Salmon eggs

Over 100 years ago, in 1910, workers began construction on the first of two hydroelectric dams that would eventually be built on the Elwha River, on Washington State’s Olympic Peninsula. Before the dams were built (the lowest just five miles from the river’s outlet on the Strait of Juan de Fuca), the Elwha was home to robust populations of several species of Pacific salmon. After the dams were built, most of the river was cut off from the ocean – salmon could no longer migrate back to freshwater to spawn, reproduce, and nourish the streams where they were born.

Last August, three years after the largest dam-removal project ever conducted in the U.S. began, the last section of the uppermost dam was demolished, and just weeks later, salmon were back in the upper Elwha.

Scientists anticipate that salmon will continue to follow their migrations upstream and recolonize the upper Elwha River; as they do so, some will encounter a mysterious population of fish living in Lake Sutherland, a small lake connected to the Elwha River by a creek that comes in above the location where the lower dam used to be.

Those mysterious fish are Oncorhynchus nerka, also known as sockeye salmon or kokanee. Sockeye salmon and kokanee are distinct populations of the same species that tend to either migrate to the ocean and return to freshwater streams to spawn, a trait scientists call ‘anadromy’ (sockeye salmon), or spend their entire lives in freshwater (kokanee). The population of Oncorhynchus nerka in Lake Sutherland was landlocked by the Elwha Dam for a hundred years, but researchers were not sure whether their ancestors were sockeye salmon trapped above the dam when it was built, or kokanee that might never have migrated to the ocean and back at all.

Now, a team of scientists believes they have the answer, and they came to their conclusion based on a humble clue – the size of the eggs the Lake Sutherland Oncorhynchus nerka produce.

As the researchers recently reported in the journal Ecological Research, they compared the Lake Sutherland fish eggs to eggs produced by several populations of sockeye salmon and kokanee from Alaska, Washington, British Columbia, and New Zealand. Kokanee eggs tend to be smaller than those of sockeye salmon (just as kokanee adults tend to be smaller than sockeye salmon adults). The Lake Sutherland fish themselves were typically about a foot long, the same size as the adults of the kokanee populations and half as big as the sockeye salmon adults.

Their eggs, however, were much larger than the kokanee eggs – and well within the range of the sockeye salmon eggs. The growth of the adult fish in Lake Sutherland appears to have been limited by their inability to access the ocean, and, based on the size of their eggs, it’s likely that the Oncorhynchus nerka in Lake Sutherland are descendants of sockeye salmon.

The scientists note that this has “immediate relevance to the restoration” of salmon in the Elwha River, because “it would mean that other traits linked to anadromy might also remain in the population, facilitating the resumption of anadromy in this population.”

Now that the dams on the Elwha River have come down, several species of salmon will once again be able to migrate upstream to spawn – and some Oncorhynchus nerka may finally be able to make it downstream to the ocean, before returning to freshwater to start the cycle anew.

The Elwha Dam in October 2011, about a month after the removal project began. 

(Image by Sam Beebe via Flickr/Creative Commons license)

Seeds, pits, and eggs

When I think of biological dispersal, the first few examples that come to mind all involve plants and seeds – the white puff of a dandelion head scattered by a gust of wind (based on the main photograph illustrating the Wikipedia page on dispersal, I’m not the only one who thinks of dandelions first); cherry pits spread far and wide by the lucky birds who found a tree laden with ripe fruit; even the squash plants sprouting from seeds tossed in a backyard compost pile.

But many organisms, animals as well as plants, disperse themselves, or are dispersed by the environment in which they find themselves: wind, or water, or humans move them around.

A team of Japanese researchers recently published a study in the journal Ecological Research detailing the dispersal of a particular type of coastal stick insect found in Japan and Taiwan, Megacrania tsudai, also known as Tsuda’s giant stick insect. Members of the genus Megacrania are large insects, usually between four and five-and-a-half inches long as adults. Only female Megacrania tsudai are found in the wild – they reproduce via parthenogenesis, an asexual process during which the young develop from unfertilized eggs. Megacrania tsudai eggs look like tiny, brown potatoes (pdf), the right size and shape for a dollhouse dinner plate, and adult females lay them often – the insects the scientists studied typically produced one egg per day.

The researchers were interested in whether or not Megacrania tsudai eggs, which are buoyant in seawater, can survive extended periods of time in the ocean – as a coastal species, they suspected that the insect might use ocean currents as a means of dispersal. They found that even after over a year spent floating in seawater, eggs hatched at the same rate as unexposed eggs (about 60% of the eggs eventually hatched, regardless of how long they had been in the water).

They also found that prolonged exposure to seawater delayed development – eggs not exposed to seawater began hatching after 120 days, while those floating in seawater for over a year didn’t begin hatching in substantial numbers until after 150 days, and some eggs were still in a pre-hatch development phase after 200 days.

In their paper, the scientists discuss the dangers Megacrania tsudai larvae and adults face as a terrestrial species living in coastal areas that regularly experience typhoons, strong winds, and floods. “However,” they write, “one or a few M. tsudai eggs are laid every day, and these eggs possess seawater tolerance and have a high variation in egg period duration. All of these characteristics are advantageous for an insect species living in a coastal forest habitat. Therefore, even if adults and larvae are completely lost due to natural disturbances, the eggs are an effective means of recovering and maintaining their population since they hatch normally after seawater exposure[.]”

Dispersal is an important way for populations to endure in the face of challenging local conditions. Megacrania tsudai eggs, just like dandelion seeds and cherry pits, can survive voyages beyond the realm of possibility for their progenitors.

Adult Megacrania tsudai on screw pine leaves in Taiwan; screw pine leaves are a primary food source for the insect. 

(Image by Bettaman via Flickr)