When it concerns intrapopulational variation in run time and arrival at spawning site i have the impression that is is a portfolio effect that decrease the instability in the population. But could an early arrival to the breeding grounds also mean an advantage in acquiring a prime spawning position? Do you know of any studies on the subject?
I can't say anything specific about Atlantic salmon but they may be similar to Chinook salmon in this regard, although the fact that Atlantic salmon do not die after spawning may be a factor. For Chinook salmon, the female will guard the nest as long as she can but when she dies, there is nothing to stop another female from spawning in her site and potentially harming the eggs of the first female. So, spawning position is only important as long as the female can/will defend it. Also, spawning early is not always a good thing for Chinook salmon. It does mean that the eggs get the development advantage of the warmer early autumn water and subsequent earlier hatching and swim up. However, if the fry swim up too early in the spring, they can easily be flushed downstream by flood waters that they are too small to withstand. I'm not sure what you mean by "a portfolio effect that decrease the instability in the population."
When spawning time is restricted and males are polygamous, males typically arrive earlier than females in the breeding area as they do in salmon and trout (Jonsson & Jonsson 2011). This should maximize mating opportunities by males as hypothesized by Morbey 2002. She used arrival timing models to predict this under a range of competitive scenarios. For instance, the work by Anderson et al. (2013) indicates that early arrival on the spawning ground gives a selective advantage.
References
Anderson. J.H. et al. (2013) Reproductive success of captively bred and naturally spawned Chinook salmon colonizing newly accessible habitat. Evol Appl 6: 165-179.
Jonsson, B. & Jonsson, N. (2011) Ecology of Atlantic salmon and brown trout: Habitat as a template for life histories. Fish and Fisheries Series 33, Springer, Dordrecht, The Netherlands, 708 + xxii pp.
Morbey, Y.E. (2002) Protandry models and their application to salmon. Behav Ecol 13: 337-343.
Thank you both for great answers!
Timothy, with portfolio effect i mean that there may be several factors that makes the population less vulnerable to disturbances. A way to not put "all the eggs in one basket"
Cheers!
/Anna
Bror is correct. Trout and salmon males typically arrive on the spawning grounds earlier the females. here is a link to one of our old studies on sea trout showing this.
http://link.springer.com/chapter/10.1007/978-94-011-5090-3_31#page-1
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