Fishing mortality of mako sharks ten times higher than fisheries’ estimates

Date: September 5, 2017

Source: Mongabay
Author: Neha Jain

  • For the first time, researchers used satellite tags attached to the fins of 40 juvenile shortfin mako sharks to directly quantify fishing mortality in the Northern Atlantic.
  • Over the course of three years, 12 (30 percent) of the sharks were harvested, mostly by longline fisheries from five countries.
  • Fishing mortality was ten times higher than estimates based on catch data reported by the fisheries, and 15 to 18 times higher than the rate associated with maximum sustainable yield, suggesting substantial overfishing.

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.

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