Marine Fish Species: Biology, Habitats, and Migratory Patterns

Gadidae Family

Fish with a compressed body, three dorsal fins, and a chin barbel on the lower jaw.

• Cod
• Haddock
• Whiting
• Saithe

Merluccidae Family

Fish with an elongated and slightly compressed fusiform body, two dorsal fins, and no chin barbel on the lower jaw.

• Maruja
• Hake
• Forkbeards

Cod

A demersal fish found in the cold waters of the North Atlantic, between 50 and 75 degrees N. They inhabit depths beyond the continental shelf, up to 500 or 600 meters. Cod undertake significant migrations between feeding and spawning grounds. Depending on their habitat in colder or less cold waters, sexual development and maturity will be more rapid or delayed.

Cod Spawning Grounds

• Iceland
• Newfoundland
• Labrador
• Lofoten Islands
• North Sea
• West Greenland
• Faroe Islands

Spawning occurs in shallow waters, 20 to 120 meters deep, where large concentrations of adult cod gather, seeking favorable conditions. Depending on the latitude, this takes place from February to October. Once fertilized, eggs rise to the surface where they are dispersed by currents.

Incubation Period

The incubation period depends on water temperature: it can be 10 days at 8 degrees Celsius and 40 days at 0 degrees Celsius.

Whiting

A smaller gadid fish living in less cold waters.

Saithe

A gadid fish found in the North Atlantic.

Hake

Merluccid fish found between 60° N and 20° N, and also in the Mediterranean Sea. During reproduction, they concentrate in large groups on the edge of the continental shelf in water temperatures around 10°C. The eggs are small and equipped with a small fat droplet that allows them to remain afloat. After hatching, the larva is born with the yolk sac still attached, which provides nourishment during the first days of life before they begin feeding on plankton. They live on the surface until they are 3 to 4 cm long, then transition to living at the bottom, ending their pelagic life stage. Fry concentrate in shallow areas, exhibiting rapid growth and reaching sexual maturity around four years of age, when they measure about 40 cm. As demersal predators, they separate from the bottom at night to feed. As they grow, they seek deeper waters.

Scombridae Family (Mackerel and Tuna)

They have a fusiform body with a smooth surface and very small scales, which may be absent in some species. Large gill slits indicate very active respiration, and their crescent-shaped caudal fin signifies they are powerful swimmers. These are oceanic fish that tend to undertake large migrations. Mackerel exhibit the shortest migration distances, while bonito and tuna undertake the most extensive migrations. Their vertical distribution is related to the thermocline; sexually mature, larger individuals tend to remain below it, while smaller or mixed-sized individuals at different stages of sexual maturity are found above it.

• Mackerel
• Bonito
• Tuna

Tuna

Migratory oceanic fish, very sensitive to temperature conditions, are not found in waters below 10-12°C. Tuna are only coastal during certain periods of their existence, their presence on the continental shelf coinciding with the start of the migration season.

Tuna Migration and Spawning

They winter in deep Atlantic waters. Sexually mature individuals approach the Gulf of Cadiz in spring, appearing on the surface to complete their maturation process before moving to the Mediterranean to spawn. These appearances in the Cadiz area have been known since ancient times, leading to traditional trap fishing.

In spring, they pass into the Mediterranean in large groups from the Strait, swimming parallel to the shore to spawn in areas like Sardinia, Tunisia, Sicily, and other southern islands. Spawning occurs from late May to early July in waters around 18°C.

Growth and Maturity

They exhibit rapid growth, gaining approximately 4 kg per year. They can reach a maximum size of 3 meters and a weight of 500 kg, becoming sexually mature at 4 years.

After spawning, adults return to deep waters, from where they disperse across the Strait to the coasts of northern Europe.

Tuna Fishing Methods

• Longline fishing
• Trolling
• Purse seine
• Traditional traps (Almadraba)

Traditional Trap Fishing (Almadraba)

Large fixed net systems are placed near the coast along the tuna's annual migration route. The placement and configuration of these nets guide tuna into a final chamber. When this chamber is lifted, tuna are captured with hooks from surrounding vessels. Major traps are located in the southwest of the Iberian Peninsula, Cristina Island, etc.

Bonito

Bonito are found in the Mediterranean, off the coasts of North Africa, between January and April. They exhibit rapid growth, reaching sexual maturity at two years of age when they measure about 60 cm.