bands, creating a sculling motion with the tail. Vertical fins provide stability, while paired pectoral and pelvic fins assist with maneuvering and hovering.

Speed: Tuna, billfish, and some sharks can achieve burst speeds of up to 50 mph, though sustained cruising typically ranges from 5 to 10 mph.
Orientation: While most fish swim horizontally, species like the seahorse, the shrimp fish of the Indian Ocean, and certain African catfish swim in a vertical posture. Some eels are also capable of swimming backward.

Do fish chew their food?

Chewing is physiologically difficult for fish because it would interrupt the flow of water over the gills necessary for respiration. Instead, feeding mechanisms vary by diet:

Carnivores (e.g., sharks) use teeth to grasp and swallow prey whole.
Bottom dwellers (e.g., rays) possess flat teeth to crush shellfish.
Herbivores often lack jaw teeth but have specialized pharyngeal teeth (grinding mills located in the throat) to process plant matter.

How can you tell a fish’s age?

Scientists determine age by counting growth rings on scales or otoliths (ear bones). Similar to tree rings, these structures deposit annual layers. Summer rings are typically wider and whiter, while winter rings are narrower and more translucent.

How long do fish live?

Lifespans vary dramatically by species. The orange roughy is known to live for over a century, while the Acadian redfish in the Northwest Atlantic can survive for more than 40 years.

Ecology and Human Interaction

Why do fish sometimes have a strong odor?

Truly fresh fish are nearly odorless. A "fishy" smell is a byproduct of decomposition caused by improper storage or preservation, not a natural characteristic of the animal.

Is there much salt in fish?

Salt content varies. While shark meat contains high levels of salt (isotonic with seawater), over 240 species of fish are naturally low in sodium and are recommended for salt-restricted diets.

Are all fish edible?

While most sport-caught fish are edible, some species possess organs that are inherently toxic to humans. Additionally, fish can accumulate toxins from their diet or environment, rendering them unsafe for consumption.

Will a fish caught and released die anyway?

Survival rates depend on several stress factors:

Pre-existing Stress: Spawning or elevated water temperatures weaken fish before capture.
Exhaustion: Prolonged "playing" time depletes energy reserves.
Injury: Deep hooking in the gills, eyes, or throat causes significant damage.
Handling: Rough handling, squeezing, or excessive air exposure.
Depth: Fish caught from deep water often suffer from barotrauma.

If a fish appears lethargic upon release, hold it gently in the water until it regains equilibrium and swims away independently. Proper handling significantly increases post-release survival rates.

The Secret Lives of Fish: Essential Facts from NOAA Fisheries

Dimensions and Diversity

Biology and Physiology

Ecology and Human Interaction

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