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Historical ecosystem modelling of the upper Gulf of California (Mexico) : following 50 years of change Lozano, Hector

Abstract

Ecological conditions in the upper Gulf of California (Northwest Mexico) have deteriorated significantly over the past seventy years through the removal of nutrients and freshwater from the Colorado River. Acting at the same time, uncontrolled exploitation has brought at least three fisheries to collapse and several endemic species, such as giant Gulf croaker (Totoaba macdonaldi) and the vaquita porpoise (Phocoena sinus), near to extinction. This motivated a study to quantify the ecological impacts attributable to diversion of the Colorado River on the dynamics and interactions of the upper Gulf of California (UGC). In the absence of baseline studies for the pre-diversion period, this work has combined scientific information, newly-gathered local knowledge from fishers (LFK), and information from industrial and artisanal fisheries to reconstruct for the first time models of the upper Gulf of California for three key periods: 1950, 1980 and 2000. Reconstruction of these past ecosystem states are based on mass-balanced models (130 species considered in 50 functional groups) generated using the Ecopath and Ecosim approach and software. Dynamic simulations, with ecosystem model parameters fitted to all known biomass and fisheries data (historical time-series of catch-at-age data and biomass surveys since 1970s reported by the Institute of National Fisheries, Mexico), were used to track and emulate changes caused by water diversion and fishing within the limits set by fluctuations in climate such as the El Niño events of 1983, 1993 and 1997. The UGC is an ecosystem to be highly-dependent on detritus-benthic components. Biomasses appear to be largely controlled by lower trophic levels. A highly diverse artisanal fishery targeting a wide range of species, impacts all levels of the food web. These effects need to be taken into account to be considered in the management plates in the region. During the last five decades, change in trophic level indicated important impacts of fishing, resulting in an average decline in the trophic level of 0.02 trophic level/decade. Also, there was a notable loss in the abundance of detritivores during the last 50 years, with a reduction of 64% in the total estimated biomass of all the groups located in trophic level 2 and 2.5. This loss is affecting the incorporation of organic matter to higher trophic levels of the food web, a critical function in any ecosystem. Overall, numerous changes in the energy flows and production rates were estimated during the research, indicating a large reduction in the energy and dynamics of the upper Gulf. These changes affect the structure and function of this ecosystem, limiting its ability to recover from future natural or human forces. Simulations suggested that river flows of only 1% of the undepleted level may produce increases of around 10% in the total biomass of the UGC, reflecting the enormous role of the Colorado River in the productivity in the region. This is particularly important considering that this region provides 10% of the annual Mexican landings (INEGI, 2001), and provides a preliminary accounting of the ecological and economic losses for Mexico during the last five decades since American river dams were built. Simulations of the effect of reduced river flows and environmental (El Niño) influences on the UGC ecosystem revealed the superficiality of our knowledge of the quantitative process simulated in the models and reconstruction of past states of natural aquatic ecosystems like the upper Gulf of California. It is evident that any attempt to restore the UGC must be initiated with a parallel study of the climate change in its influence in the region. This research suggested that human intervention can trigger rapid and significant changes in the structure and function of small marine ecosystems such as the upper Gulf of California. In the future, human interventions in river deltas, such as damming rivers and water abstraction, must be considered as one of the major forces affecting the steady state evolution of these ecosystems, impacting their natural processes, productivity, biodiversity and the human communities that depend on them for food, work and culture.

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