Why have oceanic shark numbers fallen by 71% in about 50 years?
A 2021 study in Nature found that oceanic shark and ray populations had fallen by 71% in about 50 years, as fishing pressure on them rose 18-fold.
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Oceanic shark numbers have fallen mainly because humans have caught far more of them than their slow-breeding populations can replace. According to a 2021 study in Nature, resulted in a 71 percent global decline in oceanic sharks and rays over the preceding 50 years. on these sharks and rays has risen 18-fold since 1970, and the study's lead author, Nathan Pacoureau, said there was no doubting that overfishing was responsible. Three-quarters of the species in this group are now threatened with extinction due to overfishing. Note what the figure covers: the oceanic sharks and rays the study tracked, not every shark everywhere.
Why can sharks not keep up? Sharks generally reach only after many years and produce few offspring compared with other harvested fish, so harvesting them before they reproduce severely affects future populations. In 2008 it was estimated that nearly 100 million sharks were being killed by people every year through commercial and recreational fishing. Most shark fisheries have little monitoring or management, and rising demand for shark products adds pressure.
Some species have fallen further. Some have been depleted by over 90 percent over 20 to 30 years, and the oceanic whitetip and the scalloped and great hammerheads are classified as . Sharks in tropical waters declined faster than those in temperate zones. People argue about which part of the fishing matters most: some groups say accounts for an estimated 50 percent of all sharks taken, others say the fin market is the main cause.
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A steady yearly loss adds up
y = 1 · 0.99ˣ
Challenge · Set the yearly share kept so the curve ends near 0.3 after 50 years, a fall of about 70 percent.
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0/3
Recap
Sharks have few young and mature late, so fishing at today's levels outpaces the time they need to recover.
💡 A trick to remember it · Slow to grow, slow to breed: sharks cannot refill a sea as fast as nets and hooks empty it.
Surprising fact · A survey of 371 reefs in 58 nations found no sharks on almost 20 percent of the reefs.
Connects to
- 🏖️ Do shark nets and drum lines make beaches safer, and what else do they catch?
- 🕸️ What happens to an ocean when its sharks disappear?
- 🥣 What is shark finning, and how does the shark fin trade work?
- 🏝️ How do tiny animals build a whole reef?
- 🪷 Why does something that doubles daily look harmless until the last moment?
Sources (4)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.