San Francisco Estuary mysid abundance in the fall, and the potential for competitive advantage of Hyperacanthomysis longirostris over Neomysis mercedis

Historically, the native mysid Neomysis mercedis was a key source of food for fish in the San Francisco Estuary (SFE). Following the introductions of the over-bite clam and many invasive zooplankton species in the mid-1980s, populations of native zooplankton species plummeted, including N. mercedis. In July 1993, Hyperacanthomysis longirostris, an invasive mysid, was first documented in the SFE, and it quickly became the most abundant mysid in the upper estuary. Since 2011, the California Department of Fish and Wildlife has sampled mysid abundance throughout the upper SFE concurrently with their Fall Midwater Trawl, which targets juvenile Striped Bass. We analyzed data from these mysid trawls to investigate environmental correlates of N. mercedis and H. longirostris abundance. We found that H. longirostris was found at higher temperatures significantly more often than N. mercedis, and there was a trend towards greater frequency of H. longirostris at higher salinities. Increases in water temperature over time, coupled with H. longirostris’s smaller size at maturity and lower food needs may partially explain its success in the SFE.