Ocean surface slicks create superhighways for fish

In Hawaii, bands of smooth water in the upper ocean known as surface slicks create nursery habitats for more than 100 species of commercially and ecologically important fish, according to a new study.

While it was assumed that these calm waters served as a refuge for fish, the researchers were amazed by the diversity of marine larvae they found living in the surface slicks. 

Study lead author Dr. Jonathan Whitney is a marine ecologist at NOAA and a former postdoctoral fellow at the Joint Institute for Marine and Atmospheric Research (JIMAR) at UH Mānoa. 

“We were shocked to find larvae of so many species, and even entire families of fishes, that were only found in surface slicks,” said Dr. Whitney. “This suggests they are dependent on these essential habitats.”

The research is shedding new light on how tiny fish larvae survive in the open ocean as they dodge predators and navigate ocean currents until they reach their adult habitats. 

For their investigation, the team conducted more than 130 plankton net tows in surface slicks and their surrounding waters along the leeward coast of Hawaii Island. 

Despite the fact that the surface slicks only covered around 8 percent of the ocean surface in the study area, they were found to contain 39 percent of the surface-dwelling larval fish. The slicks also contained more than 25 percent of the study area’s zooplankton, which are eaten by larval fish, and 75 percent of the floating organic debris such as feathers and leaves.

Overall, the study revealed that larval fish densities were more than seven times higher in surface slicks compared to the surrounding waters.

The experts determined that ocean slicks serve as a nursery habitat for marine larvae of at least 112 species. The remarkable diversity of fish found in slick nurseries represents nearly 10 percent of all fish species in Hawaii, including jacks, triggerfish, mahi-mahi, crabs, and shrimp.

“These ‘bioslicks’ form an interconnected superhighway of rich nursery habitat that accumulate and attract tons of young fishes, along with dense concentrations of food and shelter,” said Dr. Whitney. “The fact that surface slicks host such a large proportion of larvae, along with the resources they need to survive, tells us they are critical for the replenishment of adult fish populations.”

According to the researchers, surface slicks play a role in maintaining healthy and resilient coral reefs. The slicks also create foraging hotspots for larval fish predators.

“These hotspots provide more food at the base of the food chain that amplifies energy up to top predators,” said study co-author Dr. Jamison Gove. “This ultimately enhances fisheries and ecosystem productivity.”

“In an earlier study, our surface slick mapping suggested strong along-coast connectivity of ocean habitats. In our latest study reported here, we populated those satellite-based slick maps with the billions of animals, organic debris, and microplastics that make up the slicks,” said study co-author Greg Asner.

“Our findings are part of an important story forming around the role of biological surface slicks in maintaining coral reefs. The sheer biodiversity and biomass of the slicks, combined with their oceanic movement along the shore, form a superhighway for species that connects and effectively generates an interconnected, regional reef ecosystem.”

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer