Effects of the 1995 and 1998 mass mortality events on the spawning biomass of sardine, Sardinops sagax, in South Australian waters

Abstract

This paper estimates and compares the effects of the mass mortalities of Sardinops sagax that occurred in South Australian waters in 1995 and 1998. After the 1995 event, the spawning biomass of S. sagax in South Australian waters fell by over 75% from approximately 165 000 tonnes to approximately 37 000 tonnes. No juvenile mortality was observed during 1995 and the population recovered quickly, with spawning biomass reaching 147 000 (70 000 to 234 000, 95%CI) tonnes in 1998. After the mass mortality event in October-November, spawning biomass fell by over 70% to 36 000 (19 000 to 67 000, 95%CI) tonnes; significant numbers of juveniles were also killed. The mortality of juveniles in 1998 and the recent increase in the abundance of Engraulis australis suggest that the population may recover more slowly from the 1998 mortality event than did after 1995. The initiation of the two largest mono-specific mass mortalities of fish ever recorded in South Australian waters within a period of less than four years suggests that the timing and location of the events was non-random. Both events occurred since 1993-1994, when large-scale tuna farming began in South Australia, and less than 250 km from Port Lincoln, where large quantities of untreated imported frozen S. sagax are fed to caged tuna. The introduction of untreated imported frozen fish products into the marine environment may be one of the mechanisms that has facilitated the range shifts of pathogens that have been associated with the increased frequency of mass mortalities due to disease in the ocean.