Over the past four years, Texas' flounder population has boomed, and with it, the number of flatfish caught by Texas anglers.

Between Sept.1, 2011, and Aug. 31, 2012, Texas anglers caught and kept more than twice as many flounder as they had over the same period the previous year, according to creel surveys of almost 30,000 anglers interviewed by state coastal fisheries staff. Every Texas bay system, from Sabine Lake to the Lower Laguna Madre, this past year saw more flounder taken by anglers than the year before. And the 2010-11 catch was up from the year before.

Those consecutive record years of flounder catches by anglers have happened despite much more restrictive flounder fishing regulations that include a daily bag limit of five flounder (half what it was in 2008) for 11 months of the year and a two-flounder daily bag limit during November.

There is a good reason for the increased catches. Over the past four years, the abundance of adult flounder in Texas bays has shot up, as has the number of juvenile flounder.

"In several bays, we've seen more than double the number of flounder in the gill net surveys," said Mark Fisher, science director for Texas Parks and Wildlife Department's coastal fisheries division.

In 2008, Texas' southern flounder population seemed fated to flatlining.

While speckled trout and redfish numbers were holding their own or even improving, according to surveys of the fisheries and the catches of recreational anglers, the third member of Texas' inshore fishing recreational fishing triumvirate continued a population decline that had lasted for three decades.

Between the early 1980s and 2008, the relative abundance of flounder in Texas bays, as gauged by TPWD gill net surveys (which track adult fish) and bag seines (which track abundance of juvenile flounder) declined by half.

In the Lower Laguna Madre, TPWD coastal fisheries crews went a whole gill net season without catching a flounder, said Fisher.

"You can't get any lower than zero," he said.

Catches of flounder by recreational anglers, tracked by TPWD surveys of anglers returning from fishing trips, mirrored the decline. Despite fishing pressure on the coast increasing by 50 percent between the 1980s and 2008, flounder catches fell.

Reasons for the long, steady decline vexed fisheries managers. Despite the halving of flounder bag limits for recreational and commercial anglers in 1996 (from 20 per day to 10), the increase of minimum length to 14 inches, plus the reduction by an estimated 70-80 percent in the number of juvenile flounder taken as by-catch by the shrimp fishery, the flounder population continued declining.

Evidence indicated the decline was tied to low recruitment of young flounder into the fishery. There just weren't enough young flounder being produced and surviving to adulthood to replace the ones taken out of the fishery.

Several factors at play

Some of the problem was the continued loss of estuarine habitat in the bays, where juvenile flounder live, feed and hide from predators until large enough to survive in the bays.

Some was the loss of increasing percentages of the adult female flounder that produce the eggs that become those little flounder. Those female flounder were at their highest vulnerability to being caught during autumn, when adult female and male southern flounder migrate from bays to the Gulf of Mexico, where they spend the winter and spawn.

The autumn migration sees flounder move down channels and along shorelines, funneling toward the Gulf. As they move along these well-defined paths, they are concentrated and more easily targeted by anglers.

In Texas, the autumn migration of southern flounder begins in October and peaks in November. Illustrative of how vulnerable the flatfish are during the migration, which draws tens of thousands of anglers to the coast, about 60 percent of the flounder caught by Texas anglers over a calendar year are landed between Oct. 1 and Dec. 31. And most of them - as much as 25 percent or more of the annual catch - were taken during the 30 days of November.

But there was something else at play. Warming temperatures.

Southern flounder, the most abundant flatfish found in Texas, are members of a fish family that thrives best when it has access to cool or even cold marine waters.

It is not a coincidence that flounder are rarely found farther south than Texas and the highest numbers of flounder in Texas are in upper-coast bay systems where water temperatures are more moderate than along the lower coast.

Southern flounder spawn in the Gulf of Mexico during December and January, when water temperatures are coolest. The fish are "free spawners," meaning a female releases her eggs, which are fertilized by a troop of males - usually six or more - that follow the female.

Those fertilized eggs and resulting larvae, which float inshore and end up in estuaries, do best in a fairly narrow range of water temperatures. When water temperatures are higher than about 60 degrees, survival of eggs and larvae is much reduced.

"There's a very direct relationship between how cold it is in January and the number of juvenile flounder we see in June," Fisher said.

Warming up in Gulf

Over most of the past decade, the Gulf of Mexico has increasingly higher water temperatures during winter. Those increased temperatures have been documented by TPWD and other scientists who have monitored temperatures for decades.

"For 15-20 years, we had warmer and warmer winters. It just didn't get as cold," Fisher said.

Those warmer winters tracked with the declining recruitment of young flounder. There was nothing TPWD could do about warmer winters and the impact the weather had on flounder recruitment. So it addressed the only factor it could - angler harvest.

In 2009, with the flounder decline continuing, TPWD fisheries managers moved to reduce the harvest of adult flounder by tightening bag limits and imposing a two-flounder limit in November and banning gigging flounder during the month. The move, with took effect in Sept. 1, 2009, was predicted to reduce harvest of adult flounder by about 25 percent in the first year of the regulations.

But by increasing the number of adult flounder making it into the Gulf to spawn, managers said, Texas should see higher recruitment of young flounder. If things worked as statistical models indicated, Robin Riechers, now TPWD director of coastal fisheries, told the Texas Parks and Wildlife Commission in 2009, the biomass (total weight of flounder) in Texas waters could increase 80 percent within six years.

In what has proved to be a happy piece of serendipity, Texas has seen generally cooler winters over the past few years. And with those cooler winters has come much increased recruitment of flounder.

Flounder are fairly short-lived creatures, reaching adulthood in about two years and seldom living more than six years. So the fruits of a good spawning and recruitment year for flounder show up on the end of anglers' lines just a couple of years later.

And that's what Texas' flounder anglers have been enjoying over the past couple of years.

In 2010-11 and 2011-12, Texas anglers caught and kept more flounder than they did under the much more liberal bag limits in effect before 2009.

"Those good recruitment years are reflected in harvest," Fisher said. "That's what we're seeing with flounder. It's been a really rapid turnaround, and that's pretty neat."

shannon.tompkins@chron.com