by Eric Ettlinger
Every spring MMWD biologists count coho and steelhead smolts as they migrate from Lagunitas Creek to the Pacific Ocean. These smolts have survived despite floods, predators and competition for limited habitat, and their numbers tell us a great deal about conditions in the watershed. This year’s smolt emigration provided us with enormous insights, as well as renewed confidence that the Lagunitas Creek coho population is recovering after two years of very low numbers.
MMWD began monitoring smolts in 2006 using a rotary screw trap. The trap is essentially an aluminum raft with a large, rotating funnel that catches smolts as they’re swept downstream. The funnel has internal baffles so fish going into the funnel can’t come back out. Every morning from late March through May we remove fish from the trap and count, measure and release them back into the creek. We also mark up to twenty fish per day with a small fin clip and release them upstream to estimate how many fish we’re catching, and more importantly, how many we’re missing.
During the peak of the coho emigration in late April the trap was catching nearly 300 coho smolts per day, and it was clear that this emigration would be the largest yet seen. When the trap was removed at the end of May, we had captured over four thousand coho smolts, and we estimated that 8,315 coho had left Lagunitas Creek. Over the years we’ve gathered strong evidence that Lagunitas Creek couldn’t produce more than about 6,000 coho smolts, which it has done in three of the last seven years. How did Lagunitas Creek produce over 8,000 smolts this year?
The answer seems to be an unusually small steelhead population. This year’s steelhead emigration was smaller than average, and in the last seven years whenever steelhead numbers went down, coho smolt numbers went up (click here for graph). Steelhead seem to be able to outcompete coho for habitats that provide protection from high flows, shelter from predators and good feeding opportunities through the winter. Steelhead generally live in the creek for two years before migrating to the ocean (compared to one year for coho), and these second-year steelhead are larger and can likely dominate the best habitats. We couldn’t simply reduce our steelhead population to benefit coho, since both species are federally protected. We need to enhance habitats in Lagunitas Creek in ways that benefit coho more than they benefit steelhead. This may be the most important insight gained from counting smolts seven days a week, two months out of the year, for the last seven years.
Thank you for the update, Eric. I know it is unwise to assume, but I assume the judge in recent marin county decision, re:Stewards, Spawn and the San Geronimo Valley Moratorium was aware of your report? And NOAA and Cal. Fish and Game? Pls advise. Barb Scott boomerangbarb@gmail.com
Thanks for your question, Barb. We haven’t yet finalized our 2012 smolt survey report, so it’s unlikely that the judge or the regulatory agencies are aware of the survey results, yet. As for how these numbers relate to the controversy over development in the San Geronimo Valley, the picture isn’t entirely clear. San Geronimo Creek is a very important stream for juvenile coho salmon, and in three of the last seven years half of the coho smolts leaving the Lagunitas Creek watershed originated in the San Geronimo Valley. This year, however, the smolt count for San Geronimo Creek was below average, and these smolts made up only 14% of the watershed smolt total. That said, fish numbers are highly variable and are difficult to attribute to habitat conditions, so I would be very reluctant to base land use decisions on the results of any one fish count.
Thnx for reply. Part of controversy arises by frustration of valley residents who are so thoughtful, aware, and active in protection of watershed, and feel, quite correctly, I believe, that private and public monies being spent on lawsuits and regulations is a fraud, and lining the pockets of those fueling dissention. Just read Russell Chatham’s glorious prose in California Fly Fisher…oct 2012 issue..and can only hope that the 2 watersheds will eventually return for pleasure of fish and fishermen combined.