Amid the coronavirus outbreak that sidelined almost all other field work this spring, a handful of Maryland biologists were dispatched to the Potomac River for an emergency project.
Their task: Capture smallmouth bass so their eggs could be transported to a hatchery with the hope that their offspring could help rebuild the river’s flagging population of the prized sport fish.
“The angling community is in an uproar, and rightfully so,” said Josh Henesy, a freshwater fishery biologist with the Maryland Department of Natural Resources, who was with the crew — practicing social distancing on a boat — while trying to collect egg-bearing fish.
Smallmouth bass are, by far, the most popular sport fish in the nontidal portion of the Potomac above Great Falls. But they’ve suffered from poor reproduction every year since 2007. Without intervention, state fishery managers worry the future of the recreational fishery, valued at $23 million a year, could be in jeopardy.
State officials say the cause of the decline isn’t overfishing because the bulk of the activity is catch-and-release.
A new study by the U.S. Geological Survey’s Leetown Science Center in West Virginia and the Maryland Department of Natural Resources offers another explanation: a changing climate. Increased rainfall driven by a warming planet is causing parts of the river to routinely flood in May and June, during and immediately after the smallmouth bass spawning season.
Heavy flows scour eggs from the nesting areas where they were released and also move recently hatched larvae into areas where their odds of survival are poor.
On average, precipitation in the Bay region has been increasing for the past century with much of the increase coming during intense storms as opposed to gentle, soaking rains. In the Potomac, peak spring flows have increased 4.3% per decade from 1970 through 2010, according to the study that was published in the journal Ecosphere.
“We’re concerned that rare events are becoming less rare,” said Than Hitt of the U.S. Geological Survey and lead author of the study.
The impact of intense rainfall is further compounded by development, the study said, which speeds stormwater runoff from the land into the river.
Nor are smallmouth bass the only species being affected. The study, which analyzed 43 years of data from the Potomac River, found that 13 of 28 fish species experienced significant changes in abundance that appear to be related to higher springtime river flows.
Of those, nine increased in abundance and four decreased. Those that increased tended to be small, short-lived fish such as banded killifish and mosquito fish, which are capable of spawning several times during the spring. That gives at least some of their offspring a greater chance to find optimal flow conditions.
Declining species were larger, long-lived fish that tend to delay spawning, have fewer offspring and provide more parental care.
“The reproductive biology of these species is telling us that conditions in the river are changing,” Hitt said. “And that is important because they have been there for many generations. To see such important changes over the last 43 years gives us pause.”
Other factors, such as whether a species was native or introduced, or changes in water quality, did not explain the trends in reproduction, Hitt said.
The fact that the DNR has initiated a five-year stocking program to try to stabilize the smallmouth bass population in the Potomac shows the significant impact of shifting flow patterns on fish. “What we are seeing now is that natural reproduction may not be able to provide a stable population,” Hitt said.
The study was only possible because the DNR launched a seine net survey to monitor the river’s smallmouth bass reproduction in 1975. They not only maintained the survey over time, but identified and counted more than 244,000 fish collected over the years, ultimately resulting in a long-running data set that covered 28 species.
As a result, Hitt said, “we can put smallmouth in the context of many other species. The story they are telling is much more powerful when you see that it is not just one species showing a trend, but rather many species in the fish community.”
Also of concern were sharp declines in some ecologically important species such as river chub and creek chub. Those species are ecological engineers — they physically move stones around in the river to build nests where they release eggs. Those structures are important spawning sites for other species as well.
“Where they disappear, you can lose other species that rely on those mounds,” Hitt said. “That could have rippling effects throughout the ecosystem.”
The changes are not all bad. The small species that are thriving under the new conditions aren’t sport fish, but they provide food for those species — including the voracious flathead catfish that has turned up in the Potomac in recent years.
“We have a lot of predators in the river now,” Henesy said. “So having a strong forage base is needed to support the amount of predation now in the river.”
But with more than a decade of poor reproduction, the population of adult smallmouth bass is starting to suffer, and Henesy said he hopes the hatchery-stocking effort changes the trajectory. After all, he has seen first-hand the smallmouth population change over time. “I learned to fish on the Potomac, with smallmouth bass.”