Sharks and Rays: "Catastrophic" 50-year Decline

This article by Brice Louvet is an interesting example of how a popular scientific publication (Science Post) can re-introduce a little extra detail and precision to a press agency text (AFP) in order to recapture some of the importance of a newsworthy research project on the topic of marine biodiversity.

Article source: "Déclin “catastrophique” des populations de requins et de raies ces cinquante dernières années", by Brice Louvet, Science Post, 29/01/2021.

We know for a fact that overfishing is the main reason behind the extinction of many species of marine animal. However, tracking the changes in each species’ conservation status remains a difficult task, especially when it comes to the large marine predators that swim in international waters. 

Global Review

In a recent study, researchers from Canada’s Simon Fraser University (SFU) examined two indicators to chart the progress made in recent years in terms of improving marine biodiversity, in the context of the "Aichi Biodiversity Targets." As a reminder, these targets are part of the “Strategic Plan for Biodiversity 2011-2020”, adopted in 2010 as part of the international Convention on Biological Diversity. 

The first data source used was the “Living Planet Index” (LPI), which measures changes to the world's biological diversity. They studied 57 LPI datasets relating to 18 species of oceanic sharks and rays. The second indicator was the “Red List Index,” which monitors changes to the extinction risk of various species, in this case of all 31 species of oceanic sharks and rays. 

Shocking Results

As expected, the results were not good. In fact, research found that the global population of oceanic sharks and rays had dropped by 71% since 1970. Many species were affected, from manta rays to hammerhead sharks. 

The most shocking example of this decline is that of the oceanic whitetip shark. This species is hunted for its fins and has seen its global population fall by 98% over the past sixty years. The main author of the study, Nick Dulvy, told the AFP: “it’s much worse than other animal populations we’ve been looking at [such as terrestrial mammals]. It’s an incredible rate of decline.” 

Overfishing and "Bycatch"

It should be noted that sharks and rays are particularly susceptible to such population declines, as they have a relatively low rate of reproduction and they mature slowly.

To explain these massive reductions, the researchers point to an 18-fold increase in relative fishing pressure. The study also notes that the use of longline and seine fishing techniques has doubled over the past fifty years. The main drawback of these techniques is that species that aeren’t being targeted end up getting caught (this is known as "bycatch"). 

The researchers also believe that the decline in numbers of the affected species can only be reversed if concerted efforts are made to improve marine conservation. As a precaution, they say that strict bans and catch limits based on scientific data are urgently needed to help these species to recover.

Translated by Charlotte Hebbourn and Chloé Letellier.

Editing by Sam Trainor.

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