• Harriet

Oceanic Shark and Ray Populations Decline by 71%. Let's Dive In!

Updated: Feb 4

Many of us have seen the headlines and they are concerning, to say the least.

"71% population declines of oceanic shark and ray populations in the last half a century."

"77% of oceanic shark and ray populations are now threatened with extinction."

We’re referring to the paper published in Nature on 27th January titled 'Half a century of global decline in oceanic sharks and rays’. The paper analysed 31 oceanic shark and ray species total (hereafter referred to as sharks) using IUCN Red List data and Living Planet Index records spanning from 1970 - 2018. So, what did they find?

  • During the 50 year study period, it was noted that the largest species were effected first, followed by declines of consecutively smaller species.

  • The 77% threatened species refers to 24 of the 31 species classified as vulnerable, endangered or critically endangered on the IUCN Red List. The more typical statistic we hear is that a third of all shark and ray species are threatened but that refers to ALL species, not just oceanic species.

  • All but one of the 31 species showed declines in abundance, the exception was the smooth hammerhead (Sphyrna zygaena).

  • The Indian Ocean is the worst affected by population decimation, with declines of 84.7% reported.

  • Devil ray populations declined by 85% in some areas.

  • Sharks habituating tropical areas were worse affected than those in temperate areas.

  • These are said to be conservative estimates as UUI catches are not taken into account, and are prolific among shark and rays catches.

In the period of time the study assessed (1970-2018):

  • global fishing with longlines and seine nets, of which sharks are particularly susceptible, has doubled.

  • relative fishing pressure on shark and ray populations has increased 18-fold (relative fishing pressure refers to the exploitation of fish stocks relative to the number of fish left).

  • recorded shark and ray catches have tripled.

The cause of these declines?

As we have seen, shark populations are rapidly plummeting, but why? It may not come as a surprise to hear that fishing pressures have increased 18-fold since 1970. The authors suggest that it is due to this overexploitation of shark populations by the fishing industry that is to blame. Put simply, sharks are being killed at too high a pace, at too high an intensity, for reproductive output to keep up with, and thus their numbers sink lower and lower. After considereding maximum sustainable yields, IUCN’s listed threats for each species, and relative fishing pressures over time the authors came to the conclusion that...

"Strict prohibitions and precautionary science-based catch limits are urgently needed to avert population collapse."

In fact, on studying the Living Planet Index, they highlight that fishing pressures were already unsustainable even 50 years ago. If we are listening to scientists, can we truly be surprised by the results shown here? There is a ray of hope though. We know fishing restrictions and science based catch limits work in improving population numbers. Population stabilisation and subsequent increase in northwest Atlantic (NWA) hammerheads have been reported, owing to strictly enforced quotas throughout their US range. There is more good news for great whites and porbeagles too.

A paper released in 2014 assessed fishing legislation amongst other things in relation to abundance of white shark populations and found "growing evidence that legal protections for white sharks in the NWA and elsewhere around the world have been effective.” They went on to say "Population declines appear to have been halted and populations may now be stabilized or growing in several regions.” Porbeagle population stabilisation and potential growth due to effective fishing regulations being enforced has also been evidenced.

We know that enforcing and ensuring adherence to appropriate fishing regulations works.

Of the 31 assessed species, according to the IUCN every single species main threat was listed as ‘biological resource use’ specifically ‘fishing and harvesting aquatic resources’. It comes as no surprise that the worst affected areas, such as the Indian Ocean, are those with the fewest fishing regulations in place. The authors reemphasise that...

"strict measures to prohibit landings and minimize bycatch mortality ... are urgently needed to halt declines and rebuild populations.”