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Listen to part of a lecture in a marine biology class.
Professor: Another group behavior among fish that might be related to schooling, at least for some species, is something we see with a lot of ocean dwelling fish. It’s an attraction to floating objects, a phenomenon we call "fish aggregation behavior."
    Aggregation behavior has been documented in more than 300 fish species. Hundreds or even thousands of individuals will congregate under a floating log or tree branches or drifting algae, any chunk of debris really, whether naturally occurring or human made. It’s as if floating objects act as magnets for fish. The fish are attracted to them and just hang out there for extended periods. What purpose does the behavior serve? Does anyone have any ideas? Mike?
Student 1: Maybe the objects provide cover from predators, sort of hide them from birds flying overhead.
Student 2: Or could they contain food, like organisms that grow on the floating debris, like a floating buffet table for fish?
Professor: Both seem plausible hypotheses. Any other ideas, anyone? Ok, well, before we get to those, let’s start with one of the first hypotheses researchers ever considered, which was developed on the basis of the behavior of tuna, and that’s the "meeting point hypothesis." This hypothesis holds that tuna aggregate as a prelude to forming schools. Isolated individuals meet up and when there’s enough, they swim off in a close-knit group: a school. You’ll remember that schooling diminishes a fish’s chance of being singled out by a predator, and it helps fish detect food and find a mate among other things. Now tuna do form schools after congregating under floating objects, but what really supports the hypothesis is evidence that schools of tuna that form beneath floating objects seem to be larger than schools formed elsewhere from free-swimming tuna. So there’s a correlation between school size and aggregation, but the meeting point hypothesis for other species has been challenged recently by a group of researchers in Spain, and their argument is quite strong, I’d say. They point out that more than 80% of fish found aggregating around floating structures are juveniles. Schools, on the other hand, consist mainly of adult fish. So aside from a few species, like tuna, very few aggregating fish end up forming into schools. The data show that the meeting point hypothesis is pretty limited. Yes, Mike?
Student 1: So what about the other hypotheses? Is there evidence for those?
Professor: Ok. The "shelter from predators" and "food supply" hypotheses... well, juveniles of all species are more vulnerable to predators than adults are, and in many species fish develop coloration during juvenile stages that mimics the floating object they’re attracted to, like some species have dark bars on a yellow background, which helps them blend in with drifting algae. In most cases, when these juveniles become adults, their coloration changes and they swim away. They lose that particular camouflage along with their instinct to aggregate, so floating debris does seem to function as protection from predators for the juveniles of many species.
    As for food supply, floating objects really help with this because they drift, and as they drift, they become havens for tiny invertebrates, providing a ready meal for juvenile fish as they drift along in ocean currents with the object. Now ocean currents tend to converge at various points, and plankton, which juveniles also eat, also tend to collect in pockets at these locations. So for aggregating juveniles going with the flow, so to speak, enables their survival because nourishment can be difficult to find in the open ocean. So most fish aggregation seems to provide some benefit related to food supply or predation, particularly for juveniles.
    However, for tuna the meeting point hypothesis seems like a better explanation, because it’s not just juvenile tuna that aggregate. The adults do it, too. "Their aggregation behavior does not go away as the fish mature. This fact, by the way, is not lost on the commercial fishing industry. Commercial tuna fishers regularly deploy artificial floating objects known as Fish Aggregating Devices, or FADs. The size, color, and shape of the FAD don’t seem to matter or whether they’re free floating or anchored, like a buoy. FADs are so effective that almost two thirds of tuna catches are made at FAD sites.
Student 2: They catch juveniles, too?
Professor: Well, tuna fishers are interested in adult fish, not juveniles, so they tend to be careful about where they place their FADs, like they usually don’t put them near coasts where most juvenile tuna live.