This paper was a collaborative effort with current and former graduate students of lead PI, Kerstin Wasson, NSF REU interns, UCSC field interns, ESNERR research staff, and folks from the University of Virginia, University of Georgia, CDFW, CSU Monterey Bay, Oregon State, Hopkins Marine Station, and the Smithsonian Environmental Research Center.
Watsonville, California – August 20, 2019- As they munch on plants and burrow through the soil, crabs are wreaking havoc on salt marshes around the country. Or are they? Turns out it depends on where you look, according to a recent study published in the journal Ecology. Conducted by the NOAA-sponsored National Estuarine Research Reserve System, this assessment explored how crabs are affecting the nation’s salt marshes.
California’s Elkhorn Slough Reservecollaborated with scientists from 14 other sites in 13 coastal states to conduct this study. They found crab burrows in just under half of the plots they sampled. In only four estuaries did researchers find evidence that crabs were having a negative effect on marsh vegetation. This indicates that it is unlikely that crabs pose a major threat to U.S. salt marshes at a broad scale.
“Elkhorn Slough was one of the four sites where crab burrow density showed a significant negative relationship with marsh vegetation,” said Kerstin Wasson, Research Coordinator at the Elkhorn Slough Reserve. “Crabs burrow into the lower marsh and bank edge, giving it the look of swiss cheese—more holes than anything else. Even so, the effect of elevation was stronger than that of crabs across the entire marsh. Our marshes are drowning faster than they’re getting attacked by crabs, which is why we’re investing in major marsh restoration initiatives to raise elevation.”
There are many other types of crabs that live in estuaries, including those that support commercial fisheries. This study focused on the types of crabs that live in tidal creeks and salt marshes. At first glance, the study appears to contradict recent experiments that demonstrated that crabs can cause dramatic die-off of marsh vegetation. It turns out, however, that when it comes to crab research, location is everything. These experiments are labor intensive, involving the installation of many cages. As a result, researchers often position cages where they see lots of crab burrows, which tends to be at lower elevations and near tidal creek banks.
This study systematically sampled salt marshes along transect lines that extended from the uplands down to the edge of tidal creeks. Transects are straight lines along which samples are taken at fixed, predetermined intervals to minimize bias. Few crabs were found in most places, but they were abundant near tidal creeks in some estuaries— the same sorts of places where past experiments have been conducted.
“Patterns in nature differ across scales,” notes Wasson, the lead author of the study. “What is true at one scale may not be true at another. We found that crabs can cause a lot of harm at a local scale, in some parts of some marshes, but they don’t seem to be a main cause of marsh dieback at a national scale.”
While crabs did not have a strong overall influence on salt marshes, elevation did. Lower parts of the marsh are submerged more often and tend to have poorer overall health. This study suggests that rising sea levels associated with climate change pose a bigger threat to the nation’s salt marshes than do crabs. However, since the study found that crabs are more abundant, and cause more harm, at low elevations, climate change will also impact the effects that crabs have on marshes.
“We’ll be seeing more crabs in our marshes as waters rise,” says Dr. Kenny Raposa of the Narragansett Bay Reserve in Rhode Island, another lead scientist of the study. “The health of our marshes will be challenged by rising seas and increasing crab burrows at the same time.”
Restoration measures that help marshes stay higher, where they are less likely to be susceptible to rising seas or crab burrows, will be essential to protect the salt marshes that provide economic benefits to communities nationwide. Raposa and Wasson are leading another collaborative study across eight Reserves that is examining the effectiveness of adding soil to raise marshes to help them survive in the future.
“With scientists following consistent protocols that monitor coastal plants at 29 sites around the country, the Reserve system is providing high quality data and science that communities need to manage our changing coasts,” says Rebecca Roth, the executive director of the National Estuarine Research Reserve Association. “Reserves are places with scientists that can detect early signs of threats and then test strategies to make communities safer and more resilient.”
About the National Estuarine Research Reserve System
The National Estuarine Research Reserve System (NERRS) is a national network of 29 sites dedicated to the effective science-based management of coastal and estuarine environments. Established by the Coastal Zone Management Act in 1972, each Reserve is a partnership between the National Oceanic and Atmospheric Administration (NOAA) and a U.S. state or protectorate, university, or nonprofit. NOAA provides funding and national guidance for each Reserve. The local partner provides a place and vision for the Reserve’s work and administers programs in support of Reserve goals.
A cohort of newly recruited crabs have moved into the Hester marshes. When visiting the site you can see crab burrows of migrant adults (indicative by the size of the individuals and the burrow sizes) and in the small branching tidal creeks small burrows blanket the shallow creeks.
Percent cover is a common measurement taken by marsh ecologists…
When collecting percent cover data we often use a gridded quadrat like the one pictured in panels A &B. At each intersection point we place a narrow rod and record all the cover types that the rod makes contact with at the canopy, sub-canopy and benthos.
As keystone predators, the Southern Sea Otter (Enhydra lutris) plays a critical role in maintaining the stability and health of the ecosystem in which they live.
A trophic cascade discovered by Dr. Brent Hughes (Hughes et al. 2013) showed that there was an indirect positive effect of otters on eelgrass bed health. Elkhorn Slough eelgrass (Zostera marina) beds have been slowly recovering from a 10 hectare loss of eelgrass habitat following the construction of Moss Landing Harbor.
A TYPICAL FIELD DAY
A good day on the slough, or any outdoor adventure for that matter, starts with protection from the elements—sunscreen, a hat, and multiple layers. Weather in the slough changes hour by hour. Many days start cold and foggy, with the fog casually burning off through the day, and then just as the sun graces you with its presence the wind picks up—making that extra layer you packed worth it. Pack a good lunch, sit back, observe and record. The greatest lesson we learned through planning these days is that timing is everything for kayaking in the slough, which we picked up on quickly through trial and error. We made the mistake of heading out early morning on the windiest of days and discovered that the usually mellow, lake-like water of the slough had become a rough and turbulent channel blanketed in whitecaps. As we struggled to stay in one place long enough to observe any otter, we watched as beached kayakers were rescued by boats and towed back in. We quickly called it quits, but as soon as we started battling our way back, my (AK Clemons) paddle snapped in half. I managed to power row myself back to the harbor after multiple attempts of trying to connect our kayaks. After that, we learned to check the wind forecast prior to going out in the field.
Restoration efforts of eelgrass habitat led by Kat Beheshti, PhD Candidate at UC Santa Cruz, have been widely successful. We joined Kat’s research team in early 2018 and participated in her 2018 restoration project. This year’s goal is to enhance restoration success via plot size while concurrently monitoring sea otter usage within the proximity of our newly transplanted plots. We are interested in integrating the trophic cascade that Dr. Hughes discovered in the existing beds of Elkhorn Slough and apply it to our restoration plots to see whether there is the capacity for otters to enhance restoration success.
OUR EXPERIENCE AS PROJECT LEADS
Our goal for this project is to assess sea otter activity in and around our 2018 restoration plots. Through observing otter activity in proximity to our plots, we hope to quantify the impact otters may have on restoration success. Otter activity that we are interested in includes: feeding, forage diving, mating, in transit, and whether otters present are mom and pup pairs. Proximity of the otter to the plot is recorded using a GPS. Our mode of transportation: kayaking. As project leads, we drafted a schedule with the goal of completing 40 hours of otter observations that span across all of our 2018 restoration plots. We organized a team of currently enrolled and recently graduated UCSC students to complete this task from June 2nd-16th.
THE NITTY GRITTY
Each field day includes four hours of otter observations. We arrived at the slough roughly two hours before high tide. That way we could let the rising tide carry us through the slough, easily reaching our furthest observation point by high tide. We could then turn back with the change of tide, allowing us to effortlessly glide back to the harbor, only occasionally being thwarted by the afternoon wind. Sampling in this way allowed us to record each area twice in a day, once on our way out and again on our way back in. This sampling method combined with the changing times of high tide from week to week allowed us to compile a record of otter behavior at various times of day—from early in the morning to early evening. We successfully designed a sampling protocol that would account for the temporal and spatial variability in sea otter usage in relation to our restored plots.
One of the most rewarding consequences of this work is our heightened understanding of the dynamics and structure of Elkhorn Slough. We witnessed massive algal production, learned where in the channel was the best to flow with the tide, expected the unexpected with the weather, and observed a variety of marine mammal activity throughout varying habitat types.
We’d like to thank Kat and the Wasson Lab for giving us this opportunity and for involving in their many slough projects!
While on her honeymoon in France, grad student Kat Beheshti’s husband was served tuna steak on a bed of steamed pickleweed!
Now the image she sent to the folks at ESNERR while abroad is being used on tours as a tool for public outreach demonstrating how, what may seem like a common marsh plant is actually cultivated and harvested for consumption in other parts of the world!