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?

Abstract

Worldwide overfishing has caused the collapse of populations of excessively exploited marine fish. The coastal demersal sciaenid fish “pescadinha”, Macrodon atricauda, has been intensely fished since the 1960s along the southern Brazilian coast, resulting not only in decreasing abundance but also in increasing growth rates and decreasing age and length at first maturity. We analyzed the time series of several population indicators, such as catch per unit effort (CPUE), age composition of the stock, total and natural mortality and exploitation rate to investigate the long-term impact of fishing on the mortality, age structure and biomass of M. atricauda. Furthermore, we modeled the time-trajectory of the total biomass under an assumption of constant recruitment and discuss the limits of resilience and the risk of collapse of the fishery. The CPUE (kg/day at sea) has decreased almost 50%. Total mortality increased from 0.5 yr−1 in the almost unexploited stock in the 1950s to around 1.9 yr−1 in the 2000s, while the age structure changed dramatically: fishes in the landings were not older than 9 years old in the 1960s, 7 years old in the 1970s and no fish over 5 years old has been observed since the 1990s. Taking into account the growth changes, a deterministic model estimated a reduction of 67% in total biomass over five decades. In the last 30 years, the stock has withstood high exploitation rates (between 0.5 and 0.7 yr−1) and has suffered a steady decline in biomass but has not collapsed, probably due to life-history traits that favor resilience: early maturation and rapid individual growth. However, the stock is at risk of collapse, taking into consideration the present non-stabilized level of high mortality, changed age structure and life history, low biomass and high exploitation rates. A precautionary approach suggests the need to reduce fishing effort