Magellanic penguin as an indicator of a large marine ecosystem pelagic food web status. (PLENARY SPEAKER)

CIANCIO, JAVIER

Conferencia; 8th World Seabird Twitter Conference; 2022

World seabird union

As marineecosystems are harvested or modified by anthropogenic or natural factors, thereis an increasing demand for indicators of their health or status. ThePatagonian Shelf is a large marine ecosystem of more the one million squarekilometers, and is inhabited by many species of conservation interest. Withinthis ecosystem, seabirds are central in the aesthetic of the landscape, and theyhave also key roles in the food web due to the large biomass of prey theyconsume, and vast the extension of the ocean they use. The Magellanic Penguin (Spheniscus magellanicus) is widelydistributed in the Patagonian coast, and during breeding, this species feedsmainly on schooling fish. Argentine anchovy (Engraulis anchoita) is the dominant prey in the North, and Fuegian sprat(Sprattus fuegensis) in the South. Thesetwo latter species are also prey of many predators, and are central linksbetween the base of the food web and upper predators. Like most forage fish,both species display large natural interannual recruitment fluctuations; and, dueto the extensive area in which this ecosystem expands, large efforts arenecessary to assess their abundance. We explored, during eight breeding seasonsand a latitudinal range of 10 degrees, the changes in Magellanic penguin isotopicniche as a function of interannual changes in forage fish abundance, whichcould be in turn be used as an indicator of its abundance. We based ourpredictions on the simple assumption that during years of low forage fishabundance, their role as prey is not occupied by another single species, and consequentlypenguins have to cover their energy needs based on a diverse diet. Additionally,we looked at the trophic level of penguins as indicator of fish size classconsumed. Magellanic penguins showed large isotopic niche fluctuations among breedingseasons, explained by the contribution of forage fish in their diet.Furthermore, the size of the isotopic niche was found to be negativelycorrelated with forage fish biomass estimated from acoustic surveys and withtheir contribution in the diet of birds. We also defined an isotopic niche sizethreshold, which indicates low abundance of forage fish in nearby feedinggrounds. The trophic level of Magellanic penguins increased with latitude,which corresponds to the size-distribution of both main forage fish species. Weare presenting here a very simple and easy to implement indicator to monitor thestatus of the base of the food web in one of the most productive oceans in theworld. This index has the potential to be used as a low-cost supplementarymethod in this vast area during years when no forage fish surveys are available.