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)(2000)(2001)(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.

Age and changes in growth of the king weakfish Macrodon atricauda (Günther, 1880) between 1977 and 2009 in southern Brazil

“…In the same region, growth changes have been observed for the other abundant demersal sciaenid fishes (Haimovici, 1998): Micropogonias furnieri, between 1976and 2002(Haimovici and Ignácio, 2005; Umbrina canosai, between 1976 and 2001 (Haimovici et al, 2006a) and Cynoscion guatucupa, between 1976 and 2002 (Miranda and Haimovici, 2007). In all these cases as for M. atricauda, the changes were larger for the older fishes and were attributed to decreases in abundance due to fishing pressure.…”

Long-term changes in the age structure, mortality and biomass of the king weakfish Macrodon atricauda (Günther, 1880) in southern Brazil: Is it resilient enough to avoid collapse?

“…Changes in total mortality occurred during a period in which the yields of the overall continental shelf demersal fish abundance decreased sharply in southern Brazil (Haimovici, 1998;Vasconcellos et al, 2006). The decrease in the abundance of M. atricauda and its potential competitor Cynoscion guatucupa (Miranda and Haimovici, 2007) on shrimps, squids and small fishes along the southern Brazil inner shelf (Martins, 2000), may have increased the food availability for pescadinhas, which is consistent with the observed increase in weight by age (Cardoso and Haimovici, 2011). An increase in mortality since the start of the industrial fishery, and the changes in the age composition of the stock have consisted of three stages (Fig.…”

Density‐dependent changes in the feeding behaviour of Macrodon atricauda of southern Brazil

“…The decrease in size at maturity was interpreted as an effect of the selective fishing pressure (Cardoso & Haimovici, ). In contrast, the changes in growth and in the age at maturity were interpreted as density‐dependent effects, resulting from the decrease in abundance of M. atricauda (Cardoso & Haimovici, ), and also of striped weakfish Cynoscion guatucupa (Cuvier 1830) (Miranda & Haimovici, ), its main competitor for shrimps, squids and small fishes along the inner shelf of southern Brazil (Martins, ). This interpretation was based on the hypothesis of increased food availability.…”

“…These changes may be a result of the intense fishing pressure on the demersal fish stocks along the continental shelf of southern Brazil, which have caused a decrease in abundance in the last five decades (Haimovici, ; Haimovici et al ., ). The decreased abundance of fish stocks may have resulted in a lower availability of fishes and a higher availability of A. longinaris as prey for M. atricauda , probably due to reduced intra‐ and interspecific competition for food caused by the decreasing abundance of teleosts (Cardoso & Haimovici, ), particularly its main competitor, C. guatucupa (Miranda & Haimovici, ).…”