A chronicle of collapse in two abalone stocks with proposals for precautionary management

摘要

Two populations of greenlip abalone, Haliotis laevigata, in Backstairs Passage and Avoid Bay, collapsed during 30 years of fishing. The former population was monitored from 1980 to 1999, and the latter from 1987 to 1999. Both populations showed strong spatial contraction as their respective subpopulations in open habitat failed first compared with those in inlets, bays and around islands, where recruitment was higher. Stock-recruitment curves are gently sloping or indeterminate, indicating weak density-dependence in the one case and high variability in the other case. From data gathered during the decline of these populations we review various fishery indicators which may be useful for monitoring greenlip abalone populations and for predicting collapse. Total catch was correlated with relative spawner abundance from survey data, and thus appears to be a useful indicator of abundance in this fishery. Other useful indicators are densities and size compositions, from which annual recruitment strength and total mortality can be derived, and survey evidence of spatial contraction of fished areas. Egg-per-recruit analyses are also useful to estimate the extent of departure from predetermined sate levels of egg production. Given the fuzzy knowledge ofthe status of populations, several indicators are best used in concert to determine management strategies. We propose a "troublespot thermostat" approach in which a suite of indicators, sequentially triggered off, call for increasingly severe managementresponses. In this way declines can be detected and arrested, and timely steps taken to avoid collapse.