Assessing the South African sardine resource: two stocks rather than one?

Sardine <i>Sardinops sagax</i> distributed off the west and south coasts of South Africa have traditionally been assumed to comprise a single well-mixed stock for assessment and management purposes. New research, however, lends weight to the possibility of two stocks in this region. A precautionary management approach thus needs to consider the impact of management decisions on the hypothesised two individual stocks as well as on the resource as a whole. As a first step in this process, Bayesian assessments of South African sardine are presented, which compare results for the traditional single-stock hypothesis with those that follow from a new two-mixing-stock hypothesis. Recruits from the west stock are assumed to move to and remain part of the south stock in annual pulses of varying size. This movement is estimated to be appreciable, and to take place from a substantially more productive west stock to the south stock. This immigration makes a greater contribution to the south-stock biomass than do years of above-average south-stock recruitment. Importantly, this two-mixing-stock hypothesis is shown to be consistent with the data available. Further alternative sardine stock-structure hypotheses suggested by the most recent data are discussed.

分布于南非西南沿海的沙丁鱼（Sardinops sagax），在传统的资源评估与管理工作中，被认定为一个充分混合的单一种群。然而，最新研究为该区域存在两个种群的可能性提供了有力佐证。据此，预防性管理策略需同时考量管理决策对这两个假设种群，以及整体渔业资源的影响。作为该研究进程的首要步骤，本文开展了南非沙丁鱼的贝叶斯（Bayesian）评估，将传统单一种群假设的评估结果与新提出的双混合种群假设的评估结果进行对比。研究假设西部种群的补充群体将以年度脉冲式迁移的方式进入南部种群并成为其组成部分，迁移规模每年不等。据估算，该迁移体量可观，且主要从生产力显著更高的西部种群流向南部种群。相较于南部种群自身高于平均水平的补充年份，此种迁入对南部种群生物量的贡献更为突出。值得注意的是，该双混合种群假设已被证实与现有数据一致。本文还讨论了由最新数据提出的其他可选沙丁鱼种群结构假设。