Futuristic Seafood: Raising Delicious Cobia and Pompano—Inland!

Two saltwater superstars-cobia and Florida pompano-are regarded by connoisseurs as being some of the world's best seafood. Both cobia (pronounced COE-bee-uh) and pompano (POM-puh-no) have firm, mostly white flesh that's perfect for grilling, pan-frying or baking.

Now, Agricultural Research Service (ARS) agricultural engineer Timothy J. Pfeiffer, fish nutritionist Martin A. Riche, and fish biologist Charles R. Weirich-all based in Ft. Pierce, Fla.-are determining how to best raise cobia and pompano inland, hundreds of miles from the nearest ocean or bay, in huge tanks of fresh or only slightly salty water. The tanks are part of what's known as a "recirculating aquaculture system," or "RAS," in which water is cleaned and used again and again.

These systems offer the potential to reduce discharge of everyday fish-farm effluent to as little as 3 percent-or less-of the total amount of water used each day. Fish wastes and unused food collected in the system could be recycled as nutrient-rich compost.

But much more remains to be discovered about the needs of the saltwater fish that would be reared in the tanks. And many engineering details must be worked out. In an experiment with 2,400 juvenile pompano, the scientists showed that it's indeed possible to raise this oceanic species-from juvenile to market size-in water that's only slightly salty.

In this case, the water had a salinity of only 5 parts per thousand, as compared to the 35 parts per thousand in most oceans. Now the scientists want to make the system practical, profitable, and energy efficient for all stages of inland, low-salinity production of cobia and pompano.

What to feed the saltwater fish during the various stages of their captive lives is, of course, one of the most pressing questions. In a study with 50,000 pompano larvae, fish biologist Weirich and colleagues learned more about the feeding habits of these baby fish. For the study, the researchers used digital photography and image-analysis software to observe and record the larvae, which, in their earliest days, are transparent and look something like tadpoles.

The scientists determined the optimum size classes of two kinds of live prey-brine shrimp and tiny organisms called "rotifers." At various stages of their growth, the little fish are capable of catching and eating these snacks as they swim by. During this growth phase, the upper and lower jaws of the larvae, and their miniature digestive systems, are still developing. The fish are born with their mouths closed, and the size of the opening, or "gape," that develops determines the size of the food-in their case, rotifers and brine shrimp-that they're able to nab and swallow.

In new studies, Weirich and colleague Riche are determining how to wean the larvae off of these live feeds more quickly. That's because cultivating live feeds for the infant fish is more costly and labor-intensive than simply giving them dry feeds. Those feeds won't appear on their menu until the fish have made the transition from larvae to juvenile.

Moving juveniles from the nursery tanks to the world of the grow-out tanks-where they'll develop to market-ready weights-poses the question of how many of these young fish can be put into a tank without overcrowding them.

Overstocking can cause stress and may increase risk of disease, reduce growth rates, and create other problems that can bite into profits. Weirich, Harbor Branch Oceanographic Institute colleague Paul Wills, and Riche found that pompano stocked at the rate of 200 fish per tank weighed about 1.5 pounds at harvest, making them about 10 percent heavier than pompano stocked at a rate of 400 per tank.

But a cobia study by Weirich and Wills showed no significant difference in their survival, or weight at harvest, regardless of whether the fish were stocked at a low (35 fish per tank), medium (70 fish), or high (106 fish) density. At the end of the study, all the cobia weighed about the same. Each yielded two generously sized fillets.

No matter what the stocking rate, finfish won't flourish in recirculating tanks if there are life-threatening problems with the quality of the water. To keep the recirculating water clean and to limit risk of disease, the researchers are determining the most cost-effective use of what are known as "biofilters." These devices contain bacteria that convert the toxic ammonia in fish waste into less-toxic forms (nitrite and nitrate). In other work, they're evaluating alternative methods to efficiently remove solids-such as waste and uneaten feed-from the system.

Pfeiffer is leading studies of what is known as "airlift technology" to power the system's water-treatment components. Airlift systems can cut energy costs because they use less electricity than conventional centrifugal pumps for moving water.

Pfeiffer, Riche and Weirich work for the Arkansas-based ARS Harry K. Dupree Stuttgart National Aquaculture Research Center, and are stationed at Florida Atlantic University's Harbor Branch Oceanographic Institute in Ft. Pierce.