Water quality and physical features of goliath
grouper nursery habitat in the
Ten Thousand Islands
Anne-Marie Eklund, Steve Wong, Jennifer Schull
NOAA-Fisheries, Southeast Fisheries Science Center,
Miami, Florida
Matt Finn
Huckleberry Fisheries, Goodland, Florida
Chris Koenig and Felicia Coleman
Florida State University, Tallahassee, Florida
The objectives of this project were to characterize the
essential habitat for juvenile goliath grouper in their
historical center of abundance, the Ten Thousand Islands of
southwest Florida. All harvest of goliath grouper has been
prohibited in U.S. waters since 1990, and the species is a
candidate for the U.S. Endangered and Threatened Species
List.
We compared the abundance of juvenile goliath grouper
from 1999-2000 in natural tidal passes, or rivers, to their
abundance in channelized canals (Figure 1). These rivers
and canals link the upstream freshwater system of the Big
Cypress Basin to the system of bays that empty into the
Gulf of Mexico through a series of channels around
mangrove islands.
Most of the area is completely
undeveloped and protected, yet it is downstream from areas
in the Big Cypress Basin that have been subjected to
massive changes in water delivery over the years. This
area will be affected by the activities of the Southern
Golden Gates Estates restoration project (SGGE).
The natural rivers should provide optimal microhabitat for
the juvenile goliath grouper, including mangrove overhangs
along eroded shorelines and rocky depressions in tidal
passes.
Canals, on the other hand, tend to have
straightened shorelines with little to no eroded banks and
mangrove overhangs. Since they are dredged, canals are
also of relatively uniform bathymetry, lacking the natural
depressions that rivers contain. We hypothesized that the
physical features of the two habitat types would differ and
that the goliath grouper would be more abundant in the
natural rivers.
In each river/canal, we used 40 crab traps and 10 fish traps,
with each trap placed 0.05 nautical miles (nmi) apart.
Beginning in 2000, we deployed YSI datasondes to
continuously measure temperature, salinity, dissolved
oxygen and depth during the time that the traps were
soaking. The amount of eroded (vs. depositional or
straight) shoreline was measured by taking GPS waypoints
at the beginning and end of each section of eroded
shoreline and measuring the distance between the two
points using GIS ArcView software. The heterogeneity of
the bottom (i.e. the presence/absence of rocky holes and
other obstructions) was estimated by taking a depth reading
every 0.1 nmi along each side of the river/canal. The
change in depth from each reading was then calculated and
averaged for the entire river/canal.
A total of 687 juvenile goliath grouper were caught in the
nine rivers and canals that were sampled from 1999-2000.
There was a lot of variability of catch among the rivers,
making comparisons of rivers and canals more difficult that
we had initially thought. Goliath grouper CPUE and total
catch were highest in Little Wood River, Palm River, and
Blackwater River. Very few goliath grouper were caught
in the Wood, Pumpkin and Whitney Rivers, however, due
to anoxic conditions in these rivers. While two of the
canals, Faka Union and 92 West Canal, had lower CPUE
and total catch of goliath grouper than in several rivers, 92
East Canal had higher CPUE and total catch than many of
the natural rivers.
Our hypothesis that natural rivers would provide better and
more habitat than canals would for juvenile goliath grouper
was based on the assumption that the shorelines and
bathymetry would be vastly different between rivers and
canals.
Rocky holes and mangrove undercuts should
provide optimal habitat for goliath grouper in the form of
shelter from current and an ideal location for ambush
predator activities. Our visual assessment of Faka Union
Canal bore out that hypothesis, since the canal is
completely straight with no eroded shorelines and a
uniform bathymetry. The two other canals, however, are
not completely straight and the slight meanders have
resulted in some shoreline erosion, providing undercut
habitat for goliath grouper and other fish. In fact, the 92
East Canal had more eroded shoreline than all of the natural
rivers, except the Little Wood River. However, none of
the canals had heterogeneous bathymetry, as expected.
Optimal conditions for juvenile goliath grouper appear to
be a combination of the presence of eroded shorelines and
depressions along the bottom, high tidal amplitude and tidal
currents, salinity variation within an acceptable range (so
that the area does not become totally fresh or hypersaline)
and no extended periods of anoxia. The Little Wood and
Palm Rivers had the best combination of all of these
features, and consequently yielded consistently higher
numbers of goliath grouper. What may be more important
to determine, however, is why some of the rivers in the Ten
Thousand Islands provide more of the optimal habitat and
conditions than the other rivers, particularly since the rivers
are adjacent to one another and are all downstream from the
same Big Cypress Basin.
The SGGE may have direct impacts on the habitat of
juvenile goliath grouper in the Ten Thousand Islands.
Precious little is known concerning the animals in the
natural rivers and canals in the area, with the bulk of the
research activities taking place in upland and freshwater
areas. However, the restoration activities will surely affect
areas downstream. The Wood River, for instance, may be
starved of upstream water flow, due to the presence of the
Faka Union Canal. The SGGE project includes plugging
the vast network of canals that lead to Faka Union in order
to restore a more natural sheet flow. Undoubtedly, the
Faka Union Canal and the adjacent riverine systems will be
altered by these changes.
The Whitney River has enough eroded shoreline and
bathymetric complexity to be considered a river with
optimal physical habitat for goliath grouper. The lack of
water flow and periods of anoxia, however, led to high
mortality of organisms found in our traps and a very low
catch rate of fish.
Should the sheet-flow be restored
throughout much of Big Cypress Basin, areas like the
Whitney River may be restored for goliath grouper and
possibly other species of fish to have more optimal habitat
in which to reside and grow.
Figure 1. Six natural, tidally influenced rivers and three canals in the
Ten Thousand Islands of southwest Florida, U.S.A. Canals on either
side of U.S. Highway 92 are labeled 92 Canal West and 92 Canal East.
The dots on the river and canal transects designate locations where fish
traps and crab traps were placed to catch juvenile goliath grouper,
Epinephelus itajara, from 1999-2000.
Eklund, Anne-Marie, NOAA-Fisheries, Southeast Fisheries
Science Center, 75 Virginia Beach Drive, Miami, Florida
33149, Phone: 305-361-4271, Fax: 305- 365-4102,
anne.marie.eklund@noaa.gov.