Gill Net Surveys: An in-depth look at a Fisheries Management Tool

Everett Johnson

Gill Net Surveys: An in-depth look at a Fisheries Management Tool

My first-ever glimpse of a gill net came when a friend found one stashed in the tall grass on Matagorda Island and called the game warden. My next encounter came when I hung one with a topwater during a night wade. We never heard who hid the net in the grass, but there was no mistaking who placed the net I snagged with my Super Spook, it was festooned with TPW-Coastal Fisheries logo stickers. Hooking that net got me to thinking about a couple things First, I expected the net to hold the reason for the lack of fish on my stringer. But alas, save for some hardheads, a mullet, and a rat red, the end of the net I could see was empty. Second, what was it doing there? Everything I'd ever heard was negative; gill nets nearly wiped out our fish before they were outlawed. So what was the deal here?

Partial explanation came during one of the Spotted Seatrout Work Group sessions. A large part of the data presented was gill net-based. Desiring to know more, and at invitation from then Coastal Fisheries Director Hal Osburn, I contacted Norman Boyd. Norman is TPWD's Eco-System Leader for San Antonio Bay. Norman and his crew at Port O'Connor welcomed my visits and questions and soon became friends and eager helpers in my quest for knowledge. The gill nets I would learn, though potentially very destructive, have a legitimate place in modern-day fisheries management.

I accompanied Norman's crew to put out two nets and returned for the pickups early the following morning; that was in May 2003. Listening to a field technician explaining gill net sampling is one thing, seeing brings greater understanding. I wrote a piece for this magazine's June '03 issue entitled "Keeping Track of a Fishery" in the effort to dispel what I perceived to be a gross misconception of the program by the general angling public. Four years later, after reading many internet message boards, discussions with fishermen, and reading an article in another magazine, I can see that the misconception is still alive.

So here we go again. But before we go too far, I would like to draw an analogy. In my opinion, managing a coastal fishery could be likened to managing a giant warehouse retail store, except with almost none of the ordinary tools. Let's take a look at what the managers are up against:

-Your warehouse is huge, several thousand square miles huge, and it has literally thousands of doors through which customers can enter.

-Like warehouse outlets, customers are members; at least in the sense that they have a license to shop there. And if membership brings privilege and expectation, having a license can bring more.

-Imagine taking inventory blindfolded; it's kind of tough to count what you can't see. None-the-less, customers have the notion that you can make a quick tour every so often and somehow know what you have in stock.

-Your warehouse is open 24/7/365. You have nearly 1-million customers and they give no warning as to when they might show up. To complicate matters, your customer base is growing at a rate of about 2,000 each month.

-Some customers will refuse to check out even if you ask politely to see what they are taking.

-Events you cannot control can seriously reduce your stock.

-Many of your customers are self-proclaimed experts and nearly all of them think they could do a better job of running the store.

-And what about the future should you set limits or maybe close during certain seasons to ensure there will always be stock on the shelves? What will the customers say about that?

So my analogy is a little far-fetched you say; but is it really? Let's go back to the beginning, back to a time when there was almost no management as we now know it, back when nobody had an accurate inventory of what was out there. Here's how it got started.

In the early 1970's, Texas' coastal fisheries were managed more from a commercial aspect than for recreational purposes. Recreational fishermen enjoyed the same bounty that provided the commercial fishermen a paycheck, but the catch soon dwindled and nobody was making a haul. Something had to be done, but what? The hue and cry from the recreational sector said, "Stop the commercial harvest!" However, the managers had no basis. They needed stock assessment data to change the program and the data would be put to the legal test by commercial fishing lobbies. Early methods to obtain such data were borrowed from inland fisheries, but ponds and lakes are a world away from the coast.

In the beginning it was hoped that trammel nets could do the job, but in application they are highly dependent on human effort. Electro-shock techniques do not work in saltwater and poisoning portions of the bays to obtain data, as was common then in some inland fisheries work, was seen quickly as inappropriate for the task at hand. TPWD needed a method of making stock assessments that would produce unbiased data every time it was used, no matter where it was used, and no matter who performed the survey. In short, what is needed is "fisheries independent" data, data collected in a consistent and unbiased manner rather than from anglers or commercial fishers. Data that can be skewed by the variations in each person's ability to find and catch fish is termed "fishery dependent" and is of less value in measuring fish population statistics such as trends in abundance and size.

Gill nets are a fishery independent sampling method. Early gill net sampling was conducted with a variety of gear styles and set locations were not always selected in a statistically sound manner, so even these data would not stand a good test. Finally, in about 1975, the gill net sampling program underwent rigid formalization. First the net was standardized. A survey net is made of 150 feet of four mesh sizes (3", 4", 5", and 6") strung into a 600 foot tool. The net measures four feet from top to bottom and is fitted with floats at the top and weights at the bottom to keep them standing, provided the water is deep enough. This is the standard gill net used by TPWD staff in all Texas bays and estuaries.

To set the net, the three inch mesh is anchored on the bank and it is joined by four inch, then five inch and last comes six inch mesh that gets anchored 600 feet offshore. The net is always set perpendicular to the shoreline.

Set locations are selected randomly. The bay is divided into checkerboard grids, 1.0 nautical mile per side. Grids that contain shoreline are candidates for net sampling. Channels, passes and other navigable waterways that enter the bay are excluded as survey locations. This is important because the net can become a hazard to navigation and passes, as we all know, can concentrate fish traffic which could skew the data. Unlike a commercial fisherman who seeks to catch all he can, the biologist places nets randomly to obtain samples of whatever fish are present in that area. The net crews make records of water temperature, salinity, turbidity as well as any other local conditions and weather phenomena that might be pertinent.

The Texas coast is divided into eight major management regions; Sabine, Galveston, Matagorda, San Antonio, Aransas, Corpus Christi, Upper Laguna Madre and Lower Laguna Madre. Gill net sampling is conducted in each region over a 10-week period in spring and fall, with 45 net sets made in each season. There are also 20 net sets made in East Matagorda Bay in spring and fall. Nets are set in the last daylight hour before sundown and pick up begins at sunrise.

The time a net actually spends in the water is very critical in the calculation of CPUE, which stands for, "catch per unit of effort." The total number of each species captured is divided by the hours the net spent fishing. TPWD says the five year coastwide average CPUE for spotted seatrout is slightly less than 1.0, meaning the nets catch less than one trout per hour on average. The CPUE for red drum is almost the same. Spring nets often produce more trout while fall numbers are higher for redfish.

The catch is handled carefully and quickly as possible as many of the fish are still alive and may be healthy enough to survive the whole ordeal. The way a gill net works, a fish enters the web but cannot make it through. When it tries to back out its gill plates or another body part becomes ensnared in the webbing. Naturally, some fish die in the net. Survival rates in the early part of the spring season and the late part of the fall season are actually higher than you might imagine. Some days as many as half the trout and reds get released alive. How long a fish can live in the net is strongly related to water temperature and dissolved oxygen content and varies between species. The net's design with the three inch placed at the bank and the six inch offshore actually contributes toward the selectivity of what is caught as well as the survival prospects of the catch, even if it does come at the price of some fish getting through or around the net. The key is that the same net design, used in the same manner over the years will produce a CPUE which can be compared from year to year. This allows biologists to detect changes in fish abundance and size with a certain statistical reliability.

When the catch is coming in, the technicians are meticulous in their attention to detail. The first nineteen fish of each species from each mesh size is measured. Weighing was once part of data collection but since the formulation of today's highly accurate length/weight charts, weighing is no longer considered necessary. After 19 of any species is measured within a given mesh size the rest are only counted. Survivors are released, dead but edible fish are saved for donation to charitable organizations, fish that are dead and deemed inedible are discarded back into the eco-system. Some of the specimens are retained for otolith sampling where the "ear stones" are removed for aging in the laboratory. The age of fish compared to their length is one of the attributes managers must consider as they make their stock assessments.

After watching a crew work a net that held mostly hardheads, gafftop and mullet; I had tons of questions. What was wrong with the net? Was this a bad set? Why didn't it catch more trout? Does this mean there are no trout here? Why are you counting all those hardheads? Eventually I could sense the crew's patience wearing thin and I shut up. They were up to their elbows in catfish and all I wanted to know was why we hadn't caught more trout. I was learning that the business end of fisheries management is hardly glamorous.

My latest gill net endeavor took place last month. Norman's crew had placed two nets and the pickup crew included Norman, Theresa Krenek and Dr. Larry McKinney. Larry is the Director of Coastal Fisheries and while picking up nets is not within the normal scope of his present job, "quality control," as he termed it, still is. His performance on deck told of many days as a field biologist. While he and Norman were busy with their "picking and grinning" - slang for getting fish out of the net - I had a chance to listen, ask questions, and learn about fisheries management.

Our first net was a surprise; from having fished that location many times I would have bet it was full of trout. It held only a six. The big surprise was the Spanish mackerel and jack crevalle; I would have never guessed it. But as Norman says, "the net only catches what is there."

Our second net held a lot of fish, all kinds of fish. Trout, reds, black drum, mackerel, catfish, jacks, a few reds, and even a stingray and a blacktipped shark had found the webbing. This one took a while. The fish here were in better shape, meaning they were probably caught later than in the first net. The tide that was sweeping this shoreline had only been running for a short time. By comparison, our first net was in a cove where the current had less effect. Many were counted and measured and released. Some went into the floating retaining net, being given a chance to recover if they could. I did not get the actual count but I would say something close to half of the trout and reds made it through. The mackerel seemed to fair the worst of all.

TPWD has been conducting gill net surveys and compiling data for over 30 years. Their data base is huge, the largest ever amassed by any agency. Outside peer review from the American Fisheries Society declares it the best in the country and a model other states should emulate. This data, tied together with other fishery independent sampling such as bag seine sampling for juvenile fish and trawl samples, and also fishery dependent sampling (dockside creel surveys) enables TPWD to accurately assess relative abundance and size trends of all the recreationally and commercially important species.

Dr. McKinney commented about the importance of accurate data collection. "It's all about credibility; credibility to our constituents, our commissioners, our governor and our peers in other agencies. Texas has built the best program in the country based on the best data available. This data base has stood the test of scientific review and legal challenges. This is what we base our management decisions on, and this why we must strive to maintain the integrity of our data collection procedures."

Most recreational anglers have little understanding of all that's involved in managing our fisheries. The greatest part of what they know is what they have been told or read and unfortunately much of that is flawed. "Those gill net surveys waste too many fish; that needs to be stopped. They'll write me a ticket for one undersized fish and then turn around and kill thousands with their nets." I cannot tell you how many times I have heard arguments like this, and I too used to wonder if the method was too destructive, too expensive in the number of fish that it killed. Let's just take a minute here and look at some numbers.

Let's say TPWD's gill net program places 90 net sets in your favorite bay each year. The gill net CPUE is slightly less than one trout per hour on average. We can therefore calculate that since the average time any given net spends fishing is 12 hours; your bay receives approximately 1080 hours of net sampling effort each year. This adds up to roughly 1080 trout being caught in the nets. Some of the trout survive, 25% survival rate is probably close enough for what we're doing here, so in reality what we have is about 800 trout and maybe slightly fewer redfish being killed in the nets. Is this too expensive? Is this too high a price? How does this compare with the number we harvest recreationally? How often do you see three anglers bringing 15 or 20 trout and a couple of reds to the cleaning table? Doesn't take long to realize that the number killed in TPWD's gill nets is but a tiny fraction of what we take, now does it?

In reality, the nets can indeed be destructive, but we all knew or suspected that in the beginning. My purpose here has been to inform our readers that despite its deadly effectiveness in the hands of a skilled commercial fisherman, nylon monofilament webbing can still be a highly specialized and useful research tool the way TPWD puts it to work. Everything I've written here is honest fact, obtained firsthand in the field working with the biologists; nobody is trying to pull the wool over anybody's eyes.

When I accompany Norman's team on a pickup and see a nice trout hanging dead in the net I cannot help but wish it didn't have to happen. I rationalize those dead fish, though, as part of the cost of good management. Some will no doubt say that it would be better if they died on hooks rather than tangled in nylon webbing, but I'm not sure either death is more noble or useful than the other. I will leave you to form your own opinion. All I ask is that you will be fair in your judgement and remember that the fisheries we enjoy today are the product of a lot of hard work.

And for that we can thank the dedicated field staffers in yellow slicker pants, picking and grinning, and loving these bays as much as we do.

Editor's Note: Special recognition is due Norman Boyd, TPWD Eco-System Leader for San Antonio Bay. Norman has been a great help to me over the years and a wonderful teacher. Thanks for all your help and your dedication to the fisheries.