This year’s coho salmon run is coming to a “disappointing end” after a robust start early this winter, according to Eric Ettlinger, the aquatic ecologist for the Marin Municipal Water District, a result he attributed to above-average ocean mortality. The coho have made 110 gravel nests, known as redds, so far this winter; that’s about half of the average. (Counting redds is the main way the water district tracks the fish.) But coho live in three-year cycles, he said, so it’s critical to consider the redd numbers in light of this cohort’s past. (The fish use their tails to make nests in the creek where they lay eggs; those hatch, then the young grow in fresh water, migrate to the ocean and eventually return to spawn the next generation.) Three years ago, adult coho made 137 redds. That means the number of nests didn’t drop drastically. But Mr. Ettlinger was disheartened because in early 2013, those 137 redds produced a high number of smolts, or young fish—7,500—that left the freshwater streams in the Lagunitas Creek watershed for the open ocean. He hoped they would return in force late last year to make lots of new nests. But according to district calculations, it appears that just three percent of those smolts survived. (Eight percent is considered average in California.) He concluded that many fish must have died in the ocean. The district works to provide habitat and off-creek havens from high flows and predators for the young fish, but it cannot control what happens to the coho once they leave the watershed and spend over a year in the Pacific. “The only thing we can control, the only thing we have an influence on, is the number of fish that leave Lagunitas Creek, so we’re trying to maximize that,” Mr. Ettlinger said.