Scientists theorize that a lack of oxygen in the water is what’s killing steelhead trout in the Pescadero Lagoon. However, their recently released report concludes that more data is needed before specific solutions can be recommended.
The report comes more than three years after an independent science panel convened to provide expertise in support of possible management and restoration decisions for the lagoon and marsh. The formation of the panel, which worked for free, followed years of annual fish kills in the estuary. These mortality events have occurred in late fall, when the return of rainfall causes the lagoon to overflow, rupturing the sandbar blocking the marsh from the ocean. Dead fish have washed onshore following these events.
The Pescadero Lagoon Science Panel established two possible causes for the fish kills. Scientists believe that such breaches mix once separate fresh and saltwater layers in the lagoon. That leads to oxygen demand from biological materials and chemical compounds like hydrogen sulfide.
The demand depletes the lagoon of oxygen, from top to bottom, for days. Additionally, when the mouth to the estuary reopens, the water level drops below the marsh surface. Scientists believe the water coming out of the mud and vegetation in the marsh trickles into the lagoon, and that it’s also full of chemical compounds that demand oxygen.
“We think there are two reasons, but we don’t know which is doing the job,” said Professor John Largier, an expert in estuarine and lagoon physical processes at University of California, Davis. Largier is a member of the panel. “There is enough data to know there is a problem, but not enough to figure out how to solve the problem.”
The lagoon and marsh serve as habitat for many species. Several are considered endangered or threatened, including steelhead trout, red-legged frog and tidewater goby. The goby, a type of fish that is tolerant of oxygen-deficient water, was not found to be a part of the fish kills.
The system is managed by the California Department of Parks and Recreation.
“State Parks and the other agencies that partnered in putting the panel together are happy to have a common understanding of the lagoon’s function,” said Chris Spohrer, acting district superintendent for the Santa Cruz district of California State Parks. “It’s making it easier for us to cooperate on work moving forward.”
According to the study, the annual fish kills are a relatively new phenomenon dating back to the 1990s. But that’s one of the uncertainties incumbent of a lack of data. Largier says scientists don’t know whether the problem dates back further and just went unnoticed for years. Regardless, the report establishes that a series of human activities over the last 150 years makes finding a baseline difficult. Several changes in the lagoon environment over that period are noted, including ones that researchers believe have occurred but lack data to confirm. They include changes in artificial breaches, breaks in the levee and the reconnection of marshes in the 1990s for restoration.
“Part of the reason for listing all those things is to say, ‘It’s a pretty complicated system that’s been messed with in quite a lot of different ways. Don’t just look at one cause,’” Largier said. “In some ways the system is offering lots of habitats and in that way it’s healthy. But the number of fish that died during these breaches is really not a symptom of health.”
At the conclusion of the report, the panel offers commentary on several management actions that could diminish these mortality events, including pre-emptively breaching the sand barrier and closing the mouth in spring. However, it stops short of making a scientifically based recommendation. Scientists raise several questions that they say need to be answered to inform any possible action, including determining how important fish kills are to the affected population and how freshwater inflow has changed over the years.
“In spite of ongoing concerns with quasi-annual fish kills in Pescadero Lagoon, and the volumes of spoken and written comment, there is a paucity of critical data, an absence of strategic monitoring, and a general lack of analysis of key issues that need to be validated and articulated prior to management action,” an excerpt from the report reads. “Ultimately the community of stakeholders needs to develop a common set of objectives based on a common vision, which will rest on well-founded scientific understanding of how the system works and the availability of actionable scientific information.”
Largier says he and other panel members are already getting to work on answering some of the questions they posed. They have received funding from a regional board that will go toward determining which of the two theorized causes is responsible for the fish kills.
“We’re collecting new data this year in a way that we think will allow us to be pretty clear about what’s going on,” he said. “We’re looking to make the system as good as it can be.”