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
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
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.