by Eric Ettlinger
Every year at this time the salmon and steelhead of Lagunitas Creek begin the most dangerous chapter of their life cycle: the marine phase. After growing for just over a year in the creek, coho fry begin migrating downstream to Tomales Bay. Juvenile steelhead migrate as well, after generally spending two or even three years in the creek. On the way both species become smolts, developing adaptations for salt water and changing their appearance from spotted and colorful to the silver coloration characteristic of ocean salmon. They will spend the next year and a half to two years in the Pacific Ocean, and the lucky ones (less than ten percent) will then return to Lagunitas Creek as spawners.
MMWD biologists have been monitoring this migration since 2006 to assess how many smolts the watershed is producing. From mid-March to June we operate a rotary screw trap near Point Reyes Station to catch some of the smolts as they swim downstream. The trap operates 24 hours a day, seven days a week, and biologists check it every morning. The trap consists of a rotating cone suspended between two pontoons. Fish that swim into the wide end of the cone are directed down into a large metal box. Baffles in the rotating cone prevent them from swimming back out. Biologists collect them in the box, weigh and measure them, and send them on their way. A subset of fish are marked and released upstream of the trap to estimate the percentage of fish being caught. This enables us to extrapolate the total numbers of fish migrating to the ocean. We’ve estimated that as many as 7,000 coho smolts are produced in Lagunitas Creek, although this year that number is likely to be less than 2,000.
Monitoring smolts has taught us that coho salmon numbers often decline sharply during the winter. We used to believe that the coho salmon population was limited by a lack of habitat during the summer, such as deep pools with lots of shelter. We are now refocusing our conservation actions to enhance the kinds of habitats coho fry need during the winter, such as calm backwaters and side channels. Hopefully these efforts will enable more salmon and steelhead smolts to migrate to the ocean, and in turn, increase the numbers of spawners coming back.