Finally water, finally sockeye
This fall it felt like summer just wouldn't quit. After one of the driest summers on record the high pressure and dry weather persisted stubbornly on the coast through September. In a typical fall, we see sockeye spawning on the Central Coast starting around mid-September and peaking near the end of the month. On one tributary of Namu Lake which we've counted every year since 2011 fish normally begin spawning in early-September and we expect to see between 500 and 800 sockeye spawning by the end of the month. Not so this year. By September 25th fewer than 100 fish had entered the tributary, and only 26 sad looking fish remained alive in the few remaining pools of the drought stricken creek. The rest had been eaten by hungry bears and eagles who were presumably feeling the scarcity of salmon. The low initial numbers as well as a trend towards poor returns of sockeye coastwide this year had us worried that the dismal counts we were seeing were part of a serious trend towards population collapse. Fortunately it seems we were wrong.
After weeks of watching the weather we finally got some serious rain, and the sockeye responded in turn. The next day as the rivers dropped we walked the same tributary of Namu Lake where we'd seen just 26 live fish two weeks before. This time the river was full of life, with splashing sockeye jockeying for position on the spawning grounds, and a frenzy of predators and scavengers converging on the creek to partake in the bounty. In total we counted 1157 spawning sockeye that day, a drastic improvement from the weeks before, bringing the total estimated spawner escapement for Namu a lot closer to what we might call a 'normal' run.
What we saw this year speaks to the way that salmon have evolved to confront and respond to variation in climate. While the trend is towards warmer, drier summers and early falls, over the previous 10,000 years there have almost certainly been warm, dry years like this one. Fortunately for the sockeye, once they are in Namu Lake they can mediate the temperature they experience by moving up or down in the water column. Thanks to stratification the surface of the lake can be as warm as 18 or 20 degrees C in the fall, while the water at 15 meters depth is much cooler, typically 6-8 degrees C. As ectotherms (cold blooded animals) they can slow down their metabolism or their sexual maturation by simply moving into the colder waters in the depths of the lake. Of course they can't wait forever, so finally getting rain was a relief, both to the fish and the people who count them.
We're mostly wrapped up counting for the season now. We have one more round of counts and a trip to Koeye Lake to put everything away for the season, and then it's back to the lab to analyze the movement and survival data we've been collecting over the last three seasons. Stay tuned for more updates, and keep doing your rain dance of choice, the fish could use a little more water yet.