Deciphering how species interact with other organisms and their environment is the bread and butter of ecologists. For rare or threatened species, understanding which factors cause populations to boom or bust isn’t just academic; the answers can mean the difference between sound population management and species extinction.
These concerns have loomed large regarding the northern tidewater goby. A spotted fish about the size of a thumb drive, this federally endangered species occurs only in California estuaries, often alongside the far more common three-spined stickleback.
Because the two fish are essentially the same size, and eat the same aquatic invertebrates, scientists have worried that the stickleback is literally eating the goby’s lunch. One lab study has already shown that when stickleback flourish, goby numbers suffer.
But scientists at UC Santa Cruz suspected the fishes’ habitat might play an equal if not more important role. “These fish don’t exist in a vacuum. There’s strong environmental fluctuations where they live,” says Ben Wasserman, a former Ph.D. student at UC Santa Cruz and now a postdoctoral researcher at the University of Connecticut. “That challenges the ability of experiments to properly account for all those other factors that could influence their interactions.”
An extreme environment
The only type of habitat the goby occurs in is bar-built estuaries. Separated from the ocean by a sandbar, these estuaries resemble stagnant lagoons for much of the year. But what truly makes these estuaries extreme environments is their propensity to breach. When enough rain has fallen, the water dammed behind the sandbar will shove the barrier aside. What was once a brimming waterway then drains catastrophically into the ocean. This can leave fish and other lagoon residents high and dry for hours to days.
“Whenever they breach, it’s a dramatic shift in the water and oxygen and salinity conditions for the resident organisms,” says Eric Palkovacs, a UCSC professor of ecology and evolutionary biology and Wasserman’s graduate advisor.
As a graduate student at UC Santa Cruz, Wasserman was determined to study the goby and stickleback in their natural environment. He collected data on goby and stickleback numbers from 2014–2020 and, with a technique called empirical dynamic modeling, he was able to factor in how estuary conditions affected fish populations. He and his collaborators report in the journal Limnology and Oceanography that the state of estuary waters, not competition from another fish species, is the most important predictor of fish numbers at any given time.
“I hope this shows the promise of this technique in a wider variety of applications, like population management and conservation,” says Wasserman. “There are many cases when available population monitoring data should allow us to extract more information.”
Perfect spot for a study
Wasserman studied interactions between the two fish at Younger Lagoon Reserve. Located at the western edge of Santa Cruz, it is one of the 41 protected landscapes in the UC Natural Reserve System.
As a textbook example of a bar-built estuary, Younger proved an ideal setting for Wasserman’s research. The stickleback and the tidewater goby are the lagoon’s only two fishes. Its small size made it easy to sample comprehensively. Its staff collect a bevy of environmental data, including temperature and rainfall, and even maintains a camera pointed at the lagoon mouth that documents the exact dates of breaches.
Wasserman first needed to track the populations of each fish over time. Once a month over seven years, he and teams of volunteers submerged wire mesh fish traps around the lagoon’s shore. Roughly the size and shape of a watermelon, the traps had inward-facing funnels on each end.
“When fish encounter the mesh, they swim along it, and the funnel drops them off in the middle of the thing. Since they have to make a U-turn to get out, they mostly don’t figure out how to escape,” Wasserman says.
The researchers also took a snapshot of water conditions on each trip, measuring oxygen, temperature, and salinity readings.
Visiting regularly over seven years, Wasserman developed a feel for the lagoon’s behavior. “I knew Younger’s rhythms, to the point where in a storm one day, I said, it’s going to breach. And I put on my rain gear and went out and waited. I saw that first trickle across the sand, and then as soon as the soft sand got soaking wet it was like a dam blowing out. And in two hours I watched a channel erode probably ten feet deep and drain the water that had taken all fall to accumulate,” he says.
Getting to know your estuary
To analyze his field data, Wasserman applied new methods developed to forecast complex systems such as stock markets. Biologists first used empirical dynamic models (EDMs) to manage commercial fisheries. EDMs can also factor in other parameters such as water temperature and chemistry as well as the number of days since the estuary breached.
Older modeling methods also require the scientist to guess at the nature of the relationship between factors. For example, goby numbers could drop linearly as stickleback numbers rise, or rise exponentially at some later date, or curve over time as seasonal temperatures change. The algorithms in empirical dynamic models take that guessing off the table. They can define relationships based on raw data. “When there’s so many potential influences out in nature on your focal organisms, this helps you follow whatever signal there is in your data, and pulls that to your attention rather than you applying a certain assumption,” Wasserman says.
Wasserman applied one type of EDM, convergent cross mapping, to determine whether his time series data could predict influences on population. “We asked, what can predict our focal species, the goby? Is it stickleback abundance, or any of those other things, like temperature or salinity or days since breaching? And then we asked the same question for the stickleback.”
The one factor that turned out to predict more goby was lots of stickleback.
Wasserman used a second type of EDM, the s-map, to determine whether environmental conditions affect interactions between the two fish. This approach revealed that stickleback numbers only affect goby numbers in spring.
“If the stickleback have a good spring, the gobies are going to have a good summer,” Wasserman says. He suspects both fishes are responding in the same way to a common environmental driver, such as spring productivity. Here, the additional sunlight from longer days encourages the proliferation of algae, which in turn boosts populations of the aquatic insects both fishes eat.
Influence of environment
Similarly, the one factor that affected stickleback numbers was a recent lagoon breach. “We would go out there during severe breaching events, and the stickleback would be really struggling. All or most are dead or struggling to get enough oxygen,” Palkovacs says. The gobies were able to survive even when the lagoon had transformed into a mudflat. “The gobies might be lying on the sand or in the pickleweed after a breach. But if you were to go up and poke them, they would wiggle away and find a pool.”
The difference in the two fishes’ responses likely stems from their respective ecological niches. Tidewater goby are specialists that have evolved to cope with seasonal breaching. They can tolerate the hypersaline conditions of late fall, extremely low levels of dissolved oxygen, and even the complete drying of the lagoon.
Stickleback, on the other hand, are generalists. They occur in a wide variety of habitats across the Northern Hemisphere, from tidal streams to nearshore coastal waters. In Younger Lagoon, their populations soar when conditions are good, but may crash and sometimes disappear after a breach.
The take-home lesson for wildlife managers? Reducing stickleback numbers are unlikely to help the goby. The most important factor for its survival appears to maintaining the breaching cycle in their estuarine habitats. Unfortunately, many of California’s bar-built estuaries have disappeared as a result of human activities. Their sandbars are often kept open artificially to form harbors, while upland development, culverts, or bridges can disrupt their filling and breaching cycles.
“In my previous ecological research, I was looking at the fish or the bird or the bug or the plant. This environment forced me to see the importance of the physical process setting the stage,” Wasserman says.
For Palkovacs, the study also underscores the value of field research. “These fish have all the characteristics that might lead you to assume they’re competing, and even lab experiments show that. Our point is that you you can’t necessarily go from lab experiments to understanding what’s happening in the wild. We need to know from actual ecosystems what’s happening.”
