Persistence of depleted abalones in marine reserves of central California

We evaluated the persistence and possible recovery of two depleted marine mollusks, the red (Haliotis rufescens) and black abalone (H. cracherodii), in central California, USA. Monitoring over 32-years did not reveal increasing or decreasing trends in red abalone abundances and sizes over the past three decades, in the absence of harvesting. Comparisons between marine reserves protected for at least 25 years and sites with open public access showed significant difference in size structure for black abalone, with individuals greater than 8 cm in shell length comprising 14–37% of animals in reserves and 2–11% at open-access sites, and a trend for greater abundances of red abalone within reserves. Despite no increasing trends, protection in one of the no-take reserves, the Hopkins marine life refuge (HMLR), has led to persistence of red abalone populations over multiple generations, at average densities of 0.2individuals/m2. At other locations, both within the HMLR and elsewhere, red abalone densities are lower than at the location where long-term studies were conducted (av. 0.03animals/m2), and an order of magnitude lower than for black abalone (av. 0.4 animals/m2). These results suggest that continued fishery closure and protection in no-take reserves are effective tools for allowing persistence of abalone populations, though there are no signs of recovery to levels comparable to those preceding fisheries collapse. Such failure to recover is most likely associated with high natural mortality and possibly continued illegal take, but not with processes underlying low abalone population levels elsewhere, including food or habitat limitation, recruitment failure, or disease. Linking current structure and trends to specific processes is a crucial first step towards devising focused strategies for conserving and re-building depleted marine populations.

我们评估了美国加利福尼亚中部两种数量减少的海洋软体动物——红鲍（Haliotis rufescens）和黑鲍（H. cracherodii）的存续情况以及可能的恢复情况。在32年的监测中发现，在没有捕捞的情况下，过去三十年里红鲍的数量和大小没有呈现出增加或减少的趋势。对至少保护了25年的海洋保护区和公众可自由进入的区域进行比较后发现，黑鲍的大小结构存在显著差异，壳长大于8厘米的个体在保护区内占动物总数的14% - 37%，在公众可自由进入的区域则占2% - 11%，并且保护区内红鲍的数量有增多的趋势。尽管没有增长趋势，但在一个禁捕保护区——霍普金斯海洋生物保护区（HMLR）内的保护措施使得红鲍种群历经多代得以存续，平均密度为0.2只/平方米。在HMLR内和其他地方的其他区域，红鲍的密度低于进行长期研究的区域（平均0.03只/平方米），且比黑鲍的密度低一个数量级（平均0.4只/平方米）。这些结果表明，持续关闭渔场以及在禁捕保护区内进行保护是使鲍鱼种群得以存续的有效手段，尽管没有迹象表明其能恢复到渔业崩溃前的水平。这种未能恢复的情况很可能与高自然死亡率以及可能持续存在的非法捕捞有关，但与其他地方鲍鱼种群数量低的潜在原因无关，这些原因包括食物或栖息地受限、繁殖失败或疾病。将当前的结构和趋势与特定过程联系起来，是制定有针对性的保护和重建数量减少的海洋种群策略的关键的第一步。