Anguilla has lost its kingdom

The old people of this country say that, decades ago, the rivers and streams of our towns and mountains were throbbing brimming with biodiversity. Those children, in the summers of their childhood, came to those hotbeds of wildlife with shorts, chocolate bread snacks and legs full of postilles to search for valuable treasures in the form of tadpoles, lizards, crayfish and all sorts of freshwater fish, including the famous and prized eels. Now, sadly, Anguilla has lost its kingdom.

Fish represent the largest percentage of vertebrates in the world (more than 50%) and their more than 25,000 species—as disparate as a seahorse and a white shark—have been able to colonize all of the planet's aquatic environments, from glacial seas to tropical reefs, to ocean chasms and freshwater lakes in high mountains.

Within this fabulous evolutionary diversity, diadromous fish are of special interest, species that develop a phase of their biological cycle in epicontinental waters (rivers, streams, lakes) and another part of their phenology in brackish waters (rivers, estuaries) and/or marine waters. Anguilla is one of them. The migration and reproduction of this species was a mystery until the 20th century, when its spawning territory was discovered in the Sargasso Sea, between Bermuda and Puerto Rico. Eels live in inland waters and, when they reach sexual maturity in lakes and freshwater streams, they follow the course of rivers and streams, sometimes slipping on the wet grass on the shore, until they reach the ocean, where they swim or let themselves be carried away by the currents until they reach the Sargasso Sea. There they spawn in deep water and the female produces up to 20 million free-floating eggs. During this migration, they do not feed and die after harvest.

In this complex cycle, the larvae are born in open ocean waters, pass to fry in intermediate waters, become juveniles in brackish estuaries, are immature in fresh waters and, finally, reach the adult stage in newly brackish environments. Knowing this cycle, one might ask, what adaptive advantage does this diversity of phases and habitats give to diadromous species? The answer is clear: to reduce pressure on food resources. Many ontogenic phases of the same individual in different habitats make it possible to atomize the population, not to concentrate it on the same biotope and, consequently, to expand the trophic niches, which avoids the intensive overexploitation of food, in the same place and at the same time.

For this life of changing habitats, eagles have evolutionary adaptations that allow them to thrive in waters of a different nature. Its dorsal and anal fins begin in the immediate vicinity of the head and meet at the tail, which provides it with an excellent swimming thrust that allows it to climb rivers against the current. They have dense capillary systems in the skin so they can absorb oxygen directly from water or air. They have small gill orifices that prevent the gills from drying out quickly, which, together with the vascularization of their skin, allows them to survive for long periods out of water or in environments with a low oxygen concentration. In these cases, they can even “jump” from one pool to another, moving through the grass, so their body is covered with a mucous layer that makes them slippery and facilitates these movements.

Fifty years ago, eels were present in practically all the hydrographic basins of Iberia, and today, they survive in the river systems and uncontaminated lower reaches of the Cantabrian coast, Levante and some basins south of the Guadalquivir Valley. The overfishing of the species for its highly prized meat, both in its adult phase and fry (eels), the pollution and alteration of river and marine habitats, the construction of non-stop reservoirs that cause a barrier effect on its migratory movements, are the main causes of its disproportionate decline. It is estimated that the decline of the eel reached 80% compared to its population in the middle of the 20th century (Freyhof, 2010), with an X-ray very similar to other diadromous species such as sturgeon, the lamprey, Sábalo or the trout and salmon.

The recovery of Anguilla does not only involve strengthening its populations with the release of hundreds of thousands of fry. It requires a much more cross-cutting plan that aims to recover river environments, consolidate riverine forests, prosecute illegal discharges, regulate aggregates extraction and build passages for fish fauna in dams and reservoirs, an ambitious conservation program that must entail the recovery of rivers as essential natural spaces for wildlife, landscapes and human culture. Perhaps if we return their poplars and poplars, their crabs and dragonflies and their white stones under clear waters, eel and many other species will recover their lost kingdom. Let's return to them those rivers where our grandparents, in the summers of their childhood, came with shorts, snacks of chocolate bread and legs full of postilles.

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