Exercising Fish to Improve Growth Rates–
Fish who swim more grow faster, scientists report
Researchers:
Mary Sue Lowery
Professor of Biology
University of San Diego
E.: slowery@sandiego.edu
T.: (619) 260-4078
Kevin Kelley
Associate Professor
Biological Sciences
CSU Long Beach
E.: kmkelley@csulb.edu
T.: (562) 985-4294;
(562) 985-4845 (lab)
Mark Drawbridge
Senior Research Scientist
Hubbs-SeaWorld Research Institute
San Diego, Calif.
E.: mdrawbridge@hswri.org
T.: (619) 226-3943
Chris Peters
Research Assistant
Hubbs-SeaWorld Research Institute
San Diego, Calif.
E.: cpeters@hswri.org
T.: (303) 941 6623
Tools:
Revised:
September 5, 2008
White seabass in flume. Credit: Chris Peters
September 5, 2008
Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334
In aquaculture, feed costs can represent 50 percent of the total expense of rearing fish to harvestable size. So, it might seem odd that scientists are experimenting with raising California yellowtail in tanks in which the fish must continuously swim against a steady current. After all, we know from the Beijing Olympics of the huge energy requirements of feeding a swimming machine such as Michael Phelps, who reportedly inhales 10,000 calories a day, about five times that of an average sedentary person.
Rearing such Phelpsian fish would be economically untenable, of course. Who could afford the groceries? But as recent California Sea Grant research shows, there may be a middle ground – a low-intensity training regime applied for a set period during a fish’s juvenile stage. Within this window, when fish are already in a growth spurt, the benefits of exercise on their health and growth could outweigh additional operating expenses.
The key discovery motivating this idea: moderate aerobic exercise can lower a fish’s bloodstream level of the stress hormone cortisol. In yellowtail, better known in sushi-eating circles as hamachi, less cortisol translates into faster weight gain, which has obvious bottom-line benefits including, potentially, higher filet yield.
“Cortisol tells the body to conserve energy and not do energy-expensive things like grow,” explains Kevin Kelley, a marine biology professor at Cal State University, Long Beach, a co-investigator on the project with biology professor Mary Sue Lowery of the University of San Diego. “By lowering cortisol, you enhance growth rates. This is the punch line of the study. Exercise is about stress management as much as it is about being fit.”
In the experiments, juvenile yellowtail swimming at speeds corresponding to about 60 percent of their maximum aerobic capacity had a 17 percent greater body mass after 34 days of conditioning, as compared to couch potato control fish living in a motionless tank. This translated into “fit” fish weighing 60.3 grams at the end of the study, about the weight of a Snickers candy bar, vs. 52.6 grams for the unconditioned control fish.
Raceways in which experiments were conducted. Credit: Christina S. Johnson
Exercise – induced by subjecting fish to a steady head-on current, an aquatic version of a treadmill – was also shown to promote weight gain in white seabass, a type of drum and a popular sport fish in California. Growth effects were not observed, interestingly, in California sheephead, a long-lived, sequentially hermaphroditic nearshore reef species.
More experiments are needed to explain the uneven growth response, but it may have to do with the plodding growth of sheephead, as compared with that of yellowtail, says former Sea Grant Trainee Chris Peters, now a research assistant at Hubbs-SeaWorld Research Institute in San Diego. Between the ages of 80 days and 150 days, sheephead increase their body mass by less than 1 percent a day, compared with about 6 percent for unconditioned yellowtail. For such slow-growing species, it may take a longer period of conditioning to detect the growth effects of exercise, he says.
Consistent with this idea is the observation that sheephead did improve their aerobic capacity in response to exercise. In other words, they could get fit, if not “buff.” Fitness was measured by examining the fraction of slow-twitch red to fast-twitch white muscle fiber. In sheephead, and also yellowtail and white seabass, exercise led to a greater proportion of red muscle fiber. A similar training effect is observed in long-distance cyclists and runners.
“Maybe we are not enhancing growth rates in every case,” Peters says. “But in every case, we are making them better swimmers.”
The Hubbs-SeaWorld Research Institute currently raises white seabass for stock enhancement. Mark Drawbridge, a senior research scientist at HSWRI and a co-investigator on project, believes that conditioning juvenile white seabass prior to release – the fish are released when they are about 30 centimeters long – might boost their survival rates in the wild by improving their ability to forage and evade predators.
In terms of aquaculture, induced exercise is envisioned as being temporary, something done for a brief time to make the fish stronger, prior to transferring them to open-ocean net pens, where currents cannot be controlled.
“There are people who think you can raise fish in land-based tanks their whole life, but we are skeptical,” Drawbridge says. “There are such high energy costs and space requirements.”
Net pens also have the benefit of superior water quality and large size. “The fish are happier,” Drawbridge says. “However, those who do operate tanks would likely benefit by exercising their fish.”
HSWRI is experimenting with rearing unconditioned yellowtail in net pens in Mexico, but to date no conditioned fish have been released into these pens. The scientists would like to investigate whether early exercise has sustained benefits for adult fish in terms of size, survivorship and disease-resistance, and whether exercise improves feed conversation rates, which potentially could offset any energy costs associated with having to generate water flows.
“Exercise seems to have benefits, but how much does it cost?” Lowery asks. “That would be the next step to this study.”
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