Trout and sea trout: one, three or dozens of species?

Evolution is still going on. We can become painfully aware of it by falling prey to germs that are insensitive to the antibiotics we ingest. Or by observing massive insect attacks on agricultural and forest plantations, newly immunized against pesticides used in the EU.

Trout and sea trout through the eyes of anglers, naturalists and… minister

High school textbooks illustrate the modern effects of natural selection efforts with the example of the birch cricket moth, in which melanistic forms adapted to the dust-blackened trunks of birch trees in industrialized areas have emerged. Equally instructive, yet overlooked in the education program, are the present-day transformations of salmonids proper and pee-eaters. Let’s discuss them using the example of brook trout, lake trout and sea trout (sea trout). These are highly sought-after and seemingly well-known fish. They are valued by anglers. They are loved by gourmets.

For conservationists, they are living indicators of the state of the environment, and at the same time keystone species. Their role in economy and culture is confirmed by legal acts, especially a whole separate Decree of the Minister of Agriculture of May 2021 dedicated to watercourses for sea trout and European eel. The legislator treats the three ecological forms (morphs) of sea trout as three species, although according to many researchers they all represent one species. According to many studies to date – despite striking differences in lifestyle – there are no noticeable genetic differences between them!

Trout, salmon and something in between….

It is generally accepted that Salmo trutta is a single species breaking up into several forms that differ in biology and ecology, yet are mutually prolific. Only in the ice age did it separate from the related Atlantic (noble) salmon Salmo salar,when reproductive isolation joined geographic isolation. Already hundreds of years ago, it was noticed that in some rivers and lakes of Europe appeared a fish with characteristics exactly intermediate between the typical brook trout and the typical Atlantic salmon. It was very appropriately called Lachsforelle, which Kluk interpolated as salmon trout. The father of modern classification of organisms and binary nomenclature Linnaeus – though a creationist – explicitly called it an intermediate link between trout (forelle, fario) and salmon (lachs) [7, 9].

Brown trout from mountain streams were described by Linnaeus as a separate species Salmo fario. He classified fish similar to them, flowing into the sea every year, as Salmo eriox. He also remembered a third fish, very similar to the two mentioned above, but not identical, found in lakes. He named it Salmo lacustris.

All of them differ from the noble salmon: A more stocky, cylindrical habit; the arrangement of the bones of the gill lids; the triangular toothing of the blade of the blade (in salmon the blade will be pentagonal and not toothed); a wider tail cap; the number of scales from the lateral line to the adipose fin (14-19 in sea trout, 11-15 in salmon); a less developed lower jaw hook in spawners; and details of coloration and skeletal structure. In the 19th and 20th centuries, they were treated as a separate species of Salmo trutta, called trout or trout. Note how subjective, smoothly transitioning these characteristics are! Later genetic studies confirmed that Atlantic salmon interbreed with sea trout, hence the presence of individuals that cannot be precisely classified [1, 4].

One? Three? How about dozens of species?

Later ichthyologists have either broadly or narrowly embraced Salmo trutta. In the first view, S. trutta is a species widely distributed in Europe, Asia Minor and Central Asia, and northern Africa, and introduced on all other continents except Antarctica. In the second, narrower view, S. trutta is actually a swarm of small species, endemic to particular river basins and lakes. Separate species will be the much-loved by Polish anglers marmorata Salmo marmoratus from Slovenia, carped S. carpio from Garda, and at least five species endemic to this Balkan Tanganyika – Ohrid:

1. Balkan salmon Salmo balcanicus,
2. Salmo lumi salmon (lumi trout) ,
3. S. aphelios salmon ,
4. Letnica (ochrid trout) S. letnica,
5. ochrid salmon (beluga, beluga) S. ohridanus [19].

Salmo trutta caught in Poland is considered to belong to one nominative subspecies, but belong to three ecological forms, which are also morphs (color forms):

1. Sea trout(Salmo trutta morpha trutta), which spends most of its life in the Baltic Sea, but trumpets in rivers and mountain streams;
2. The brook trout(Salmo trutta morpha fario), a sedentary (tuvad) form that does not leave trout country in the mountains and is sometimes introduced into lowland streams;
3. Laketrout (Salmo trutta morpha lacustris), which live permanently in lakes and sometimes enter tributaries to spawn [2, 5, 6].

The power of evil against one! But up to three times a piece!

A different life cycle means slightly different mechanisms of disappearance. Since all three ecotypes need different protective measures, the legislator was right to subject them to separate regulations (protective period and size, daily catch limit, other detailed rules for angling, designation of watercourses particularly important for the preservation of the stock, e.g.: needing to be made unobstructed).

Let’s protect sea trout from the Tatras to the Baltic Sea, from Wdzydze to Wigry

The most significant lake trout refuge in Poland remains the Kashubian sea – Lake Wdzydze (Szerzawa) with its tributaries Trzebiocha, Wda and Pilica, located in Bory Tucholskie, in the Kościerskie district of Pomorskie Voivodeship. The population from Wdzydze remains valuable in that it is fairly original. Meanwhile, the Baltic sturgeons of the Oder and Vistula rivers are descended from specimens from Canada (more precisely, from the St. Lawrence River), the famous Atlantic salmon of the Drava River originate from the Dvina (Daugava), and the Pejspusk (Vištytis) whitefish are found in Wigry, since the true Wigry whitefish have become extinct (dissolved into the gene pool of the Pejpus ones). S. trutta m. lacustris is preserved in the Hancza and Shurpils.

In contrast, it has disappeared in Wigry, Bialy Wigry, Sajno or Sajenko. It had already disappeared from Wigry during the establishment of the national park in the 1990s, at the same time as the much more common – seemingly – catfish. It returned thanks to stocking and biomanipulation. Since Hańcza is a reserve and Wigry a national park, all the more so Wdzydze – as a source of domestic lake trout for the renaturalization of both lakes of the Suwalsko-Augustowskie Lake District – should be at least a reserve and/or a protection circuit [13, 14].

Brook trout and lake trout are typical examples of species completely dependent on stocking. Hence the Conservation Dependent category on the red lists. The same can be said of eel, salmon, steelhead and even grayling. Protecting sea trout will be one of the most important activities of the new LIFE for Seaside project (LIFE Przymorze). The proposal was submitted in September 2024. LIFE Przymorze plans, among other things, to protect the last remaining spawning grounds and restore historical ones in the PLH320017 Natura 2000 Trzebiatowsko-Kołobrzeski Pas Pas Nadmorski area, reduce tourist pressure, limit sewage discharges, as well as more effective anti-poaching measures [1, 2, 6].

In the article, I used, among other things. z:

1. Bagliniere J., Maisse G., Watson J. 1999 Biology and Ecology of the Brown Sea Trout. Springer Praxis Books.
2. Bartel R. 1988 Trouts in Poland. Pol. Arch. Hydrobiol. 35, 321-339.
3. Borzęcka I. 2010. Classifying Vistula and Pomeranian sea trout populations using discriminant functions based on selected scale characters. Arch. Pol. Fish., 18, 123-131.
4. Charles K., Guyomard R., Hoyheim B., Ombredane D., Baglinière J. 2005. Lack of genetic differentiation between anadromous and resident sympatric brown trout (Salmo trutta) in a Normandy population. Aquatic Living Resources, 18(1), 65-69.
5. Cios S. 2003. Remarks on the occurrence of trout, sea trout, salmon and grayling in Polish waters in former times. PZW Scientific Yearbooks, 16, 17-32.
6. Cios S. 2008: is there a problem, or isn’t there? Has the sea trout already displaced the trout? Prz. Fish, 33, 5, 42-45.
7. Kluk J. 1780. Domestic and Wild Animals, Peculiarly Domestic, Natural History of the Origins and Management: the Necessary and Useful of Domestic Animals, Raising, Propagating, Curing Diseases, Catching Wild Animals, Taming, and Consuming them: Harmful while Exterminating. Printing House of the XX Scholium Piarum, Warsaw.
8. Radtke G., Dębowski P. 1996 Trout from Lake Wdzydze, Salmo trutta m. lacustris L., in the years 1951-1995. Zoologica Poloniae, 41(Supl.), 99-104.
9. Radtke G., Dębowski P. 2016. spawning nests of sea trout from Lake Wdzydze as an indicator of its population status. PZW Scientific Yearbooks, 29, 5-21.
10. Sušnik S., Snoj A., Wilson I., Mrdak D., Weiss S. 2007 Historical demography of brown trout (Salmo trutta) in the Adriatic drainage including the putative S. letnica endemic to Lake Ohrid. Molecular Phylogenetics and Evolution. 44 (1): 63-76.