Visual surveys are used to estimate relative abundance and to monitor the size class distribution of economically important fish species in coral reef areas of the Florida Keys National Marine Sanctuary.
The Fisheries-Independent Monitoring (FIM) program at the Fish and Wildlife Research Institute (FWRI) is a long-term project designed to evaluate fishery resources in Florida. The program uses a technique called stratified random sampling (SRS), which is a statistical means of resolving the complications caused by variations in habitat, to provide valuable information to fisheries managers on relative abundance, size structure, distribution, habitat use, and recruitment.
In 1999, the FIM program extended its statewide sampling effort to include the coral reef areas of the Florida Keys National Marine Sanctuary (FKNMS). The FKNMS, with an area of 9,500 square kilometers (2,800 square miles), is one of our country's largest marine sanctuaries and features the only living coral barrier reef in North America.
The sanctuary includes the coastal and oceanic waters immediately surrounding the entire Florida Keys and has a diversity of coral reef, hardbottom, and sea grass habitats. This complex ecosystem supports many important reef-fish fisheries, primarily species of snapper, grouper, and grunt. The Final Management Plan for FKNMS developed in 1996 by the U.S. Dept. of Commerce identified our FIM program as a major component of the Sanctuary's long-term fisheries monitoring program.
In contrast to the estuarine waters and species sampled in other areas of the state, the program in the Keys operates in mostly open waters monitoring reef tract fishes. This shift in emphasis required the development of new sampling methods and techniques. Instead of the more customary net-based sampling procedures like seining or trawling, the FIM program in the FKNMS uses divers to collect data on species' abundance and size structure of the populations. Underwater visual censuses are an efficient, nondestructive way to observe and study fish communities in environments that do not allow the use of nets. Visual surveys in the Keys follow the standard FIM stratified-random sampling (SRS) design.
Sampling operations are being conducted in four zones of the FKNMS (Figure 1). Zone A is the northernmost and easternmost region of the Sanctuary adjacent to Key Largo and covers John Pennekamp Coral Reef State Park on the Atlantic side of the Keys. Zone B extends from the southwestern end of Key Largo along the rest of the Upper Keys to the southern tip of Everglades National Park just north of Long Key; the Atlantic side of Zone B extends over the reef tract. Zone C circles the Middle Keys from Long Key to Big Pine Key. Zone D surrounds the Lower Keys (Big Pine Key to Key West). For stratified-random sampling, the zones are subdivided into grids of cells; each cell has an area of one square nautical mile. Habitat type within each cell is identified using the Florida Keys Benthic Habitat Geographical Information System, and cells containing coral reef, including margin reef and patch reef habitats, are then randomly selected for survey sites.
Two visual sampling methods have been used by the FIM program: point counts and transect surveys. Point-count surveys are conducted by a stationary scuba diver who conducts a census of an area equivalent to a cylinder with a five meter radius for a time period of three minutes. Transect surveys are conducted by a diver swimming a given distance (30 m) and monitoring fish within five meters on either side. The minimum criterion for horizontal underwater visibility during a survey is eight meters. During transect surveys, divers count and assign to five centimeter size intervals, all snappers, groupers, hogfish, triggerfish, butterflyfish, angelfish, whitespotted filefish, and bigeyes. During point counts, the same information is recorded for these species and for all grunt species as well; grunts are too numerous to survey accurately on a transect survey. Estimates of abundance are then calculated as the number of fish per 100 square meters of a sampled area. The data collected by visual census are summarized and analyzed for all fish species counted and measured.
Both point counts and transects were used together for the first two years (1999-2000) of the FIM program in the sanctuary. During this initial period, three point counts and three transects were conducted on each of 26 sample sites each month. Comparison of the results showed that point counts were the better technique for visual surveys. The point count method allows divers more observation time, thereby providing more accurate counts and size estimates. Point counts have the added advantage of being logistically easier to execute. Therefore, a new sampling protocol for the visual surveys was initiated beginning in 2001: the transect method was discontinued, the number of sites was increased to 39, and four point counts were done at each sample site instead of three.
Sampling in the sanctuary is conducted monthly from April through October and usually discontinued for the winter, as strong trade winds in late fall and in winter reduce water clarity. However, the winter of 2000-2001 was an exception; unusually calm weather allowed continued sampling from November through March.
FIM's program of visual censusing provides a unique source of information on populations of economically valuable reef fish species. Results from this study are coupled with life history studies being conducted concurrently on many reef fishes to establish important population parameters like growth rates, size and age of maturity, and reproductive potential. Together, visual surveys and life history studies provide a powerful tool for monitoring the status of the populations of these highly valued species in the waters of the Florida Keys.