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Fisheries-Independent Monitoring Using Stratified-Random Sampling

Map of Florida showing blue shading in current sampling zones and orange shading in past zones.

Florida estuaries sampled by the Fisheries-Independent Monitoring program (1989 – present). Current (Blue) and previously (Orange) sampled estuaries are indicated, along with years sampled.
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The Fisheries-Independent Monitoring program (FIM) conducts stratified-random sampling to estimate fish abundance and population trends in seven estuarine regions around Florida.

The effective and timely management of Florida's marine and estuarine fisheries resources requires the collection of a variety of information on many species. To provide information on fish population trends, the Fisheries-Independent Monitoring (FIM) program was established to survey fishery resources in Florida estuaries. The FIM program initiated sampling in 1989 in the Tampa Bay and Charlotte Harbor estuaries. Sampling surveys are currently conducted in seven estuarine regions of the state: Apalachicola Bay, Cedar Key, Tampa Bay, Sarasota Bay, Charlotte Harbor, Indian River Lagoon, and Northeast Florida. FIM also conducts similar surveys supported by special funding in St. Andrew’s Bay, the Florida Everglades, and Florida Bay.

The FIM program uses a stratified-random sampling (SRS) design, an approach which distributes sampling effort among habitat types and directs greater sampling effort into habitats with higher variability in catches in an effort to reduce variability in the data. With the SRS design, FIM divides each estuary to be surveyed into zones based on hydrological and logistical characteristics. The different habitat types (for example: water depth, seagrass beds, shore type) available within each zone are then identified as strata. The FIM program conducts monthly sampling at sites randomly selected from the strata available within each zone.

A variety of techniques and sampling gears are used by the FIM program to ensure that the wide range of species, sizes, and ages necessary for stock management are sampled during each monthly survey. Smaller fishes are collected with both a 21.3-meter seine and a 6.1-meter otter trawl. The FIM program uses the 21.3-meter seine in water depths of 1.8 meters or less, while the trawl typically samples water depths of 1.8 or greater. Larger sub-adult and adult fishes are collected using 183-meter haul seines in water depth of 3.0 meters or less. The two seine types sample shoreline habitats and open bay areas with both seagrass and non-vegetated bottom habitats. The trawl is typically deployed in deeper, open water habitats but is also deployed over deeper seagrass habitats.

On the left, two people standing in the water pull a net together while a third person is on a boat in the background. On the right two people standing at the back of a boat release a net into the water.

FIM scientists use haul seines (left) and trawls (right) to collect a variety of species of fish and invertebrates. The use of different types of sampling gear allow the FIM program to sample both shallow and deep waters within the estuary across a variety of habitats (i.e., sea grass beds, mangrove shorelines).

With each gear deployment, FIM program scientists record data that describe the physical features, such as water quality and habitat types, of the sampling site and the fish community collected. Recorded physical features include measurements of the type and quantity of submerged (seagrasses, bottom type, oysters, etc.) and shoreline (mangroves, seawalls, emergent vegetation, etc.) habitats at each sampling site. Measured water quality parameters include temperature, pH, salinity, and dissolved oxygen. All species of fish, shrimp and crabs collected in each gear deployment are identified and counted and a representative subsample is measured. Fish greater than 75 mm are examined for external abnormalities (tumors, ulcers, parasites, etc.) and abnormal specimens are returned to FWRI’s aquatic heath scientists for further analysis. Tissue samples for mercury concentration, ageing, reproductive condition, diet, and genetic studies are taken from selected fish and analyzed by scientists, both within and external to FWRI (state and federal wildlife agencies, universities, etc.).

Analyses of the FIM program data are used by fisheries managers to assess abundance trends for resource species, define essential fish habitat, and describe life-history parameters such as age, growth and age of maturity. Since the FIM program’s gears and sampling techniques tend to target juvenile and sub-adult fishes, the abundance trends are a valuable forecasting tool for future adult stocks. Fisheries managers use these FIM data as well as other fisheries data to assess the overall well-being of fish populations.

Three line charts that show declines in number of fish caught by researchers in 2010 before numbers rise in the years after.

Abundance trends of common snook (8 – 24 inches Standard length) before and after the 2010 Cold Snap in Florida. These FIM data document the collapse and subsequent recovery of common snook populations in these estuarine areas.

The FIM program data are frequently used to assess the impact of environmental disturbances such as red tides, extreme cold events, and oil spills. For instance, a severe cold event in January-February 2010 resulted in a large fish kill of snook, which is a very cold sensitive species, in Florida estuaries. In 2010, the FIM program data indicated that snook abundance was lower than any year since sampling started (1996). This information led to an emergency closure on the harvesting of snook until they recovered.

Management measures, such as the one-fish-per-person, per-day bag limit, and annual closed seasons protect snook during vulnerable times such as cold weather and spawning. Other measures, including minimum and maximum size limits protect juvenile and older breeding fish within the population.  These management measures contribute to rebuilding and protecting snook populations on both coasts. Snook numbers currently meet or exceed FWC management goals and a limited harvest season for snook is allowed. Visit the snook regulations Web page for specific East coast and West coast seasonal closures and maximum and minimum size limits.

Three line graphs that show a sharp drop in number of red drum and spotted seatrout collected. Snook had a smaller drop.

Abundance trends of common snook, red drum, and spotted seatrout before and after the 2005 Red Tide event in Tampa Bay.  Red drum and spotted seatrout populations were more affected by the red tide event than snook. These FIM data documented the initial impact and subsequent recovery of each of these species.

Another example of an environmental disturbance that was assessed using FIM program data was the prolonged red tide event of 2005. Although not resulting in immediate management actions, the effect of this red tide event is evident in the FIM program abundance trends for several economically important species (snook, red drum, and spotted seatrout) in Tampa Bay. Data analyses such as these provide valuable tools for making informed and timely management decisions.

Sound management is required to preserve the health of Florida's marine environment for our present and future enjoyment. The FIM program’s stratified-random sampling design and extensive fisheries surveys provide abundance estimates and population parameters needed to determine stock sizes and trends.  These data and analyses are vital for determining appropriate fisheries management measures and to assess the effectiveness of those measures after they are enacted.