NANFA-- "straying"

Jay DeLong (
Sun, 27 Feb 2000 20:47:57 -0800

> Another issue that has not received any DETAILED consideration is
> the extent to
> which straying ocurs in the wild. Pacific salmon, it is known,
> stray from one
> run to another, therefore mixing up the genes and keeping the runs strong.

That's what some would have you believe, but this assessment isn't accurate.
The definition of straying is when adult fish return to a stream other than
the one in which they were reared. Obviously, straying has occurred
historically because there wouldn't be anadromous fish in so many streams up
and down both coasts. Individual straying fish were the colonizers, and
over many generations their offspring adapted to the stream, forming the new
run (stock). But when a stream contains a unique stock, subsequent straying
to that stream (or of those fish to other streams) weakens the existing
stock rather than strengthening it. In nature straying rates are low--
probably 1-3%-- so natural hybridization occurs at very low levels.
Hatchery fish, however, can stray at significantly higher rates. This
occurs through several formerly common hatchery practices, such as using
out-of-basin eggs or fry. When these fish return as adults, they may not
return to the stream they were reared in, and instead seek out the stream(s)
their parents came from. Imprinting on water chemistry is not the only
factor that determines where a fish returns-- genetics is more important
than was once thought. And what often happens is that they enter other
streams in the area because of the conflicting cues.

Another cause of straying is that some salmon hatcheries use salt water
rearing pens, and the fish returning to these sites haven't imprinted on a
specific freshwater stream. So they mill around the pens until the urge to
spawn is great and they enter nearby streams randomly. If these streams
have a wild stock, the straying fish spawn with them.

Jay DeLong
Olympia, WA

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