Monday November 26, 2012

While many people have observed salmon swirling about in West Coast rivers during the spawning season, it’s not always apparent what is actually happening beneath the surface. Like many other mating animals, salmon exhibit spawning behaviors. Females travel up river to find an appropriate location to create their redds or nests. Once a female finds substrate of suitable size with adequate flow to provide oxygen for her eggs, she will begin creating her redd. Female salmon will turn on their sides and flick their tails several times to move the substrate downstream, piling it upon itself. This digging also dislodges and clears any fine sediment from the substrate, which can smother eggs. The female will guard this redd position within the stream until a suitable male approaches her and she will often fight off other females for this position.

Females frequently show size selectivity (Berejikian et al. 1997) in their male partners, selecting larger, more fit, dominant males over the smaller males. While this one-on-one pairing does happen, multiple males often attempt to spawn with a single female. Dominance is determined by the competing males (either through female selection or fighting between males), but the dominant male may not be the single contributor of milt (male gametes) to the unfertilized eggs. The non-dominant males are referred to as ‘satellite males’, and usually do not occupy a position within the redd, but move around in close proximity (Esteve 2005). While smaller males are less likely to compete for the dominant male position, they do fight other smaller males to ensure they are as close as possible to the redd (Berejikian et al. 2010).

In this video, the dominant, larger male positions himself close to the female Chinook salmon. He attempts to maintain close proximity to her, especially when she is releasing her eggs. By staying as close as possible to her, he is trying to prevent other males from fertilizing the eggs. However, this video documents how these satellite males “sneak” in close to the female to release milt just as she is depositing her eggs. Males that perform this action are referred to as “sneaker males”. These non-dominant, smaller males position themselves close to her vent, thereby increasing their chance to fertilize her eggs and pass on their genes. The dominant male can’t fight off all the competing males, thus offspring from a single redd can be fathered by more than one male. The sneaker male strategy is not particularly successful, according to research conducted by Berejikian et al. 2010. The researchers found that jacks (smaller males) were the last male to enter into a mating and only sired about 20% of the offspring, based on the genetic make up of the sampled offspring. The order in which males join the spawning event may affect their success, with the first adult male typically having better success in passing on his genes. But then again, 20% is better than nothing.

This post featured in our weekly e-newsletter, the Fish Report. You can subscribe to the Fish Report here.

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