Warmer winters trigger changes in Texas bays
A couple of degrees of temperature and a few inches of water might seem hardly worth noticing.
But those slight changes are having and stand to have effects — subtle and significant, understood and still uncertain — on the state’s coastal bays and the estuaries, the marine life tied to them and the people who enjoy those resources.
Almost 20,000 acres — 31 square miles — of salt marsh, the oyster grass-dominated matrix of wetlands that is the supercharged engine driving inshore coastal fisheries and providing recreation for more than a million anglers and livelihood for tens of thousands, disappeared from estuaries along Texas’ coast between 1990 and 2010.
Over the same 20 years, the amount of black mangrove in the shallows rimming the state’s coastal estuaries grew by approximately 4,000 acres, much of that growth in areas along the upper coast, where the tropical shrub had not been previously documented. In many cases, the expansion of the tropical mangroves has come at the expense of the more productive temperate salt marsh.
The population of southern flounder, one of the most popular sportfish in Texas bays, steadily has evaporated over the last three decades or so, with relative abundance of the flatfish during recent years falling to 70 percent or more below levels of the early 1980s.
In contrast, the relative abundance of gray snapper in Texas bays and estuaries as gauged by Texas Parks and Wildlife Department gill-net surveys saw an almost sixfold, coastwide increase between 1993 and 2006. Some bay systems saw as much as a 20-fold increase in this tropical species often called mangrove over little more than a decade. In most of those bays, gray snapper were historically wholly absent or very rarely encountered.
Facts don’t lie
Those changes and several others are directly tied to the effects of recent climate change. And nowhere are the effects of climate change in Texas more easily seen or more clearly documented and verifiable than in the state’s bays and coastal estuaries.
Constant change is normal and necessary — even crucial — for these dynamic ecosystems. But the changes, causes and rates of those changes to Texas coastal marine communities over the last few decades have been extraordinary, profound and undeniable. The culprit is warmer temperatures (specifically, warmer winters) and rising sea level.
Argue and debate the causes of climate change all you want, but the hard, documented facts are that, over the last three decades or so, the average temperature of Texas bay waters has steadily climbed. Summer water temperatures in the bays and Gulf of Mexico off Texas are not getting significantly warmer; winter temperatures are not getting as cold.
Sea level along the Texas coast also has inched up, with that rate increasing. Relative sea-level rise on the Gulf Coast has been twice as high as the global average. Sea level on the Texas coast has risen 5 to 17 inches in the last century and is accelerating. Current predictions estimate a sea level rise of 1 to 4 feet along the Texas coast by the end of this century, barely 80 years away.
That rising sea level is accelerating erosion of Texas coastal shoreline, now at about 10 feet a year. That erosion gnaws at estuarine shorelines and inundates low-lying salt marsh, transforming it to open water.
The loss of salt marsh and adjacent intermediate marsh is a blow to a bay’s productivity. Those marshes, dominated by oyster grass, other spartina species and a mix of other thick, low-growing, salt-tolerant vegetation are the nursery ground and refuge of almost every marine animal in the bay systems — finfish such as speckled trout and redfish, mullet and bay anchovies, and silversides, shrimp, blue crabs, periwinkles snails and marine worms.
While much of the loss of Texas salt marsh comes from erosion and drowning by rising sea level, some of the loss is coming as black mangrove expands its range, crowding out and replacing salt marsh. That range expansion is a direct result of warming winter temperatures.
Black-mangrove march
Black mangrove is a tropical and subtropical plant, intolerant of freezing and even near-freezing temperatures. A halfcentury ago, mangroves, which grow in shallow, saline water, were found only in the southern reaches of the Texas. They were most common in the Lower Laguna Madre, with scattered stands found as far north as Aransas Bay on the mid coast. Through the 1980s, regular freezes kept black mangroves at bay, killing the pioneering stands that crept north.
But over the last quarter century, as winter temperatures have warmed, the plants have marched north. Between 1990 and 2010, the amount of mangroves on the Texas coast increased 74 percent.
That expansion continues apace. Today, extensive stands of black mangrove dominate estuaries as far north as Matagorda Bay and have extended to Galveston Bay and even into Louisiana. About 10 percent of the loss of Texas salt marsh from 1990 until 2010 is attributed to mangroves crowding out the shorter vegetation.
The change from salt marsh to mangroves created a very different habitat for marine life — one that advantages some but disadvantages others. Research indicates mangrove stands are much less productive than salt marsh, especially for marine life such as blue crabs, shrimp and many small fish. Predation rates are higher in mangrove stands than salt marsh. A mix of the two habitats appears to be beneficial to estuarine life, but the long-term impacts of the conversion of salt marsh to mangrove remain uncertain.
What is not uncertain is the warming Texas waters’ impact on southern flounder. The western Gulf Coast is the edge of the southern flounder’s range. And that is tied to water temperature. Flounder require relatively cool water temperatures to successfully reproduce.
The adult fish, which exit bays in autumn and winter in the Gulf of Mexico, spawn in the open Gulf. But successful spawning and survival of fertilized eggs and larval flounder requires water temperature in a narrow range around 62 to 64 degrees Fahrenheit. Warmer temperatures result in poor survival of flounder eggs and larvae, and favor survival of males over females.
Over the last three decades, as winter water temperatures have warmed, southern-flounder spawning success and recruitment of young flounder into the bay fishery have been poor. While a cool — what used to be normal — winter a couple of winters ago gave way to a good flounder spawn, most recent years, including this past winter, saw very poor spawning success.
If the trend of warmer winters continues, Texas’ flounder population is fated to founder.
The warming bays have had the opposite effect on some marine species. Just as the tropical/subtropical mangroves have taken advantage of warmer winters along the Texas coast, so have some species of tropical and subtropical marine life. Gray snapper — mangrove snapper — are one of those species, moving north as Texas winters have warmed. Forty years ago, mangrove snapper, much like their namesake shrub, were encountered in Texas bays only as far north as the Lower Laguna Madre. But over the last two decades, the fish steadily expanded their range north and now are common along the mid coast and found even into Louisiana coastal waters.
Then there is snook. Snook, one of the most sought-after inshore sport fish, are a tropical species with a range long restricted to the southern tip of Texas. But over the last two decades, snook, like mangrove snapper, have extended their range up the Texas coast. Last year, TPWD crews found snook in every bay system in Texas. Some bay system, especially those on the mid coast, have seen significant increases in snook numbers. This autumn, TPWD coastal fisheries crews encountered several common snook, including some fish well over 30 inches, in the Matagorda Bay systems. And anglers this summer reported landing dozens of snook from inside the West Galveston Bay system.
Even West Indian manatees, the ponderous tropical marine mammals, have found their way to Texas warming coastal waters. Over the last two decades, manatees have become common visitors to bays on the lower half of the Texas coast and have been encountered as far north as the Galveston Bay system.
Hard freeze overdue
Texas bays and the life in and around them have changed over the last three decades or so, becoming more tropical or at least less temperate and seeing water levels increase — both results of long-term changes in climate.
Historically, the Texas coast has seen a major coastwide freeze event — one strong enough to drop water temperatures into the 40s or even 30s for days, killing significant numbers of fish, including almost all tropical species bold enough to poke their noses into Texas waters, and push black mangroves back to their place in semitropical South Texas — every 10 to 15 years.
It has been almost 30 years since the most recent such event on the Texas coast. Since then, the long-term trend has been toward warmer winters and higher water levels. As long as that holds true, expect those seemingly small increases in water temperature and sea level to make huge changes in Texas bays and the life in them.