History of The Brown Trout
Just when the divergence between the Atlantic salmon(salmo salar), and the brown trout(salmo trutta), occured is unknown, but there is large scientific evidence showing that they have a common ancestor. Recently found chromosome differences between the Atlantic salmon(56-58) and Salmo trutta(80) clearly separate the salmon and brown, but the similarity of the markings of the juveniles and parr (yearlings and fry) are strong evidence of a recent heritage.
Behnke’s recent research show that both brown and salmon have the same amount of DNA(weight). If the brown trout evolved by partial chromosome doubling, it should have 40% more DNA. Because the amount is approximately the same for both, it seems that the brown trout is the ancestral form and the Atlantic salmon is derived from it, by fusion of some of the chromosomes. The salmon has many long one-armed chromosomes, apparently a fusion of two-arm brown chromosomes.
The brown trout is an extremely polymorphic species. This fish adapts very quickly to the environment that it lives in. During the ice age (prior to the great flood), most of the British Isles and Northern Europe were covered with ice, and almost all freshwater fishes would have been eliminated in the regions of glaciation. There are a few isolated brown trout populations that are still present today up in the mountains. They have found these isolated distinct populations by measuring the genetic marker LDH eye enzeme.
The ice forced the brown trout southward, permitting these forms to invade the Mediterranean, Black, and Caspian seas. The glaciation was not continuous, however, and alternate advances and retreats of ice (along with the brown trouts repopulation in accesible locations) and separate geographical areas, most likely resulted in long periods of genetic isolation, allowing genetic differentiation to take place. Thus, the different types of brown trout.
CLASSIFICATION
There is considerable genetic diversity among brown trout populations of north-western Europe with any indiviual population containing only a limited part of the genetic variation present in the species(Freguson 1989). There are even distinctive, reproductively isolated sympatric populations present(in Sweden, Ireland, and Spain). This is why, at one time, there were as many as 50 separate “species” of brown trout.
Lets look at the history of the classification. Carlolus Linnaeus, in his historical work “Systema Naturae”, published in 1758, named three types of trout in Sweden by the binomial nomenclature now universally used; S.trutta, the trout of large rivers; S.fario, the trout of small brooks; and S.criox, the migraratory sea trout. Then in Gunthers 1866 “Catalog of Fishes in the British Museum” described 10 species of brown trout from the British Isles that deserve special attention because of recent phylogenetic and taxonomic studies.
These are; the river trout(s.fario), sea trout(S.trutta); great lakes trout(S.ferox), Loch Leven trout(S.levenensis), Welsh black-finned trout(S.nigripinnis) see picture, the Irish gilaroo,(S.stomachicus), the western sea trout(S.cambricus), the eastern sea trout(S.brachypoma), the Galaway sea trout(S.gallivensis), and the Orkney sea trout(S.orcadensis). By 1930 David Jordan, Carl Hubbs, and others argued that the separate species should all be accorded one name “Salmo trutta L.” and had been accepted by virtually all taxonomists as the proper name for the various forms and descriptions of brown trout. So, that’s the end of our story, wrong…
In 1932, L.S. Berg, a highly respected Russian authority of fishes of Eurasia, agreed that all forms of brown trout deserved to be recognized as belonging to one species, but saw enough differences in geographical subdivisions to recognize six subspecies stemming from geographical separation: northern and western Europe(Salmo trutta trutta), Black Sea and trib.(S.t.caspius), Caspian Sea and trib.(S.t.caspius), Mediterranean region(S.t.macrostigma), Lake Garda, Italy(S.t.carpio), and Sea of Aral and Amu Dar’ya River(S.t.aralensis).
INTRODUCTION TO NORTH AMERICA
Brown trout (salmo trutta) are not native species to North America. The first documented introduction og brown trout was on April 11, 1884, J.F.Ellis stocked 4,900 brown trout fry(von Behr strain) into Michigan’s Pere Marquette River. After this initial distribution in 1884, distribution of brown trout was swift and wide. The first North American introduction of Loch Leven trout, Salmo trutta levenensis (a lake form), appears to be made in Long Pond near Saint John’s, Newfoundland, in 1884. The sea run strain (S. t. trutta) was also introduced around this time but, the only currently known strain exists in Nova Scotia and Massachusetts.
After the first North American introduction of the Lock Leven (see picture) trout occured in 1884, brown trout were introduced into every province except Prince Edward Island. Incredibly, there was little or no attempt to keep the Lock Leven and the von Behr strains isolated or distinct. Goverment and private fish distribution records listed both types, but widespread shipment from one hatchery to another (crossbreeding), and the introduction of both strains into the same waters apparently resulted in the merging of the original distinguishing characteristics. Perhaps we should now discuss the differences between the two strains. . The von Behr trout, (S. fario) lived in small streams, were brightly coloured, and rarely exceeded the lenght of 12 inches.
In contrast, the Loch Leven trout, (S. levenensis), was a lake-dwelling form, silvery gray with black spots, reaching a size of 18 lbs. If, as reported, the von Behr and Loch Leven strains have been widely interbred and broadly distributed, and if the brown trout has a plastic genetic ability(polymorphic), I’m not surprised that North American brown trout are, in appearance and life history, similar to practically every form originally described in Europe. All three different types of brown trout were introduced into our waters during the 1800’s, but recently C. Krueger and B. May discovered that the populations are becoming genetically differentiated. This is extremely important for fisheries managers. They studied the allozyme data of brown trout from Lake Superior and came to some interesting conclusions.
The differentiation among hatchery stocks was 2.2 times greater than that observed among the 8 samples from wild populations. Similarly, the differentiation between the two groups (hatchery and wild) was also larger than that observed amomg samples from wild populations. They also found that the level of differentiation observed in Europe appeared to be greater than that observed in Lake Superior, but the amount of differentiation was highly significant because of their so recent introduction. The existance of multiple brown trout stocks in Lake Superior implied that reproduction-isolating mechanisms occur among populations of brown trout. They are also quickly adapting to their enviroment(polymorphic) through genetic selection.
Ryman (1981) noted a general tendency of brown trout from European waters “to aggregate into close and genetically distinct populations.” They concluded that the genetic differences could be due to 1) to rapid rate of population differentiation since stocking through the effects of small founding populations coupled with assortative mating based on precise homing behavior, or 2) to the partial preservation of the original genetic characteristics of the different European stocks that were introduced into the basin. Personally, I think that both factors are present.
By Wayne Sheridan
