By Naití Morales – ESMOI, Universidad Católica del Norte.
The great predators of the sea, such as sharks, tunas, swordfish, bonitos and others which are called “top predators”, are all those species which are found at the top of a food chain or trophic level. This means that they are the major predators in their habitat and, in general, don’t have others that prey on them. They represent a major component of marine ecosystems, playing an important role in the regulating the top-down population of prey immediately below them in the food chain. The constant decline in the populations of “top predators” is creating a growing concern for their conservation and control of the indirect effects that their disappearance could cause.
There is strong evidence that indicates that a reduction in, or disappearance of, “top predators” causes a series of changes in the composition and diversity of species within an ecosystem. If one component of the food chain changes substantially, all the species in the chain will be affected in what is called a cascade effect. For example, if we remove the sharks, the populations of intermediate predators will increase and cause a decrease in the population of herbivores. As the population of herbivores decreases, there is no one to eat the algae and the ecosystem will change from one dominated by corals to one dominated by algae.
The loss of the so-called “top predators” has been a result of human action. Overfishing is considered the principal cause of the decrease of these species in the world, since, as a result of their biology (late sexual maturity, few young per litter, long gestational periods, etc.) these species are more vulnerable than others to high levels of fishing, which easily leads to a population collapse and eventual extinction.
As a result of its isolation, the marine biodiversity of Rapa Nui has seen few studies. Nonetheless, those few that have been published show a high level of endemism and an important impact on the fisheries in the area. In the waters off of Rapa Nui, the effects of fishing are evident. It is now difficult to find sharks and other large fish. In contrast, on Motu Motiro Hiva,“islet which looks toward Hiva”, which is the name given to the small island of Salas y Gomez by the Polynesians on their oceanic adventures and located 400 kilometers (250 miles) from Rapa Nui, the populations of “top predators” represent 43% of the biomass of the area. The leader is the Galapagos shark, or Māngo (Carcharhinus galapagensis), followed by the Yellowtail Amberjack, or Toremo (Seriola lalandi), the Black Jack, or Ruhi (Caranx lugubris), and the White Trevally, or Po‘opo‘o (Pseudocaranx cheilio). This high level of predators is an unequivocal sign of good health in the ecosystem.
In the year 2010, the government of Chile created the Motu Motiro HivaMarine Park, covering an area of 150 km2 (58,000 sq. mi.) in order to save the local biodiversity from future threats. However, the population of top predators within the park is still in danger. The discovery of specimens with hooks in their mouths, combined with the possible presence of fishing boats within the area, lead us to believe that illegal fishing is still a major problem which needs a solution.
The majority of top predator species are active swimmers and migratory, which offer great challenges to those who would study them and propose plans for conservation. ESMOI, the Millennial Nucleus for Ecology and Sustainable Management of Oceanic Islands, is currently studying the migratory patterns and connectivity among the top predators of Rapa Nui and Motu Motiro Hiva, in order to determine if the populations in both places are connected and, at the same time, verify if the Marine Park is effective for their protection.
During the last CIMAR-21 Islas expedition, on board the Chilean Navy vessel “Cabo de Hornos”, 33 Galapagos sharks and 19 Yellowtail Amberjacks were tagged with conventional markers, known as spaghetti, for marking and recapture. The range of total length obtained in this sampling was between 76 and 170 cm (30 and 67 inches) for the Galapagos shark and 69 and 124 cm (27 and 29 inches) for the Yellowtail Amberjack, ranges which are within the usual observations for these species. Among them, only the largest ones, 4 sharks and 1 amberjack, were implanted with satellite markers (MiniPat). The satellite markers will give us the routes that these individuals travel during one year. At the same time, we did genetic analysis to determine the relationships between the Rapa Nui population and the Motu Motiro Hiva group.
Since the Galapagos shark and the Amberjack have a world-wide distribution, our next objective is to determine the relationship between the populations in the Easter Island eco-region (Rapa Nui and Moto Motiro Hiva) and those of other zones of the Pacific Ocean, such as the Galapagos Islands in Ecuador, the Isla del Coco in Costa Rica and Hawaii in the USA, to determine how these species with a migratory capacity of only a few hundred kilometers (miles) have managed to dominate almost all the oceans of the world.