Energy flow and trophic structure of Galápagos shallow rocky reef systems along a gradient of productivity and artisanal fisheries

Abstract

This thesis aims to better understand how changes in primary production, temperature and fishing intensity within a biogeographic regional influence the trophic structure and the diversity and distribution of key species In the Galapagos Marine Reserve. Changes in the marine ecosystem influenced by El Nino and fisheries have been extensively evident in Galápagos. However, alterations resulting in the food chain such as changes in energy flows, trophic interactions and system structure influenced by these factors have been poorly described. Trophic models for the period 2004-2008 were constructed using the "Ecopath with Ecosim" software to describe differences between the three major biogeographic regions in the Galápagos Marine Reserve: the west cold upwelling system - Bolivar Channel, the south-central mixed system - Floreana and the far-north tropical system - Darwin and Wolf. Information required for the construction of these trophic models were mainly based on five years of underwater ecological surveys, oceanographic monitoring and population censuses carried out by the Charles Darwin Foundation and the Galapagos National Park. Additional information was obtained from other sources such as literature, reports and other trophic models. The description of differences in the system structures, energy flows and ecological characteristics of each biogeographic region were based on quantitative ecosystem parameters estimated by Ecopath. Additionally, the effects of artisanal fisheries and El Nino on the Galápagos ecosystems were explored with Ecosim to understand the mechanisms behind the observed changes in the ecosystem dynamics. In conclusion, the three systems compared in this study showed clear differences in their oceanographic settings as well as in their fishery impacts. The selectivity of target species by the local artisanal fishery plays an important role in structuring the benthic community. However, direct effects are spatially variable, depending in part on the proximity to fishing ports. Conservation strategies need to take into account the biogeographic functionality and structural differences of each system to improve the effectiveness of the actual spatial management plan in the Galápagos Marine Reserve. In addition, further studies are recommended to improve the models for future exploration of management strategies. The current fishing-monitoring program must be evaluated and adapted to produce better geo-referenced data, which will provide a robust input to evaluate the fishing effect in the Galápagos Marine Reserve.