COS 18-3
Sink or swim: Steelhead trout performance in an intermittent estuary
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 the majority of the State’s estuaries are intermittently closed due to the formation of coastal sandbars at the river’s mouth. This geomorphological event can lead to drastic changes in habitat quantity and quality which affect numerous ecosystem processes including fish production. Our goal is to understand how shifts in ecosystem state affect steelhead growth performance in the Pescadero Estuary, on the central California coast. Our approach has been to monitor estuarine state (inlet fully open to partially open to closed) and water quality each month from July 2011 – December 2013. Additionally, we seined the estuary over a 2-5 day period each month to capture, measure, and mark individual juvenile steelhead trout (Oncorhynchus mykiss).
Results/Conclusions
We conducted our steelhead trout mark-recapture study from July-November 2012 and again from May-December 2013. Estimated steelhead population size for the 2012 study varied across multiple orders of magnitude and was highest in July 2012, rapidly decreased after the mouth closed in August 2012 to a low of 65 individuals in November 2012 before the mouth breached during a large storm event. Steelhead growth was extremely high during the 4 month rearing period, approaching nearly 1 mm/day. Year 2013 was marked by both a drastically different sandbar regime and performance and population trends. Steelhead growth was much lower and less variable in 2013 compared to 2012. Population levels were also more consistent in 2013 versus 2012 with less intermonth variability. While these results suggest that the lagoon can be high quality rearing habitat for steelhead, an acute mass kill has been observed in 13 out of the last 18 years at the time of sandbar breach, indicating that the Pescadero lagoon can, at times, be harmful habitat for steelhead and other organisms. We will conclude by discussing hypotheses regarding mechanisms behind the kill and ongoing conservation efforts in this system.