This summer Dr. Breitburg investigated whether comb jellies, usually found in low oxygen bottom waters, are down there hiding out from sea nettles, which may prefer the surface waters where the zooplankton lives. Sea nettles are seen in great numbers in surface waters because that's where there is plenty to eat. Sometimes, what seems like a massacre of ctenophores may be triggered by low oxygen in the bottom water, which drives prey and predator together in mid-level depths of the water where there is more oxygen. When sea nettles are slowly cruising the bottom, they may be the most efficient predator. Why this happens may be of only passing interest to delighted beachcombers and fisherman who chance upon this surprise bounty for the crab pot.
Many fish can tolerate low-oxygen water.
But the uptilt of oxygen-poor water may also produce massive kills of some bottom fish trying to escape and, without swim bladders, finding swimming at the surface too difficult for evading swiftly diving birds. Dr. Breitburg says so far nobody knows. Some people think that jellyfish didn't use to be so abundant, she says. While Dr. Breitburg has not personally examined historical records and whatever fisherman wrote about their workday life, such records may contain evidence of changes in jellyfish densities. By Robert C. Radcliffe.