Pacific Science Center

Bringing science to life.

BOLD: “In Our Home Waters”

Introduction

Puget Sound receives freshwater from a variety of streams and rivers, varying in size, flow regimes, and fish communities. The majority of these waterways host populations of one or more of the salmonid species, including salmon, trout, and char species native to the region. Given the complexity of ecology and behavior shown by the native salmonid species listed below, it is essential for scientists to correctly identify them. The species can be difficult to classify during certain stages in their life histories, despite the significant differences in their behavior and ecology. The species highlighted in BOLD: “In Our Home Waters” are compared as follows, including the juvenile stage they are most often confused:

  • Pink & Chum Salmon- smolt
  • Coho & Sockeye Salmon- smolt
  • Cutthroat & Rainbow (Steelhead) Trout-smolt
  • Dolly Varden & Bull Trout-parr

In his art, Joseph Rossano introduces the cutting-edge technology of DNA barcoding for biodiversity science by engaging the public in the beauty of the fish species and the complexities of their identities.

Pink & Chum Salmon– smolt

Pink Salmon (Oncorhynchus gorbuscha)

Pink Salmon (Oncorhynchus gorbuscha)

Chum Salmon (Oncorhynchus keta)

Chum Salmon (Oncorhynchus keta)

Juvenile, or smolt, pink (Oncorhynchus gorbuscha) and chum salmon (O. keta) are easily mistaken for each other in the field for many reasons. Both migrate to sea at or shortly after they emerge from the stream gravel where they were incubating through the winter. They are both slim in shape, adapted for swimming in open water.

The pink salmon are initially smaller than chum salmon because the eggs of pink salmon are smaller than those of chums. However, the pink salmon grow faster and soon equal the size of the chum salmon. The species co-migrate along the shorelines of Puget Sound, coastal British Columbia and southeastern Alaska, where they abound. During juvenile stages in their early marine period, the two species still look very much alike. It is not until they are full grown adults that pink salmon develop their distinguishing features: smaller scales and dark spots on their tails and backs.

Coho & Sockeye Salmon- smolt

Coho Salmon (Oncorhynchus kisutch)

Coho Salmon (Oncorhynchus kisutch)

Sockeye Salmon (Oncorhynchus nerka)

Sockeye Salmon (Oncorhynchus nerka)

Coho salmon (Oncorhynchus kisutch) and sockeye salmon (O. nerka) smolts can also be challenging to distinguish from each other. Individuals of both species commonly reside in freshwater for a year prior to seaward migration. The sockeye salmon reside, with few exceptions, in lakes, while the coho salmon are typically found in streams. Sockeye salmon tend to have slimmer bodies than the coho salmon but as the coho undergo the remarkable transformation from parr to smolts, readying to migrate seaward, they become slimmer as well.

Coho salmon have a series of bars and stripes used in territorial displays directed towards members of their species, which make them very recognizable in streams. However, both coho salmon and sockeye salmon smolts become silvery and so the species converge in color as well as shape as they migrate to sea. In addition, these smolts migrate downstream at about the same time of year, and become difficult to identify before the coho salmon develop their spots as adults. Sockeye salmon remain unspotted in adulthood. Notwithstanding the similarity in appearance, the marine distributions of the two species are very different: coho will remain primarily in coastal and inland waters, while the sockeye migrate to the open ocean.

Cutthroat & Rainbow Trout- smolt

Cutthroat Trout (Oncorhynchus clarki clarki)

Cutthroat Trout (Oncorhynchus clarki clarki)

Rainbow Trout (Oncorhynchus mykiss)

Rainbow Trout (Oncorhynchus mykiss)

Coastal cutthroat trout (Oncorhynchus clarki clarki) and anadromous rainbow (steelhead) trout (O. mykiss) commonly spend 1-3 years rearing in streams before migrating to sea as smolts, though representatives of these species often do not migrate to sea at all. As newly-emerged fry the two species are entirely indistinguishable from each other by external examination. Biologists traditionally make no effort to identify them as anything but “trout” until the end of their first summer in freshwater.

As the fry grow larger they become somewhat easier to distinguish: the cutthroat trout tend to have longer jaws, extending back past their eyes, and develop the red color under their throat that gives them their name. The spotting patterns often differ as well, but spotting varies greatly in individuals of both species. Even as smolts the two species are difficult to distinguish.

Adding to the challenge, the two species do hybridize naturally and the crosses may resemble either the male or female parent or seem intermediate. However, their visual similarity does not reveal their considerable differences in range and behavior. The cutthroat have much shorter marine migrations than the sea-run rainbow trout or steelhead, as they are commonly known. Coastal cutthroat trout are confined to North America, from northern California to southeast Alaska whereas steelhead are distributed farther south in California, and all around the Pacific Rim to Russia.

Dolly Varden & Bull Trout – parr

Dolly Varden (Salvelinus malma)

Dolly Varden (Salvelinus malma)

Bull Trout (Salvelinus confluentus)

Bull Trout (Salvelinus confluentus)

Dolly Varden (Salvelinus malma) and bull trout (Salvelinus confluentus) are commonly referred to as trout, but they are more properly called char, sometimes spelled charr. Fish in this genus have light spots on a dark background, in contrast to the dark spots on a lighter background that characterized Pacific salmon and trout. These two species were not distinguished by scientists until 1978, when bull trout were recognized as different from Dolly Varden.

The Dolly Varden are distributed much farther north than bull trout and only Dolly Varden are native to Asia. However, the species’ distributions overlap and the visual similarities of juveniles make them very difficult to distinguish from one another. Both reside in streams and lakes, and both migrate to sea in some populations, further complicating their study.

Written and researched by:

Professor Thomas Quinn
School of Aquatic and Fishery Sciences
University of Washington
Seattle, WA 98195

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