Chinook and Jack Salmon – Why Are Chinook and Jack Salmon Biting Off at Bonneville Dam?

Fishery biologists at Bonneville Dam are struggling to comprehend why spring chinook counts have reached record-setting heights this year. A large part of this run appears to be 3-year-old males that return early for reproduction – something fishery biologists cannot account for without further investigation.

Scientists speculate that jacks use an internal assessment of ocean conditions when making their fateful decision to spawn early.

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Each spring, thousands of salmon (jacks included) migrate out into the ocean for one or more years before returning home rivers to spawn and begin breeding. Males typically become mature spawners at four years of age.

However, some female salmon reach sexual maturity earlier than expected – this phenomenon is known as precocious maturation.

Don Larsen of NWFSC’s Biological Sciences program is conducting extensive studies of these mini-jacks to gain more insight into their early development, and determine whether hatcheries contribute to higher rates of early maturation than naturally occurring populations. He and his team collect plasma samples from every spawning male salmon they catch to measure levels of 11-ketotestosterone which indicates precocious maturing male salmon.

Their study revealed an interesting phenomenon; jacks’ sperm outperformed that of dominant hooknose males. It is speculated that this advantage lies with its ability to locate eggs faster, and various factors are believed to contribute to it.

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Jack salmon utilize their life history tactics to gain access to females without competing with full-size males for mating rights, taking up satellite positions around a herd and sneaking up to release their sperm as eggs are laid. Furthermore, these smaller fish mature at an earlier stage than full-size males so are less vulnerable to marine mortality [5].

Although reproductive success (RS) for jacks versus full-size males varies among populations, jacks can still contribute significantly to wild salmon spawning potential. Studies conducted at Auke Creek Coho population show jacks contributed over 23% of adult returning offspring with single hereditary traits governed both environmental and genetic influences [6]. By comparison, hatchery-origin males fare significantly worse in this same population [7-9].

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Some male chinook salmon reach much larger sizes than their spawning partners, earning them the name of “jack salmon.” Jacks typically display humpback backs, long jaws with sharp teeth, a darker hue with greenish undertones, and dark coloration with greenish tintings – they may fetch as much as $100 at roadside sales!

Even though jack salmon contribute significantly to future generations of salmon, they are frequently undersampled in enumeration surveys (e.g. counting towers, temporary weirs, aerial surveys). Their small size often results in them being disregarded when estimating returns from wild populations.

Scientists understand that jacking is determined by both additive and nonadditive genetic effects; however, their work could eventually help improve salmon population forecasting accuracy.