Tiny, Translucent Shrimp Reclaims New Stretch of Olema Creek

By Science Communication Specialist Jessica Weinberg McClosky, San Francisco Bay Area Inventory & Monitoring Network

December 2023 - Last fall, as the coho and steelhead monitoring crew was counting juvenile fish in Olema Creek, they spotted a familiar species in an unexpected place. Normally, they see tiny, translucent, California freshwater shrimp (Syncaris pacifica) in the lower two miles of the creek. Now, crewmembers were seeing them almost a half mile farther upstream than their previously recorded upstream extent.

California freshwater shrimp seen in October. These endangered invertebrates recently expanded their range in Olema Creek, overcoming obstacles to move farther upstream than biologists had found them previously.

NPS / Michael Reichmuth

The monitoring team has been documenting Olema Creek’s federally endangered California freshwater shrimp population for years during their annual fish surveys. Their observations fill in gaps between less frequent shrimp inventories, adding detail to an impressive story of range expansion.

In the early 2000’s, the monitoring team found the shrimp to be uncommon residents of just the lowermost reaches of Olema Creek, well north of the town of Olema. By 2018-2019, the shrimp had expanded south near the town. Then in 2021, the team found them farther south/upstream by a half-mile. Still, this summer’s continued range expansion took biologists by surprise.

In 2021, the shrimp had reached an obstacle—a bridge over the creek. Below the bridge, the creek was artificially narrow, with hardened banks causing water to rush through a channel dozens of feet in length. California freshwater shrimp aren’t champion swimmers, so biologists assumed the bridge was as far as they could go on their own. They were mistaken. Having somehow conquered the rushing water, the shrimp are now continuing their upstream march.

Records are slim, but biologists suspect that historically, California freshwater shrimp occupied most of the Olema Creek watershed. So what caused them to largely disappear? And why are they now recolonizing higher reaches?

Female California freshwater shrimp carrying eggs. This species is the only surviving member of its genus, and one of just four members of its Jurassic-era family currently found in North America.

NPS / Michael Reichmuth

Olema Creek was extensively disturbed throughout the 20^th century. Some sections were straightened, others were impacted by logging or cattle grazing, and there was even a pesticide spill in 1993. Although the creek has not undergone a major restoration, policies implemented over the years seem to have given the creek the space it needs to heal. Logging has ceased, and cattle are no longer permitted in the creek channel. Stream-side vegetation has re-grown, woody debris has re-accumulated—creating pockets of slower-moving water, and water quality has improved. The shrimp seem eager to take advantage.

That’s good news for the species’ resilience. California freshwater shrimp are the only surviving members of their genus, and one of just four members of their Jurassic-era family currently found in North America. As new challenges—mostly climate change-related—arise, their resiliency will be key. As will the coho and steelhead monitoring team’s ability to help keep tabs on the shrimp. For example, the team was able to initiate a shrimp rescue in 2021 during an extreme drought. More generally, the latest data on the shrimp’s growing range will also help Point Reyes and Golden Gate protect the shrimp’s hard-won habitat gains for generations to come.

For more information

• Pacific Coast Science & Learning Center California Freshwater Shrimp webpage
• San Francisco Bay Area Network Salmonid Monitoring webpage
• Contact Fishery Biologist Michael Reichmuth or Aquatic Ecologist Darren Fong

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