In a previous article in Fish Bytes, I wrote about the five different migration

patterns of bony fishes with emphasis on those having the anadromous

type as demonstrated by seven species of the Genus Oncorhynchus;

Chinook, coho, chum, pink, and sockeye (Pacific Ocean salmon) and

steelhead (rainbow trout) and sea-run cutthroat trout. Worldwide, native

species of the genus Oncorhynchus are associated with the North Pacific

Ocean and the coastal regions of North America and Asia. During the

Miocene epoch, about 15-20 million years ago, a common ancestor of all

the extant species in the genera Oncorhynchus and Salmo (e.g.,Atlantic

salmon and brown trout) diverged into two groups; one group evolved in

the North Pacific Ocean to become the genus Oncorhynchus and the other

group evolved in the North Atlantic Ocean to become the genus Salmo. By

10 million years ago an Oncorhynchus ancestor had diverged into two

lineages, one lineage undergoing speciation leading to today’s Pacific

Ocean salmon species. This was completed by the end of the Miocene, 5

million years ago. The five species of eastern Pacific Ocean salmon had 5

million years since the end of the Miocene for intraspecific diversification

much of which occurred from glacial refugia during the Pleistocene (from 2

million to 10,000 years ago) and in the constant changing ecosystems.

The other lineage became the North Pacific Ocean and Western North

America trout species. The trout lineage led to two evolutionary lines; one

became the rainbow trout lineage and and the other became the cutthroat

trout lineage. Through geographic and reproductive isolation, each of these

trout lineages has undergone further evolution and genetic divergence to

produce subspecies. An important feature of the five species of eastern

Pacific Ocean salmon is, to date, no species has diverged into subspecies.

This indicates anadromy provided each of the Pacific salmon species with

the opportunity for genetic variation into specific stocks and populations

within a species. However, sufficient reproductive isolation and natural

selection has not occurred within a Pacific salmon species’ geographical

range that would be necessary for the evolution of subspecies . Because

straying does occur during the spawning migration, a mature adult salmon

does not necessarily always undergo homing to its natal stream. The

2. common feature is the salmon life cycle involving anadromy with the North

Pacific Ocean providing food and room for growth and fresh water for

more effective spawning with protection of fertilized eggs and alevins in the

gravel of a redd. Although the species of Pacific salmon in the western

North Pacific Ocean and eastern Asia may intermix with the same species

of the eastern Pacific Ocean salmon, they segregate during the spawning

migration to the home stream.

Whether a Pacific salmon species is likely to evolve into subspecies during

the current Anthropocene epoch, positioned within the Holocene, remains

to be seen. This unofficial geologic time period, perhaps from the middle of

the 1800s, began with industrialization. There has been the increasing

global human population’s activities and its demands on the use of

ecosystem resources. The more recent significant impacts on the earth’s

climate and ecosystems could create conditions where subspeciation

might occur. Some salmon scientists consider the kokanee, a

non-anadromous sockeye salmon, as a subspecies. However, there is no

such official designation with a subspecific epithet for kokanee, i.e., the

kokanee remains with sockeye as O. nerka. Examples of subspecies are the

coastal rainbow trout/steelhead, O. mykiss irideus and the interior

watersheds’ redband trout/steelhead, O. mykiss gairdneri. Similarly, the

coastal cutthroat trout/sea-run cutthroat, O. clarki clarki and the west slope

cutthroat, O. clarki lewisi.

Clearly, the Pacific salmon have a long evolutionary history that is in parallel

with the geological history of their historical range. The future history of

these species now depends on the magnitude of changes to their

ecosystems during the Anthropocene. Salmon have proven to be resilient

organisms, but resiliency has its limits when an organism can no longer

adapt to relatively rapid changing features within otherwise life supporting

ecosystems.

December 22, 2021

David Beatty