Fish that hang out in surface waters
Surface waters provide a huge, but nearly featureless place for fish to live. In bottom waters there’s vegetation like seagrass and mangrove roots, there’s rock outcrops, oyster reefs, docks and pilings, and even artificial reefs. But up at the surface, 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% 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 certain stages of their life. These fish, which use most of, or the entire water column are referred to as pelagic fish. Examples include jacks, herring, mackerel, tunas, dolphin (mahi-mahi), and billfish.
Pelagic fish tend to have sleek, often torpedo shaped bodies, that allow for continuous and fast swimming. Most of these fish school or at least form lose travel groups. And both small prey and large predator fish often hide in plain sight by being silvery in color. Silver 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!)
Sight is important too!
Most surface feeding fishes are sight feeders 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 at night schools tend to break up simply because they can’t see each other?
One of the more interesting aspects of fish that hang out 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. Let’s face it 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 diner!
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.
Pelagic fish are often on the move
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, from one side of the ocean to the other.
Upwellings bring cold, nutrient-rich deep water to the surface, and 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 young larval fish 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.