What causes bonnethead sharks to migrate?

Bonnetheads have strong preferences regarding temperature.

Research Need

The bonnethead (sphyrna tiburo), a small shark species, lives along the Southeast coast. Fishers are likely to find bonnetheads in North and South Carolina estuaries from April through November. Beacuse its habitat often faces significant temperature changes, the bonnethead migrates south during cold seasons to seek warmer waters and returns northward to the same sites year after year (a phenomenon known as “site fidelity”). 

The specifics of what motivates this organized migration, however, remain unknown. Scientists knew that if they tracked these animals for several years, while monitoring sea surface temperatures, they might be able to better understand their migratory behavior.

What did they study?

Researchers focused on South Carolina’s North Edisto River, a common migration destination for bonnetheads. The team tagged 44 bonnetheads at the North Edisto site from 2015 to 2018 and continued to track their activity until 2023. Each shark was fitted with a small, battery-powered passive acoustic tag that enabled scientists to track its location.

What did they find?

Despite traveling across the Southeast coast during colder seasons, all but one shark returned to the North Edisto site every summer. 

Furthermore, average seasonal temperatures affect how far bonnetheads migrate, with warmer-than-average temperatures corresponding to shorter distances and colder-than-average temperatures corresponding to longer distances.

In general, the sharks overwintered in the nearshore Atlantic Ocean waters from Georgia to central Florida. 

The sharks showed a strong preference for water temperatures within the range of about 62° F to 72° F. Even if areas outside this range were easily accessible, the sharks avoided them.

What else did they find?

Some sharks displayed social behavior, traveling in the same groups more than once. This reflects studies across shark species that show sharks tend to bond with other sharks they commonly encounter.

So what?

Because temperature appears to have the greatest impact on bonnethead shark migration, climate change creates high stakes for this species — and potentially others. As waters continue to warm, temperature-sensitive species will have trouble finding optimal habitats, which will cause significant disruption to the patterns and geographical ranges of their migrations.

Reading

Keller, Bryan A., et al. “The spatiotemporal effect of sea surface temperature on the seasonal migrations of the Bonnethead, Sphyrna Tiburo.” Environmental Biology of Fishes, 27 Dec. 2024, https://doi.org/10.1007/s10641-024-01632-0.

The text from Hook, Line & Science is available to reprint and republish at no cost, but only in its entirety and with this attribution: Hook, Line & Science, courtesy of Scott Baker and Sara Mirabilio, North Carolina Sea Grant.