INORGANIC TURBIDITY AND THE FAILURE OF FISHERY MODELS

Models for the prediction of fishery production and/or harvest based o n primary production, algal biomass, or nutrients and morphometry have been effective in many lakes and reservoirs. Lake Chapala, Mexico's l argest, is located on the Rio Lerma, one of Mexico's principal rivers. It was made a reservoir in 1903 by the construction of a hydroelectri c dam on the out-flowing Rio Santiago. For the first half of this cent ury Lake Chapala was famous for its native white-fish (Chirostoma luci us) fishery. This fishery has collapsed. The present fishery consists of small Chirostoma species and the introduced Oreochromis. During the past 17 years the water level has fallen by over 3 meters. Now very s hallow, clay resuspension creates low visibility (Secchi < 1 m). Nutri ents are abundant with total phosphorus exceeding 1 mg l-1 and inorgan ic nitrogen exceeding 0.5 mg 1-1. Photosynthesis is limited by light a nd especially the unfavorable mixing depth to photic depth ratio. Mode ls based on phytoplankton production or biomass underestimate the fish ery by about one order of magnitude while a morphoedaphic model overes timates the fishery to the same extent. We sought to explain alternate pathways to support the realized fishery. Experiments suggest that ba cterial prodution, with a bypass of the microbial loop, may offer a pa rtial explanation. Management practices to increase the fishery based on an increased autotrophic base to food chain would fail without cons ideration of bacterial processes in this highly turbid ecosystem.