Snook - prized by anglers for decades because of their acrobatic leaps on
the end of a fishing line and their ability to evade capture more often than
not - are one of the most important catches in Florida's saltwater
recreational fishing industry. Their popularity, however, has a downside:
fishing pressures have placed them on the state's list of "species of
special concern" and resulted in the need for fishing restrictions and
careful monitoring. "It's a very valuable species," said Dr. Kenneth Leber,
Director of Mote's Center for Fisheries Enhancement. "The saltwater fishing
industry brings $5.4 billion into the state's economy each year. And at the
rate it's growing, we're going to need even stronger protection measures in
the future." Right now, on Florida's west coast, anglers are allowed to keep
one snook per day. "The angling population is going to double; what do you
do? Reduce that to half a fish per day?" Leber asked. Such considerations
have led researchers to study essential fish habitats and how to protect
them so fish can maintain their numbers.
Other efforts - those at Mote Aquaculture Park
- focus on growing marine species in captivity so they can be released to
help replenish wild fish stocks. Wild snook spawn naturally from May through
September when groups of males and females gather in passes and release eggs
and sperm into the water column. Once fertilized, the eggs float on top of
the water until they hatch into larvae. To get eggs for Mote's snook
breeding program, researchers have to head out into the field and collect
eggs and sperm as the fish prepare to spawn. "We're entirely dependent on
the natural environment," Leber said. "So when there's something like red
tide, it makes it difficult to nearly impossible to gather the eggs so we
can rear snook for experimental stocking programs. We need a way to
successfully control the situation so that we're not subject to the whims of
Mother Nature. That's where Mote Aquaculture Park comes in. Building on
studies by Drs. Yonathan Zohar and John Stubblefield at the University of
Maryland's Center for Biotechnology, and those performed by Dr. Cheng-Sheng
Lee at the Oceanic Institute in Hawaii, Dr. Kevan Main and her staff have
grown snook to maturity spawned them in captivity. "People have captured
mature snook and had them spawn in tanks a short time later, but until now,
they have not been able to grow younger snook to maturity and have them
spawn in captivity," said Main, director of Mote's Center for Aquaculture
Research and Development. "Maturation is really the key here." Main reported
the team's results at the World Aquaculture Society's annual meeting in
February 2007. "I think one reason snook spawning has not been successful is
because people have been working with small tanks," she said.
A key to getting snook to spawn in captivity has also been the ability to
control other environmental elements. Mote's 14,300-gallon snook tanks are
housed inside "barns" at the aquaculture park where Matt Resley, senior
biologist, can adjust water temperature and lighting, mimicking water
temperatures, sunrise, sunset and lunar cycles. Conditions were adjusted to
mimic winter for two months, spring for one month and then summer for four
months. The snook were sampled to determine their maturation status - that
is, whether sperm and eggs were present and whether they were the
appropriate size for spawning. When the time was right, the female snook
were given a low-dose, slow release hormone to provide the final push needed
for them to spawn. "There are so many environmental factors that play into
snook spawning in the wild," Resley said. "We can't replicate all the
factors, but we can control the key elements. That gets them really close to
spawning, but it doesn't give them that final push."
Zohar found in studies of other species that fish spawn only after a
chemical in their brain tells their bodies to release certain hormones. In
captive fish, that chemical may be missing and the fish won't spawn. Zohar
subsequently synthesized a gonadotropin-releasing hormone (GnRH) that
provides the final cue. "It actually acts on the thyroid, which regulates
hormones," Resley said. "And it's a protein so it breaks down quickly in the
fish. Because it acts on the fish brain and not the egg, it's not
transferred to the larvae." The next steps are improving egg collection
techniques in the tanks, enriching diets of breeding fish with vitamins and
expanding to new marine species, such as sea trout and red snapper. "Closing
the life cycle for production of marine species for both food and for
restocking is a key element to making the aquaculture industry successful,"
she said. "These studies are moving us closer to our goals of creating a
viable industry in Florida and providing new sources of food to meet the
increasing demand for seafood."
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