A recent Potomac River study links extreme rainfall to declining fish populations

Smallmouth bass are one of several Potomac River Species threatened by Climate Change and increased rainfall. photo courtesy of USFWS Mountain-Prairie/flickr.

Washington, DC is an important landmark in many ways. It’s our nation’s capital, the largest population center in the Potomac River region, and the point where our river changes from freshwater to tidal. It’s also the border between the habitats of two different, but beloved, bass species.

Many non-anglers have heard of the largemouth bass (or at least of Big Mouth Billy Bass, the singing robot fish…) that lives in the tidal waters below Great Falls, but fewer know of the ever-so-slightly smaller-mouthed species that dominates sport fishing in our fresh, upstream waters.

It’s time to pay attention to the smallmouth bass (Micropterus dolomieu). Recreational fishing for smallmouths in the upper Potomac River is a $23 million a year industry. Now, the fish and all that it represents are threatened by the climate crisis.

Let’s look up-river to see how we can help save this indispensable local legend…

Small mouths, big deal

A great Blue heron and a smallmouth bass. Photo courtesy of Eric BEgin/FLickr.

A smallmouth bass. Image courtesy of the NOAA Great Lakes Environmental Research Laboratory/Flickr.

You don’t have to be an avid angler to care about the smallmouth bass. Their maxillaries (a.k.a. upper jaw flaps, a.k.a. mouths) may be small, but their charm, and impact, are huge.

Did you know?

Smallmouths aren’t native to the Potomac River, but they aren’t invasive or harmful. Originally from the Upper Mississippi River, Great Lakes, and St. Lawrence River systems, smallmouth bass have been introduced to many rivers throughout the world. They were first brought to our region “in 1854 when a B&O railroad employee brought 20 fish from the Wheeling River in West Virginia to the C&O Canal Basin in Cumberland, Maryland” (MD DNR). They are now an established part of the ecosystem in the Upper Potomac River and South Branch Potomac River.
If you’ve heard of the brown bass, bronzeback, smallie, bronze bass, or bareback bass, you have heard of the smallmouth bass. Like many widespread species, they go by many nicknames.
They build nests! Like other members of the sunfish family, smallmouth bass build nests in order to reproduce. The males build the saucer-shaped nests in the the sand, gravel, or rubble, usually within 150 feet of where he built his nest the previous year. Then the mating pair perform their “spawning ritual” above the nest, after which the female leaves (often to spawn with another male…) and the male stays to guard his eggs.
Healthy smallmouth bass = clean water. Like many of us, smallmouth bass are picky about where they live. They like cool, clear streams with moderate currents and clear lakes. Cool and clear are the key words, these fish can’t tolerate high temperatures or pollution. That means they’re an excellent indicator of when our streams and rivers are healthy - and when they’re not.

Smallmouth bass, image courtesy of DC Nature

Solving the mystery of the smallmouth

One thing is certain when it comes to smallmouth bass conservation - the species, and the fishery it supports, is in jeopardy. But why? Smallmouth bass reproduction in the Potomac River has been declining since 2007, but the cause has been unclear until recently.

There are lots of threats to our local ecosystem that are undoubtedly affecting bass - invasive catfish and snakeheads outcompeting them for prey, chemical runoff like pesticides and fertilizers compromising their immune systems, and outbreaks of blue-green algae to start - but none of these seem to be the culprit behind the flagging population.

A US Geological SURVEY Scientist investigates the effects of chemical pollutants on smallmouth bass in the Shenandoah river. image courtesy of the chesapeake bay program.

Climate change is the real criminal, according to a new study by the U.S. Geological Survey’s Leetown Science Center in West Virginia and the Maryland Department of Natural Resources. Specifically, the increased rainfall caused by a warming planet is leading to flooding and heavy river flows that ruin the reproductive chances of smallmouth bass.

Average precipitation has been increasing in the Chesapeake Bay region for 100 years and more and more of that precipitation is coming from intense storms. Furthermore, springtime water flow in the Potomac has increased 4.3% every decade from 1970 - 2010.

Smallmouth bass spawn from April - June, ideally in waters between 58 to 70°F. Flooding and violent water flows in late spring and summer scour their eggs from the carefully built nests we mentioned above and disperse them into areas where the sensitive fish stand a much lower chance of survival. Increasing local temperatures aren’t helping their reproduction either.

Spring storms are threatening other local fish species as well. According to the same study, 13 of 28 species monitored in the Potomac over the past 43 years experienced declines in abundance seemingly related to increased springtime river flows.

So what can be done to shore up not just smallmouths, but all the fish in the upper Potomac River?

Saving smallmouths

Tackling the climate crisis seems a lot to take on. But, like the male smallmouth bass guarding the eggs in his rocky nest, we can protect our local fish and give them the best chance to weather the storm. Here’s how:

Anglers, keep up with best practices! This recent study shows that overfishing, as previously thought, is not the cause of smallmouth declines. Rather, the angling community is already doing its part by practicing catch-and-release fishing and following size, creel, and possession regulations. Learn more here.
Protect our upstream lands. The impact of intense rainfall is increased by development, which allows polluted stormwater to speed into our streams and creeks without being slowed or filtered by soil. To date, Potomac Conservancy has protected over 16,000 acres of local land from development. Find out how you can support our lands program here.
Get vocal for clean water! One thing should be clear now - smallmouth bass like clear water. Sign up to take action with our advocacy team to help pass local laws that create smart development, healthy forests, and fight the climate crisis.