Historical marine population estimates: Triggers or targets for conservation? The dugong case study

Abstract

Recent studies have estimated the historical abundance of large marine vertebrates to determine potential targets for conservation. We evaluated this approach using 1990s aerial survey estimates of dugong abundance and an estimate of the decline in dugong numbers since the 1960s based on changes in the catch per unit effort of dugong bycatch in a government shark control program on the east coast of Queensland, Australia. This analysis indicated that the catch rate of dugongs caught in shark nets, at six locations between latitudes 16.5° S and 28° S, declined at an average of 8.7% per year between 1962 and 1999. This represents a decline to 3.1% of initial catch rates over the sampling period. If the changes in the populations sampled by the shark nets and aerial surveys were equivalent, this result suggests that the region supported 72 000 dugongs in the early 1960s compared with an estimated 4220 dugongs in the mid-1990s. The seagrass habitat in the region is currently insufficient to support 72 000 dugongs, suggesting that our hindcast estimate may be an unrealistic target for recovery. Nonetheless, the evidence of serious dugong decline from the shark-net data and other sources has triggered significant conservation initiatives. This case study indicates that comparisons between historical and contemporary estimates of the abundance of large marine vertebrates can be powerful qualitative triggers for conservation action, but that quantitative targets for recovery require systematic testing of the assumptions underlying hindcast estimates, scientific evaluation of the current carrying capacity of the ecosystem for the target species, and consultation with a broad range of stakeholders. For some species, it may be more productive to set anthropogenic mortality targets that are designed to enable the population to recover to its optimum sustainable population than to set recovery targets per se.