February 12: Day five – juvenile fish take refuge in the Sargasso Sea

Yesterday we found a juvenile fish in our Sargassum seaweed samples. The small fish, about four centimeters long, had mottled coloration of browns and spots that perfectly camouflaged it in the seaweed. When the scientists identified the fish as a type of sea chub—a species normally found in shallow water environments around reefs—I wondered how this small fish made it so far offshore to the open ocean. Was this an unusual sighting or something more common, and was it perhaps somehow related to the Sargassum itself?

This topic sparked an informative discussion with Kathleen Sullivan Sealey and Jeff Drazen who shared their expertise and helped find relevant publications for me to read. It was a good task for the day as more rough weather had changed our plans, sending us even further south to the next science station.

A juvenile sea chub found in Sargassum. More than 80 different species of juvenile fish associate with Sargassum seaweed.

A juvenile sea chub found in Sargassum. More than 80 different species of juvenile fish associate with Sargassum seaweed.

Was the chub unusual? A fisheries guide published by the Food and Agricultural Organization of the United Nations reports that young yellow and Bermuda sea chubs are often found among floating Sargassum. And it’s not only sea chubs—other researchers report that more than 80 different species of fish have been found associated with Sargassum seaweed, most of them juveniles. It turns out that the life history for the majority of marine fish involves a pelagic (open ocean) stage in which sheer numbers and luck determine the survival rate. Floating Sargassum may be an important nursery habitat that improves the odds a juvenile fish will make it to adulthood.

Typically an adult marine fish can spawn hundreds of thousands of potential offspring that drift with currents as fertilized eggs. Over several days and weeks the larval fish develops, gaining muscles and bony structure that give it mobility. At this point, the young fish has some control up and down in the water column and becomes an effective predator on other plankton, all the while still drifting with the currents itself. The juvenile fish also begins to search for its adult habitat, using smells and sounds to find it. This is a vulnerable time as it changes from a mostly transparent larvae to a juvenile that can be seen by predators. A patch of Sargassum can give the young fish a refuge from predators and a place to find food while it matures.

As I watched the turbulent waves and wide open blue sea where there is little place to hide, it was easy to imagine how a patch of Sargassum may help young fish survive and be an important habitat for fish in the area.

— Debbie Nail Meyer