Eel stocking always in fashion! The fry of this amazing species are introduced into our lakes, rivers and coastal waters of the Baltic Sea from spring to autumn. Private entities compete in this field with social organizations (mainly the Polish Angling Association) and state institutions (including the Ministry of Agriculture, Polish Waters, the Institute of Inland Fisheries) [1]. The species is still on our tables, despite the fact that it has become much rarer globally than many of the fish protected under Polish regulations and/or the Habitats Directive. One of the main reasons for its disappearance in European waters remains the lack of operable fish ladders [1]. Anguilla anguilla requires peculiar transitions, e.g.: eel ladders, different from most fish.
As far back as the ancient Greeks…
The migrations of these serpentine fish have fired the human imagination for thousands of years. Aristotle believed that they arise from the bowels of the earth, which is usually interpreted as self-management. It’s possible, however, that Stagyrates was thinking more of the eelgrasses, or blindworms, that live in the silt, whose true nature (lamprey larvae) was not elucidated until the 19th century. Benedict Dybowski. Until two centuries ago, the most recent opinions of scientists were as inaccurate as the ruses of the fishermen of the time, who considered all European eels to be males and took Zoarces viviparus, known to them from the estuaries, as their mates. It is for this reason that they are still called eelpouts/mother eels in many languages today (German: Aalmutter, English: eelpout) [2].
The correctness of these notions seemed to be confirmed by some dissections of eels (not to mention observations of cooks), since the parasitic worms found in their bodies were taken for the fetuses of these serpentine fish.
The eel buries itself in the slime at the bottom of the lakes for the winter with its poops, and it stays dormant all winter. In the spring, from the lakes to the rivers it comes out and flows with the water. Final, and these recent sightings have shown that the eel fetus vividly releases its fetus into the world instructed his readers Stanislaw Jundzl in Zoology Briefly Collected [2].
Eel and the Polish case
The dichotomous nature of European eels and male gonads was discovered by Polish ichthyologist Szymon Syrski (1829-1882), director of the Natural History Museum in the then Austrian city of Trieste [2]. In Shmidt ‘s Wanderings of Fishes, translated from Russian and published by KiW in 1950, he is listed as an Austrian scientist. The contemporary PWN Encyclopedia reports only that Syrski detected male gonads in Adriatic fish of the genus Serranus ; he made the first bathymetric map of the Adriatic Sea. With prominent naturalists it was like with bivalve fish in our country: there were many more of them than is commonly believed. Perhaps because few people mention them/they?
Dark everywhere, deaf everywhere….
The eel undertakes a unique catadromous-type migration, that is, from freshwater to the sea for breeding. Always 4 days after the full moon, from the beginning of summer to the end of autumn, females run down rivers and lakes, sometimes crawling ashore along the way. In the estuaries, they are joined by males, after which they swim to the Sargasso Sea – the only body of water whose boundaries are defined only by ocean currents (the Canary, Gulf and North Sea currents) and not by land. An analogous migration from the rivers and lakes of Greenland, North and Central America to the same body of water is undertaken by the related American eel Anguilla rostrata.
Not much shorter are the migrations of the Japanese eel A. japonica, flowing thousands of kilometers from the depths of China, Korea and Japan towards the underwater Mount Suruga near the Marianas. Also, the Australian eel A. australis, with its 4,000-count. km migrations from Australia and New Zealand to the Coral Sea, near New Caledonia, the length of the route beats most catadromous species [3-8].
For several years there have been increasing reports that not only males, but also some female European and Japanese eels do not go up rivers and streams, possibly undertaking only shorter food migrations. These matters remain as dark as the autumn nights of eelgrass runoff or the inside of their burrows, because the data from chemical analysis of otoliths and fatty acids turned out to be in open contradiction with parasitological data, and it is by these three methods (based on otolith chemistry, fatty acid ratios and the fauna of parasitic worms) that one determines, as a standard, where and how long a particular individual fish has lived [3-6].
40 million years of evolution, 2,000 years of speculation
Disputes about the timing of the Anguilla fish migration are not silent. Soviet researchers, followed by some Polish ones [e.g., 7-9], believed that the current pattern of these migrations was formed more recently, only after the Pleistocene glaciers receded. That’s when the spawning grounds of American and European eels, previously located off the coasts of North America and northern Africa, were supposed to come into contact. In contemporary, high-impact scientific articles, the prevailing view is of a much earlier start to these migrations, dating back some 40-50 million years.
They were supposed to have developed during the time of the Tethys Ocean, with a completely different climate, different distribution of continents and sea currents. Initially, they were as short as those of modern Indo-Pacific island eels, about 100 km each, to extend over a quarter of the globe and a period of 2-3 calendar years [3-6]. However, we still don’t know why – contrary to common sense and the principle of Ockham’s razor – our eels are returning to the Sargasso Sea, instead of switching to breeding in freshwater, preserving pelagic eggs, as the Lota lota burbot, the world’s only fully freshwater codfish, did. Or rub themselves in estuaries, as many grandmothers and galaxies tend to do.
Real and imaginary superpowers
Eels not only undertake unique migrations and have a peculiar shape. They are also distinguished by their ability to crawl ashore and capture prey inaccessible to other predatory fish, their close relationship of biological rhythms with the phases of the moon, and finally their blood, which is poisonous to mammals [3, 4, 7, 9]. The commonness of the English name eel in the names of other fishes causes our A. anguilla to be erroneously attributed to the characteristics of a couple of other snakehead fishes. The superpowers of eels in question continue to fire the imagination of journalists and educators.
Sometimes, however, one should restrain one’s fantasies so as not to resurrect old superstitions, much less spread misinformation, such as in the article about the superpowers of eels. Not only was it illustrated with photos of tropical, venomous moray eels, but it also attributed to our fish the characteristics of a couple of other species, such as the unrelated Amazonian electric eel, the tunicates(Electrophorus spp.).
Eel protection in Polish law
Finally, it is worth detailing how the protection of these serpentine fish. The eel is protected not by the Law on Nature Protection, but by a series of international agreements, such as the Washington Convention, the Bonn Convention or the Community Conservation Plan, and at the national level by the National Conservation Plan and the Decree of the Minister of Agriculture on. bi-environmental fish, crucial to the economy. In addition, as befits a species permitted for angling and fishing, it is protected by conservation periods and dimensions, catch limits, minimum mesh size of fishing nets, , possibly reserves and parks.
In the article, I used, among other things. From the works:
[1] Dębowski P., Bernaś R., Skóra M. & Morzuch J. (2016). Mortality of silver eel (Anguilla anguilla) migrating downstream through a small hydroelectric plant on the Drawa River in northern Poland. Fisheries & Aquatic Life, 24(2), 69-75.
[2] Samek A. (1992). Natural history covering animals description…. KAW, Warsaw.
[3] Arai T. (2020). Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798. Heliyon, 6(10).
[4] Durif C.M.F., Arts M., Bertolini F., Cresci A., Daverat F., Karlsbakk E. … & Browman H.I. (2023). The evolving story of catadromy in the European eel (Anguilla anguilla). ICES Journal of Marine Science, fsad149.
[5] Gross M.R., Coleman R.M. & McDowall R.M. (1988). Aquatic productivity and the evolution of diadromous fish migration. Science, 239(4845), 1291-1293.
[6] Righton D., Aarestrup K., Jellyman D., Sébert P., van den Thillar, G.E.E.J.M. & Tsukamoto K. (2012). The Anguilla spp. migration problem: 40 million years of evolution and two millennia of speculation. Journal of Fish Biology, 81(2), 365-386.
[7] Szmidt P. (1955). Fish migration. KiW, Warsaw.
[8] Fish migration. (1976). w: Little zoological dictionary: fish. Wiedza Powszechna, Warsaw.
[9] Zalachowski W. (1997). Fish. PWN Scientific Publishers, Warsaw.