Final answer:
Since the 1950s, many of the ocean's large predatory fishes have disappeared due to overexploitation from commercial fishing, with some regional shark populations declining by 90% or more. This decline has been part of a broader process of fishing down the marine food webs and has been aggravated by high bycatch rates from indiscriminate fishing practices.
Step-by-step explanation:
The question relates to the significant decline of large predatory fishes in the oceans since the 1950s. Since that time, overexploitation due to intensive commercial fishing activities has led to severe declines in populations of ecologically significant marine vertebrates such as sharks, tunas, and sea turtles. Specifically, some regional shark populations have suffered declines of 90% or more. This dramatic decrease also reflects a process known as 'fishing down marine food webs', wherein fishermen target increasingly smaller fish species as the larger ones become scarce.
Moreover, with marine stocks monitored by the FAO being fully exploited or overexploited, and 24% being depleted, the current biomass of these fish populations is much lower than it should be to maximize their sustained yield. The effects of such overexploitation are profound, as many fish, mammals, and sea turtles face the risk of extinction, and the oceanic ecosystems are being fundamentally altered.
It is to be noted that global bycatches, the catch of non-target species, correspond to 25-65% of the total catch, exacerbating the decline of already vulnerable species through indiscriminate fishing methods.