During most of their lives, eulachon (Thaleicthys pacificus) spend their days at a depth of about 100 metres, just off the sandy bottom of the continental shelf, eating krill and larval shrimp and trying not to get eaten themselves, or caught in a shrimp-trawl net. In late fall, the mature four-year-olds start to school up, ready to move toward the spawning grounds.
Unlike salmon that home accurately to their natal rivers, eulachon home to a region—the one they imprinted on in their early days as they drifted in the sea near the mouth of the river where they hatched. By New Year’s Eve our school of eulachon, containing maybe five tons of fish—just one school of many—is dodging halibut 50 kilometres from the mouth of the river. These days the Nass River has the largest eulachon run of the 14 rivers in BC that have (or had) regular runs.
Now it’s March, and the eulachon are massing in the salt water that lies under the fresh water where the river meets the sea. While travelling, they’ve absorbed the minerals out of their long sharp teeth, which are now gone completely. Together with stored fat, the minerals are used to mature their gonads so eggs are ripe and free-flowing, and milt (sperm) is ready to blow.
Everyone is here for this party. Halibut and salmon are gorging on the oily fish. Seals, sea lions, otters, gulls, eagles, mergansers—even wolves and bears—are trying to get some, craving the nutrition after a long hungry winter. The Nass could have close to 2000 tons in a big run. In addition to what is eaten by other fish, the wildlife around the river might take several tons themselves. And those pesky humans will take another few hundred tons, leaving, in a good year, enough to replenish the stock.
Some males run upriver ahead of the others, and are said to be checking the conditions. Several runs will follow, using the tide—sometimes under the ice in February, sometimes later into April or May. But now, on a big rising night tide during the lowest river flows of the year, our school of eulachon surf up the river, using the reversed flow to help them up as far as possible. Eulachon, like other anadromous fish, try to prevent marine predators from preying on their eggs, but aren’t strong enough to venture very far above the tidal influence.
A frenzy of sexual activity begins. By the millions the eulachon locate glides—areas of faster moving current—to spawn. The females wriggle close to the bottom, accompanied by one or many males. The surface of the females is slippery and very smooth, while the males are bumpy and less slippery. This may help them identify who they are spawning with in the darkness. The males have a distinctive ridge down their midline and extraordinary ventral fins that may help hold the female to his side for the few seconds of their act.
Within a few hours, hundreds of tons of eulachon will have spawned, turning the entire river white, like watery cow’s milk. The females
disappear downstream immediately, but males stay around, likely hoping to spawn with any less punctual females and exhaust the last bits of their milt and energy before washing out to die.
The fertilized eggs become sticky and sink out downstream in clean sandy areas, forming a mat of eggs that resists being moved. The three-week incubation period may be the only peaceful time in an eulachon’s existence—that is if the egg is anchored well enough to survive the blast of spring runoff.
A few weeks later a tiny, half-centimetre-long larva hatches out at night to begin its perilous existence as a delicious food item. Fueled by a yolk sac that will last a few days, the little larva drifts downstream and begins to float around, together with millions of other larval fish and the rest of the microscopic life in the estuarine circulation. Over a period of days to weeks the larvae gradually grow and drift with the current away from the river; this is where they imprint to the region, rather than the site where they were born. Over the next months they gradually swim out to the rearing grounds on the continental shelf, where they will spend most of the four years of their life growing—and trying to avoid being eaten themselves—before returning to the river to start the process over again.