Hotspots within hotspots? Hammerhead shark movements around Wolf Island, Galapagos Marine Reserve

Abstract

Are pelagic species such as sharks and tuna distributed homogenously or heterogeneously in the oceans? Large assemblages of these species have been observed at seamounts and offshore islands in the eastern tropical Pacific, which are considered hotspots of pelagic biodiversity. Is the species distribution uniform at these hotspots or do species aggregate at a finer spatial scale at these sites? We employed three techniques to demonstrate that the aggregations of scalloped hammerhead sharks, Sphyrna lewini, and other pelagic species were confined to the southeastern corner of Wolf Island in the Galapagos Marine Reserve. Coded ultrasonic transmitters were placed on individuals at this site and at another aggregation site at Darwin Island, separated from Wolf by 40 km, and they were detected by monitors moored at the southeastern corner of Wolf Island and rarely by monitors deployed at other sites around the island. Hammerhead sharks, carrying depth-sensing continual transmitters, were tracked for two-day periods in a vessel and shown to reside a disproportionately large fraction of their time at the southeastern corner. Visual censuses were carried out seasonally at the eight monitor sites at Wolf Island, recording the abundance of one species of tuna, four species of jacks, and a number of other species. The highest diversity and abundance of these species occurred in the southeastern corner of the island. Our results support the use of hammerhead sharks as indicator and umbrella species for pelagic hotspots on a fine scale.

Governability assessment of the Galapagos Marine Reserve

“…Marine species in the GMR are either resident or transient, depending on the nutrient supply from the ocean currents, temperature, and current strength (Galápagos Conservation Trust 2013). Their distribution is uneven with high concentrations of marine taxa (e.g., sharks, stingrays, and sea turtles) in pelagic zones of deep waters depression and sea mounts around Isabela, Fernandina, and Wolf (Hearn et al 2010;Galápagos Conservation Trust 2013). These marine species vary in their importance to different sectors, and in terms of how well they are managed, as shown in Table 1.…”

First record of the smooth hammerhead shark (Sphyrna zygaena) in Saint Peter and Saint Paul Archipelago: range extension for the equatorial region

“…Nonetheless, according to the author, the record requires confirmation. Smooth hammerhead sharks may seek shelter and food in island ecosystems, such as scalloped hammerhead sharks, which usually form aggregations in these environments (Hearn et al, 2010;Bessudo et al, 2011;Ketchum et al, 2014).…”

Extensive use of mesopelagic waters by a Scalloped hammerhead shark (Sphyrna lewini) in the Red Sea

“…Juveniles of this species inhabit coastal waters, while adults are found in groups or as solitary individuals further offshore [8,9]. For the Indian Ocean, the species' maximum observed total length (TL) was reported as 316.8 cm and males and females in this region have been observed to reach maturity between 180-189.9 and 220-240 cm TL, respectively [10].…”