Triennial migration and philopatry in the critically endangered soupfin shark (Galeorhinus galeus)

Globally, one-quarter of shark and ray species is threatened with
extinction due to overfishing. Effective conservation and management can
facilitate population recoveries; however, these efforts depend on robust
data on movement patterns and stock structure, which are lacking for many
threatened species, including the Critically Endangered soupfin shark
(Galeorhinus galeus), a circumglobal coastal-pelagic species. Using
passive acoustic telemetry, we continuously tracked 34 mature female
soupfin sharks, surgically implanted with coded acoustic transmitters, for
seven years via 337 underwater acoustic receivers stationed along the west
coast of North America. These sharks and an additional six were also
externally fitted with spaghetti identification tags. Our tagging site was
a shallow rocky reef off La Jolla (San Diego County), California, USA,
where adult females were observed to aggregate every summer. Tagged
soupfin sharks were highly migratory along the west coast of North
America, between Washington, USA and Baja California Sur, Mexico. However,
every three years, they returned to waters off La Jolla, California, where
they underwent gestation. This is the first conclusive evidence of
triennial migration and philopatry (‘home-loving’) in any animal, which is
apparently driven by this species’ unusual triennial reproductive cycle.
Females of other shark and ray species with triennial reproductive cycles
are also likely to exhibit triennial cycles of migration and philopatry.
At least six (15%) of our tagged soupfin sharks were killed in commercial
gillnets in Mexico. Policy implications. Identifying multiennial migratory
cycles of mature females can reveal hidden stock structure in the form of
discrete breeding cohorts, which are spatially and temporally segregated
as they cycle through different reproductive phases. Accounting for this
complexity may improve the performance of spatially structured stock
assessment models, particularly when fishery removals are spatially
heterogeneous, as well as inform the spatiotemporal design of
fishery-independent surveys. In the US, the soupfin shark is neither
actively managed nor recognized as a Highly Migratory Species; however,
given the highly migratory behavior we report, this designation should be
revisited by the US Pacific Fishery Management Council. Finally, given the
extensive fishery removals in Mexico, any future management must be
internationally cooperative.