Persistence of depleted abalones in marine reserves of central California

摘要

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/mpo. 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/mpo), and an order of magnitude lower than for black abalone (av. 0.4 animals/mpo). 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.