It’s no secret that widespread overfishing is driving many shark species to extinction. Many of these apex predators are ensnared incidentally as bycatch in longline fisheries targeting tuna or swordfish. Shortfin mako sharks — the fastest sharks in the ocean — are among the shark species that are frequently kept even when caught as bycatch because of the high market value of their meat.
Still, we may have been underestimating how many sharks are being snared by longlines, which can stretch dozens of miles and have thousands of baited hooks at regular intervals.
Now, direct satellite tracking of juvenile shortfin mako sharks reveals the mortality rate from fishing is 10 times higher than estimates calculated using catch data reported by fishers, raising concerns about overfishing in the western North Atlantic and the sustainability of current fishing practices.
“This was way above our expectations. We were quite shocked actually,” Mike Byrne, an assistant professor at the University of Missouri and lead author of a study published this month in the Proceedings of the Royal Society B that used satellite tracking to estimate mako shark mortality rates, told Mongabay. (Byrne was a postdoctoral fellow at Nova Southeastern University during the study.)
“The shortfin mako is among the most vulnerable and valuable shark taken in high seas fisheries,” Sonja Fordham, founder and president of Shark Advocates International, a project of The Ocean Foundation, and Deputy Chair of the IUCN Shark Specialist Group, told Mongabay. Fordham, who was not involved in the present research, warns that this study “represents the first of a few alarm bells now sounding for North Atlantic mako sharks.”
Byrne explains: “Heavy fishing mortality on the young sharks limits the number that eventually make it into the breeding population, and given how long it takes this species to mature (females take around 19 years to reach maturity) and how slowly they reproduce (a triannual reproductive cycle with an average of 8 to 10 pups per litter), this can limit the ability of the population to recover.”
Nick Dulvy, co-chair of the IUCN Shark Specialist Group and Canada Research Chair in Marine Biodiversity and Conservation at Simon Fraser University, told Mongabay that, “Prior to this study we thought fisheries for this species were sustainable, now we have to question that. It appears that the new estimate of fishing mortality means the fishery is unsustainable and the population is at risk of decline.” Dulvy was not part of the study.
Overfishing has already been blamed for threatening more than half of the native shark species in the Mediterranean with extinction, according to a 2016 regional assessment. Shortfin mako sharks are globally categorized as Vulnerable by the IUCN Red List, but in the Mediterranean Sea, they are Critically Endangered.
Fisheries’ reported catches versus real-time tracking
While studying the movement ecology and habitat use of shortfin makos, Byrne noticed that many of his tagged sharks were caught by fisherman as bycatch. “It was pretty hard to ignore,” he claimed. Mako sharks’ habitat overlaps with that of commercially important species such as tuna. Even if they are released after being caught by longliners, mako sharks, like many other shark species, fare poorly — almost a third of healthy satellite-tagged makos died even after being released, according to recent research.
“Deaths are hard to see in the ocean, often we estimate death rates indirectly,” Dulvy said. Currently, fishing mortality calculated from stock assessments is based on indirect data — that is, catches reported by fisheries — which is inaccurate due to underreporting.
As a result, Byrne and his team directly tracked the fate of 40 mako sharks — mostly juveniles — in the North Atlantic from 2013 to 2016 by attaching satellite-linked radio tags to their dorsal fins. This technique, known as satellite telemetry, allows the tag to communicate with satellites and transmit the sharks’ locations to the researchers whenever the sharks swim at the surface, exposing their fins in the air. Byrne and team’s study represents the first time satellite telemetry has been used to quantify fishing mortality in mako sharks, an open ocean pelagic species. Harvests were detected if a tag consistently reported from the same location on land or when a tag was constantly heading towards a coastal port, implying that the shark may have been onboard a vessel.
Over the course of three years, 12 of the sharks were harvested (30 percent of the total sharks tracked for the study). Using statistical models, Byrne and his colleagues estimated that mako sharks have a 72 percent chance of surviving a year without being harvested, which translates to a 28 percent chance of makos being killed by fishing every year — a frighteningly high estimate, according to Byrne. Using the survival estimate, the researchers calculated the instantaneous fishing mortality representing the fraction of the mako population taken by fishing, and compared this with the rate obtained using the most recent available stock assessment (from 2012) based on fisheries’ data.
The instantaneous mortality rate derived from tagging the sharks was ten times higher. Even the lowest mortality estimate was higher than the fisheries’ highest estimate. And the authors believe that this may still be an underestimate, as some fishers may have destroyed the tags at sea upon capture without reporting the shark’s death.
The tracks of some of the sharks tagged in this study, along with other makos tagged this year, are available at almost real-time from the Guy Harvey Research Institute and Nova Southeastern University.
The study revealed substantial overfishing for this mako population, with a 15-to-18-times higher mortality rate than that associated with the maximum sustainable yield—the maximum catch that can be taken from a fish population without depleting the population. When fishing mortality is above this threshold, explains Byrne, “the population cannot replace itself and crashes.”
During the study, the sharks entered waters under the sovereignty of 19 different countries and were harvested mostly by longline fisheries from five countries, the top three being Canada, Mexico, and the United States. In six of the harvests, the researchers were able to contact the fishermen from the vessel in which the sharks were caught to confirm the vessel type.
The study demonstrates that satellite telemetry can directly measure the survival and mortality of individuals, including the location of capture, eliminating the dependence on inaccurate catch data reported by fishermen. Byrne said this “could go a long way towards helping fisheries’ managers assess and manage these populations.”
In conventional mark-recapture studies used to quantify species survival, where non-electronic tags are attached to individuals who are released and re-captured in the future, researchers can’t know the fate of sharks they don’t see again, Byrne said. “Did they live a long healthy life and just avoid being recaptured? Did they die before they could be recaptured? Maybe the animal was recaptured but the fisher never reported it.”
Because these studies take many years to get a sample size large enough to perform analyses and require voluntary reporting by fishers, they are not used in stock assessments by the International Commission for the Conservation of Atlantic Tunas (ICCAT), an inter-governmental fishery organization that consists of 50 countries as well as the European Union.
Using satellite telemetry, studies can also be completed in a shorter time period with smaller sample sizes. “You could potentially wrap up a study in a year… then repeat the study in other years to get snapshots of how fishing mortality changes through time,” Byrne added.
Setting catch limits
In the Atlantic, there are currently no annual catch limits for mako sharks, Byrne noted. He believes that we should start to “consider implementing annual total allowable catch limits for mako sharks, just as there are for other commercially valuable species such as swordfish and tuna.” Dulvy also stresses the importance of establishing precautionary catch limits based on scientific studies to ensure that fishing of this species is sustainable.
“It’s now urgent that officials from mako shark fishing countries — particularly top producers like Spain, Morocco, Portugal, and Japan — muster the political will to agree international limits that immediately and dramatically cut mako fishing pressure and reverse these troubling trends,” Shark Advocates International’s Fordham said. The next ICCAT meeting will be held in Morocco in November, where she expects “to see proposals to establish the first international limits on mako fishing.”
Future studies should include different geographical regions as well as different sizes of sharks, Byrne said, in order to get a more complete picture of the extent of fishing mortality in the North Atlantic. He also recommends that shark studies using satellite telemetry consider addressing survival and mortality to guide management and conservation efforts. And since sharks are highly migratory, swimming long distances and traversing the waters of many countries, this opens the door for coordinated international partnerships to conduct research and conserve shared shark populations.
- Byrne, M.E., Cortés E., Vaudo, J.J., Harvey, G.C. McN., Sampson, M., Wetherbee, M.B., & Shivji, M. (2017). Satellite telemetry reveals higher fishing mortality rates than previously estimated, suggesting overfishing of an apex marine predator. Proceedings of the Royal Society B: Biological Sciences, 2017; 284 (1860): 20170658. doi:10.1098/rspb.2017.0658
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- Campana, S.E., Joyce, W., Fowler, M., & Showell, M. (2016). Discards, hooking, and post-release mortality of porbeagle (Lamna nasus), shortfin mako (Isurus oxyrinchus), and blue shark (Prionace glauca) in the Canadian pelagic longline fishery. ICES Journal of Marine Science, 73(2), 520-528. doi:10.1093/icesjms/fsv234