Based on biomass, more sharks live in the waters surrounding the northernmost islands of the Galapagos archipelago than anywhere else in the world. The islands of Darwin and Wolf are home to 11.2 tonnes (12.4 tons) of sharks per hectare (2.5 acres), according to new findings published in PeerJ this week. The findings will help inform the government of Ecuador as it develops the “no-take” reserve that was just announced in March.
Overfishing has depleted shark and other top predatory fish populations around the world, reducing their biomass by more than 90 percent. Now, one in four species of cartilaginous fish is threatened with extinction because of overexploitation and habitat loss. Even remote locales have been severely impacted. There are, however, a few hotspots for sharks remaining: Costa Rica's Cocos Island National Park, Chagos Marine Reserve in the Indian Ocean, and Darwin and Wolf in the far north of the Galapagos Marine Reserve.
Darwin and Wolf are the tops of eroded, extinct submerged volcanoes. These small islands, located less than 40 kilometers (24.8 miles) apart, are heavily influenced by the warm Panama current that comes from the northeast. Unlike other islands in the archipelago, which is dominated by the cold equatorial counter-current, the waters of Darwin and Wolf range from 22.5 to 29 °C throughout the year.
During November of 2013 and August of 2014, a team led by Pelayo Salinas de León from the Charles Darwin Research Station conducted two expeditions to Darwin and Wolf to estimate the abundance of shark and predatory fish assemblages. The team surveyed the fishes at seven sites around the two islands using traditional visual censuses as well as a diver-operated stereo-video system equipped with cameras.
Their estimates revealed the largest reef fish biomass ever reported: 21.7 tonnes (24 tons) per hectare at Darwin and 11.4 tonnes (12.6 tons) per hectare at Wolf, or 15.9 tonnes (17.5 tons) per hectare on average. And 73 percent of the total biomass (or 11.2 tonnes per hectare) was sharks, including scalloped hammerheads, Galapagos sharks, and blacktips.
The presence of sharks and top predators indicates a healthy marine ecosystem. But the team also found low amounts of predatory reef fishes such the leatherbass and sailfin grouper. These long-lived fish grow slowly, making them especially vulnerable to overfishing. Despite being part of the Galapagos Marine Reserve, the two northern islands weren’t fully protected until the announcement of the Darwin and Wolf Marine Sanctuary in March 2016.