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Interactions among groupers and sharks at grouper spawning grounds in the US
Virgin Islands |
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Dr.
Brad Wetherbee
- Department of Biological Sciences, University of Rhode
Island Dr.
Mahmood Shivji Dr.
Ben Victor - Ocean
Science Foundation Dr. Richard Nemeth- Center for Marine and Environmental Studies, University of the Virgin Islands Click here for a video summary
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| Populations
of several species of groupers found off Florida and throughout
the Caribbean have declined drastically over the past few
decades to the point where groupers are among the species of
reef fish for which there is the most concern by agencies tasked
with management of coral reef resources.
These population declines have led to enactment of
restrictions on targeted fisheries, establishment of marine
protected areas at spawning sites and other locations, and even
listing by NMFS as species of concern with consideration for
possible listing as threatened or endangered species under the
ESA. These measures
are intended to reduce fishing pressure on groupers and enhance
recovery of their stocks. Management
priorities include identification of essential fish habitat
including grouper spawning sites.
Although much emphasis has been placed on ecosystem-based
management of marine resources, one aspect of the ecology of
groupers on their spawning sites that has received little if any
attention is their interaction with predators.
Anecdotal reports of increased shark abundance at grouper
spawning sites during spawning activity, coupled with ecological
tenets concerning the ability of predators to limit population
growth of prey, suggest that shark predation on groupers at
their spawning sites represents a constraint on population
growth, and in this case a limit to the rate of recovery of
grouper stocks. Despite
the logic of such assumptions, other instances of a perceived
increase in predator abundance have actually been tied to the
presence of more human observers studying the prey species at a
particular time rather than an influx of predators from any
considerable distance.
This project is aimed at identifying the species of sharks
associated with grouper spawning sites off St. Thomas, USVI.
The Red Hind Bank Marine Conservation District and a similar
refuge at the nearby Grammanic Bank contain spawning sites used
by red hind, Nassau grouper, tiger grouper and yellowfin
grouper. Fishers and researchers have noted an increase in
the abundance of a number of species of sharks at these
locations during peak spawning season. The assemblage of
sharks observed at these sites includes a variety of species
that vary in size, reported feeding habits and documented
movement patterns (nurse, Caribbean reef, lemon, bull, great
hammerhead and tiger sharks). This project will survey the
sharks that occur at these spawning aggregations to document the
relative abundance of shark species at the sites and will
utilize acoustic transmitters and an array of automated
receivers to passively monitor site fidelity, residence and
movement patterns characteristic for the major species of shark
that are present. The use of depth sensitive transmitters
for both sharks and groupers will enable investigation of
vertical movements of sharks in the water column in relation to
similar movements of grouper during well-synchronized temporal
spawning events. Receivers placed within the refuge and at
adjacent sites will address the question of whether there is
actually an observable increase in shark activity at the sites
during the spawning aggregations, whether sharks present are
local sharks that are constantly present or that move a short
distance to the sites, or whether sharks are moving into the
area from greater distances. Such information will
ultimately enable assessment of the potential scale of shark
predation on groupers at spawning aggregations. Our study
will also provide baseline data for tracking changes in shark
abundance as grouper populations increase and for comparison
among spawning aggregations throughout the Caribbean.
We have recently begun deploying pop-up satellite transmitters on tiger sharks at the spawning site. Click the picture below for a short video of a tiger shark carrying a Microwave Telemetry Inc. satellite transmitter.
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