Sexy Salmon

I grew up in Alaska and have strong memories of going to rivers in the late spring/summer for the Pacific salmon, Oncorhynchus spp., runs. These salmon are anadromous – they migrate by the thousands up from the ocean to freshwater rivers to spawn (breed and lay eggs). They’re semelparous, meaning that after they breed, they die. Breeding happens late spring through fall, so I was surprised about the fall spawn of our native Atlantic salmon.

While Atlantic salmon, Salmo salar, are generally anadromous (again, migrating from saltwater to freshwater for reproduction), here in Vermont our salmon are all landlocked, meaning that juveniles remain in freshwater (e.g. Lake Champlain) and return to the sea (source).

Unlike Pacific salmon, they’re iteroparous (meaning they breed multiple times during their life). Though functionally most individual Atlantic salmon are semelparous, dying after just a single reproductive cycle – roughly 90% of the individuals spawning in a year are spawning for the first time. At maturity – around 3 or 4 years old for males, 4 to 5 for females – Atlantic salmon move up into the rivers surrounding their resident lake as early as September to breed. Breeding can occur as late as December.

Breeding takes place in cool, well-oxygenated streams. Females will clear out several shallow depressions in the river bottom, called redds, by beating her tail against the bottom to remove all silt and debris. These lighter colored redds can be seen pretty easily from the shore, contrasting with the darker river bottom. Eventually the female will lay her eggs in the gravel with a male at her side. The male simultaneously releases milt (seminal fluid that contains the sperm), fertilizing the eggs externally. The eggs incubate in the gravel for the next 5 or so months before hatching.

Males and females can be somewhat tough to identify by size and color, though most mature male Atlantic salmon have kypes, a hooklike projection on the lower jaw (this is visible on the larger salmon in the video above). The kype seems mostly to function as a signal to other males, and it appears that males with bigger kypes are more likely to breed (source, source). There are also about 3 males to every 1 female at the spawning grounds (source), so probably means that if you see a salmon, chances are it’s male.

With such skewed sex ratios, it’s maybe not surprising that there are different reproductive strategies amongst the males. In Atlantic salmon, there are sneakers and fighters (sneakers are male fish that “pretend” to be female to be nearby other females during copulation to “sneak” in some sperm. This only works because fertilization of the eggs is external). Having sneakers within a population acts on a check on fighter males (who fight each other to be near the female when she lays her eggs), effectively limiting the benefits of investing in larger body size. So when fighters are present, males tend not to get really big, and there’s less of a difference in size between males and females (source). But the farther north you go, the fewer sneakers there are, and as a result the greater the difference in size between males and females.