THE ANIMAL FILES

Rainbow trout

Oncorhynchus mykiss

A river resident and a sea traveller can share one species name.

Rainbow trout include freshwater residents and migratory steelhead forms.[1]

Follow the evidence ↓
Rainbow trout
Image: Jonathunder · Commons · CC BY-SA 3.0
More than one life history
Resident and migratory forms[1]
Gravel as a nursery
Eggs settle between gravel[1]
Colours change with context
Silver forms can lose the obvious stripe[1]

More than one life history

Some rainbow trout remain in fresh water, while steelhead forms move between marine feeding areas and freshwater spawning sites. Smolting prepares young fish for the transition toward sea life.[1]

Gravel as a nursery

Females prepare a gravel nest by moving material with the tail. Eggs enter the spaces between stones and are covered as more gravel is displaced. Newly hatched fish initially use a yolk sac before feeding outside it.[1]

Colours change with context

Freshwater adults often carry dark spots and a reddish lateral stripe, while lake and migratory forms can be much more silvery. Juveniles show vertical parr marks.[1]

A COMMON MIX-UP

Is steelhead always a separate species from rainbow trout?

No. Here it names a migratory life-history form within the rainbow trout species. A common name can describe a way of living as well as a taxon.[1]

TRY A LITTLE REASONING

Why should a spawning survey examine the gravel as well as adult fish?

Reveal the explanation

Eggs and newly hatched stages use the stream bed. Adult presence does not by itself show whether the conditions needed for those stages are available.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Rainbow trout (cited revision)
  2. NCBI Taxonomy — Oncorhynchus mykiss

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