Biophysical interactions between fish larvae and surface slicks enhance a tropical island ecosystem

Our understanding of how marine fish larvae are influenced by and interact with ocean features remains limited. Here we present results from a 3-year interdisciplinary study aimed at characterizing the biophysical interactions with surface slicks, a common and prevalent oceanographic feature in coastal waters of the Hawaiian Islands and around the world. Using data from 134 neuston tows inside slicks and in ambient water outside slicks, we show that slicks (driven by a variety of underlying physical mechanisms, including but not limited to internal waves) form a mosaic of interconnected habitat that accumulates and structures a huge diversity of plankton and micronekton. Through a combination of convergent flow and active behavior, slick nurseries host dense aggregations of eggs, larvae and juvenile fishes from 50+ families originating from shallow coral reefs, epipelagic, and mesopelagic habitats. We provide evidence that larger more mobile larval and juvenile fish, found nearly exclusively in slicks, are actively targeting these habitats to capitalize on concentrated prey-resources and shelter from predators. Using remote sensing analysis, we are able to scale-up these observations across our 1,000 km²study area in West Hawaii, revealing that slicks cover ~8% of the ocean surface habitat yet contain ~40% of all neustonic larval fishes, and up to 90% of pelagic juvenile fishes. The behavioral preference for larger/older larval fish to actively select slicks has major implications for transport. Biophysical interactions with slicks drive changes in pelagic community structure that translate to higher abundance, greater diversity and increased trophic complexity. We postulate that slicks play a previously underappreciated but critically important role in enhancing larval supply and ecosystem productivity in tropical marine systems.