Effects of selective fishing on a small scale multi-species and multi-gear freshwater fishery in the Magdalena River Basin (Colombia)

Abstract

Classical management has not been able to stop the 65% decrease in fishery production during the last 40 years in the Magdalena-Cauca River Basin. To analyze the effects of selective fishing of multiple species and small scale fisheries we addressed temporal changes at fishing level and the response of fishermen. The fishery reduced production and CPUE (catch per unit effort standardized), decreased the large sizes and growth rates of Prochilodus magdalenae and Pseudoplatystoma magdaleniatum, changed the abundance of trophic levels (decreased carnivores and increase of detritivores, omnivores), and increased exploitation rates. The fishermen have responded by implementing self-control measures, diversifying fishing gear and mesh size, including new species and sizes in the catch with a higher cpue (catch per unit effort) of small sized fish, adjusting the fishing effort to the abundance. We conclude that selective fishing has had ecological effects and fishermen have empirically self-regulated to optimize the cost-benefit ratio of their activity, developing a fishery that is more in line with ecosystemic structures. We address the balanced harvest strategy as a management alternative.