Cold therapy puts fish in the mood

Seabass are promiscuous, but if the atmosphere’s not just right, they’re not in the mood. Dim the lights and chill the air, and suddenly they’re raring to go.

At the National Institute for Oceanography and Fisheries in Alexandria (NIOF), researchers have been experimenting with controlled environmental conditions in order to increase the sexual appetite of marine hatchery fish. Last August they successfully mated five broodstock pairs of seabass by tricking them into believing it was winter, their traditional mating season.

“It was the first time in Egypt to successfully spawn seabass during the summer,” says Mohamed Abdel Razek Eissa, head of NIOF’s aquaculture department.

The achievement is a milestone for Egypt’s aquaculture industry. While the experiment needs to be replicated and refined for commercial use, hatchery operators could soon be applying the technique to produce marine finfish seed year-round, obviating the need for collecting fry from the wild.

Hatcheries currently supply less than five percent of the fry used as seed for Egypt’s marine aquaculture projects. The remainder–more than 80 million a year–are collected from estuaries and brackish lakes along Egypt’s northern Mediterranean coast. The annual collection has put aquaculture in direct competition with traditional fishermen, who blame the growth of fish farming for their declining catch.

In an effort to protect wild stocks, the government has issued new regulations prohibiting the collection of marine fry from the sea. Once the moratorium comes into effect in 2013, all mariculture projects will be required to use hatchery-raised fry as seed stock.

The approaching deadline has researchers scrambling to make broodstock more productive–in essence, to give fish more mojo.

“We have just three years to increase our hatchery capacity,” says Eissa. “If we can spawn broodstock more than once a year, it would double or triple the amount of fertilized eggs.”

During a recent visit to the NIOF research facility, Eissa showed off the “nuptial chamber” where seabass couples mated last summer. Outside the mercury was at 33C; inside the sealed, air-conditioned chamber the temperature was nearly 20 degrees cooler.

The water temperature of the four fiberglass tanks inside was gradually brought down to 15C and the lighting adjusted to mimic the sun low on the horizon. The simulated winter conditions triggered a hormone in the brains of the fish brain, causing physiological changes and reproductive urges to kick in.

The five broodstock pairs produced the usual quantity of eggs–about 600,000 in all–but only 110,000 were fertilized. Eissa attributes the low fertilization rate to chemical imbalances resulting from the out of season spawning. He feels confident, however, that further experimentation would help to increase the survival rate.

More problematic is the high cost of refrigeration.

“It is very expensive to decrease the air and water temperature during the hot summer months,” says Eissa, pointing to a bank of wheezing air conditioners.  “It is really only economical to induce seabass to spawn in the spring or the fall.”

But even that has enormous commercial value. Spreading out the spawning period over three seasons would allow mariculture projects to acquire new hatchery fingerlings to replace those that have grown into larger fish, increasing the efficiency of their production lines.

Researchers have also noted that several species of marine finfish used in Egyptian mariculture reproduce under “less intensive” conditions. Sea bream and sole, which instinctively spawn in fall and spring respectively, become fertile when water temperatures hover around 20C.

“They may be good candidates for summer spawning,” Eissa suggests.




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