Date Published: March 12, 2019
Publisher: Public Library of Science
Author(s): Elena Nalesso, Alex Hearn, Oscar Sosa-Nishizaki, Todd Steiner, Alex Antoniou, Andrew Reid, Sandra Bessudo, Germán Soler, A. Peter Klimley, Frida Lara, James T. Ketchum, Randall Arauz, Even Moland.
Many species of sharks form aggregations around oceanic islands, yet their levels of residency and their site specificity around these islands may vary. In some cases, the waters around oceanic islands have been designated as marine protected areas, yet the conservation value for threatened shark species will depend greatly on how much time they spend within these protected waters. Eighty-four scalloped hammerhead sharks (Sphyrna lewini Griffith & Smith), were tagged with acoustic transmitters at Cocos Island between 2005–2013. The average residence index, expressed as a proportion of days present in our receiver array at the island over the entire monitoring period, was 0.52±0.31, implying that overall the sharks are strongly associated with the island. Residency was significantly greater at Alcyone, a shallow seamount located 3.6 km offshore from the main island, than at the other sites. Timing of presence at the receiver locations was mostly during daytime hours. Although only a single individual from Cocos was detected on a region-wide array, nine hammerheads tagged at Galapagos and Malpelo travelled to Cocos. The hammerheads tagged at Cocos were more resident than those visiting from elsewhere, suggesting that the Galapagos and Malpelo populations may use Cocos as a navigational waypoint or stopover during seasonal migrations to the coastal Central and South America. Our study demonstrates the importance of oceanic islands for this species, and shows that they may form a network of hotspots in the Eastern Tropical Pacific.
Oceanic islands and seamounts provide important habitats in the pelagic environment for many marine species, often resulting in biological “hotspots” characterized by a greater diversity and abundance of pelagic life . They can be thought of as “border environments” where reef-associated communities interact with a suite of open water species of different trophic levels, from planktivorous fishes to top predators . Sharks in particular, often aggregate at oceanic islands. However, these areas may not necessarily be, energetically speaking, their most important habitat . While these locations may provide refuge, cleaning, or navigation reference points [4–9], sharks may be particularly vulnerable here to fishing gear targeting their aggregations.
Scalloped hammerhead sharks form large aggregations at several locations around Cocos Island, similar to Galapagos, Malpelo and other oceanic islands in the Eastern Tropical Pacific [17,18,19,24]. The results of our tagging studies suggest that these aggregations are fluid, such that individuals do not remain at the island constantly throughout the year or season, but associate with the island mostly during daytime hours, throughout the year, punctuated by absences. In addition, that sharks should return to Cocos after absences of nine months or greater is worthy of note, and indicates the importance of the island to this species.