Fish assemblage shifts track ecosystem states in a Central California intermittent estuary

Background/Question/Methods

Intermittent estuaries are estuaries whose connection to the ocean is severed for a period of time due to physical blockage. They are especially common Mediterranean-climate regions like California where approximately 70% of the State’s estuaries are intermittently closed due to the formation of coastal sandbars at the river’s mouth. In addition to blocking fish migration routes to and from the ocean, this geomorphological event can lead to drastic changes in water chemistry and quality. Our goal is to understand how shifts in ecosystem state affect fish assemblages in a small intermittent estuary along the California coast, Pescadero Estuary. Our approach has been to monitor estuarine state (inlet fully open to partially open to closed) and water quality each month beginning in July 2011. Additionally, we seined the entire estuary (including multiple hauls) over a two day period each month to quantify assemblage shifts.

Results/Conclusions

Typical of icthyological assemblage patterns in estuaries throughout the world, the monthly catch data was often dominated by a single species, but the particular species varied considerably among months (max percent dominant species: 100%, min: 60.5%, mean: 79.9%). Species that dominated the catch in at least one month include steelhead trout, threespine stickleback, staghorn sculpin, and topsmelt. Three different guilds of fish have been observed thus far to occupy the intermittent estuary: anadromous, euryhaline-fresh, and euryhaline-marine. Standardized catch rates (number of fish per seine haul) varied by over two orders of magnitude (max: 191.3, min: 3.1, median: 20.3), with the largest catches observed during open state and the lowest catches observed during the partially open state (i.e., sandbar formed but tidal overwash sometimes enters the lagoon). Diversity values (Shannon, Evenness) were highest (1.10, 0.56 respectively) soon after a mouth breach-related event, that rapidly altered the assemblage through a combination of mortality (observed) and movement (likely, but not quantified). Shannon Diversity and Evenness values were lowest (0.00, 0.17 respectively) when unusually dry conditions resulted in the re-formation of the coastal sandbar during the typically wet winter months. Our preliminary results suggest that a combination of life history traits, seasonality, and mouth condition largely describe the species assemblage patterns observed.