Traditional methods of monitoring shortfin makos Isurus oxyrinchus in the western North Atlantic have greatly underestimated the impact of fishing on their populations. A new study published in the Proceedings of the Royal Society shows that fishing mortality (the rate at which sharks are killed by fisheries) for shortfin makos is actually 10 times higher than previous assessments indicated. Michael Byrne and Mahmoud Shivji from the Nova Southeastern University’s Guy Harvey Research Institute (GHRI), together with their co-authors, employed satellite telemetry data as a fisheries-independent tool for monitoring mako sharks with near real-time tracking, allowing them to see directly how many were captured.

Shortfin makos are long-lived, highly mobile sharks whose habitat overlaps with that of commercially targeted tuna and billfishes. The result is that this shark, listed as Vulnerable on the IUCN Red List, is taken as by-catch in these fisheries and often retained to be sold for its high-value meat. To manage the species adequately, its populations need to be monitored. The trouble is that the majority of current population estimates rely on data reported by fishermen themselves. These ‘fisheries-dependent’ data are often unreliable, being based on catch reports that may be misrepresented.

This study tracked 40 sharks tagged with satellite-linked radio tags (SLRTs) over a period of three years. The tag is attached to the shark’s dorsal fin and connects to satellites to provide an estimate of the shark’s location every time the fin breaks the surface. Byrne and his colleagues found that these individuals swam into the Exclusive Economic Zones of 19 countries and were harvested in fisheries belonging to five countries. This result highlights the importance of cross-border cooperation to manage mobile species. Of the sharks tagged in this project, 30% were caught and killed. This suggests that the population is experiencing a state of overfishing – critical information needed to underpin conservation management decisions. Satellite tagging programmes, according to the researchers, have the potential to generate more than just information about where and how sharks move; they can inform more accurate estimates of shark fishing mortality than the traditional fisheries-dependent methods do.