Eddies found to enhance survival of fish in sub-tropical waters

A new U.S. study finds that young fish reared in nutrient-rich eddies in the Straits of Florida grew faster and had a survival advantage compared to their counterparts outside eddies.

The findings, resulted from a study which sequentially sampled tropical fish from their larval stages to their settlement in reefs and published in Proceedings of the National Academy of Sciences (PNAS), confirms earlier hypothesis that swirling eddies in the ocean are beneficial to organisms such as larval fishes residing within them.

"Eddies upwell nutrients and provide a high-productivity environment that gives larval fishes growing there a head start on survival," said Su Sponaugle, principal investigator on the study and a marine biologist affiliated with both Oregon State University and University of Miami.

"In cooler springtime waters, when larval fish are growing more slowly, the difference between fish raised inside or outside of eddies is small. By August, when warm waters elevate fish growth rates, food becomes scarce and larval fishes residing inside eddies are more likely to survive."

In their study, the researchers collected larval fishes both inside and outside of eddies, focusing on three species - bluehead wrasse (Thalassoma bifasciatum), bluelip parrotfish (Cryptotomus roseus) and bicolor damselfish (Stegastes partitus). They determined the daily growth rates of the fish through examination of their otoliths, or ear stones, and found that those raised within the eddies had substantially higher growth rates than fish captured outside the eddies.

A few weeks later, they sampled young juveniles that had settled to nearby reefs and were able to determine that almost all of those that survived to the juvenile stage had growth patterns similar to larvae from eddies.

Fish raised inside of eddies have different growth signatures in their otoliths than those raised outside eddies, explained Kathryn Shulzitski, lead author and assistant scientist at University of Miami. "This is the first time we have been able to sample fish throughout their larval upbringing offshore to their life as juveniles on the reef and see which fish had a survival advantage."

The researchers theorize that larval fish residing outside of eddies either starve to death or become sufficiently weak that they are more susceptible to predators.

"If there are areas where eddies predictably occur, these could be considered pelagic nursery areas that would warrant higher levels of protection from human interference," said Robert Cowen, director of Oregon State University's Hatfield Marine Science Center and a co-author on the study, about what resource managers and fish population modelers could learn from the findings.

"Further, the role of these eddies should be incorporated into modeling efforts, which inform decision-makers," Cowen said, "the influence of eddies may become even more important with warming oceans." Endit