One question is whether we are witnessing an irreversible jellyfish takeover of the seas or just a natural waxing of the jellyfish population that will eventually wane. Scientists don’t know. But finding the answer is crucial to keeping the oceans safe for every sort of undersea life.
One reason for the mystery is that jellyfish, lacking skeletons or shells, leave little trace in the geologic record. Another is that they have not been studied much; fishery biologists have long ignored jellyfish as a small nuisance.
One recent study of the existing scientific data found “no robust evidence for a global increase in jellyfish.” The researchers noted that populations oscillate in 20-year cycles, and an expansion that began during the 1990s is only making it seem as if jellyfish are taking over. Another study, which looked at not only recorded data but also anecdotal reports from experienced fishermen, found that jellyfish populations have increased beyond what natural cycles can explain.
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