Salmon Reproduction: the Spawning Process

Right, guys and gals, hold on to your hats!  Today we’re talking about sex.
When you’re a salmon, your spawning strategies pretty much depend on whether you’re male or female.  Females compete with other females to acquire, prepare, and defend nesting sites.  Males compete with other males for chances to spawn with females.  This type of competition is called intrasexual, meaning within the same sex.  But salmon also compete intersexually (between sexes).  They do what’s called “mate choice”, where one individual picks his/her mate based on traits such as phenotype (“Look at how big that guy’s hump is!”) and behavior (“Aw man, her tail waggle brings all the salmon to the yard”).
Females generally invest more in their offspring—they put more energy in gonads and produce fewer gametes (eggs).  In fact, 20-25% of the female’s energy is devoted to gonadal investment.  The relationship between body size and egg size varies a great deal.  The reason for this is an energy “cap”.  The female can’t afford to use too much of her energy for reproduction…otherwise she won’t have enough energy for herself (and remember, migration is energy-expensive!)  So there are two reproductive strategies here: devote your energy to making big eggs or devote your energy to making lots of eggs.  You can’t get the best of both worlds (big and many eggs)—you simply don’t have the energy!  So basically, the female salmon choice is: bigger but fewer eggs (big eggs are more likely to survive) or smaller but many more eggs (the more eggs, the more chances for offspring survival…sort of a “flooding the market” strategy).  In populations, if mortality is size-selective, bigger eggs tend to be favored; if it’s not, more eggs tend to be favored.
Biologists like to say, “Sperm are cheap, eggs are expensive.”   And because eggs are expensive (i.e., the female puts in a greater investment), females are often “choosier” than males.  Of course, in salmon, the female can’t do all the choosing.  Once she picks a nest site (redd), she won’t move too far away from it.  So she can only choose from the males in her vicinity.  Meanwhile, males don’t choose spots; they are free to wander among the different redds.  In this way, they have some choice in the matter as well.
So Sally Salmon is coming up the stream.  She’s exhausted after migrating 900 miles and she just wants to find a suitable place to nest.  What is she going to look for?  She’ll generally want a site with knee-deep water, water velocity from 0.1-0.7 m/sec, and substrate with less than 10% of the substrate material less than 1mm.  If the substrate material is too small, her eggs will be covered and will suffocate.  If water velocity is too slow, the same thing will happen because the water won’t be transporting enough oxygen to the eggs.  But if the water velocity is too fast, the eggs might get swept away.  Similarly, if the water is too shallow, the sunlight and heat will destroy the eggs; too deep and the eggs probably won’t get the nutrients and oxygen they need.
Generally, pool tailouts are favored spawning sites. 
I have mad MS Paint skillz.
Sally’s found a suitable site…now what?  She begins digging behavior—laying on her side and using her tail to dig a hole.  This winnows out the fine material which might suffocate eggs and also produces a bit of a tailspill (like what you get when your puppy digs a hole in the garden: a hole and a pile of dirt next to the hole).  Digging behavior is also a signal to male salmon: “Hey!  I’m ready to spawn!”.  Sammy Salmon sees this behavior, thinks Sally is a pretty cute (and fertile) salmon, and comes over to mate.  There’s some chemical and physical communication going on between Sammy and Sally at this point, and it’s critical.  If Sally’s eggs are exposed to water too long, they become unviable (i.e., no babies possible).  Sammy’s sperm must be released right with the eggs so that fertilization can happen right away.  Sally and Sammy do a “mating dance”.  They wriggle up together and at the exact moment (hopefully) release the eggs and sperm.  It all happens in just a couple quick seconds.  (check out this video of spawning coho salmon at the UW Big Beef Creek research station)
Sammy swims off.  But this is only one egg pocket!  Sally still has more eggs!  She moves a tiny bit upstream and creates a new egg pocket (and the tailspill from the new egg pocket hides the old one, which is a bit of defense against predators, scour, etc) and then repeats the process with Stanley Salmon.  Over the period of a day or two, Sally will make multiple egg pockets, each with a decreasing number of eggs.
Why does Sally need more than one egg pocket in her redd?  Well, she might be physically unable to squeeze out all her eggs at once.  But aside from that, high egg density likely reduces survival, so spreading the eggs out is beneficial.  She also doesn’t want to “put all her eggs in one basket”, if you’ll forgive the axiom.  She’s minimizing risk by producing several pockets—that way, if one pocket has bad luck, she’s still got others that might survive.  Finally, this strategy allows for multiple mates.  This promotes diversity and also minimizes risk.  For example, what if Sammy had bad sperm?  If she had only mated with him, none of her eggs would produce offspring.  But because she also mated with Stanley, then even if Sammy’s sperm were bad, she still has a chance at producing offspring with Stanley’s sperm.
Well, Sally’s all spawned out.  Most of her energy is gone and she is getting ready to die.  How does she spend the last days of her life?  Nest guarding from other females, of course.  She put so much effort into that redd that she doesn’t want any other salmon messing it up.  There’s been no quantitative study on the benefits of nest guarding (so far), but it’s likely that it serves some purpose.  All females do it, and those that arrive early actually do it longer—i.e., the females that arrive earlier to spawn specifically set aside some extra bodily energy so that they can survive longer and protect their eggs from the females that arrive later to spawn.
After guarding her eggs for a few more days, Sally’s done her duty.  She dies, and her carcass enriches the surrounding environment, contributing substantial amounts of nitrogen and phosphorous (which are often limiting nutrients) to the stream environment.  And Sally’s eggs lay buried safe in the gravel, slowly consuming their egg sacs and developing into alevin.

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