Most of the world’s oceans are filled with vast areas of blue nothingness. The open ocean is more or less a desert in terms of fish per square mile; that’s why the few special places where underwater structures force ocean currents to the surface become such fish havens. Several types of offshore structures hold fish in such large numbers that they are veritable fish factories. One of the most common is the steep canyon, where fish congregate along steep underwater walls that act as underwater fences and stack up bait.
Oceanic seamounts and pinnacles also hold lots of fish, because their vertical hills, mountains and underwater peaks create current upwellings that carry algae- and nutrient-rich water from below to the surface. Solar radiation then triggers algae blooms, which feed the forage species that attract, and become food for, our favorite game fish.
But in the world of fish-attracting features, none is more common, interesting or more productive in terms of aggregating game fish than floating debris, or flotsam. “Flotsam” is a peculiar word that was originally associated with floating pieces of ship wreckage. However, in the sport-fishing world, flotsam is defined as anything found floating in the sea — a common log, a discarded wooden pallet, nets, ropes or rafts of sargassum weed. No matter what the floater is made up of, it still represents a unique opportunity for life to take hold in a really big, empty sea.
After floating at sea for weeks, almost any object begins to form its own ecosystem, which begins to attract fish and other life from all different levels of the food chain. In some parts of the world, fishermen create fish-aggregating, or fish-attracting, devices (FADs) by using anchored trash like pallets, netting and palm fronds to mimic the fish-attracting quality of free-floating debris.
Although the presence of food around debris is certainly one of the main reasons that fish congregate around floating objects, it’s not the only one. Scientists have found that certain fish use objects to help orient themselves in what is otherwise a vast desert of watery terrain lacking any discernible landmarks.
Dr. Kim Holland of the Coconut Island lab of the Hawaii Institute of Marine Biology performed an exhaustive study on small tuna associated with FADs off Oahu, Hawaii. Holland placed radio tracking devices on tuna and followed them for several days. What he found was almost beyond comprehension. The tuna would leave the FAD as the sun got to midmorning levels and go out on forage hunts. They would generally travel as a school and were commonly recorded at distances of up to 14 miles away from the FAD, before returning to the same FAD as darkness fell. These young 1- to 2-year-old tuna made this journey every day.
It’s nearly unimaginable to think that a fish with a brain the size of a pea could leave a floating object in the middle of a vast blue world, go around chasing bait species from the surface to below 500 feet over a 24-hour period (covering an area of many square miles), and then steer themselves right back to their “home” FAD every time. It nearly defies reality, especially given that we as anglers and captains can find a piece of flotsam, mark it on a GPS, note the current speed and direction, move a few miles from the flotsam and never find it again.
As these tuna got bigger (typically above 25 pounds), they tended to move away from the FADs and start a different cycle of life as open-ocean school fish, usually bonding to porpoise schools in a symbiotic feeding relationship. This amazing study by Dr. Holland and his team just scratches the surface of how fish can navigate in their world.