Changes in the population structure, growth and mortality of striped weakfish Cynoscion guatucupa (Sciaenidae, Teleostei) of southern Brazil between 1976 and 2002

Abstract

Changes in the population structure, growth, and mortality of the striped weakfish Cynoscion guatucupa of southern Brazil were studied based on data collected from commercial landings in Rio Grande between 1976 and 2002. Mean length in the trawl fishery decreased abruptly while mean ages decreased steadily in recent years. Most abundant age classes in recent landings were 2 and 3 years old in the paired-trawl catches, one to three for otter and double rig trawls, and 5 and 6 years old in the gill net catches. Oldest fishes caught were aged 18 years and no fish over 14 years old was caught since 1985. The growth rate tended to increase over the course of the study, especially in the last analysed period (1999–2002). The total mortality instantaneous coefficient Z calculated from the paired trawls data catch curves increased from 0.36 in 1976 up to 0.92 in 2002 and the exploitation rate E increased from 0.31 up to 0.73 if a natural mortality coefficient M of 0.25 is assumed. The changes were attributed to the increase of the fishing on the striped weakfish stock, shared by Brazil, Uruguay and Argentina and are suggestive of overfishing.