A special open ocean fauna occurs around what little structure is found in the open sea. Floating bits of seaweed (usually sargassum), jellyfishes, siphonophores, and driftwood almost always have fishes associated with them. Many flotsam-associated fishes such as filefishes and jacks are the juveniles of inshore or pelagic species; others such as sargassumfishes and driftfishes are found nowhere else, attesting to the reliability of occurrence of such objects. Flotsam also serves as an attractor for large predators, such as sharks, dolphinfishes, tunas, and billfishes (Gooding & Magnuson 1967); a single log will commonly have more than 400 tuna of 5 kg each associated with it, often involving several species (Sharp 1978). It has been suggested that concentrations of flotsam are indicators of regions of high productivity in the open sea because the flotsam accumulates at the top of vertical circulation patterns (Langmuir cells) that also concentrate nutrients and zooplankton (Maser & Sedell 1994). The mechanisms by which pelagics locate floating objects and their importance to fishes that do not feed around them remain a matter of conjecture (see Fig. 20.6).
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