Editor’s Note: This interesting piece and link comes from our Program Chair and fisherman of the world, Ross Beatty. He brings our attention to an unusual phenomenon in steelhead and salmon reproduction. It’s all about perpetuating the genes for the future! LKH
On a recent steelhead trip I caught a juvenile steelhead around 16” long. The guide referred to it as a “precocious parr” because it was sexually mature and had never gone to sea. I thought this was interesting, so I did a little googling and found an article by Douglas Watts, who has studied this behavior in Atlantic salmon. I have included an out take where he talks about how these parr have found a way to successfully mate with mature salmon. It is a long paper, so I doubt most people would want to read it through, but it is a phenomenon that I wasn’t aware of before…. Click here for the link to the entire paper.
A fascinating aspect of Atlantic salmon is the existence of precocious parr.
Douglas Watts – retrieved 2/5/2018 from the above link
Atlantic salmon live in the streams where they were born until they are two years old. In the spring of their third year, when they are 6-7 inches long, they turn bright silver, their kidneys and other organs undergo a profound change to let them live in saltwater, and they head out to sea for two years, whereupon they return to their natal stream to mate and spawn as 8-15 pound adults. But some male Atlantic salmon take a different path. In the fall of their second year, they become sexually mature while still only the length of a dollar bill. Because baby salmon in freshwater are called "parr" (a very old Scottish word), these prematurely sexually mature males are called precocious parr.
Like many animals, male Atlantic salmon aggressively compete with each other for the right to mate with females. Male salmon compete by "claiming" a female as she is digging her nest and then trying to drive all other interested males away from the nesting site. While male salmon do not bite each other, they will use their heads and snouts (which become curiously enlarged and curved at spawning season) as battering rams to "head butt" a particularly obstinate competitor. As a rule, the larger male tends to win these competitive displays and the smaller male (or simply less aggressive male) moves away to find another available female or to wait on the sidelines for a rematch. In cases where three or more males are vying for one female, these competitive matches are tumultous, with the male salmon chasing each other up and down a pool and in the shallows. This frenzy can continue for several days, especially if additional males arrive in the area after being driven off by other males at nests up or downstream, or as fresh males arrive from the ocean. The male battles only end when all the females in the area have spawned.
Enter stage right our little friends, precocious parr. Precocious parr are sexually mature male Atlantic salmon, but are only the length of a dollar bill. They are only two years old (rather than four), and have never gone to the ocean. Adult male salmon which have gone to sea and back are big fish, anywhere from 28-44 inches long and weighing from eight to 40 pounds. They have swam from their home rivers more than 2,000 miles to their marine feeding grounds near Greenland and back, growing from 7 inches long to nearly 3 foot long in just two years. Most of their compatriots on this long migration did not survive, but were eaten by larger ocean predators at some point in their journey. These large males are the veterans, the survivors, are in the peak of condition, have a tummy full of milt and only one objective: to win a female salmon against all competitors and to pass along their genetic legacy.
So how does a precocious salmon parr that weighs a few ounces and is barely the length of an adult salmon's tail have any chance of competing for and winning a female? Isn't this totally wacky? Precocious male salmon parr do this by using their tiny size as an asset. Their secret weapon is as comical as it is effective. Here's the secret: during all of the time the big, giant male salmon are chasing each other around, fighting and vying between each other for "possession" of the female salmon and her nest, the precocious parr wait in the wings for the big males to be preoccupied with fighting other and then stealthily swim into the nest itself and sidle up alongside and underneath the female's abdomen, much like how a remora swims underneath the belly of a shark. Then they wait.
The only time precocious parr mate with a female salmon is when there were also large males around, and a large male with the female. In these cases the precocious parr sidles up underneath the female's belly, waits for the female and large male to simultaneously emit eggs and milt and then the precocious parr emits his (much smaller) package of milt at the same time, which then settles into the nest with the eggs and the large male's milt. Interestingly, even though the tiny parr has a lot less milt to squirt onto the eggs than the large male, his abdomen is much closer to the eggs, because he positions himself underneath the female which places him just a few inches above where the eggs are deposited.
Photo from USGS.gov