An outgoing tide and light north wind behind a November cool front dropped water level in Brays Bayou several inches lower than normal, exposing a band of muddy clay usually submerged.
"You can see the burrows they make," said Lance Robinson, nodding toward the bare bank as he maneuvered a center-console boat up the bayou where it flows through MacGregor Park near the University of Houston.
Dozens of holes, most about the circumference of a one-pound coffee can and as much as three feet deep, riddled the exposed bayou bed.
Robinson, along with fellow Texas Parks and Wildlife Department coastal fisheries staffers Jan Culbertson and Matt Johnson, soon would have in hand some of the holes' engineers.
At a bend in the bayou, where clusters of abandoned shopping carts carried by flood waters created what passes for habitat in this channelized and "civilized" section of Brays, the three fisheries managers began retrieving an array of nets and wire traps set the day before.
Among the dozen or so species of fish caught in the webbing and wire cages were several fish that looked like props from the sci-fi Alien movies.
Speckled gray with a shovel-like head, sucker-like mouth, hard-to-spot beady eyes, spectacularly exaggerated dorsal and pectoral fins and covered with thick, hard scales, each of which sported a sharp peak, the fish looked otherworldly.
"Plecostomus," Robinson said, untangling yet another of the obviously out-of-place fish from a gill net. "They're thick in here."
They aren't supposed to be.
Hypostomus plecostomus, more commonly called armored catfish or "sucker" catfish, are native to the Amazon Basin in South America. But they've established a thriving expatriate population in Houston's bayou system.
Baby plecos are sold in aquarium stores. Often called "cleaner" fish, plecos use their sucker-like mouth to feed on algae growing on aquarium glass and rock.
But plecos, which can grow to a couple of feet in length, can become too large for typical aquariums.
"Then people dump them in the nearest waterway," Robinson said of the armored catfish. "They obviously do quite well around here."
The holes in the bayou bank are made by the armored catfish. They are "cavity nesters," using their shovel-like heads to burrow into banks, creating holes in which they spawn and protect their young.
Judging from the number of holes along the bayou and the number of plecos of all sizes TPWD has captured in sampling nets, the population is booming. And so, too, are the erosion problems all those holes in the bank create.
"Plecos" are far from alone as immigrants to Houston's bayou system and many other waterways in the state.
"There have been 100 or so non-indigenous fish, shellfish or mollusks documented in Texas waters," said Robert Howells, research scientist with TPWD's Heart of the Hills Fisheries Research station near Uvalde and widely respected authority on invasive aquatic species. "Not all of them have become established. But a lot have."
More species are coming each day, it seems. With them come a plethora of potential problems impacting the environment, economy and public heath.
Houston has the uncomfortable position of being a center of the increasing invasion into Texas waters by non-native aquatic species. Southeast Texas has a warm, almost tropical climate, making it possible for tropical species with low cold tolerances to survive.
The Houston area is veined with freshwater waterways and sits close to Galveston Bay, a marine environment.
Thousands of ships each year — about 6,300 foreign vessels — dock at the Port of Houston, pumping out ballast water often holding a variety of aquatic organisms. Those organisms are dumped into an area where fresh water (from the Buffalo Bayou/San Jacinto River) and marine water (Galveston Bay) meet. An organism easily can find water to its liking.
Although Texas has a list of prohibited aquatic species, many states allow importation of species Texas prohibits. With Internet sales and overnight shipping, it is easy for people to, knowingly or unwittingly, have illegal species transported into Texas, where some find their way into public water.
Houston's diverse culture, one of the city's best features, also adds to the problem. Ethnic food markets use the global economy and overnight shipping to import live aquatic species for customers yearning for traditional foods.
The result is a rocketing of the number and variety of non-indigenous species entering the area.
"I have seen some things in Houston that I'd seen only in reference books," Howells said of exotic fish and other aquatic life. "It is astonishing what's coming into the area."
TPWD game warden John Rao concurs.
"There's no telling what's coming into the Houston area right now or what's already here," he said.
Rao enforces state regulations governing live organisms prohibited in Texas because of their real or potential danger to the native natural resources or human health.
Visiting businesses dealing in live aquatic resources — aquarium supplies, food markets, aquaculture operations — keep Rao busy.
There have been others, too.
The discovery of a couple of northern snakehead in a small pond in Maryland two years ago made national news and triggered an expensive attempt to eradicate the non-indigenous fish. The snakehead, a particularly nasty beast of a fish from Asia, is highly aggressive and a voracious predator. Fisheries managers fear an established population of snakeheads would prove the ruination of many native species.
A year before the Maryland snakehead issue blew up, Rao had discovered and confiscated dozens of live snakeheads in and around Houston.
"It would boggle people's minds if they knew all the (exotic aquatic wildlife) coming into this area," Rao said. "It's a catastrophe waiting to happen."
Non-indigenous aquatic species pose grave environmental threats, fisheries managers say. Some insight into those problems can be seen in exotics already established in Texas waters.
The Asian grass carp, a voracious vegetarian, has changed the face of some Texas waters by devouring native aquatic vegetation. That vegetation serves as a filter of suspended particles, oxygen recharge, nursery habitat for young fish, and habitat for forage species such as minnows and invertebrates.
Waters where grass carp have become established (including the Buffalo Bayou system, San Jacinto and lower Trinity) have seen their aquatic habitat greatly altered and fisheries negatively impacted.
Exotics often are more aggressive than native fish species and can out-compete them for available food and habitat. Predation also is an issue. Both can reduce or even eliminate some native species.
But the introduction of diseases or other pathogens into native fish populations is perhaps the most sobering prospect of the increase in non-indigenous fish, fisheries scientists say.
"You're looking at the potential of introducing diseases native fish have no immunity to," Robinson said. "It could be like what happened with the Indian population in this county when they were exposed to smallpox for the first time."
About $100 billion is spent in this country each year to address impacts of non-indigenous species, Robinson said.
Some of those impacts are tied to human health. In a recent incident, a particularly virulent South American strain of the bacteria vibrio parahaemoliticus, a relative of cholera, was tied to more than 400 cases of serious human illness across 13 states. The bacteria arrived through ballast water and was introduced to humans through oysters.
Currently, Texas has no coordinated action plan to address issues surrounding non-indigenous, aquatic "nuisance" species.
But staff of agencies such as TPWD are collecting information on the distribution and impact of alien aquatics.
Robinson, who heads TPWD's coastal fisheries programs along the upper coast, is working with the agency's inland fisheries division on a project focusing on non-indigenous fish in the tidal bayous around Galveston Bay.
The ongoing project, with funding help from the Galveston Bay Estuary Program, is aimed at quantifying the problem.
"We're looking at it as a base-line study — what's out there and, from that, trying to come up with some kind of risk assessment," Robinson said.
Using a variety of sampling gear — electrofishing boat, gill net, trammel net, hoop net, perch traps, minnow traps — placed in Greens, Buffalo and Brays bayous, the fisheries crews have cataloged a thriving and diverse population of alien species — grass carp, German carp, tilapia, armored catfish, pacu, Rio Grande cichlids and even Amazonian red-tailed catfish.
"It's really opened our eyes to the extent of exotics," Robinson said.