Large marine predators are key elements in ocean ecosystems, acting as regulators of ecosystem services and many have high economic and social value. As such, their rapid and widespread decline as a result of overexploitation and other human impacts has aroused great concern among both scientists and policymakers. In response to this situation, resources and efforts have been put into compiling data to develop functional models of their ecosystems. These models, based on ecological processes, are crucial for drawing up guidelines that allow for the sustainable exploitation of ecosystems, while also ensuring the conservation of endangered species. To build these models, however, there is a lack of detailed information, e.g. on diet, energy flows and the distribution of predators and prey.
This lack of knowledge is obvious in the oceanic region of the Madeira archipelago, including the Selvagens. The aim of this project is, for the first time and in an integrated way, to describe the structure and functioning of these pelagic marine ecosystems. By building a multidisciplinary team, we intend to investigate and model the trophic networks involving the marine megafauna of this oceanic area, including aerial and underwater predators: pelagic seabirds, tunas, marlin (targeted by commercial or sport fishing), turtles and marine mammals. These charismatic species will be used to obtain information on the distribution and ecological importance of their prey, which includes fish and cephalopods, but also crustaceans and jellyfish. The geographical area covered in the project includes the vast 1.5 million km2 region used by one of the most studied seabird species in the region: the shearwater. Once data is compiled for other seabird species as well as data from the tuna fishery, this area will probably be extended to include an important fraction of the 4 million km2 of the Portuguese EEZ.
In this oceanic region, the areas with the highest densities of seabirds are still unknown. In this way, we will complement the already extensive database on shearwater feeding areas with tracking data for most of the other bird species (Black-winged Teal, Common Nuthatch, Chickadee and Calcamar), using GPS devices and geolocators. Spatial analysis techniques and remotely obtained oceanographic data will then be used to produce predictive distribution models.
The diet of seabirds tracked with GPS, as well as that of tunas and marlin with known catch locations, will be used to determine the distribution and ecological role of different prey in these subtropical ecosystems. Conventional and molecular methods (DNA-barcoding) will be used simultaneously to identify prey from the stomachs of live animals as well as animals from fisheries. Knowledge of the trophic ecology of these ecosystems will be deepened using mixing models and estimates of community metrics derived from the analysis of stable isotopes in predator and prey tissues. The heterogeneity of the predator community studied will make it possible to sample marine prey in a horizontal (coastal-pelagic) and vertical (epi vs mesopelagic) gradient, as well as in different water masses and bathymetric gradients.
A significant impact of human activity on the oceans is marine pollution, even in remote locations. The levels of pollutants in this region are almost unknown, but it is known that potentially toxic trace elements bioaccumulate and amplify along trophic chains. Therefore, their quantification, especially in species consumed by humans, is relevant. With a multi-species group of predators and prey, this study provides the right conditions for documenting and interpreting geographical patterns in the occurrence of trace elements (Hg and other contaminants), as well as for identifying trophic enrichment processes.
This innovative proposal will make it possible to: 1) obtain an integrated perspective of the distribution and abundance of charismatic megafauna and their prey in a subtropical ecosystem, and identify the ecological and oceanographic variables that determine their spatial partitioning; 2) obtain predator diet data and develop complementary methods aimed at understanding oceanic trophic networks, including the little-known role of mesopelagic species; 3) use georeferenced predator diet information to describe the distribution, abundance and ecological role of prey species; 4) describe the structure, functioning and resilience of the ecosystem, combining distribution, diet and biomarker data, with the potential to be used as a tool in fisheries management models and in the designation of Marine Protected Areas.
