Sustainability demands vigilance: evidence for serial decline of the greenlip abalone fishery and a review of management

摘要

The greenlip (Haliotis laevigata) abalone fishery in the Western and Central Zones of South Australia comprises about 74 distinct metapopulations. Although the total catch has been maintained since 1979, analysis of catch and effort data for the twozones at a fine scale in 49 map code areas shows that 39 areas are in decline; of these, 28 areas, with declines exceeding 50 percent, are considered serious. In the Western Zone, the proportional catch from 23 inshore reefs has declined from approx 69to approx 34 percent of the total catch over 20 y of fishing; whereas, that of 10 distant reefs has increased from approx 26 to approx 50 percent of the total over the same period. Fishing mortality is 0.4-0.6 on inshore reefs and 0.2-0.4 on distant ones, indicating serious maldistribution of effort. The frequency of declining populations reaching the 50 percent decline point peaked in the years 1989 to 1990; a subset of the same group of populations reaching the 80 percent decline point peaked in 1995to 1996, indicating a continuing decline trajectory, despite quota imposition and reduction. Multiple regression analysis of the proportional rate of decline of 38 populations in the Western Zone showed that the rate of decline was inversely correlatedwith distance from port and with a closure index that measures the extent to which the population is bounded by land. The closure index is considered to estimate crudely the extent to which coastal topography retains larvae within the population. Comparison of the spatial extent of fishing areas in eight populations mapped in 1978 to 1979 and again in 1996 to 1999 showed strong spatial contraction matching the decline in catch. Spatial contraction occurred from deeper to shallow water leaving relict subpopulations around headlands and in bays, consistent with inferences on the effect of coastal topography on abalone larval retention. In the Central Zone, with six divers and 12 reefs fully fished over a shorter time period, the declines are less certainand less severe. The catches of five reefs have declined by >50 percent and increased substantially on one remaining productive reef. An overfishing hypothesis best explains the long-term decline in the catch of many reefs and the transfer of fishing todistant or remaining reefs. The history of management is reviewed and shows that, over the lifetime of the fishery, management has moved from control by government with little consultation with industry in 1967 toward self-management with endorsement bygovernment from about 1995. Since 1995 management has been unresponsive to declines in the fishery, and possible reasons are explored. We advocate application of the precautionary approach by management at the scale of the metapopulation to arrest the declines and the establishment of recovery plans for areas in serious decline.