Julie Brown, University of Miami
Fishermen have long known that floating objects in the ocean have a natural tendency to aggregate fish. They have been using natural Fish Aggregating Devices (FADs) for decades, usually logs. There are many theories to explain fish’s fascination with floating objects. One is that the FAD provides the only visual stimulus in the open ocean, where there is otherwise an endless void of blue. Another hypothesis is that floating objects tend to accumulate in convergence zones, where moving bodies of water come together. Convergence zones are relatively productive areas where particles can accumulate, which is good for fish trying to feed or spawn. FADs could be visual markers for these barely detectable changes in oceanic conditions. In modern times, fishermen take advantage of this phenomenon by placing artificial FADs in the ocean, and return when the object has accumulated a satisfactory amount of fish to harvest.
Fad design and technology
The earliest FADs were made of natural materials, such as logs, bamboo, and palm fronds. These materials, of course, degrade over time, and fishermen have upgraded to a variety of man-made materials such as netting. Modern FADs can either be anchored to the bottom, or drifting. The rest of this article will focus on drifting FADs.
Today, many FADs for commercial fishing are equipped satellite-linked echosounder buoys, which let fishermen remotely estimate the fish biomass around the FAD. They can accurately time their fishing operations to maximize harvest, and minimize search time. These ecosounders can only provide information about biomass, however; they cannot tell the size or species of fish. Scientists have demonstrated that there is a higher incidence in bycatch (non-target species or undersized tuna) when fishermen set their gear around FADs, as opposed to free-swimming schools of Tuna
Photo credit: http://www.joostvanuffelen.com
Increasing trend in fads
In the past decade, the use of FADs has increased dramatically in the Tuna Purse Seine industry. In the Eastern Pacific alone, the number of FADs deployed by this industry jumped from 8,006 to 13820 in seven years. And this only includes reported FADs. The actual number is probably higher, because the regional fishery management organization, IATTC (Inter-American Tropical Tuna Commission), does not require smaller vessels to report FAD use. The Pew Research Center
estimates that FAD use is also increasing in other oceans, although not quite as dramatically. The use of FADs now dominated the Tuna Purse Seine industry, which accounts for over 60% of the world’s tuna landings.
Around 1999, scientists began to question whether FADs could alter the biology and ecology of pelagic fish species (Marsac, Fonteneau et al. 2000). The ecological trap theory suggests that FADs, which are often seeded in offshore areas, attract and retain fish to areas that aren’t suited to their survival and reproduction. In fact, when tuna associated with a FAD were sampled, and 85% of them were shown to have empty stomachs, while only 25% of tuna from free-schools had empty stomachs (Ménard, Stéquert et al. 2000). Other studies, however, show conflicting evidence that the presence of a FAD has no effect on the body condition or lipid content of tuna (Robert, Dagorn et al. 2014). The verdict is still out for this hypothesis, and more studies are needed.
Tuna fishing becomes significantly more efficient (more biomass harvested per “haul”) when FADs are used to congregate the schools. The schools are easier to locate, and less time is spent searching for fish. Free-swimming schools tend to be more sparsely distributed, travel faster, and are harder to spot. In other words, this technology increases the Catch and decreases the Effort. Because scientists use an index called Catch Per Unit Effort (CPUE) to estimate the status of fish stocks, an error in this calculation could occur if the technological advancements are unaccounted for. This phenomenon is called stock hyperstability, and it doesn’t just occur in the Tuna industry. Generally, we assume that CPUE and stock abundance follow a linear trend over time. Basically, if it becomes harder to catch as many fish, it’s because there are proportionately less fish available. FADs, however, congregate the few remaining fish, and they are easier to catch than they would otherwise be. This can mask the effects of overfishing.
Marsac, F., A. Fonteneau and F. Ménard (2000). Drifting FADs used in tuna fisheries: an ecological trap? Pêche thonière et dispositifs de concentration de poissons, Caribbean-Martinique, 15-19 Oct 1999.
Ménard, F., B. Stéquert, A. Rubin, M. Herrera and É. Marchal (2000). Food consumption of tuna in the Equatorial Atlantic ocean: FAD-associated versus unassociated schools. Aquatic Living Resources 13(4): 233-240.
Robert, M., L. Dagorn, N. Bodin, F. Pernet, E.-J. Arsenault-Pernet and J. L. Deneubourg (2014). Comparison of condition factors of skipjack tuna (Katsuwonus pelamis) associated or not with floating objects in an area known to be naturally enriched with logs. Canadian journal of fisheries and aquatic sciences 71(3): 472-478.