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

Desdemona Despair

By Neha Jain
5 September 2017

(Mongabay) – 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. [more]

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

ABSTRACT: Overfishing is a primary cause of population declines for many shark species of conservation concern. However, means of obtaining information on fishery interactions and mortality, necessary for the development of successful conservation strategies, are often fisheries-dependent and of questionable quality for many species of commercially exploited pelagic sharks. We used satellite telemetry as a fisheries-independent tool to document fisheries interactions, and quantify fishing mortality of the highly migratory shortfin mako shark (Isurus oxyrinchus) in the western North Atlantic Ocean. Forty satellite-tagged shortfin mako sharks tracked over 3 years entered the Exclusive Economic Zones of 19 countries and were harvested in fisheries of five countries, with 30% of tagged sharks harvested. Our tagging-derived estimates of instantaneous fishing mortality rates (F = 0.19–0.56) were 10-fold higher than previous estimates from fisheries-dependent data (approx. 0.015–0.024), suggesting data used in stock assessments may considerably underestimate fishing mortality. Additionally, our estimates of F were greater than those associated with maximum sustainable yield, suggesting a state of overfishing. This information has direct application to evaluations of stock status and for effective management of populations, and thus satellite tagging studies have potential to provide more accurate estimates of fishing mortality and survival than traditional fisheries-dependent methodology.

Satellite telemetry reveals higher fishing mortality rates than previously estimated, suggesting overfishing of an apex marine predator