Long Term Monitoring

The Fish and Wildlife Research Institute (FWRI) Freshwater Fisheries Research section began the Long-Term Monitoring Program for Florida’s freshwater resources in 2006. What started as a focus on fish data for lakes has expanded to include river systems and aquatic habitat. Researchers’ primary objective is to track freshwater fisheries and habitat trends over time using standardized methods that help ensure the integrity of the data collected across the state. This information can then be used to assess the overall health of Florida freshwater resources, direct research efforts, and make management decisions.

Each year, biologists collect data on the fish community and sport fisheries of more than 50 lakes and rivers statewide. They add this information to a large database where it is available for use in future studies or evaluations conducted by researchers and fisheries managers. As of 2019, the database contained over 3 million records, and the number will continue to grow as the project continues.

Data Collection Methods

Biologists with FWRI, as well as other FWC divisions, use a variety of methods to sample freshwater fish communities. The type of equipment used depends on the target species, sampling location, and habitat. Researchers most commonly use two types of equipment to collect fish data in Florida’s lakes and rivers:

• Electrofishing is used to collect fish in shallow water, 2-6 feet deep. The most common species are Florida bass (Micropterus salmoides) and bluegill sunfish (Lepomis macrochirus), which have been collected in every lake sampled. This gear is also used to assess sport fish populations in the spring.
• Trawling is used to collect fish in open water during the fall. The trawl boat pulls a net suspended near the lake bottom as it moves forward. The net dimensions are configured to target young black crappie (Pomoxis nigromaculatus), and using this gear helps fisheries biologists assess populations of this popular sportfish.

In addition to length and weight measurements, fish collected during sampling events are also observed for abnormalities. Any notable health conditions, such as sores or lesions are recorded in order to monitor fish health trends.

Since the beginning of the Long-term Monitoring Program, water quality and details about the amount and types of plants present have been recorded with each fish sample.

However, in 2015, a formal aquatic vegetation monitoring program was put in place. During summer months, boats equipped with sonar mapping devices criss-cross each lake along evenly spaced transect lines. Biologists also collect plant samples at points along the transect lines using a plant rake. This information allows biologists to produce detailed plant maps for approximately 50 lakes each year.

Importance of Monitoring Data

Using the above methods, biologists monitor and measure the following fish metrics:

• Species diversity – an index of the number of species and abundance of each that make up a fish community
• Species richness – a component of species diversity that represents the number of different fish species present in the community
• Catch rate – an index used to estimate the relative abundance or density of fish, and can include values for all fish (total catch rate) or groups of fishes (e.g., catch rates of sport fishes or nonnative fishes)

The long-term monitoring project is another opportunity for scientists to monitor freshwater fish communities that are valuable resources to the state of Florida. State, federal and academic personnel use the data collected to develop management strategies and conduct related research as scientists produce a more complete picture of long-term trends. Additional information like water quality and plant data are used to help explain changes observed in the fish community or fishery.

Though we tend to think of fish as being “saltwater” or “freshwater” there are fish that can handle both fresh- and salt-water. Some like salmon, American eels and American shad move from saltwater to freshwater or vice-versa to mate and grow to maturity. These fish are called “diadromous” and the movement is a normal and essential part of their breeding and life cycle. Others, like tarpon, snook, mullet and redfish tend to move between fresher and saltier waters based on other factors, usually food.

How can they do it?

Part of it has to do with the fish itself. Some fish species are “stenohaline” meaning they have a narrow range of salt tolerance. Goldfish, pompano, many sharks and pike are examples of stenohaline fish. Their bodies can only handle a small range of salinity in the surrounding waters. Freshwater stenohaline fish tend to regulate this by putting out a lot of dilute urine and by actively taking in ions through their gills to keeps the water in balance. This process is called “osmoregulation.” If they did not, they would absorb water and lose ions like sodium, potassium, and chloride. Stenohaline marine fish, on the other hand, produce small amounts of concentrated urine and shed ions through their gills. In another form of osmoregulation, marine fish can actually drink seawater. If marine fish didn’t, they’d lose water and become dehydrated.

Euryhaline fish have bodies that can adapt to a wider salinity range. They can produce concentrated or dilute urine, their gills can take in or excrete ions and they may or may not drink the surrounding water. It all depends on the water they’re in. The water itself also has something to do with it. Not all freshwater is really “fresh,” and the St. Johns River is no exception. When ocean waters receded and uncovered the central landmass that is now Florida some 10,000 years ago, residual seawater became trapped in pockets of what is now our aquifer. Florida’s springs tend to have more salt and minerals in their water than others. Because of this, the water in the springs, runs, lakes, and river tends to have more ions and salt than freshwaters in the panhandle or in a state like Georgia. Salt Springs off of Lake George is a great example of this with salinities as high as 4 parts per thousand. Silver Glen Springs has salinities as high as about 1 parts per thousand. This makes it easier for euryhaline marine fish to osmoregulate.

Another advantage to Florida’s “fresh” waters is that most of Florida’s water comes from limestone aquifers. As water percolates through the ground and into the aquifer it slowly dissolves the limestone. This in turn causes our water to contain ions of calcium and carbonate, which make up limestone. These ions can be used by fishes in place of the sodium and chloride ions found in saltwater, and allow fish to maintain an ionic balance that helps them keep functioning normally.

Other variables can also determine whether a marine fish can get into freshwater or not. Some fish have a narrow temperature range they can tolerate (stenothermal) while others have a wider range (eurythermal). Some stenothermal fishes, like snook, can’t take cold water so they need warm winters to allow them to move up the Florida coast all the way to Mayport, then they can swim upriver to as far as Blue Springs in Orange City. Redfish and tarpon, which have a wider range of temperatures they can tolerate, can migrate up and down the St. Johns River during most of the year, so normal winters don’t keep them out of the river and lakes. They can be more “year-round” visitors. In fact, these fishes may be more prevalent in the springs in the winter as the warmer spring water temperatures are easier on them.

Finally, there’s food. Like people, fish go where the food is good. This means that predatory fish can come up the river when food items like shrimp, menhaden and anchovies, or mullet run up the river. These prey items are usually coming upriver themselves for food as well. Once in the river the predators may or may not be feeding on the items they were following and could transition over to other forage. As most anglers will tell you, Florida’s waters have ample sources of food, so it’s not odd for saltwater fish, once they’ve made their way into freshwater, to stick around and snack on the abundant forage.