Surface waters provide a huge, but nearly featureless, place for fish to live. In bottom waters, there’s vegetation like seagrass and mangrove roots, rock outcrops, oyster reefs, docks and pilings, and even artificial reefs. All of those provide places for fish to hide from predators and find food for themselves.
But up at the surface, other than a little floating debris, there’s pretty much just water. Lack of habitat diversity in this region limits the number of species that can occupy it to less than 2 percent of all known fish species.
Very few fish spend their entire life at the surface. Most fish associated with surface waters use this area opportunistically for feeding or during only certain stages of their life. These fish, which use most or all the water column, are referred to as pelagic fish. Examples include sharks, jacks, herring, mackerel, tunas, dolphin (mahi) and billfish.
Pelagic fish tend to have sleek, often torpedo-shaped bodies, which allow for continuous and fast swimming. Most of these fish school or at least form loose travel groups. And both small prey and large predator fish often hide in plain sight by being silvery in color. Silver reflects and scatters the incoming light from above.
Some open water fish are counter-shaded, meaning they are dark on top and light on the bottom. Being dark on top helps them blend into the bottom when viewed from above, and being light on the bottom helps them blend in with the sunlit sky when viewed from below. This can help them avoid detection by predators and potential prey alike.
Most surface-feeding fishes hunt by sight, so light is important for helping them find prey. Light is also important because fish need to see each other in order to school. Did you know that at night, fish schools tend to break up simply because they can’t see each other?
One of the more interesting aspects of fish that spend time at the surface is their tendency to hang around floating objects such as drifting debris, jellyfish and floating seaweed. One reason for this appears to be that floating objects – also called flotsam – are something different in a big empty space, and fish are just attracted to that. We’re kind of like that too. Who hasn’t gone out of their way to find out what that thing was bobbing on the surface? And, since it aggregates fish, flotsam often provides a service, similar to a drive-through restaurant!
For many species, and especially certain juvenile fishes, flotsam can also provide important protection from predators. In fact, the abundance of jellyfish or drifting weeds in a region may positively impact the survival of some young-of-the-year fish species. One study, conducted back in 1972, found that 54 different species occupied floating Sargassum “weed lines” off Florida.
Most pelagic fish move freely from one area to another in search of food, or for spawning, or when environmental conditions change. Some large predators, such as tuna, can cover thousands of miles, swimming from one side of an ocean to the other.
Upwellings are areas where cold, nutrient-rich deep water comes to the surface. They provide important feeding areas for pelagic fish, because the nutrients allow for blooms of tiny microscopic plants, which in turn support tiny microscopic animals. These tiny plants and animals are collectively referred to as plankton. Plankton provide food for small fish … and you know the rest of the story!
For some large migratory fish, such as bluefin tuna, the overall migration pattern tends to follow a triangular pattern based on their spawning patterns. The adults migrate in a direction opposite that of surface currents. They are headed towards areas suitable for spawning and close to waters that support high concentrations of plankton. Plankton will be critical to the survival of their offspring.
After spawning, adult fish return to their feeding areas, and the larval fish drift in the currents before “settling” in nursery areas. As they grow, the juvenile fish migrate to the adult areas. Interestingly, this pattern reduces competition for food among the different life stages of a fish species, because the different ages live in different areas.
Betty Staugler is the Charlotte County extension agent for the Florida Sea Grant Program. Contact her at email@example.com or 941-764-4346.