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."