Along the central and southern Californian shore, seasonal sandbars occasionally cut off estuaries from the ocean and form temporary freshwater lagoons at the mouths of some coastal streams. The lagoons are prime habitat for young stream-born steelhead to pack on weight before heading out to sea after the sand bars breach.
Unless, of course, the fish go belly up before they get the chance, as was the case in 2011 when 235 suffocated steelhead carcasses were pulled from Pescadero Lagoon in San Mateo County. It was the 11th straight year that adverse conditions in the lagoon killed off some number of steelhead, a threatened species under the federal Endangered Species Act.
The fish kills appear to be the result of a bottom layer of anoxic water that quickly mixes into the water column when the sandbar breaches in the winter and opens the lagoon’s connection to the ocean. In an effort to prevent a die off this year, a coalition of state and federal environmental agencies is working together to force an early breach in the sandbar to bring relief to the fish while collecting data to help build a long-term management plan.
“When it’s breaching on its own, that’s when we’re seeing these fish kills,” said Devin Best, a natural resource management specialist with National Oceanic and Atmospheric Administration, one of the partner agencies. “This one particular lagoon is the anomaly of many others where normally we’re promoting natural breaching. But in this one in particular, we’re saying, ‘No, we need to do some sort of management of it because everybody is loosing too many fish on these natural occurrences.'”
The annual fish kill in Pescadero is especially peculiar because the lagoon lies in a relatively undeveloped watershed, Best said.
“You’d think would be pretty prime shape. As far as the lagoon itself, there’s a lot of wetland habitat,” he said. “It’s one of the larger lagoons that has the least amount of anthropogenic influences. Yet we’re still seeing these fish kills that we don’t see in others that are in a much more degraded state.”
Part of the explanation is that the Pescadero watershed lies in a part of California that sees less annual rainfall than much of the coast. With less rain falling on the watershed, not enough water is flowing into the estuary and out into the ocean to counteract the tidal influences and wave action that build up the sandbar barrier at the mouth.
The Pescadero Lagoon breaches naturally when rain levels and tributary flows pick up in late fall and winter. But by that time, a wedge of dense, oxygen-depleted saline water has formed on the bottom of the lagoon and mixes quickly into the water column when the connection with the sea is reformed. This year, NOAA, the U.S. Fish and Wildlife Service, California State Parks and the California Department of Fish and Game are collaborating on a plan to keep that from happening. The agencies are hoping that by digging an artificial breach and building a connection to the ocean earlier in the year, the lagoon and its dissolved oxygen levels will freshen up in time for the natural breach.
Data collection will play an important role in the plan, including spot sampling with handheld meters and long-term installations of continuously recording sensors throughout the lagoon. The agencies will measure dissolved oxygen, salinity, and temperature before during and after the project. This is an opportunity to give all the agencies involved an better understanding the overall functioning of the lagoon and how it responds to the man-made breach.
“With the data that we’re able to collect . . . it’s really taking from what we’ve been thinking was going on and giving us the information we need to be able to develop good management skills and programs to implement out there,” Best said.
“It’s sort of an interim program right now, but the idea is if we develop this and it works based on the information we have, then we can probably implement something like this in subsequent years.”
Image: Workers survey the initial trench dug to artificially breach Pescadero Lagoon in an effort to prevent a fish kill that occurs when the lagoon breaches naturally (Credit: Sarah Swenty/USFWS, via Flickr)