|First Day Cover|
The migration of marine species is a phenomenon that manifest itself in various ways, in daily, seasonal or annual cycles.
Fish that migrate from the ocean to the river or from the river to the ocean during their life cycle are called diadromous and are classified as follows: anadromous fish (live mostly in the ocean and breed in the rivers) and catadromous fish (live in fresh water and breed in the ocean).
Most of these fish are threatened by extinction. In Portugal, the main threats are posed by the construction of obstacles to the migration routes (such as dams and dikes), destruction of spawning grounds, overfishing, illegal fishing and widespread pollution.
The disappearance of these species is a major loss for our aquatic biodiversity, with both economic, gastronomic and cultural consequences.
In order to reverse the situation, it is essential to implement the measures envisaged in the spatial planning policies, improve the quality of the aquatic habitats, ensure the continuity of the rivers, rehabilitate the natural breeding grounds, control pollution and gravel removal, re-establish the natural hydrologic systems and carry out a sustainable fishing management.
Atlantic Salmon, Salmo salar (block)
This species’ situation is critical, with less than 250 individuals, most of which consist of a subpopulation of the Minho River.
Salmons hatch in the rivers where they remain for roughly two years. They migrate to the ocean and when they reach their sexual maturity they return to the rivers to breed, to the same place where they were born.
Shad, Alosa alosa
The shad, which is an endangered species, can be found in Portugal in the hydrographic basins of the Minho, Vouga, Mondego (that holds the biggest subpopulation), Tagus, Guadiana rivers and, occasionally, in the Douro River.
The juveniles migrate down the rivers to the estuaries, where they grow and mature. At a later stage they migrate to the ocean, where they remain feeding on plankton, returning to the rivers to breed.
River lamprey, Lampetra fluviatilis
This is a critically endangered species; in Portugal it can be found in the hydrographic basins of the Douro, Vouga, Mondego, Lis, Ribeiras do Oeste, Tagus and Sado Rivers.
It is a very primitive fish, with a simple cartilaginous skeleton and a toothed, cupping shaped mouth that enables it to feed on the blood of other animals. The adults spawn in fresh water, on the bottom of the rivers, dying shortly afterwards. The juveniles migrate downriver to the estuaries and later out into the ocean where they complete their growth.
European eel, Anguilla anguilla (block)
This endangered species can be found in all the hydrographic basins of the Portuguese Mainland and in the coastal zones of the archipelagos of the Azores and Madeira; it hunts during the night and feeds on crabs, crayfish, molluscs, amphibians and fish eggs. The European Eel spawns in the Sargasso Sea, Northwest of the Caribbean. The larvae cross the ocean in the opposite direction, towards continental waters and complete their maturity in the European rivers, and later, when it is time to breed, they migrate back into the ocean.
Thin-lipped grey mullet, Liza ramada
This species, that is highly adaptable to the variation in salinity, can be found abundantly along the whole Atlantic coast. They move around in large shoals, in coastal waters, estuaries, lakes and rivers where they feed on algae, invertebrates and waste.
At the end of spring the adults and juveniles migrate upriver to freshwater environments. In the fall the adults return to the ocean to breed. The next generation of juveniles enter the estuaries by the end of winter.
Flounder, Platichthys flesus
Present along the whole Portuguese coast, particularly north of the Tagus River, the founder can be found along the littoral shore down to depths of 80 meters; it lives on sandy or muddy bottoms, feeding on small fish and invertebrate. When they reach sexual maturity (four years in the case of the females or three years in the case of the males), they migrate to the ocean to breed, in the winter and beginning of spring. The juveniles enter the estuary in the spring and beginning of the summer.
Date of Issue: 25 March 2011
Values: stamps of €0,32; €0,47; €0,68 and €0,80 and two souvenir sheets with one stamp of €1,80 each
Designer: Atelier Acácio Santos / Elisabete Fonseca with illustrations of Pedro Salgado
Printer: Joh. Enschedé
Process: 4-colour offset lithography
Size: stamps 40.0 mm x 30.6 mm
Perforation: Cross of Christ 13 x 13
Paper: FSC 110g/m2
Sheet: sheets of 50 stamps