More than a third of shark and ray species are now threatened with extinction, due to overfishing, habitat loss, pollution and climate change. This means that with increasing environmental pressure, time is running out to save the world’s sharks. As top predators, sharks are essential for maintaining the stability and health of marine ecosystems.
Yet, unsurprisingly for the species that roam far below the surface of the world’s oceans, there are still many unknowns about the impacts of environmental change.
Hard data on shark populations is needed to inform conservation strategies and manage catch limits on the high seas, especially when these ocean areas are often beyond national jurisdiction.
Researchers are therefore using new methods to improve knowledge about sharks, including the use of genomic techniques that can help uncover their genetic history. Knowing more about how shark populations change over time can provide insight into environmental pressures, the degree of relatedness between individuals and the degree of genetic diversity maintained, explained population geneticist Professor Einar Eg Nielsen. at the Technical University of Denmark (DTU) in Lyngby.
“You can see that in general a species is doing well. But that can mask the fact that it’s good in some areas and bad in others,” he said.
“You need information about populations to manage them well. But if you don’t have genomic markers with sufficient resolution, you fail to unravel the structure of the population.
That’s why the DiMaS project he co-led conducted genetic research on sharks. The initiative aimed to expand information on the recent history of sharks to help assess how they might respond to future climate change and fishing pressures.
The team focused on the shortfin mako shark (Isurus oxyrinchus), which is commercially fished but also incidentally caught as bycatch in seas around the world. The species is listed as endangered on the IUCN Red List, generally considered the definitive inventory of the conservation status of species worldwide.
The researchers amassed nearly 1,000 jawbone and vertebrae samples spanning three centuries from museums, national fisheries institutes and personal collections, including modern samples from fisheries institutes. After separating those of lower quality, they then selected half of them for genomic analysis.
“The problem with these beasts is that they’re everywhere, so it’s hard to get really good samples,” Prof Nielsen said. “But by joining forces with other institutions, we were able to get more samples so we could look at temporal patterns.”
Despite their severe decline in numbers, the resulting analysis revealed a potential cause for cautious optimism about the makos’ long-term survival prospects, as the team found that their genetic diversity had not declined by significantly in recent years.
High levels of connectivity between different shortfin mako populations may have contributed to this, said Dr Romina Henriques, formerly at DTU, co-lead of DiMaS, and now population geneticist at the University of Pretoria in South Africa.
“The fact that there is this strong connectivity suggests greater resilience,” she said.
However, things are not that simple. It appears the level of connectivity has changed over time and some historical populations may have been more isolated and therefore potentially more vulnerable, Dr Henriques said.
“What I found really interesting is that this connectivity wobbles over time, so you have some shortfin mako populations that seem more differentiated than others,” she said. “What we think is that you probably have isolated populations, but there’s quite a bit of movement.”
Another caveat is that with their potential lifespan of 30 years or more, shortfin makos are relatively long-lived. Given that fishing pressures only increased in the second half of the last century, there may simply not have been enough time for recent population declines to translate into declines. genetic diversity.
But whatever subsequent research concludes, the widespread movement of shortfin mako sharks and the connections between populations underscore the need to manage fisheries and conservation at a regional level rather than in individual areas, Dr Henriques said.
“That means conservation should not just be national, but regional,” she said. “It doesn’t matter if two or three countries decide ‘more mako fishing’ because they will naturally move away from protected areas.”
Another important area of research is how climate change may affect sharks’ ocean habitat through its effect on oxygen levels, leaving the animals more vulnerable to overfishing.
Since warmer waters dissolve less oxygen, studies suggest that climate change is depleting levels in the oceans and leading to the expansion of so-called oxygen minimum zones (OMZs). It is thought that this could, in turn, compress the areas of the ocean where many sharks, fish and other marine animals can exist into a smaller space.
“One hypothesis is that the sharks are compressed vertically into this upper layer, in a smaller and smaller volume of water, which lends itself to higher catches made by fishermen,” explained Professor David Sims, ecologist sailor at the Marine Biological Association in Plymouth. and the University of Southampton, UK.
Assuming that much of these effects are currently unknown, the Ocean Deoxyfish project led by Professor Sims is exploring the phenomenon in sharks, as well as tunas. “Ultimately what we want to do is be able to predict the distribution of sharks in a deoxygenated ocean world,” he said.
Such exploration has been facilitated by developments in tagging devices that can be attached to shark fins. “Over the past 20 years, there have been real advancements in marine telemetry – marine biologging, as it’s called – using miniature electronic devices to track sharks to inform about their movements, behavior and their ecology, as well as their interaction with the environment,” said Professor Sims.
In the early stages of Ocean Deoxyfish, the results largely confirmed the researchers’ hypothesis. In a study conducted in the OMZ in the eastern tropical Atlantic off Africa, the team found that the habitat of blue sharks was vertically compressed. Their maximum diving depth appeared to be around 40% lower than other areas, potentially increasing their vulnerability to fishing.
cycle of misfortune
However, the picture is complicated because sharks may also benefit from increased opportunities to feed in these compressed areas, as the prey themselves seek to avoid hypoxic, low-oxygen waters.
“There’s a catastrophic cycle that is self-perpetuating,” Professor Sims said. “Sharks see opportunities for food, but anglers also take advantage of the opportunity to catch more sharks per unit time.”
Ocean Deoxyfish researchers are developing increasingly sophisticated tracking beacons that will record oxygen levels in addition to shark movements and typical measurements such as temperature, pressure and depth. The data will be uploaded directly to the satellites, eliminating the need to retrieve beacons.
The new beacons will also capture video footage of what Professor Sims describes as a “bird’s eye view” of animal behavior, allowing researchers to see what they are doing as they dive. He added that blue sharks appear to have the ability to perform at least a few dives in low oxygen waters, which he says may be associated with them feeding on squid that can tolerate such waters. .
“It will be amazing to capture that and find out what they’re doing there,” he said. “What we hope is that these beacons will capture the pursuit of these deep-sea cephalopods [such as squid] in the zone of minimum oxygen.’
Ultimately, he hopes projects like his will contribute to the long-term management of sharks and other species that live in the ocean.
“Overfishing is reducing these populations to levels they have never reached before,” he said. “There must be an interaction between climate change research and fisheries management to a greater degree than there has been.”