Haddock vs Flounder: What Are The Differences?

Written by Jeremiah Wright
Published: September 9, 2022
Image Credit A-Z-Animals.com
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We all know that haddocks and flounders are both groundfish species and are considered types of whitefish, yet they have very distinct features that can help you differentiate them. The haddock is from the family of true codfish called Gadidae. The flounder comes from Paralichthyidae, a family of large-tooth flounders and one of the four flatfish families found in the Gulf of Mexico.

Today, we will learn how to compare the two groundfish species and differentiate them according to their distinctive features.

Comparing Haddock vs. Flounder

HaddockFlounder
SizeLength: 12 to 36 inches
Weight: 2 to 40 pounds (0.9 to 18.14 kg)
Length: 8.7 to 23.6 inches 
Weight: up to 22 pounds
AppearanceElongated bodies with distinguishable coloring, having blackish lines along the back and a distinctive marking between the lateral line and the pectoral finUnusually flat bodies with brown, red, orange, green, or blue coloring and two big round eyes from small stalks on the same side of its head
HabitatOn or near the bottom of lakes and seasCoastal waters near docks, reefs, or bridges
DistributionFound on both sides of the North Atlantic from Newfoundland, Cape May, New Jersey, Georges Bank, and the Gulf of MaineMostly abundant in tropical and temperate waters along the coasts of Europe, North America, Africa, and Asia
DietThese carnivores eat bottom-dwelling animals such as mollusks, crustaceans, worms, sea stars, sea urchins, and even fish eggsThey are carnivores that mainly feed on fish spawns, crustaceans, polychaetes, and small fish. As soon as they grow larger, their diet may include shrimp
PredatorsSpiny dogfish, gray seals, skates, and other groundfish such as pollock and codCod, bluefish, groupers, moray eels, stingrays, and sharks
Lifespan10 or more years3 to 10 years

The Key Differences Between Haddocks and Flounders

To help us differentiate haddocks and flounders, let’s take a look at some key features – appearance, size, diet, distribution, predators, reproduction, and lifespan. Examining all these key features will help us grasp their differences.

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Haddock vs. Flounder: Appearance

Two Haddock on green background
The haddock is easily recognized by its black lateral line running along its white side.

Solodov Aleksei/Shutterstock.com

We can easily tell the haddock and flounder apart by analyzing their physical appearance. The two whitefish species have distinct features, which can help you tell them apart.

The haddock is easily recognized by its black lateral line running along its white side. It has three dorsal fins, two anal fins, a white belly, and a distinct mark called the “Devil’s thumbprint,” which is a dark blotch above its pectoral fin.

On the other hand, besides its distinctive flat body suited for bottom-dwelling, the flounder has an interesting body structure. It is known for its ability to undergo a process called transmutation, meaning that it completely changes its body form by the time it reaches adulthood. The flounder can be brown, orange, green, or blue. It has a regular fish body at birth and will develop into a round flatfish by maturity. It also can change the positioning of its eyes. 

Haddock vs. Flounder: Size

Just-caught Haddock in someone's hand
Haddocks have a total length of 12 to 36 inches.

Piotr Wawrzyniuk/Shutterstock.com

Haddocks and flounders have approximately the same sizes but still have minor differences. Still, determining their dissimilarities is one of the easiest ways to tell the two fish apart.

Haddocks have a total length of 12 to 36 inches, weighing from 2 to 40 pounds (0.9 to 18.14 kg) by the time they reach adulthood. They rapidly grow into adult sizes, and it may take up to four years to achieve full sexual maturity.

Flounders weigh up to 22 pounds. They have a minimum length of 8.7 inches and can grow up to 23.6 inches. However, they have no maximum length, so they can still grow into a larger size than the average.

Haddock vs. Flounder: Diet

Flounder (Paralichthys) - swimming over rocks
Flounders are nocturnal carnivores that feed mostly on fish spawns, crustaceans, polychaetes, and small fish.

CT Johnson/Shutterstock.com

Haddocks’ and flounders’ diets are slightly different. They hunt for their prey around the ocean floor, such as slow-moving invertebrates or smaller fish. 

Haddocks mainly consume worms, sea stars, sea urchins, mollusks, crustaceans, smaller fish, and fish eggs, while flounders are nocturnal carnivores that feed mostly on fish spawn, crustaceans, polychaetes, and small fish. Larger flounders may include shrimp and crabs in their diet.

Haddock vs. Flounder: Distribution

The two fish can be found in different areas. Haddocks are mostly found on both sides of the North Atlantic. They can be seen from Newfoundland to Cape May and New Jersey but are most abundant in the Gulf of Maine and on Georges Bank. 

Meanwhile, flounders occupy the tropical and temperate waters in Europe, North America, Africa, and Asia.

Haddock vs. Flounder: Predators

Both fish are preyed upon by other animals, usually larger than them. Haddocks are preyed on by animals such as the spiny dogfish, skate, cod, halibut, monkfish, and grey seal. Flounders are hunted by cod, bluefish, groupers, moray eels, stingrays, and sharks.

Haddock vs. Flounder: Reproduction

Flounder - Isolated on white
Female flounders can release up to 29,000 eggs per spawn and spawn up to thirteen times each season.

IrinaK/Shutterstock.com

Haddocks and flounders also differ in how they reproduce. A female haddock releases its eggs in groups on the ocean floor, and a male fertilizes them. Annually, average-sized females can produce around 850,000 eggs, while larger ones produce up to 3 million.

Flounders reproduce with a spawn method known as “broadcast spawning,” which means that females and males release their eggs and sperm in the water at the same time. In this way, they ensure that the eggs are fertilized, decreasing predators’ chance to eat them. Female flounders can release up to 29,000 eggs per spawn and spawn up to thirteen times each season.

Haddock vs. Flounder: Lifespan

Haddocks are fast-growing species. They mature as early as 1 to 4 and may start reproducing. They can live for ten or more years, but the haddocks caught by scientists are usually 3 – 7 years old. Flounders also have short lives as they can live from 3 to 10 years.

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Haddock  vs Flounder

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About the Author

I hold seven years of professional experience in the content world, focusing on real estate, nature, and wildlife. Asides from writing, I enjoy surfing the internet and listening to music.

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Sources
  1. NOAA Fisheries, Available here: https://www.fisheries.noaa.gov/species/haddock