The decrease of large fish benefits jellyfish. So jellyfish become more and more abundant. In a recent book by FAO, Ferdinando Boero argues that increased availability of jellyfish will favour medusivorous species (see art above from Boero 2013*, page 13). These would include inter alia the ocean sunfish and leatherback sea turtle, dominating the right side of the drawing.
While I do like the graphic representation, I suspect that looking at this image alone may mislead some to infer that the overall past and present abundance of marine megafauna are about the same, i.e. that only species composition or size have changed. I would rather think that the image was not designed to make a statement about relative densities.
In addition, it remains to be seen whether jellyfish-eaters that are currently exposed to multiple anthropogenic threats (such as the leatherback sea turtle Dermochelis coriacea, presently classified as Critically Endangered in the IUCN Red List) will be capable of rebounding based on prey abundance. Boero (2013) cites some evidence suggesting that they may.
In the long run, species that benefit from jellyfish abundance might include Homo sapiens... in case we develop a widespread appetite for jellyfish burgers!
As reported in the book, jellyfish already are commercially important in some markets.
* Boero F. 2013. Review of jellyfish blooms in the Mediterranean and Black Sea. Studies and Reviews. General Fisheries Commission for the Mediterranean. No. 92. FAO, Rome. 53 pp.
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