Fisheries-Independent Monitoring Using Stratified-Random Sampling

The management of Florida's marine and estuarine fisheries resources requires the collection of a variety of information on many species. To help provide that information, the Fisheries-Independent Monitoring (FIM) program established a survey project using stratified-random sampling (SRS), a technique used to describe and compare population trends. FIM initiated stratified-random sampling in 1988 with a survey of Tampa Bay. Surveys are underway in seven regions: Apalachicola Bay, Cedar Key, Charlotte Harbor, Florida Keys National Marine Sanctuary, Indian River, Northeast Florida, and Tampa Bay. From 1992 to 1996, FIM also used this method to survey the Choctawhatchee Bay region of the Panhandle.

Stratified-random sampling, an approach designed to reduce statistical error, is a tool for managing the habitat variations that complicate data collection. Using SRS, FIM divides each selected survey region into zones based on logistical and hydrological characteristics. Each zone is then stratified, or assigned, into areas by habitat (for example; depth, seagrass beds, shore type, etc). FIM conducts monthly sampling at sites randomly selected from the strata available for each zone.

To ensure the sampling of a wide range of fish sizes and ages during an SRS survey, FIM uses a variety of techniques and fishing gear to collect fish population data . Smaller fishes are usually collected with a 21-meter seine or a 6.1-meter otter trawl. FIM uses the 21-meter seine in water depths of 1.8 meters or less; they use the trawl in water of greater depths. Larger subadult and adult fishes are collected using 183-meter haul and purse seines. Program biologists use a haul seine along shoreline habitats and a purse seine for open bay sampling. FIM also employs visual surveys, during which divers count, identify, and estimate the sizes of fishes observed, to collect data in the Florida Keys National Marine Sanctuary.

Fisheries-Independent Monitoring program scientists try to describe every possible variable for the sampled regions. Fish data collected include species, size, sex, and numbers of fish caught. For each site, FIM records habitat features such as the type and quantity of submerged and shoreline vegetation and the presence of seawalls or oyster beds. Measurements of water quality include temperature, pH, salinity, and dissolved oxygen. Scientists examine the fish collected at each sample site for any external abnormalities or signs of poor health. Tissue samples are taken from selected fish for analysis of mercury content.

Resource managers use the data from these surveys in many important ways. In addition to providing estimates of the relative abundance of many economically and recreationally important species, the data allow the development of annual abundance models of juvenile fishes. These models may be used to predict the availability of a species in the near future.

Often, the information is also useful in maintaining and creating recreational fishing regulations. Size restrictions for various fish species are largely determined by SRS data, which describe the size and age structure of populations. Finally, surveys provide information about fish distributions and habitat use to the Inshore-Marine Monitoring and Assessment Program (IMAP), a multi-agency project characterizing the ecology of inshore waters in Florida and several other states.

Sound management is required to preserve the health of Florida's marine environment for our present and future enjoyment. Using SRS, the Fisheries-Independent Monitoring program provides the numbers and information needed to determine necessary fisheries management measures and to assess the effectiveness of those measures after they are enacted.