Regional Assessment of Hard-bottom Communities in the Florida Keys with an
Emphasis on Lobster and Sponge Dynamics
Mark J. Butler IV
Old Dominion University, Norfolk, VA
John H. Hunt
Florida Marine Research Institute, Marathon, FL
Background
Nearshore hard-bottom habitat is a ubiquitous feature of the shallow waters within
the Florida Keys, yet remarkably little is known about their structure or ecological
function, a fact highlighted in recent years by questions about the possible impact of
environmental change and resource exploitation on these communities. During the
past decade, hard-bottom communities in south-central Florida Bay were altered by
widespread sponge die-offs. There are now concerns about the possible impact of
Everglades restoration and salinity change on hard-bottom communities in this
same region. The potential impact on spiny lobster nurseries are of particular
concern because of the economic importance of this species in the region. In
addition, the perceived over-exploitation of shallow water sponges by the
commercial sponge fishery has prompted a call by some for the closure of the
fishery. Resolution of this issue is hampered because there has been no stock
assessment of sponges in the Florida Keys, the population dynamics for the
pertinent species are largely unknown, and the effect of the fishery on commercial
sponges and allied species in hard-bottom habitat have never been studied.
Objectives
Recently, we began a series of field studies, laboratory studies, and simulation
modeling to better understand commercial sponge population dynamics, to
determine the impact of the sponge fishery on sponge communities, and to examine
the effect of salinity change on lobster, sponge, and octocoral survival in shallow
hard-bottom habitat. Specifically, we seek to:
(1) Conduct the first in-depth, large-scale assessment of shallow hard-bottom
community structure in the Florida Keys.
(2) Determine from repeated field measurements of tagged sponges at various
sites throughout the Florida Keys the necessary site- and size-specific
population dynamics information needed for the management and modeling of
commercial sponge populations.
(3) Conduct field studies in concert with representatives of the fishing industry to
determine the catch efficiency of sponge fishers and the by-catch associated
with sponge harvesting.
(4) Conduct laboratory studies that experimentally test the tolerance of prominant
hard-bottom-dwelling species (e.g., spiny lobster, five sponge species and two
octocoral species) to different salinities, periods of exposure, and water
temperatures.
(5) Incorporate new and existing information in a spatially-explicit simulation
model to quantitatively compare the impact of potential management
strategies on the sustainability of the sponge fishery and its impact on hardbottom community structure in the FKNMS.
Objective 1: Hard-bottom Community Surveys
In the summer of 2002, we completed a comprehensive survey of the density of
60 different species of sponges, octocorals, corals, and macroinvertebrates at 135
shallow hard-bottom sites throughout the Florida Keys. Those data, which we are
now analyzing, will be used to:
(a) characterize hard-bottom community types relative to GIS habitat
designations
(b) determine if biogeographic provinces exist for hard-bottom in the Florida
Keys
(c) ascertain an appropriate subset of sites suitable for long-term monitoring
of hard-bottom in the region
(d) determine the current status of commercial sponge populations in the
Florida Keys.
Objective 2: Sponge Population Dynamics
In the winter of 2002-2003, we established a dozen sites spanning the Florida
Keys where we are monitoring the size-specific growth and reproductive status of
three commercial sponge species: sheepswool sponge (Hippospongia lachne),
yellow sponge (Spongia barbara), and glove sponge (Spongia graminea). At a
subset of those sites we are also experimentally determining whether sponges that
are disattached from the substrate continue to grow or can subsequently reattach
to the bottom.
Objective 3: Fishery Impacts on Sponge Communities
These field experiments are planned for the summer of 2003.
Objective 4: Salinity Tolerance of Prominent Hard-bottom Species
In the summer of 2002, we completed a series of laboratory experiments in which
we tested the salinity tolerance of three size classes of Caribbean spiny lobster
(Panulirus argus), five species of sponge (loggerhead sponge – Speciospongia
vesparium; vase sponge – Ircinia campana; branching candle sponge – Ircinia
sp.; sheepswool sponge; golfball sponge – Cinachyra alloclada), and two species
of octocoral (angular seawhip – Pterogorgia anceps; purple sea plume –
Pseudopterogorgia acerosa) at typical winter and summer water temperature.
The survival of these species were tested at four salinities (15 psu, 25 psu, 35 psu,
45 psu) at winter (18oC) and summer (28oC) water temperatures. The sponges
and octocorals were subjected to two types of salinity stress in separate
experiments. In one, salinities were altered from 35 psu then held constant for the
remainder of the experiment, whereas in the other experiment the species only
experienced the target salinity for 2 days before salinity returned to 35 psu. We
also used laboratory assays to test whether juvenile spiny lobster movement
changed in response to changing salinity. Our results indicate that octocorals
were intolerant of salinity change. Although survival varied among sponge
species, all sponges were generally intolerant of low (15 psu, 25 psu) or high (45
psu) salinity even for short periods of time (2 days), and especially at summer
water temperatures. Only the smallest juvenile spiny lobster died in response to
altered salinity in our experiments; the larger two size classes experienced no
mortality after 6 weeks of exposure. However, larger juvenile lobsters respond to
changing salinity by increasing their movement, but increased movement is not
sustained at the most extreme salinities.
Objective 5: Modeling
We have completed the modeling of potential salinity impacts on spiny lobster,
sponge, and octocoral populations in the Florida Keys. We simulated salinity
conditions in the Florida Keys during an average year compared to an
exceptionally dry or wet year using salinity conditions prevalent in the region
during 1993 and 1995. Florida Bay experiences the most extreme salinities
during wet and dry years and it is in these areas in our model where lobsters,
sponges, and octocorals are most impacted. Sponge and octocoral populations in
Florida Bay experienced mass die-offs in our model under extreme salinities. The
impact of extreme salinity on juvenile lobsters was both direct (i.e., mortality or
emigration in response to salinity) and indirect (i.e., loss of sponge shelter due to
their mortality) and was projected to reduce lobster recruitment by 10-15%.
Mark J. Butler, Department of Biological Sciences, Old Dominion University,
Norfolk, VA 23529, Phone: 757-683-3609, Fax: 757-683-5283,
mbutler@odu.edu, Question 5
John H. Hunt, Florida Marine Research Institute, Florida Fish and Wildlife
Conservation Commission, 2796 Overseas Highway, Suite 119, Marathon, FL
33050, Phone: 305-289-2330, Fax: 305-289-2334, John.Hunt@fwc.state.fl.us