The Long-Term Recovery of Sponge Populations In The Florida Keys, USA
Following a Widespread Mortality
John M. Stevely and Don E. Sweat
Florida Sea Grant Extension Program, Gainesville, FL
During 1992 and 1993, widespread sponge mortalities significantly impacted
sponge populations in the Florida Keys, USA. The extent of the impacted areas
was estimated to be approximately 1,000 km2. The cause of the mortalities was
attributed to cyanobacteria blooms. It was hypothesized that the sponge mortality
resulted from clogging of the sponges’ filter feeding mechanism, bloom toxicity,
or perhaps lowered dissolved oxygen levels. However, the exact cause has not
yet been documented.
The work described here was initiated in response to concerns regarding the
ecological and fishery impacts resulting from increased commercial sponge
(sponges of the genera Hippospongia and Spongia) harvesting effort in the late
1980s and early 1990s. Beginning in 1994, the work entered a second phase:
documentation of the sponge mortality impact on sponge community biomass,
and long-term evaluation of sponge community recovery. Data on the recovery of
sponge populations in two areas has been collected on an annual basis from 1993
through 2002. Additional data has been collected within Everglades National
Park.
Project data documented a highly significant decline in sponge numerical
abundance, with an even more significant reduction (up to 90%) in sponge
community volumetric biomass. However, the severity of the mortality varied
significantly over the affected area. Sponges of the genera Ircinia, Hippospongia
and Spongia appeared to be the most susceptible to the mortality. Spheciospongia
vesparia appeared to be more resistant than many other species, but was
completely eliminated throughout extensive areas. One species, Cinachyra sp.,
appeared to be particularly resistant.
As work has progressed, a more comprehensive description of the sponge fauna in
the study area has been undertaken. Data are now collected for 30 sponge taxa
and we have a reasonable complete description of the sponge fauna and relative
abundance throughout the study area.
The 2002 data clearly document that significant sponge population recovery has
occurred. However, recovery is not complete. Recovery is not uniform
throughout the areas sampled, and several “important” species -- in terms of
contribution to sponge community biomass -- have not exhibited significant
recovery. Two species, Spheciospongia vesparia and Ircinia campana, accounted
for approximately 70% of the sponge community biomass prior to the sponge
mortalities. Significant recovery of S. Vesparia has been observed, but recovery
has not been consistent throughout the affected area. No significant recovery of I.
campana has been observed.
Project data, in general, documented recovery of commercial sponge species
(Hippospongia and Spongia). Sponge harvesting activity has been observed in
areas where commercial sponges had been previously eliminated. However, there
has not been complete recovery throughout the study area.
It is now possible to identify several sponge species that are short-lived, and that
widely fluctuate in abundance. In a sense, it may be impossible to say that these
species have truly recovered because their abundance is probably constantly
changing.
It is also now possible to follow the recovery of long-lived species that comprise
the major portion of sponge biomass. With this information, it will be possible to
truly evaluate sponge population recovery from a natural resource management
perspective.
Stevely, John, Florida Sea Grant Extension Program, 1303 17th St. W. Palmetto,
FL, 34221, Phone: 921-722-4524, Fax: 941-721-6608,
jmstevely@mail.ifas.ufl.edu., Question 4