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.