Author : Ortiz, Eufronio A. Jr.

Summary : This study analyzed the environmental problem confronting the Magat watershed in Isabela province, Philippines, with emphasis on the possible impacts of the 16 July 1990 earthquake on the reservior's fishery productivity. The interrelationship of the physico-chemical, biological, and socio-economic components of the reservior was discussed in relation to both human interventions and natural calamities in the watershed. As a result of human interventions in the watershed, the watershed had been transformed into grasslands and agricultural lands at the expense of primary and secondary forest. The 16 July 1990 earthquake represented a major perturbation moved huge volumes of soil eroded along the mountain areas of Cordillera and Caraballo mountains. This accelerated the sedimentation of the Magat reservoir. Deposition of eroded materials into the Magat riverine ecosystem reduced algal growth and reportedly caused the temporary disappearance of some freshwater fishes. The occasional deposition of erosion materials in waterways during the rainy season had caused turbidity and sedimentation of waterways resulting in the decline of fish population. Data on the quality of the reservoir before and after the calamity showed significant differences which could account for the cumulative effects of changes in the chemical and physical conditions. Declining fishery productivity of the reservoir affected the socioeconomic condition of Magat fisherfolks though a decreased fish catch of 393,478 kg in 1994 to 54,823 kg in 1991 and the culture period was longer (5-6 months up to one year).