Younger Lagoon study tells tale of two fish
Deciphering how species interact with other organisms and their environment is the daily bread of ecologists. For rare or endangered species, understanding what factors cause populations to explode or collapse is not just academic; the answers can mean the difference between good population management and species extinction.
These concerns weighed heavily with respect to the northern tidal goby. A flash drive-sized spotted fish, this federally endangered species is found only in California estuaries, often alongside the much more common threespine stickleback.
Because the two fish are essentially the same size and eat the same aquatic invertebrates, scientists fear that the stickleback is literally eating the goby’s lunch. A laboratory study has already shown that when the stickleback flowers, the number of gobies suffers.
But the UC Santa Cruz scientists suspected that fish habitat might play an equal, if not more important, role. “These fish do not exist in a vacuum. There are strong environmental fluctuations where they live,” says Ben Wasserman, a former Ph.D. student at UC Santa Cruz and now a postdoctoral fellow at the University of Connecticut. “This calls into question the ability of the experiments to properly account for all these other factors that might influence their interactions.”
An extreme environment
The only type of habitat in which the goby occurs is bar-built estuaries. Separated from the ocean by a sandbar, these estuaries resemble stagnant lagoons for much of the year. But what really makes these estuaries extreme environments is their propensity to rupture. When enough rain has fallen, the water held behind the sandbar will push the barrier back. What was once an overflowing waterway then catastrophically spills into the ocean. This can leave fish and other lagoon residents dry for hours or even days.
“Each time they break, it’s a dramatic change in water, oxygen, and salinity conditions for the resident organisms,” says Eric Palkovacs, professor of ecology and evolutionary biology at UCSC and Wasserman Certified Counselor.
As a graduate student at UC Santa Cruz, Wasserman was determined to study goby and stickleback in their natural environment. He collected data on the number of gobies and sticklebacks from 2014 to 2020 and, using a technique called empirical dynamic modeling, he was able to account for how estuary conditions were affecting fish populations. He and his collaborators report in the journal Limnology and Oceanography that the state of the waters in the estuary, and not competition from another species of fish, is the most important predictor of the number of fish at any given time.
“I hope this shows the promise of this technique in a wider variety of applications, such as population management and conservation,” Wasserman says. “There are many cases where the available population monitoring data should allow us to extract more information.”
Perfect place for a study
Wasserman studied the interactions between the two fish at Younger Lagoon Reserve. Located on the western end of Santa Cruz, it is one of 41 protected landscapes in the UC Nature Preserve System.
As a classic example of a bar-built estuary, Younger proved an ideal setting for Wasserman’s research. The stickleback and the tidal goby are the only two fish in the lagoon. Its small size made it easy to sample comprehensively. Its staff collect a wealth of environmental data, including temperature and rainfall, and even keep a camera pointed at the mouth of the lagoon that documents the exact dates of breaches.
Wasserman first had to track the populations of each fish over time. Once a month for seven years, he and teams of volunteers submerged wire mesh fish traps around the shore of the lagoon. About the size and shape of a watermelon, the traps had inward-facing funnels at each end.
“When the fish meet the mesh, they swim along it and the funnel deposits them in the middle of the thing. Since they have to turn around to get out, they usually don’t know how to escape,” says Wasserman.
The researchers also took a snapshot of water conditions on each trip, measuring oxygen, temperature and salinity.
Visiting regularly for seven years, Wasserman developed an idea of how the lagoon behaved. “I knew the rhythms of Younger, to the point that in a storm one day, I say, it will break through. And I put on my rain gear and went out and waited. I saw that first trickle on the sand, then as soon as the soft sand got soaked it was like a dam exploding. And within two hours, I saw a channel erode about ten feet down and drain the water that had taken the whole fall to build up,” he says.
Know your estuary
To analyze his field data, Wasserman applied new methods developed to predict complex systems such as stock markets. Biologists first used empirical dynamic models (EDM) to manage commercial fisheries. EDMs can also take into account other parameters such as water temperature and chemistry as well as the number of days since estuary rupture.
Older modeling methods also require the scientist to guess the nature of the relationship between the factors. For example, goby numbers could drop linearly as stickleback numbers increase, or increase exponentially at a later date, or even curve over time as seasonal temperatures change. Empirical dynamic model algorithms eliminate this assumption. They can define relationships based on raw data. “When there are so many potential influences in nature on your focal organisms, it helps you track any signal present in your data and grab your attention rather than applying a certain assumption,” Wasserman says.
Wasserman applied a type of MDE, 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 the abundance of sticklebacks, or any of these other things, like temperature or salinity or the number of days since breaking? And then we asked the same question for the stickleback.
The only factor that was found to predict more gobies was a lot of sticklebacks.
Wasserman used a second type of EDM, the s-map, to determine if environmental conditions affect the interactions between the two fish. This approach revealed that the number of sticklebacks affects the number of gobies only in the spring.
“If the stickleback has a good spring, the gobies are going to have a good summer,” says Wasserman. He suspects the two fish react similarly to a common environmental factor, such as spring productivity. Here, the extra sunlight from longer days encourages algae blooms, which in turn stimulate populations of aquatic insects that both fish eat.
Similarly, the only factor that influenced stickleback numbers was a recent breach in the lagoon. “We would go there during serious breaching events, and the stickleback would really struggle. All or most are either dead or struggling to get enough oxygen,” says Palkovacs. The gobies were able to survive even when the lagoon turned into a mudflat. “Gobies can be lying on sand or in pickles after a breach. But if you were to go up and poke them, they’d squirm and find a pool.
The difference in the responses of the two fish probably stems from their respective ecological niches. Tidal gobies are specialists that have evolved to deal with seasonal breaches. They can tolerate the hypersaline conditions of late fall, extremely low levels of dissolved oxygen, and even complete drying of the lagoon.
Sticklebacks, on the other hand, are generalists. They occur in a wide variety of habitats across the Northern Hemisphere, from tidal currents to inshore coastal waters. In Younger Lagoon, their populations soar when conditions are right, but can plummet and sometimes disappear after a breach.
The lesson for wildlife managers? Reducing stickleback numbers is unlikely to help the goby. The most important factor for its survival appears to be maintaining the break cycle in their estuarine habitats. Unfortunately, many bar-built estuaries in California have disappeared due to human activities. Their sandbars are often kept open artificially to form harbors, while upland development, culverts or bridges can disrupt their fill-and-break cycles.
“In my previous ecological research, I looked at the fish or the bird or the insect or the plant. This environment forced me to see the importance of the physical process that sets the stage,” says Wasserman.
For Palkovacs, the study also highlights the value of field research. “These fish have all the characteristics that would lead you to assume they are competitive, and even lab experiments show that. Our point is that you can’t necessarily go from lab experiments to understanding what happens in nature. We need to know from real ecosystems what is happening.